Dissertation. Michael Deilmann. Bochum, 2008

Transcription

1 RUHR-UNIVERSITÄT BOCHUM Dissertation Silicon oxide permeation barrier coating and sterilization of PET bottles by pulsed low-pressure microwave plasmas Michael Deilmann Bochum, 28 Dissertation zur Erlangung des Grades eines Doktor-Ingenieurs der Fakultät für Elektrotechnik und Informationstechnik an der Ruhr-Universität Bochum

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5 Dissertation eingereicht am: Tag der mündlichen Prüfung: Berichter: Prof. Dr.-Ing. Peter Awakowicz 2. Berichter: Prof. Dr. rer. nat. Jörg Winter

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7 Abstract Modern packaging materials such as polyethylene terephthalate (PET) offer various advantages over glass or metal containers and are gaining in importance for food and beverage packaging. PET bottles are nonbreakable and light weight compared to established materials, but they only offer minor barrier properties against gas permeation. Therefore, the shelf-life of packaged food and beverages is limited. Additionally, common sterilization methods like heat, hydrogen peroxide or peracetic acid may not be applicable due to reduced heat or chemical resistance of the plastic material. In the field of plasma sterilization, a process is developed on the basis of investigations into plasma sterilization mechanisms. Two relevant test spores for food packaging applications are considered and the capability of an optimized plasma for plasma sterilization of PET bottles is demonstrated. The achieved sterilization efficiency with treatment times below five seconds is in accordance with regulations of FDA (Food and Drug Administration, USA) and VDMA (Verband Deutscher Maschinen- und Anlagenbauer e.v., Germany). Furthermore, the developed process is characterized concerning phase and spatially resolved electron temperature and density profiles. Additionally, absolutely calibrated emission spectroscopy is applied for process optimization. For the permeation barrier coating of PET substrates, a silicon oxide (SiO x ) barrier coating based on oxygen diluted hexamethyldisiloxane (HMDSO) plasmas is investigated. To achieve homogeneous treatment of three dimensional substrates, a pulsed plasma process is developed for SiO x film deposition. A criterion correlating pulse conditions and residence times of the process gases in the packaging is deduced allowing for a process scale-up. The influence of all relevant plasma conditions, namely gas composition, flow rates, process pressure and pulsed microwave power properties is investigated in detail. Their effect on barrier properties, surface morphology and coating composition are analyzed. It can be stated, that good permeation barrier coatings have to exceed a critical thickness of 6 nm, are carbon free and as smooth as possible. Besides the coating analysis, the neutral gas composition and the ion bombardment onto the surface are considered by means of mass spectrometry. It is revealed that the deposition process of SiO x films is based on the deposition of SiO x C y H z like HMDSO fragments on the surface and oxygen etching to form SiO x. As oxidation products mainly CO 2,OHandH 2 O are observed. Furthermore, the permeation mechanisms of deposited films are analyzed revealing defect permeation as a dominant mechanism for the observed residual permeation. These defects are visualized by means of capacitively coupled oxygen etching of deposited barrier layers. Concluding, a plasma sterilization process is successfully developed and characterized for the treatment of PET bottles. It is easily combinable with a permeation barrier coating, which is developed and characterized.

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9 Contents Contents List of Figures List of Tables i iii v Introduction Surface wave excited plasmas 5. Plasma ignition by means of a Plasmaline antenna Pulsedplasmas Experimental setup 2. Reactorsetup Generatorcharacteristics Automatizationofreactorsetup Parameterranges Plasmadiagnosticmethods Langmuir probemeasurements Energymassspectrometry Opticalemissionspectroscopy Coating analysis Permeationmeasurement Fouriertransforminfraredspectroscopy Stylusprofilometry Atomicforcemicroscopy Scanning electron microscopy, Energy dispersive x-ray spectroscopy Microbiologicalmethodology Fraunhofer Institute for Process Engineering and Packaging Countreductiontest Substrate materials Polyethyleneterephthalate Silicon wafer Glass Characterization of reactor setup 3 3. Continuouswavecharacterization Electron density and electron temperature Ionenergydistributionfunctions Power influence on electron density and electron temperature Pulsedcharacterization i

10 ii Contents 3.2. Electron density and electron temperature Ionenergydistributionfunctions Influence of reactor coating on plasma parameters Plasma sterilization for aseptic filling of beverages Classification and definition of aseptic filling Demands of aseptic filling Established sterilization processes for aseptic packaging Selection of test spores and artificial contamination Mechanisms of plasma sterilization Definitionofplasmaparameters Characterization of sterilization plasma Sterilization results Sterilization of specimen holders Sterilization of PET bottles Conclusion Permeation barrier coating Mathematicaldescriptionofpermeation Plasmapolymerizationofhexamethyldisiloxane AnalysisofHMDSOvapor Analysisofions Fragment ambiguities of oxygen diluted HMDSO plasmas Pulsedcoatingdeposition DevelopmentofbarrierlayersystemonPETfoils Adhesionpromotinglayer Influence of oxygen dilution during barrier coating deposition Influence of pulse power and duration Influence of process pressure Influence of layer thickness on barrier properties Optimized plasma parameters for deposition of barrier layer system Investigationofpermeationmechanisms Analysisofcoatingdefects Discussion of permeation mechanisms Coating of bottles Conclusion...2 Summary 3 Bibliography 5

11 List of Figures. Illustration of cylinder symmetric inner and outer surface wave configuration 5.2 Electron density and electrical field for Plasmaline configuration Schematicofthereactorsystem Noise of microwave generator Power characteristic of microwave generator Scheme of Langmuir probesetup Schematicofenergymassspectrometer Ion energy distribution functions regarding aberrations Mass dependent transmission function of HIDEN EQP Permeationmeasurementsetup FTIRspectroscopysetup Penetration depth of infrared beam into coating versus wave number Gaussian fits to FTIR absorption spectrum of plasma polymerized SiO x film Polyethyleneterephthalate FoilcarrierfortreatmentofPETfoils FTIR absorption spectrum of PET and peak assignment Radial electron density and electron temperature profiles Axial Electron density and electron temperature profiles Ion energy distribution functions of cw argon plasmas Influence of power on electron density and electron temperature Time resolved electron density and electron temperature profiles Time resolved ion energy distribution functions for various process pressures Time resolved count rate of Ar + ions Time dependent electron density behavior and ion flux of Ar + ions Time dependent ion flux of Ar + ions of argon and argon:oxygen plasma Influence of SiO x coating of the reactor chamber on potentials SEM pictures of B. atrophaeus and A. niger Radial and temporal behavior of electron density Optical emission spectrum of H 2 :N 2 :O 2 plasma Time dependent reduction of B. atrophaeus and A. niger on specimen holders Time dependent reduction of B. atrophaeus insidepetbottles Illustration of four steps of permeation Boundary conditions and steady state solution of polymer film permeation Measurement and simulation of time dependent oxygen flux ChemicalstructureofHMDSO Schematic of plasma polymerization of HMDSO FragmentationpatternofHMDSO... 6 iii

12 iv List of Figures 5.7 FragmentationpathsofHMDSO Influence of electron impact energy on fragmentation pattern of HMDSO Deposition rate of SiO x coatings for various inter-pulse durations SEM pictures of SiO x coatingsonpristinepet Permeation rates for various adhesion promoting layers FTIR absorption spectrum of adhesion promoting layer Ions spectrum during deposition of adhesion promoting layer Permeation rates of SiO x coatings for various oxygen fluxes EDX investigation of layer composition for various oxygen fluxes FTIR absorption spectra of SiO x coatings for various oxygen dilutions Peak position of SiCH 3 vibrationsforvariousoxygenfluxes Ratios r SiCH3 and r SiOH forvariousoxygenfluxes Surface morphology of SiO x coatings for various oxygen fluxes Normalized mass spectra of neutrals for various oxygen fluxes Relative behavior of neutrals for various oxygen fluxes Normalized mass spectra of positive ions for various oxygen fluxes Relative behavior of ions for various oxygen fluxes Optical emission spectrum of deposition plasma Behavior of optical emission related to significant ions and molecules Permeation rates of SiO x coatings for various pulse powers FTIR absorption spectra of SiO x coatings for various pulse powers Surface morphology of SiO x coatings for various pulse powers Permeation rates of SiO x coatings for various pulse durations FTIR absorption spectra of SiO x coatings for various pulse durations Normalized mass spectra of neutrals for various pulse powers Relativebehaviorofneutralsforvariouspowers Normalized mass spectra of neutrals for various pulse durations Relative behavior of neutrals for various pulse durations Fragmentation rates of HMDSO for variation of power and pulse duration Normalized mass spectra of positive ions for various pulse powers Relativebehaviorofionsforvariouspowers Normalized mass spectra of positive ions for various pulse durations Relativebehaviorofionsforpulsedurations Time resolved relative optical emission during microwave power pulse Permeation rates of SiO x coatings for various process pressures FTIR absorption spectra of SiO x coatings for various process pressures Ratios r SiOH and r ss forvariousoxygenfluxes Surface morphology of SiO x coatings for various process pressures Permeation rates of SiO x coatings for various coating thicknesses Arrhenius plotofoxygenpermeation Activation energy of oxygen permeation for various oxygen fluxes SEM pictures of SiO x coatings on PET after etching in CCP oxygen plasma Visualizationofdifferentpermeationmechanisms SEMpicturesofdifferentcoatingdefects Permeation rates of coated PET bottles as function of pump area Permeation rates of bottle walls as function of pump area SimulatedpressuresinPETbottles...

13 List of Tables 2. Parameter ranges of microwave reactor setup Electron impact ionization cross sections of the rare gases Infrared absorption peaks in FTIR spectra of SiO x C y H z like films Decay times for various pressures in the afterglow Minimum requirements of germ reduction for aseptic packaging machines Recommended test spores for various sterilization mediums Relative intensities of fragments of HMDSO Bond energies of HMDSO and electronegativity of the elements Optimized plasma parameters for deposition of barrier layer system v

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15 Introduction Modern packaging materials such as polyethylene terephthalate (PET) offer various advantages over glass or metal containers and are gaining in importance for food and beverage packaging. PET bottles are nonbreakable and light weight compared to established materials, but they only offer minor barrier properties against gas permeation. Therefore, the shelf-life of packaged food is limited. Additionally, common sterilization methods like heat, hydrogen peroxide or peracetic acid may not be applicable due to reduced heat or chemical resistance of the plastic packaging material. Industrial scale applications cope these disadvantages using e.g. gas scavengers or blends as part of the PET melt to reduce the gas permeation or even develop multilayer bottles. These bottles consist of several layers, where at least one layer is a high barrier material, such as ethylene vinyl alcohol (EVOH), responsible for a reduction of gas permeation. The production of these multilayer bottles is challenging, because a homogeneous distribution of the high barrier material has to be ensured during the production of a bottle preform and the stretch blow molding process. Additionally, the usage of various barrier materials can lead to problems of bottle recyclability due to an increased amount of impurities within the PET raw material. In contrast, different plasma based coating processes are topic of ongoing investigations to decrease the permeation through PET for different applications like foils or bottles [, 2, 3]. Plasma polymerized layers are deposited on the packaging material as permeation barriers. E.g. amorphous carbon hydrogen (a-c:h) coatings are known to act as good permeation barriers for food packaging applications, but they tend to show a light brown color on the substrate. In comparison, plasma polymerized silicon oxide (SiO x ) coatings have advantageous properties concerning transparency, recyclability and microwave use. Thus, they are gaining in importance in industrial processes. The influence of plasma parameters on permeation properties of SiO x coatings is investigated by different groups for various setups like capacitive [4, 5, 6] or microwave plasmas [3, 7] and are employed for permeation barrier coatings. Additionally, microwave plasmas in combination with biased substrate holders [8, 9,,, 2, 3, 4] or magnetic field assisted plasma enhanced chemical vapor deposition of permeation barriers [5, 6, 7, 8] are investigated. An expanding thermal plasma can also be used for the deposition of high performance SiO x coatings on flat samples [9, 2, 2, 22, 23, 24]. Typically, these investigated plasmas are used for a continuous deposition of barrier coatings on flat samples like packaging foils by means of silicon containing monomers, e.g. hexamethyldisiloxane (HMDSO). Contrary, pulsed plasmas gain in importance for industrial processes and allow for a further degree of freedom by varying the pulse parameters. The pulsed deposition of SiO x coatings is established for diverse applications [25, 26, 27, 28, 29], which show major influence of the pulse parameters on coating properties. Therefore, the

16 2 Introduction investigation of pulsed plasmas for the deposition of permeation barrier coatings is promising. Typically, pulsed plasmas are characterized by a pulse frequency and a duty cycle. A more detailed investigation is required to understand the correlations of plasma pulsing and layer deposition for process optimization and scalability. In the field of packaging material sterilization, various methods are established mainly based on the usage of toxic chemicals like hydrogen peroxide or peracetic acid. The influence of these substances on the packaging material are not extensively investigated. Furthermore, residues of the sterilants can interact with packaging materials leading to modifications of packaging composition, color or transparency and to the production of off-flavors. Objective For the plasma treatment of bottle shaped packaging materials, a microwave driven low-pressure plasma reactor system is developed based on a Plasmaline antenna [3]. It allows for the ignition of a plasma inside bottles for various purposes based on the propagation of outer surface waves along an antenna. The objective of this thesis is to develop and characterize a combined sterilization and permeation barrier coating process for PET packaging materials. In detail, in the field of plasma sterilization, a plasma process is developed in accordance with today s regulations of validation of aseptic filling machines. Therefore, the results of investigations of sterilization mechanisms by means of low-pressure plasmas as performed by Halfmann et al. [3, 32] and the European BIODECON project [33] are adapted for the sterilization of PET bottles and the process is characterized in terms of plasma density, electron temperature and optical emission. The deposition of transparent permeation barrier silicon oxide coatings represents the main focus of this thesis. They are designed to reduce the oxygen permeation of PET packaging materials and to allow for a homogeneous substrate treatment. The influence of the deposition parameters namely oxygen dilution, pulse power, process pressure and pulse conditions on the barrier properties, layer composition and morphology are investigated. Additionally, the deposition plasma is analyzed regarding composition and surface ion bombardment and insights into the deposition chemistry are revealed. Outline of the thesis The first chapter Surface wave excited plasmas describes the realization of plasma ignition by means of surface waves and concentrates on the plasma creation by means of a Plasmaline antenna. The experimental setup is described in chapter 2 regarding the description of the reactor setup, the plasma diagnostic methods, the coating diagnostics and the microbiological methodology. Additionally, the used substrate materials are introduced. The plasma is characterized in chapter 3 considering electron density and electron temperature as well as ion energy distribution functions of continuous and pulsed argon plasmas. Chapter 4 Plasma sterilization for aseptic filling of beverages describes the development of a plasma based sterilization process based on the fundamental research of sterilization mechanisms. The capabilities of this new sterilization method are discussed and a treatment according to the requirements for validation of aseptic filling machines is designed.

17 Introduction 3 The development of a permeation barrier coating system on PET constitutes the focus of chapter 5. A mathematical description of time dependent permeation is presented allowing for the determination of permeation constants of silicon oxide coatings. The plasma polymerization of hexamethyldisiloxane as silicon containing monomer is afterwards discussed and the fundamentals of plasma enhanced chemical vapor deposition processes based on organic monomers are introduced. Thereafter, a criterion for the homogeneous deposition of barrier coatings depending on the pulse parameters of a hexamethyldisiloxane-oxygen plasma is deduced. The design of a permeation barrier coating on PET foils builds the main focus of this thesis. It is discussed in section 5.4. The influence of main plasma parameters on coating properties and plasma composition is investigated and insight into the process chemistry are revealed. It is followed by the investigation of the permeation mechanism of plasma polymerized barrier coatings regarding coating defects and bulk permeation. Finally, the results of the adaption of the developed barrier coating system for the coating of PET bottles is discussed.

18 4 Introduction

19 . Surface wave excited plasmas Electrical gas discharges are utilized for a wide range of application such as microelectronics, material processing or lightning. Various kinds of electrical coupling like capacitive or inductive are established using different frequencies ranging from direct current to several Megahertz. Beside these kinds of plasma, surface wave excited plasmas are investigated since the 97s and gain in importance for e.g. surface processing applications. Moisan et al. [34] review surface wave sustained plasmas driven in a frequency ranging from MHz to GHz. Typically a microwave frequency is used for the ignition of the plasma. Particularly, a frequency of f =2.45 GHz is widely utilized due to the usage of microwave magnetrons for other applications like microwave ovens resulting in reduced equipment costs. Surface wave sustained plasmas can be easily operated and are not affected by changes in the discharge conditions and plasma parameters [35]. With a proper designed wave launching system, the propagation of the surface wave is monomode allowing for excellent reproducibility of the plasma properties like electron density and temperature [34, 35]. Additionally, surface wave excited plasmas are characterized by an extraordinary flexibility in terms of the applied frequency, the gas pressure range and usage for manifold applications. Moisan et al. developed the surfatron in the 97s as a first simple and efficient surface wave launcher for the generation of long plasma columns at microwave frequencies [34, 36]. It consists of an evacuated quartz tube, in which a process gas is ignited by means of an electrical wave launching system adapted to the outside of the tube. The plasma is sustained due to an inner surface wave propagating along the inner surface of the quartz tube as visualized in figure.(a). metallic wave guide air (atmospheric pressure) quartz tube plasma (low pressure) metallic wave guide air (atmospheric pressure) quartz tube plasma (low pressure) (a) Inner surface wave excited plasma. (b) Outer surface wave excited plasma. Figure.: Illustration of cylinder symmetric inner and outer surface wave configuration [37]. 5

20 6 Chapter Surface wave excited plasmas. Plasma ignition by means of a Plasmaline antenna Räuchle [37] developed an outer surface wave sustained plasma called Plasmaline based on a kind of /r transformation of the inner surface wave system leading to a plasma ignition in the outer area of a quartz cylinder as shown in figure.(b). A metal rod is surrounded by a quartz cylinder building a microwave antenna for the plasma ignition in the outer volume of the quartz cylinder within an evacuated vessel. Electromagnetic waves propagate mainly within the tube along the antenna and within the plasma as radially decaying surface waves [37]. This plasma generation is comparable to the propagation of electromagnetic waves in a coax cable. The inner conductor is represented by the Plasmaline antenna and the outer is built by the reactor vessel for a cylinder symmetric system. In between these conductors, the media can be described as dielectrics influencing the wave propagation and the characteristics of the electromagnetic field. In dependence on the radius r, the axial elongation z and the azimuthal direction ϕ, a time dependent propagation of the electric field can be described by [37] E r E(r, z, ϕ, t) = E z = E (r) e i(kz ωt), (.) E ϕ where k and ω denote the complex wave number and the frequency ω =2πf, respectively. Additionally, symmetry is assumed regarding the azimuthal angle ϕ for the cylinder symmetrical system. Based on Maxwell s equations E = µ H t and H = ɛ E t + J, (.2) a wave equation (.3) is derived describing the wave propagation [37, 38] ( E ) = µ ɛ ω 2 ɛ r (r) E. (.3) Solving this equation for the wave form described by equation (.), a differential equation for the radial and axial component of the electric field, E r and E z, respectively, is derived to be [37, 38] 2 E r r ( r + ) ɛ r (r) Er ɛ r (r) r r { µ ɛ ω 2 ɛ r (r) k 2 r + 2 ɛ r (r) [ 2 ɛ r (r) r 2 ( ) ]} 2 ɛr (r) E r = r and (.4) ( ) E z = µ ɛ µ ɛ ω 2 ω 2 rɛ r (r)e r. (.5) ɛ r (r) r r Based on this description, the wave propagation in a coaxial arrangement consisting of an inner and outer conductor separated by a homogeneous dielectric ɛ r (r) =const is revealed to be ɛ r (r) =const E r r and E z = and E ϕ =. (.6)

21 . Plasma ignition by means of a Plasmaline antenna 7 The radial component of the propagating wave E r shows an /r characteristic leading to a vanishing E z as described by equation (.5). Therefore, a transversal electromagnetic wave (TEM) is propagating along the antenna for a homogeneous dielectric. In contrast, the characteristic of the electromagnetic wave becomes more complex considering a coaxial arrangement with an ignited plasma in between the two conductors. For the wave propagation within a plasma, a complex dielectric permittivity ɛ r depending on the plasma parameters has to be taken into account. The dielectric constant of a plasma can be derived to be [39] ɛ r = ω 2 pe ω 2 ( i ν ω ), (.7) where ω pe and ω denote the electron plasma frequency and the microwave frequency ω = 2πf, respectively. Additionally, the electron collision frequency ν describes the momentum transfer between electrons and neutrals. The electron plasma frequency is given by [39] n e e ω pe = 2. (.8) ɛ m e Therefore, the plasma density n e strongly influences the electron plasma frequency ω pe,the dielectric constant ɛ r and the electrical field distribution in the plasma volume depending on the spatial coordinates. By means of this relation, a critical density n e,crit can be determined leading to ɛ r = for neglected friction conditions ν = Hz [37]. For a microwave plasma with f =2.45 GHz, the critical density is n e,crit = m 3 for ω = ω pe and ɛ r =. These conditions lead to a strong influence on the wave propagation properties of electric fields due to a vanishing dielectric constant. E r, n e inner conductor quartz cylinder E r n e n e,crit r r Figure.2: Electron density n e profile and radial electrical field component E r for Plasmaline configuration [38]. The level of the critical electron density n e,crit is indicated. Figure.2 illustrates the characteristics of the radial component of the electric field E r and the electron density n e for a cylinder symmetric arrangement. Additionally, the value of the critical electron density n e,crit is indicated. As it will be determined in chapter 3, the plasma density n e exceeds this critical density n e,crit as shown in figure.2 for the investigated experimental conditions. The radial component of the electric field E r shows an /r characteristic within the space between the powered inner conductor and the quartz

22 8 Chapter Surface wave excited plasmas cylinder due to a constant ɛ r. Due to the continuity condition of the normal component of the electrical displacement field ɛ r (r) E r (r) =ɛ r (r + r) E r (r + r), (.9) discontinuities of E r are observed at the boundaries of the quartz cylinder. For the quartz cylinder, the dielectric constant is approximately ɛ r =3.8 [37, 38]. Therefore, the electric field drops within the material. The electron density n e increases for increasing radiuses r, reaches a maximum and then vanishes for further increasing radiuses. Thereby, the level of the critical density n e,crit is passed twice. As afore mentioned, the dielectric constant ɛ r of the plasma vanishes at the position of the critical density n e,crit leading to a peak of the electric field due to continuity condition (.9) at the radiuses, where n e (r) =n e,crit. Typically, this peak is observed only for the smaller radiuses, where n e (r) =n e,crit, because the electron density gets much higher than the critical density (n e n e,crit ) leading to strong shielding effects for the electrical field. For the second position, the electrical field is already vanished and typically, no second peak occurs for high density plasmas [37]. The axial component of the electric field E z can be calculated for the revealed radial component by means of equation (.5). Due to a complex characteristic of E r, the axial component E z does not vanish as previously described for homogeneous dielectric materials. Therefore, an axial component E z is present leading to the propagation of a transverse magnetic (TM) wave along the Plasmaline antenna. The axial component E z undergoes damping phenomena and decreases along the antenna depending on the applied power. Therefore, the elongation of the plasma can be adjusted by the applied power and plasma conditions. Due to the damping of the microwave power along the antenna, a nearly linear decrease of the electron density is observed and theoretically described along the axial direction of the antenna for a Plasmaline setup [37, 35, 34]. Therefore, Räuchle et al. developed a Duo- Plasmaline for a homogeneous treatment of webs. Two microwave magnetrons are used at both sides of an antenna to realize a homogeneous electron density profile along the antenna by superimposing the two sources [37, 4, 4, 42, 43, 44, 45, 46]. In addition to the described wave propagation comparable to a coax cable, there exists a series of other TM modes able to propagate along the antenna. But due to the direct coupling of the wave launcher to the inner conductor of the antenna, the coax cable mode is exited in preference according to the theoretical description within this chapter [37]. A further description of the wave penetration of a surface wave exited plasma can be given by an analysis of the skin depths δ [39, 47]: δ = c ω Im{ɛ /2 r }, (.) where c denotes the speed of light c =(µ ɛ ) /2. For low-pressure microwave plasmas, the electron plasma frequency ω pe is much higher than the applied microwave frequency ω and the collision frequency ν [47]. Regarding equation (.7) and (.), the skin depth can be simplified for ω ν to be [39, 47] δ c me = ω pe µ n e e. (.) 2

23 .2 Pulsed plasmas 9 Typically, the skin depth δ is below δ 9.5 mm for a microwave plasma at f =2.45 GHz. Therefore, an estimate for the position of the maximum of the electron density as illustrated in figure.2 is revealed..2 Pulsed plasmas Typically, electrical discharges are driven in a continuous wave (cw) mode by applying the desired frequency. Recently, pulsed plasmas gain in importance for manifold processes like etching or deposition applications and constitute a fundamental topic considered from a scientific and technological point of view. A pulsing of the applied power leads to a further degree of freedom and represents a huge advantage over cw plasmas [48]. Typically, pulsed plasmas are characterized by a pulse frequency and a duty cycle describing the repetition rate and the relation between pulse duration and inter-pulse period, respectively. Variations of these two additional external controllable parameters allow for influencing the plasma and substrate parameters, e.g. deposition rate, film composition, etch selectivity, dust formation or heat load. Furthermore, charging effects of substrates can be handled responsible for damaging of processes micro-structures. Bousquet et al. [25, 26, 49] investigate e.g. the influence of plasma pulsing on the layer properties of deposited SiO x films. They show that the pulse parameters significantly determine the film properties like film composition and refractive index of the coating. Bouchoule et al. [5] show the influence of pulse parameters on the particle formation of an argon silane plasma. They reveal a strong correlation between the pulse parameters and the dust formation within the plasma responsible for undesirable dust particles deposited on the substrate. Additionally, Wenig et al. [5, 52] investigate in detail the behavior of electron density and electron temperature during the afterglow of pulsed plasmas. Especially, the behavior of electron reheating in the afterglow is investigated responsible for measurable residual energies of electrons in the late afterglow. They show, that in pulsed discharges, the mean electron energy can be tuned to influence the plasma chemistry, whereas in cw discharges, the mean electron energy is fixed by the gas composition, the neutral gas pressure and the discharge geometry. Therefore, the electron temperature, that is correlated to the mean electron energy for an assumed Maxwell distribution of the electrons, can be strongly influenced by means of power modulated discharges. As previously discussed, the axial elongation of plasma along a Plasmaline antenna is influenced by the applied microwave power. A certain level of power has to be applied for a homogeneous plasma and surface treatment. Plasma pulsing allows for a homogeneous ignition during the pulse duration by applying higher pulse powers. Furthermore, plasma pulsing enables to reduce the mean power applied to the system by reducing the duty cycle. This capability can be important for the treatment of thermolabile materials, such as plastics or even biodegradable polymers, to reduce the heat load of the substrates [53]. Concluding the properties of surface wave exited plasmas, they are characterized by a high electron density leading to high plasma activity in terms of e.g. deposition rate, light emission and chemical activity. The power modulation of plasmas allows for a proper tuning of plasma and coating properties and gives two further parameters in terms of the duty cycle and the pulse frequency. Therefore, pulsed surface wave excited plasma are used within this thesis for the treatment of PET bottles and foils for sterilization and permeation barrier coating purposes.

24 Chapter Surface wave excited plasmas

25 2. Experimental setup The experimental setup for the treatment of PET bottles and foils is introduced in this chapter. The ignition of a plasma in a bottle is based on the propagation of an outer surface wave along a Plasmaline antenna as fundamentally described in chapter. Furthermore, the applied plasma diagnostics and the substrate analysis tools are described in this chapter, which are used within this thesis. The last section characterizes the substrate materials used for the sterilization and barrier coating process. 2. Reactor setup Diagnostic flanges (here e.g. Langmuir probe) z z process gases inner quartz tube copper tube outer quartz tube Plasmaline antenna bottle cage gas connectors Figure 2.: Schematic of the reactor system including definition of z-axis and positions of radial and axial Langmuir probe measurements (height of vacuum vessel: 4 mm, diameter of vacuum vessel: 4 mm, diameter of bottle cage: 85 mm, diameter of Plasmaline antenna: 2 mm).

26 2 Chapter 2 Experimental setup The experimental setup for plasma treatment of PET bottles and foils is schematically shown in figure 2. [54]. It is composed of a vacuum chamber with a volume of 6 l and is capable of treating various bottle sizes up to.5 l. The reactor chamber has a height of 4 mm and a diameter of 4 mm. The bottles are inserted upside down in the vacuum chamber and fixed by a metal bottle cage, which has a diameter of 85 mm for the considered one liter bottles. In case of treating PET foils, a carrier is used, which fixes the substrate at the position of a bottle wall inside the metal bottle cage. The reactor chamber can be evacuated to a base pressure of. Pa. Microwave power is applied to the system by means of a modified Plasmaline antenna. This antenna consists of a copper tube with surrounding inner and outer quartz tube. The copper tube forms the inner conductor of a coaxial wave guide and acts as a microwave antenna. The plasma extends along the outer quartz tube due to the propagation of outer surface waves and forms the outer conductor by itself [37, 55]. Furthermore, the inner quartz tube is used to provide process gases, which flow through it into the reactor system. Liquid HMDSO is evaporated as process gas for deposition of barrier coatings and is fed into the chamber as a mixture with oxygen. The gas tubes are heated to prevent condensation of HMDSO. The vacuum system is pumped by a combination of a rotary and a roots pump whereas a gate valve is responsible for pressure control. 2.. Generator characteristics A microwave power source (Muegge Electronic GmbH, Reichelsheim, Germany) provides microwave energy at f =2.45 GHz with a maximum power of P cw = 2 kw. The source is capable of being pulsed within ms t on 24 ms and ms t off 25 ms, where t on and t off denote the pulse duration and the inter-pulse period, respectively. The generator is equipped with a trigger pulse output for time resolved measurements. Generator noise 5 I / ma t / ms Figure 2.2: Noise of microwave generator. The microwave generator is not a research grade generator to be comparable to industrial scale processing applications. Therefore, a noise signal is present, which has to be analyzed prior to the measurements. Figure 2.2 shows the current signal of an electrical probe at a potential of Φ = 4 V during a cw plasma ignition to illustrate the generator noise.

27 2. Reactor setup 3 A periodic noise signal is observed. It is characterized by a frequency of f = 3 Hz (T =3.3 ms) due to the commutation of the three phase net signal (f =5Hz)bymeans of a rectifier circuit. Power characteristic For a time resolved investigation of the power characteristics of the microwave generator, the matching system is tuned to reflect nearly all power. The reflected microwave power is measured by means of a microwave detector (Muegge Electronic GmbH, Reichelsheim, Germany) connected to an oscilloscope. Exemplarily, the time resolved behavior of the microwave generator for a t on = 4 ms pulse is illustrated in figure 2.3. A transient characteristic 2.5 normalized voltage signal t/ms Figure 2.3: Power characteristic of microwave generator for t on = 4 ms and t off =4ms. is observed within ms t ms. The generator power drops twice at t =.25 ms and t =.5 ms. The initial maximum is reached after t =. ms and a second maximum is observed at t =.35 ms. These characteristic behavior has to be kept in mind for the time resolved investigations as performed in the following chapters Automatization of reactor setup The reactor setup is completely computer controlled by means of a Labview [56] program. It allows for time dependent control of all external parameters, such as gas flows, power settings and pressure control. Additionally, recipes can be defined for complex processes, which are executed computer controlled. A process protocol records all parameters for later investigations Parameter ranges Table 2. summarizes the possible parameter ranges for operation of the experimental setup for sake of clarity.

28 4 Chapter 2 Experimental setup parameter symbol range cw power P W P 2 W pulse power P W P 4 W pulse duration t on ms t on 24 ms inter-pulse period t off ms t off 25 ms working pressure p Pa p Pa HMDSO gas flow Φ HMDSO sccm Φ HMDSO 5 sccm oxygen gas flow Φ O2 sccm Φ O2 8 sccm nitrogen gas flow Φ N2 sccm Φ N2 2 sccm hydrogen gas flow Φ H2 sccm Φ H2 sccm argon gas flow Φ Ar sccm Φ Ar 2 sccm Table 2.: Parameter ranges of microwave reactor setup. 2.2 Plasma diagnostic methods 2.2. Langmuir probe measurements For the analysis of electrical discharges, the determination of the electron density n e and the mean electron energy Ēe is essential, where Ēe is represented by the electron temperature T e for an assumed Maxwell distribution of the electron. Electrical probe measurements according to Langmuir [57, 58, 59] allow for a time and spacial resolved measurement of these important parameters. The measurements are based on the analysis of a voltagecurrent characteristic of the discharge using the orbital motion limited (OML) theory for the movement of electrons and ions towards a cylindrical probe tip as described in [6]. The Langmuir probe used in this thesis is a tungsten wire with a diameter of 5 µm and a length of 5 mm. A voltage-current characteristic is measured by means of an electrical setup as schematically shown in figure 2.4 [6]. A voltage ramp is applied to the probe tip related to the continuously measured floating potential of the plasma and the probe current is measured. Afterwards, the voltage-current characteristic is analyzed and statistically evaluated as described by Wenig and Schulze [5, 62] to determine the electron density n e, the electron temperature T e, the plasma potential U pl and the floating potential U fl. low-pass filter f =Hz-kHz g floating potential amplifier V= probe tip floating probe band-stop filter probe current; < ma - ma ramp voltage -8V - +8V floating potential 6 Bit 6 Bit 2 Bit control unit Figure 2.4: Scheme of Langmuir probe setup.

29 2.2 Plasma diagnostic methods 5 Druyvesteyn [63] gives a relation of the measured characteristic of the electron current I e and probe voltage U relative to plasma potential U pl and the electron energy distribution function f e (E) 8me E d 2 I e f e (E) = Ae 3 du 2, (2.) E=eU where A denotes the surface area of the probe. The electron density n e and the mean electron energy Ēe can be determined using the moments µ k of the electron energy distribution function: n e = µ and Ē e = µ µ for µ k = E k f e (E)dE. (2.2) The electron temperature T e is revealed under the assumption of a Maxwell distribution by T e = 2 3k B Ē e. (2.3) Using this theory for the evaluation of the Langmuir probe characteristics, a phase and spatial resolved measurement of these important plasma parameters is possible. Within this thesis, the spatial measurements are performed radial and axial at position of z = 5 mm and radius of r = 28 mm, respectively (cp. figure 2.). They characterize the profiles and homogeneity of the low-pressure surface wave excited plasma. Due to the noise of the microwave generator as illustrated in figure 2.2, the measurements of the current-voltage characteristics are averaged regarding the measurement time of 2 µs per voltage step and the generator noise of f = 3 Hz. The time resolved measurements are also performed with a time resolution of 2 µs and averaging over about 2 characteristics Energy mass spectrometry Energy resolved mass spectrometry is a versatile diagnostic method to analyze the process chemistry of plasmas. In this work, energy mass spectrometry is applied for the analysis of neutrals and ions during barrier film deposition regarding their composition and to determine energy distribution functions of ions. A HIDEN EQP 3 [64] (HIDEN Analytical Ltd., Warrington, UK) spectrometer is used, which is schematically illustrated in figure 2.5. It allows for a mass selective measurement to determine the intensities of ions or neutrals versus the mass related to the charge of the species as expressed by m/z. The spectrometer is mounted at the same heights as radial Langmuir probe measurements are performed (cp. figure 2.). A sampling orifice with a diameter of d orifice =6µm separates the vacuum inside the spectrometer (typically p Pa) and the process chamber. The sampling orifice is mounted in-plain of the metal bottle cage and is electrically grounded. The system is pumped by a combination of a membrane pump and a turbo molecular pump. The HIDEN EQP is equipped with an ion extraction system for the analysis of ions produced in the plasma and an electron impact ionization source emitting and accelerating The three-character symbol m/z is used to denote the dimensionless quantity formed by dividing the mass of an ion by the unified atomic mass unit and also by its charge number (regardless of sign) [65].

30 6 Chapter 2 Experimental setup orifice axis ionization source lens extractor lens2 energy filter mass filter detector Figure 2.5: Schematic of Energy mass spectrometer HIDEN EQP 3 [64]. electrons perpendicular to the line of sight of the sampling orifice. Therefore, neutrals can be ionized inside the mass spectrometer at various conditions and be detected. For the neutral gas analysis, the produced ions are accelerated to an energy of.8 ev and drift to lens2, where they are matched to the energy filter for efficient ion transfer [64]. During neutral gas analysis, a positive potential is applied to the ion extraction system to repel positive ions of the plasma. The filament current is held at a low level, typically below µa, to prevent thermal dissociation effects. The energy filter of the EQP 3 consists of a 45 sector field energy analyzer and the mass separation is realized by a quadrupole mass filter. The detector is based on a continuous dynode electron multiplier (channeltron). The mass spectrometer is heated prior the measurements to reduce the residual amount of water inside the spectrometer influencing the signal of hydrogen and oxygen containing molecules. Additionally, the filament temperature is chosen as small as possible and constant by means of low filament currents during the measurements to prevent thermal influences on e.g. molecule dissociation. Tuning of the mass spectrometer for ion analysis In case of extracting ions produced in the plasma by means of electrical filters, effects due to different trajectories of ions inside the mass spectrometer have to be taken into account. The extraction unit of the EQP 3 consists of an electrode extractor with tuneable negative potential for positive ion analysis (cp. figure 2.5). Therefore, positive ions from the plasma are accelerated into the mass spectrometer and refocused by means of lens onto the exit aperture of the electron-impact ionization source [64]. Similar to chromatic aberration phenomena of optical lenses, focusing effects of electrostatic lenses occur due to different ion energies. This aberration is observed, if the position of the focal length of an electrostatic lens is changed by various energies of charged particles leading to beam focusing at different positions. For the investigation of ion energy distribution functions of plasmas by means of energy resolved mass spectroscopy, this chromatic aberration has to be prevented, because

31 2.2 Plasma diagnostic methods 7 otherwise ions can be lost inside the mass spectrometer influencing the obtained count rate of ions versus the ion energy. This problem is discussed in detail by Hamers [66] and successfully adapted for a determination of ion energy distribution functions [67, 68]. For a proper ion extraction by the mass spectrometer, the ion beam has to be focused in the way, that the focal point of lens is localized in the exit aperture of the ionization source for all ion energies. This configuration leads to a mostly parallel ion beam passing the entrance of the energy filter and no ion losses due to collisions with the electrode aperture at the exit of the ionization system. The given mass spectrometer allows for a proper tuning by adjusting the lens parameters of the extracting system represented by the extractor and the lens voltages. Experimentally, the optimized configuration of extractor and the lens are revealed for the EQP 3 by tuning according to the procedure described by Hamers [66]. The determined voltages of extractor and lens are V ext = 4 V and V L = 4 V, respectively. Figure 2.6 shows the influence of settings of the ion extraction system on the revealed ion energy distribution functions including and excluding chromatic aberrations. For example, a strong over estimation of ions depending on their energy is observed as illustrated in figure 2.6, when aberration is present. normalized count rate / a.u energy / ev Figure 2.6: Ion energy distribution functions for different lens settings of extraction optics: ( ) with and ( ) without aberrations. Mass dependent transmission of energy mass spectrometer For an analysis of plasma polymerization of organosilicon compounds with high monomer masses, a knowledge about the mass dependent transmission function is essential for an adequate data analysis (cp. table 5., page 62). The mass dependency of the used mass spectrometer is revealed by means of a residual gas analysis of noble gases and a comparison of the measured count rates. Therefore, the reactor is consecutively filled with one of the noble gases helium, neon, argon, krypton and xenon. The pressure inside the reactor is kept constant at p = Pa and the count rate revealed by the channeltron detector is determined. The count rate C of the detector at a specific mass is proportional to the particle density n EMS of these particles inside the mass spectrometer, the ionization current I e of the filaments responsible for electron impact ionization, the cross section of electron impact

32 8 Chapter 2 Experimental setup ionization σ ei of the element and the mass dependent transmission T EMS [69]. Thus, it can be described as C = T EMS n EMS I e σ ei. (2.4) The density n EMS and the partial pressure p EMS = n EMS k B T of particles inside the mass spectrometer has to be determined for a determination of the transmission function T EMS as follows. The number of particles Ṅ in entering the mass spectrometer from the reactor volume via a sampling orifice with area A are Ṅin = ΦA, whereφ denotes the particle flux density corresponding to thermal velocity at reactor pressure p: Φ = p 2πmkB T p m. (2.5) Additionally, the pumping system of the mass spectrometer influences the pressure p EMS. The particle flux Ṅout through the pumping system is described depending on effective throughput S eff = SL/(S + L) as Ṅ out = p EMS k B T S eff = p EMS SL k B T S + L, (2.6) where S and L denote the throughput and particle conductance, respectively [69, 7]. Balancing particle numbers Ṅin and Ṅout reveals n EMS = ( + L L S ) ΦA ΦA L for L S. (2.7) The throughput of the used pumping system is S = 36ls and the conductance of the mass spectrometer is rather small due to many installations and can be estimated to be L.5ls [69]. For the given conditions of a molecular stream inside the spectrometer, the conductance is proportional to velocity of the particles L m [7]. Using this result and solving equation (2.4), the mass dependent transmission function T EMS is determined as function of known quantities: T EMS = C pi e σ ei, (2.8) where T EMS = T EMS const. Therefore, the transmission function can be estimated by filling the reactor at a certain pressure p with gases characterized by an electron impact ionization cross section σ ei and the factor describing the isotope abundance for a certain mass as merged in table 2.2 for the rare gases. element m/z isotope abundance E ei / ev σ ei / cm 2 He Ne Ar Kr Xe Table 2.2: Electron impact ionization cross sections σ EI for single ionization of the rare gases [7] and isotope abundances. The energies used for electron impact ionization are below measurable threshold values for multiple ionization to eliminate phenomena related to production of double ionized

33 2.2 Plasma diagnostic methods 9 atoms [7]. For the used mass spectrometer and lenses settings, an exponential transmission function is determined described by ( ) m/z T EMS =.445 exp. (2.9) 22 Figure 2.7 shows the results of the determination of T EMS normalized to the transmission of helium. The exponential behavior of the mass transmission function is due to the mass separation by means of a quadrupole filter as expected and experimentally confirmed by Pecher [69]. relative transmission / a.u m/z Figure 2.7: Mass dependent transmission function T EMS (m) of HIDEN EQP 3: ( ) relative transmission of noble gases, ( ) exponential fit according to equation (2.9). Time resolved measurement For time resolved measurements of ion energy distribution functions, an experimental setup is used based on a multichannel scaler card FAST ComTec MCA-3 (FAST ComTec GmbH, Oberhaching, Germany) [72]. The multichannel scaler card is connected to the pulse output of the mass spectrometer and the trigger signal of the microwave generator to count the pulses with respect to pulse period. The time resolution used for the measurements is t = µs and typically, the signal is acquired over more than 5 pulses. A Labview program controls the lense settings of the mass spectrometer to set the required mass and energy ranges and is responsible for data storage. The energy resolution is chosen to be.ev Optical emission spectroscopy For the spectroscopic investigation of light emission from the plasma, a broadband echelle spectrometer ESA 3 (LLA Instruments GmbH, Berlin) is used. It allows for simultaneous detection of emission in the wavelength range 2 nm λ 8 nm with a spectral resolution of 5 pm and 6 pm at λ = 2 nm and λ = 8 nm, respectively, due to measurement in different orders of the spectra. The spectrometer exhibits wavelength ranges, which can not be detected for wavelengths λ 5 nm. These regions with zero efficiency are not significant for the spectral analysis within this thesis, but have to be kept in mind during data analysis. The spectrometer is absolutely calibrated using a tungsten-ribbon lamp and

34 2 Chapter 2 Experimental setup branching ratios of N 2 and NO bands as described by Bibinov et al. [73]. For the measurements with the spectrometer, an optical fiber is used for transferring the light emission from the plasma to the spectrometer. An aperture blend mounted on the end of the fibre defines the measurement cone with an aperture angle of 2.9 to determine the volume of the light emission from the plasma. For measurements, a quartz window is used at the heights of flanges used for Langmuir probe and mass spectrometry investigations (z = 5 mm, cp. figure 2.). The optical fibre is mounted in front of the quartz glass window at a distance of r = 25 mm related to the center of the reactor. Regarding the aperture of the measurement blend, a radius of r c =4.6 mm of the measurement cone at the Plasmaline is determined. It is below the radius of the Plasmaline antenna (r = 6 mm). Therefore, the measurement cone is well defined and emission from areas behind the Plasmaline in the line of sight of the spectrometers do not influence the measurement. The optical fibre is adjusted using a Laser to ensure a proper alignment. 2.3 Coating analysis 2.3. Permeation measurement For an evaluation of gas permeation through pristine and coated PET substrates, oxygen is used as test gas. Oxygen is a relevant gas for food packaging applications due to reactions with packaged food or beverages like oxidation of vitamin containing fruit juices. Compared to other gases like carbon dioxide or water vapor used for determination of permeation, oxygen constitutes a better model permeant because of its known transport properties [6]. E.g. water vapor is known to induce structural changes in SiO x coatings through stress cracking [74]. Therefore, oxygen is used for the quantification of the permeation through pristine and coated PET. The oxygen permeation rate is determined by a MOCON OX-TRAN 2/6 (MOCON Inc., Minneapolis, USA) [75] oxygen transmission rate system. The determination of oxygen transmission rates with this instrument is approved in industry and research institutes. The measurement principle is carrier gas method according to ASTM D and DIN [76]. The test substrates can be foils or three-dimensional packages like containers or bottles. Figure 2.8 schematically shows the setup for testing of foils and bottles. The MOCON OX-TRAN 2/6 is equipped with six measurement cells. Therefore, six substrates, either foils or bottles, can be analyzed in parallel. For the testing of foils, a circular test area of cm 2 is considered and pure oxygen is used as test gas. The measurements are performed at a temperature of T =23 C [76] and a relative humidity of %. The oxygen partial pressure difference used for the permeation measurement by carrier gas method is bar. All permeation rates of coated and uncoated foils are tested at these conditions. Additionally, the temperature of the measurement chamber can be varied in the range of T =2..5 C to qualify temperature influence on permeation properties as considered in section 5.5. The calibration of the measurement setup is confirmed by means of certified oxygen transmission reference films traceable to the National Institute of Standards and Technology (NIST). Furthermore, the determination of oxygen permeation through bottles is performed with a package adapter as shown in figure 2.8(b). Therefore, air is used as test gas and the inside

35 2.3 Coating analysis 2 air O 2 temperature control O 2 foil O-ring epoxy glue carrier gas carrier gas to detector (a) Testing of foils. carrier gas carrier gas to detector (b) Testing of bottles. Figure 2.8: Permeation measurement setup for determination of oxygen permeation through packaging materials [76, 75]. of the package is flushed by the carrier gas. The oxygen concentration in air is assumed to be 2%. Nitrogen with an admixture of 2% hydrogen is used as carrier gas. Before this gas mixture is feed into the measurement cells, it is necessary to remove residues of oxygen from the carrier gas as a precaution by means of a catalyst. This catalyst removes the oxygen by producing water using the hydrogen content of the carrier gas [76]. The quantification of oxygen, which is flushed to the sensor by nitrogen:hydrogen mixture is based on an electrochemical detector consisting of a nickel-cadmium and a graphite electrode, which are drenched in caustic potash. Oxygen molecules, which penetrate the detection system react at the surface of the graphite cathode under consumption of four electrons and produce 4OH : O 2 +2H 2 O+4e 4OH. (2.) The created 4OH ions produce four electrons at the porous cadmium anode according to 2Cd+4OH 2Cd(OH) 2 +4e, (2.) which can be measured as current via a calibration resistor. Therefore, each oxygen molecule, that penetrates the electrochemical detector system produces four electrons. This relation allows for a linear measurement of the oxygen permeation and a zeroing of the detector signal for different measurement cells. The permeation flux density J describes the volume of oxygen molecules Q reaching the electrochemical detector per measurement area A and time t: J = Q A t. (2.2) Therefore, an adequate unit of permeation flux density J for testing of foils can be given as [J] = cm 3 m 2 day. (2.3)

36 22 Chapter 2 Experimental setup Customary, the unit [J] = cm 3 pck day (2.4) is used for the testing of packages and the permeating volume of oxygen flux is normalized to the package surface without exact determination of this value. Regarding potential differences and small permeation leakages of the six measurement cells, both parts of the measurement cell can be flushed with nitrogen:hydrogen mixture and the detector signal is constituted as individual zero value. These values of the six measurement cells are determined prior to the measurement of foils or bottles. For the identification of the individual zero values for the analysis of packages, small glass bottles (V = 5ml) are mounted on the package adapter, which are assumed to be impermeable Fourier transform infrared spectroscopy Fourier transform infrared spectroscopy is a versatile diagnostic method to analyze bond compositions of solids, liquids or gaseous molecules. It is based on the absorption of infrared radiation due to vibrations within molecules, when they are irradiated. The frequency of molecular vibrations depends on the masses of atoms, bond forces and geometry of the molecule. Therefore, it allows for a classification of different bond structures within molecules and a distinction of various compositions. Typically, molecular vibrations are described as function of the wave number k = λ = νc of irradiating infrared beam and can be found in a range of k = cm,wherec denotes the speed of light, λ and ν the wavelength and frequency of infrared beam, respectively [77]. δ movable mirror infrared source L beam splitter L mirror substrate crystal n 2 n Θ d p substrate and detector (a) Scheme of FTIR spectrometer based on an Michelson interferometer [77]. (b) Principle of ATR FTIR spectroscopy [77]. Figure 2.9: FTIR spectroscopy setup. For the measurement of an infrared absorption spectrum, a setup as schematically shown in figure 2.9(a) is used. It consists of a polychromatic infrared source and a Michelson interferometer. A polychromatic infrared beam is emitted by the source and applied to a beam splitter with a transparency of 5%. A fixed and a moveable mirror reflect the beam and guide it to the substrate and the detector. The distance between the beam splitter and the mirrors is equal, if the moveable mirror is not displaced leading to a positive interference of the splitted beams. A periodic displacement of the moveable mirror by a distance δ leads to

37 2.3 Coating analysis 23 intensity variation in detected signal I(δ) depending on δ due to constructive and destructive interferences depending of the wavelengths within the polychromatic source, which is called interferogram. A Fourier transform of detector signal allows for a determination of fourier spectra depending on wave number k. For the analysis of substrates, the absorption spectrum can be determined with regard to a background spectrum containing the characteristic absorption of the measurement cell. The substrates can be analyzed by transmitting the infrared beam directly through the sample for an analysis of the substrate composition. For an analysis of thin films, a method based on total reflection of the infrared beam is more established called attenuated total reflection (ATR) FTIR spectroscopy. Therefore, the sample is pressed on a crystal with a high refractive index and the infrared beam incidences at an angle above the critical angle Θ c =sin ( n2 n ), (2.5) where n and n 2 denote the refractive index of the crystal and the sample, respectively [77]. Figure 2.9(b) schematically shows the setup for sample positioning of an ATR unit. The penetration depth d p of the infrared beam penetrating the sample is determined by d p = λ 2πn (sin 2 Θ (n 2 /n ) 2 ) (2.6) and is exemplarily shown in figure 2. for the given experimental conditions [77]. A strong dependency of d p on the wavelength is revealed, which is regarded in the evaluation of the spectra. In this thesis, the composition of SiO x C y H z coatings and PET is analyzed using a Bruker Vector 33 FTIR spectrometer (Bruker Optik GmbH, Ettlingen, Germany) equipped with a PIKE MIRACLE (PIKE Technologies, Madison, USA) Θ = 45 diamond ATR unit with a refractive index of n =2.4. The measurements are performed with a resolution of k =2cm and 32 scans are executed per spectrum. By reason of the transmission of the diamond ATR crystal, the spectra are presented in the range of 75 cm k 4 cm. x -6 2 dp / m k/cm Figure 2.: Penetration depth d p of infrared beam versus wave number for n 2 =.4 and n =2.4 and an incidence angle of 45 according to equation (2.6).

38 24 Chapter 2 Experimental setup absorption / a.u k/cm 2 5 (a) Measured FTIR absorption spectrum ( ) and sum of Gaussian fits ( ) of silicon oxide coating. absorption / a.u k/cm 2 5 (b) Gaussian fits to FTIR absorption spectrum of silicon oxide coating, absorption fits correlated to: SiO bonds ( ), SiOH ( ). Figure 2.: Example of Gaussian fits to FTIR absorption spectrum of plasma polymerized SiO x coating. For analysis of SiO x C y H z films, PET substrates are coated with a thickness of d 3.5 µm. Thus, the thickness of the deposited SiO x C y H z layer exceeds the penetration depth of the infrared beam penetrating the film to prevent detection of background signal from PET substrate (cp. figure 2.). Figure 2. gives an example of an SiO x coating FTIR spectrum. For a detailed quantitative analysis of bond composition, Gaussian fits to the measured absorption spectra I(k) areused described by ( ) (k µ) 2 I(k) =Î exp (2.7) 2σ 2 and the parameters σ, µ and Î are determined by least square method. The results of Gaussian fitting are in good agreement with measured absorption spectrum and allow for a

39 2.3 Coating analysis 25 quantitative analysis to compare different layer compositions as shown in section The peak assignment of different bonds relevant for SiO x C y H z film analysis is merged in table 2.3. In principle, all bond vibrations of molecules can be detected that cause a change of the electrical dipole momentum of the molecule configuration leading to absorption of infrared radiation. In the analyzed SiO x C y H z coatings, bending, rocking and stretching vibrations of Si C, Si O, O HandC H bonds are noticeable observed leading to capabilities of qualitative and quantitative analysis of film composition. All shown FTIR spectra of barrier coatings in this thesis are normalized to the peak of stretching of SiOSi (...7 cm ) Stylus profilometry For the measurement of the deposition rate of SiO x coatings, an adhesive tape protects parts of the bottle surface during the coating process and permits the determination of the layer thickness by means of a stylus profiler Veeco Dektak 6M (Veeco Instruments GmbH, Mannheim, Germany). Therewith the deposition rate can be determined along z-axis by measuring the step height between coated and uncoated regions Atomic force microscopy Atomic force microscopy is performed for investigations of the surface morphology of the deposited layers using a Veeco CP atomic force microscope (Veeco Instruments GmbH, Mannheim, Germany). The microscope is used in a non contact mode and the scan area is 3 nm 3 nm for the investigations. The mean roughness R a of the surface is determined as R a = 3 nm 3 nm nm where h(x) denotes the measured heights signal of the surface. h(x) dx, (2.8) k /cm bond molecule vibration reference 8 Si C SiCH x rocking [78, 79, 8] Si O SiO bending [78, 8, 82, 83, 8] 84 C H SiCH x rocking [78, 79, 83, 8] O H SiOH bending [78, 84, 82, 85]..75 Si O SiO stretching [78, 79, 8, 82, 83] [86, 5, 8, 87, 88] C H SiCH x bending [79, 8, 85, 87, 9] 63 O H SiOH bending [89] Si H SiH stretching [78, 79, 86, 8, 85, 9] C H SiCH x stretching [85, 9] C H SiCH x stretching [78, 79, 86, 5] O H SiOH stretching [78, 84, 82, 8, 85, 9] Table 2.3: Assignment of infrared absorption peaks in FTIR spectra of SiO x C y H z like films

40 26 Chapter 2 Experimental setup Scanning electron microscopy and Energy dispersive x-ray spectroscopy Scanning electron microscopy is used for the imaging of deposited films and endospores. The central SEM unit of Ruhr-Universität Bochum (Zeiss SMT 53, Carl Zeiss SMT AG, Oberkochen, Germany) is assigned due to the experience in SEM imaging. Additionally, energy dispersive x-ray spectroscopy (EDX) measurements are performed for an absolute determination of SiO x C y H z layer composition concerning oxygen, silicon and carbon content. Therefore, PET substrates are coated with a layer thickness of d = 5µm to prevent background detection of PET. 2.4 Microbiological methodology For capability tests of plasma sterilization for food packaging materials, microbiological techniques have to be used to grow cells, realize a proper artificial contamination of substrates and determine viable cells after a treatment. For sterilization challenge tests, typically endospores of bacteria or fungi are used. They can be described as a resistive state of the bacteria or fungi to protect against environmental stress. The state is characterized by a high resistance against most agents that would normally kill vegetative cells. E.g. a high heat resistivity of endospores and resistance against typical cleaning products like alcohols and detergents is observed. Therefore, endospores are used within this thesis for the sterilization challenge tests. The microbiological methodology of the preparation of endospores for these tests as performed in cooperation with the Fraunhofer Institute for Process Engineering and Packaging, IVV, are described in this section Fraunhofer Institute for Process Engineering and Packaging The microbiological tests are performed in close collaboration with the IVV based on the experience of the IVV in the field of evaluation and certification of sterilization processes. All microbiological work, such as growing of test spores, contamination of substrates and determination of viable spores is performed in the Fraunhofer institute to provide reproducible and reliable processes Count reduction test The count reduction test is a challenge study to evaluate the capabilities of a sterilization process. Therefore, substrates are artificially contaminated with test spores and the number of viable spores is determined after the sterilization process. Additionally, reference samples are evaluated, that are also contaminated, but not treated by the sterilization process. The count reduction test can be listed as an eight step process described by. Contamination of test substrates: Vitreous microscope slides or complete PET bottles are contaminated with a defined number of test spores by homogeneously spraying the spores on the substrate by means of a mask over an area of cm 2 or on the inner surface of a bottle. The exact amount of spores used as base contamination is determined afterwards using reference substrates, that are not applied to the sterilization process. Typically, a base contamination is used, that is two decades higher than the demanded reduction rate, e.g. a base contamination of 6 germs is used for a demanded inactivation of 4 [9].

41 2.5 Substrate materials Sending the contaminated substrates from Fraunhofer institute to Ruhr- Universität 3. Treatment of substrates: The test substrates are brought into the reactor chamber by means of a sample carrier and plasma treated. Otherwise, PET bottles are treated without the usage of a carrier. For each set of plasma parameters or a treatment time, three substrates are treated under the same conditions to allow for a slight statistical evaluation. After treatment, the substrates are packed in sterile containers and a defined amount of Ringer s/tween solution is added. The solution supports the surviving of sublethal damaged spores to identify them as colony forming units afterwards. 4. Handling of reference substrates: Three reference substrates are used to determine the base contamination. They are handled as the other test substrates except the plasma treatment. They are also packed with Ringer s/tween solution as the treated substrates. 5. Sending substrates from Ruhr-Universität to Fraunhofer institute 6. Determination of base contamination: The reference samples are used for the determination of base contamination. Therefore, dilution series of the reference samples are made, pipetted on agar plates and incubated for a few days. The number of incubated colonies is counted representing the number of base contamination on the reference sample. The unit of the number of spores is colony forming units ( CFU). 7. Determination of viable spores: Comparable to the determination of base contamination, the number of surviving spores is counted after incubating dilution series pipetted on agar. 8. Calculation of reduction rate: The logarithmic reduction rate R is determined by ( ) ( ) reference sample R =log log (2.9) CFU CFU where reference and sample denote the number of colony forming units of the reference and the treated substrates, respectively [9, 9, 92]. Furthermore, the destructionvalue (D-value) is calculated defining the time in minutes for the destruction of 9% (one decade) of the microorganisms, which is important for comparing the sterilization capabilities. 2.5 Substrate materials 2.5. Polyethylene terephthalate Polyethylene terephthalate is a versatile packaging material based on a long-chain polymer belonging to the family of polyester. It is formed by means of a reaction from terephthalic acid and ethylene glycol and allows for forming into various kinds of packaging like foils, containers or bottles [93, 94]. The chemical composition of PET is shown in figure 2.2 revealing the organic long-chain polymeric structure. Within this thesis, PET is used as substrate material for the deposition of permeation barrier coatings and sterilization tests. Three different shaped PET substrates are used for various diagnostic reasons:

42 28 Chapter 2 Experimental setup O C O C O CH 2 CH 2 O Figure 2.2: Polyethylene terephthalate. n Foils PET foils (Goodfellow GmbH, Friedberg, Germany) are used as substrate material for the evaluation of the barrier properties of plasma polymerized SiO x coatings. The foils have a thickness of 23 µm and are characterized by an oxygen permeation of J PET = 68.2 ± 2.cm 3 m 2 day. The size of the foils is 9 mm mm and they are fixed at the position of a bottle wall at z =..2 mm (cp. figure 2.) using a foil carrier as illustrated in figure 2.3. The mean value and the standard deviation of the permeation J PET is determined by 7 measurements at different positions of the PET web to cancel out variations of the substrate material, if necessary. Figure 2.3: Foil carrier for treatment of PET foils. Flat substrates Flat PET substrates (Goodfellow GmbH, Friedberg, Germany) with a thickness of mm constitute the substrate for investigations of the coating composition. They have a size of 2 mm 2 mm and are fixed at the position of a bottle wall at z =5..7 mm during treatment. Bottles Sparkling water PET bottles (Gerolsteiner Brunnen, Gerolstein, Germany) are used as example for the treatment of three dimensional packages. The bottles have a volume of liter and they are characterized by a mean oxygen permeation of J bottle =.28 ±.2 cm 3 pck day, which is determined considering 28 bottles. An analysis of the composition of the different kinds of substrates shows no differences of the used materials in terms of the bond structure. Figure 2.4 illustrates the composition of PET as determined by FTIR spectroscopy. The spectrum is dominated by C Osingleand double bonds and C H bonds correlated to the polymer composition. A good agreement of the detected bonds with the polymer structure as visualized in figure 2.2 is observed.

43 2.5 Substrate materials 29 C=O C-O C-O absorption / a.u C-H C-H O-CH C-H CH k/cm 5 Figure 2.4: FTIR absorption spectrum of PET and peak assignment according to [95, 96] Silicon wafer For the investigation of the surface morphology of coatings, silicon wafers are used as substrate materials due to their reproducible surface roughness. Wafers are cut into 2 mm 2 mm substrates and treated at the position of the bottle wall at z =5..7 mm Glass Glassy microscopy slides represent an inert material for sterilization tests eliminating possible interactions of the substrate and the test spores during the treatment. Therefore, they are used as specimen holders and artificially contaminated for the sterilization challenge tests performed in this thesis additionally to the usage of PET bottles.

44 3 Chapter 2 Experimental setup

45 3. Characterization of reactor setup This chapter contains the plasma characteristics investigation of the surface wave excited microwave plasma used for the treatment of PET foils and bottles. For the analysis within this chapter, an argon plasma is considered to show the basic influence of parameter variations like process pressure and microwave power on the profiles of electron density and electron temperature. Additionally, their influence on the ion energy distribution function is determined. The analysis is performed for continuous wave and pulsed plasmas to show the spatial and temporal behavior of the discharge. The application of the used plasma diagnostic methods for coating plasmas is hardly applicable due to coating of the Langmuir probe and the reactor chamber walls leading to strong variations of the parameter to measure as it is revealed in section 3.3. Therefore, argon is chosen as process gas for the analysis of the characteristics to determine the fundamental behavior of the Plasmaline discharge. 3. Continuous wave characterization 3.. Electron density and electron temperature The electron density n e and electron temperature T e are important parameters to describe plasmas. They are revealed by an investigation of the electron energy distribution function and T e is determined under the assumption of a maxwellian distribution of the electron energy as described in section Radial profiles The radial profiles of the electron density n e and electron temperature T e are shown in figure 3. for various process pressures 2 Pa p 5 Pa. They are determined for radii 6mm r 42.5 mm at an axial position of z = 5 mm according to figure 2.. The results for the electron density as plotted in figure 3.(a) show the expected behavior as discussed in section. as function of pressure. For increasing radii, an increase of the electron density is observed reaching a maximum of n e =4. 7 m 3 for p 3 Pa and n e =3.5 7 m 3 for p = 2 Pa. For further increasing radii, the electron density drops and reaches a value of n e < 7 m 3 close to the bottle cage, which has a radius of r =42.5mm. Forp =2Pa, the plasma is not homogeneously ignited inside the reactor chamber as it will be shown for the axial characterization. Therefore, the observations for p = 2 Pa represent a not fully ignited plasma. The revealed maximum of the electron density is much higher than the critical electron density n e,crit = m 3. Therefore, the distribution of the radial electrical field component E r can be assumed to show the characteristic as illustrated in figure.2. A strong electric field close to the Plasmaline antenna can be deduced responsible for an effective 3

46 32 Chapter 3 Characterization of reactor setup ne / m 3 5 x r/mm (a) Electron density n e kbte / ev r/mm (b) Electron temperature T e Figure 3.: Radial electron density n e and electron temperature T e profiles of argon plasma for various process pressures at plasma conditions P = 3 W, Φ Ar = sccm: ( ) p =2Pa,( )p =3Pa,( ) p = 4 Pa, ( ) p = 5 Pa. The axial position of measurements is z = 5 mm. plasma heating. For a plasma density of n e =3.5 e 7 m 3, a skin depth of δ =9mmis revealed as described by equation (.). Therefore, the electron heating occurs in a skin depth layer around the Plasmaline with a thickness of some millimeters [47]. This explains the heating zone observed by high electron temperatures T e close to the Plasmaline antenna. As shown in figure 3.(b), a peak of the electron temperature is revealed for radii 6mm r 5 mm, which correlates well with the determined skin depth of δ = 9 mm. The electron temperature remains nearly constant for r 5 mm and values of k B T e =.5eV and k B T e =.8 ev are revealed for p 3 Pa and p = 2 Pa, respectively. The process pressure strongly influences the profiles of the electron density. A shift of the position of the maximum of electron density to lower radii is observed for increasing process pressures. For higher pressures, the plasma concentrates close to the antenna and a stronger decay is observed for increasing radii compared to the density profiles of lower process pressures. Additionally, the electrons loose energy by collisions due to the higher process pressures as revealed by decreasing electron energies in the heating zone of the plasma close to the Plasmaline (cp. figure 3.(b)). The observations of electron density and temperature profiles reveal two regions with different plasma conditions. Close to the Plasmaline antenna, a region with high electron densities and high electron temperatures is observed due to strong microwave coupling mainly induced by the radial electric field component E r. In contrast, close to the bottle cage, the plasma conditions can be rather described as a kind of low density plasma. Depending on the process pressure, the expansion of the high density region can be influenced. The elongation of the electron heating zone is mainly described by the skin depth δ (cp. equation (.)). The penetration depth of high energy electrons responsible for the ionization of neutral gas atoms and molecules strongly depends on the process pressure due to electron cooling by collisions. Therefore, the spatial elongation of the plasma shrinks for increasing pressures. A variation of process pressure is a possibility to change the plasma properties of a surface wave excited plasma at a fixed distance from the heating zone to influence the degree of ions present close to the surface. The theorectical considerations of a surface wave excited

47 3. Continuous wave characterization 33 plasma as described in chapter are in good agreement with the observations revealed for the behavior of the electron density profile and the electron temperature characteristics. Axial profiles ne / m 3 5 x kbte / ev z/mm 5 z/mm (a) Electron density n e (b) Electron teperature T e Figure 3.2: Axial electron density n e and electron temperature T e profiles of argon plasma for various process pressures at plasma conditions P = 3 W, Φ Ar = sccm: ( ) p =2Pa, ( )p =3Pa,( ) p = 4 Pa, ( ) p = 5 Pa. The radial position of measurements is r = 28 mm and the end of the Plasmaline antenna is at z = mm as illustrated in figure 2.. Figure 3.2 illustrates the axial profiles of the electron density n e and electron temperature T e for various process pressures. The radial position of measurements is r = 28mm and the end of the Plasmaline antenna is at z = mm as illustrated in figure 2.. A strong decay of the densities is observed for z mm representing the volume above the antenna. The behavior of the profiles for 3 Pa p 5 Pa agree well with the radial measurements and show a rather homogeneous electron density along the antenna. In contrast, the electron density at p = 2 Pa nearly linear increases for increasing axial positions and reaches a maximum for z mm. For this pressure, the plasma does not ignite homogeneously in the reactor chamber at the applied microwave power of P = 3 W and small deviations between the radially and axially measured densities are revealed. They are assumed to be slightly changed conditions of the reactor system regarding temperature and condition of the Plasmaline antenna. The almost linear characteristic of the electron density is in good agreement with theoretical investigations discussed in [37] predicting a linear decrease of density for increasing distances to power coupling. The electron temperature shows a nearly constant characteristic along the z-axis. Only a minor drop is observed for z mm. Comparable to the radial results, the electron temperature for p =2Pa is different from the behavior of higher pressures and hardly to evaluate for lower densities due to low and noisy current signals of the probe measurements produced by the microwave generator characteristics. Therefore, for z mm the electron temperature for p = 2 Pa is not shown. The electron temperature at z = 5 mm is revealed to be k B T e =.9eV, which is in good agreement to the radial measurements. Compared to the higher pressures,

48 34 Chapter 3 Characterization of reactor setup strong variations of the electron temperature are observed which are assumed to be due to the inhomogeneous ignition of the plasma leading to variations of T e along the axis Ion energy distribution functions For surface processing applications, the energy of ions impinging on a surface is a relevant parameter influencing the interaction between the plasma and the substrate. The ions are accelerated within the sheath formed in front of a surface leading to ion bombardment. Figure 3.3 shows the distributions functions of Ar + (m/z = 4) ions versus their energy. They can be understood as velocity distribution function plotted versus the energy as investigated in detail in [97]. 4 x 5 count rate E Ar + / ev Figure 3.3: Ion energy distribution functions of Ar + (m/z = 4) ions for various process pressures at plasma conditions P = 3 W, Φ Ar = sccm: ( ) p =2Pa,( )p =3Pa, ( ) p =4Pa. A single peak structure is observed with a tail at the lower energy side. The mean ion energy is strongly dependent on the process pressure. For higher pressures, the ions loose more kinetic energy as a result of collisions due to reduced mean free paths leading to lower energy of ions impinging on the substrate. For the considered plasma conditions, mean ion energies of Ar + ions of Ē Ar +(2 Pa) = 7.8eV, Ē Ar +(3 Pa) = 6.5eV and ĒAr +(4 Pa) = 4.3eV are measured. For higher pressure conditions, charge transfer collisions predominantly lead to production of ions within the sheath. At kinetic ion energies above.2 ev, the process with the highest cross section of ion neutral collisions is resonant Ar + Ar charge transfer [98, 99]. Therefore, collisions of Ar + ions with a neutral Ar atom lead to a charge transfer before the ion hits the surface. Produced ions are accelerated towards the substrate due to the potential gradient, but do not gain the whole energy of the difference between plasma and substrate potential. Besides the reduction of the mean energy of the ions, the total amount of ions impinging the surface is reduced. As previously discussed, the process pressure influences the election and ion density of the surface wave sustained plasma [47] close to the bottle cage. Exactly this behavior is observed for increasing pressures leading to a reduction of ion bombardment of the substrates as result of a concentration of the density maximum around the Plasmaline antenna.

49 3.2 Pulsed characterization Power influence on electron density and electron temperature ne / m x P/W (a) Electron density n e kbte / ev P/W (b) Electron temperature T e Figure 3.4: Influence of various cw microwave powers W P 2W on electron density n e and electron temperature T e at plasma conditions Φ Ar = sccm and p =3Pa, r =2mmandz = 5 mm. A variation of the input power leads to a modification of the plasma properties in terms of the electron density n e and electron temperature T e. Figure 3.4 shows n e and T e for increasing cw microwave power at a radial position of r =2mmandz = 5 mm. For increasing powers W P 4 W, a density increase is revealed. It is followed by a step in the behavior of electron density leading to nearly constant densities for powers P 5 W. The electron temperature remains nearly constant over the whole power range. For powers P < 3 W, the evaluation of the Langmuir probe characteristic becomes difficult due to low densities of the plasma and the noise of the microwave generator (cp. figure 2.2). The stagnation of electron density can be explained by the surface wave excitation of the plasma. If the shielding characteristic of the plasma at a certain radius represented by ɛ r (r) = is generated, a further increase of the electric field strength does not influence the properties of the plasma. This property of a surface wave excited plasma significantly limits a proper control of the electron density close to the substrate surface. It can be stated, that a certain amount of power is required for a homogeneous ignition and a further increase does not significantly change the plasma properties for surface processing. This behavior is confirmed by investigations of coating properties for deposited films in section typically revealing a kind of step function depending on applied microwave power. 3.2 Pulsed characterization This section contains the investigations of the Plasmaline microwave discharge driven in pulsed mode. Pulsed plasma are important for surface modification applications and treatment of three-dimensional objects as it is revealed in chapter 5. They allow for influencing the process chemistry and tuning of the plasma parameters during the pulse and in the afterglow phase of the discharge.

50 36 Chapter 3 Characterization of reactor setup 3.2. Electron density and electron temperature ne / m 3 5 x t/µs (a) Electron density n e kbte / ev t/µs (b) Electron temperature T e Figure 3.5: Time resolved electron density n e and electron temperature T e profiles of argon plasma for various process pressures at plasma conditions P = 5 W, Φ Ar = sccm, t on = 4 ms, t off = 4 ms: ( ) p =24Pa,( )p = 3 Pa, ( ) p = 4 Pa. The axial and radial position of measurements is z = 5 mm and r = 2 mm, respectively. The characteristics of electron density n e and temperature T e for a pulsed discharge described by t on =4msandt off = 4 ms are shown in figure 3.5 for various process pressures. The pulse parameters are chosen due to their relevance for film deposition as is it revealed in chapter 5. The investigation of the temporal behavior is performed at r = 2 mm resulting in nearly similar densities for all considered process pressures (cp. figure 3.). The characteristic of the electron density and temperature can be divided in to three consecutive parts: (a) transient ignition behavior, (b) steady state pulse phase and (c) plasma afterglow. (a) Transient ignition behavior During the first 8 µs of a pulse, the electron density and electron temperature show a transient response following the generator characteristics as determined in figure 2.3. The electron density n e exhibits maxima at t = 2 µs and t = 36 µs and minima at t = 24 µs andt = 5 µs,whichisingoodagreementwiththe power characteristic of the microwave generator as discussed in section 2... In contrast, the electron temperature T e shows a maximum within the first 4 µs and further maxima at t = 25 µs andt = 5 µs. It exhibits minima at t = 4 µs andt = 36 µs. Therefore, the temporal peaks of the electron density and temperature are antipodal. A decrease of the electron density leads to an increased penetration depth of the electrical field due to reduced shielding effects of a less dense plasma leading to a heating of the electrons. Therefore, an increase of the electron temperature is observed opposed to the behavior of the electron density. (b) Steady state pulse phase The electron density and electron temperature remain nearly constant for 8 µs t 34 µs illustrating a steady state condition of the plasma. For p = 4 Pa, the steady state phase begins at t = 2 µs and more peaks are observed at positions correlated to the generator characteristics. (c) Plasma afterglow A plasma afterglow is observed for t 34 µs. When the power is switched off, the electron temperature immediately drops below T e ev. It is difficult

51 3.2 Pulsed characterization 37 to investigate for the later afterglow phase due to noisy signals produced by the generator characteristics as discussed in section 2... The electron density decrease occurs for increasing process pressures with longer time scales due to ambipolar diffusion of the particles to the walls [39, 52, ]. Ambipolar diffusion is reduced for increasing pressures due to reduced wall loss rates. Therefore, the characteristic decay times in the afterglow are increasing with pressure [52]. The revealed time constants τ of electron density decays are listed in table Ion energy distribution functions The time resolved ion energy distributions of Ar + (m/z = 4) are plotted in figure 3.6 for three different process pressures. The lowest considered pressure is p = 24 Pa instead of p = 2 Pa because it allows for a homogeneous plasma ignition for the chosen parameters. Besides the time and energy dependent ion fluxes, the integrals of the ion count rate over all energies are shown in figure 3.7 representing the total number of ions independent of energy. Comparable to the analysis of electron density and temperature, the three previously mentioned phases can be distinguished in the characteristic of the ion flux detected by the mass spectrometer as well. (a) Transient ignition behavior An ignition behavior of Ar + ions flux towards the surface is observed for all considered pressures. The characteristic time scales correlate with the values revealed for electron density and temperature. For the lowest considered pressure p = 24 Pa, the maxima and minima of the IEDF as shown in figure 3.6(a) are observed at similar times compared to the electron temperature. It is assumed, that during the electron density drop the electrons and ions are preferentially accelerated by reason of larger penetration depth of the electric field. Reducing the electron density and magnifying electron temperature induces an increase of the characteristic length scale of the plasma described by Debye length (λ D = ɛ k B T e /(n e e 2 )). Therefore, the ions gain more energy within an extended sheath. Additionally, the peak positions of the sum of Ar + ions (figure 3.7) correlate well with the positions of the maxima and minima of electron density. For a process pressure of p = 24 Pa, the peaks of the ion flux are precisely observable, whereas they start to blur for higher pressures. Additionally, for p = 4 Pa the transient ignition behavior is temporarily enlarged as observed for the electron density. By reason of the higher pressure, the coupling of the pulsed electrical field in the center of the reactor and the ion flux measured at the bottle cage is reduced due to shielding and collision phenomena within the plasma. Due to higher pressures, the electrons loose energy during collisions leading to a reduced electron temperature as revealed by probe measurements. Therefore, the ionization of neutral argon atoms is reduced leading to less argon ions as observed by means of mass spectrometry. (b) Steady state pulse phase During the steady state pulse phase, ion energy distribution functions similar to the cw case are observed. The mean ion energy strongly decreases with increasing pressures due to ion-neutral collisions as previously described. Furthermore, the amplitude of the ion flux is reduced for higher pressures, but the influence is not as dominant as observed for cw plasmas (cp. figure 3.3). (c) Plasma afterglow The ion energy immediately drops at the end of the microwave power pulse for t 34 µs as it is illustrated in figures 3.6(b), 3.6(d) and 3.6(f). This initial dropping phase is in accordance with the expectation that the sheath voltage collapses

52 38 Chapter 3 Characterization of reactor setup t / µs EAr+ / ev EAr+ / ev (a) p = 24 Pa (b) p = 24 Pa (detailed afterglow) t / µs 3 t / µs EAr+ / ev EAr+ / ev (c) p = 3 Pa (d) p = 3 Pa (detailed afterglow) t / µs 3 t / µs t / µs EAr+ / ev (e) p = 4 Pa EAr+ / ev (f) p = 4 Pa (detailed afterglow) Figure 3.6: Time resolved ion energy distribution functions for various process pressures at plasma conditions P = 5 W, ΦAr = sccm, ton = 4 ms, toff = 4 ms shown as overview and detailed afterglow chart.

53 3.2 Pulsed characterization 39 5 count rate t/µs Figure 3.7: Time resolved count rate of Ar + ions as integral over all energies for various process pressures at plasma conditions P = 5 W, Φ Ar = sccm, t on =4ms,t off = 4 ms: ( ) p =24Pa,( )p = 3 Pa, ( ) p =4Pa. instantaneously when the pulse ends [] leading to less ion acceleration in the sheath. Additionally, the high energetic ions instantaneously vanish leading to a drop of the tail of the ion energy distribution function. A residual number of ions exhibiting an energy of approximately E ev is observed with different lifetimes depending on the pressure. For the lowest considered pressure, ions are observable up to 35 µs after power switch-off and for p = 4 Pa, even after 8 µs ions are found. For p = 24 Pa, the ion energy even seems to increase during the afterglow. The characteristic of the generator can not be neglected considering this effect. The generator might contribute to a further power coupling. The investigations of the generator power characteristics as shown in figure 2.3 reveal no complete switch-off of power after t = 34 µs. A remaining value of about 5% of the peak value is present until t = 4 µs. For the used microwave pulse power of P = 5 W, a power of P 75 W is coupled into the plasma after the pulse switch-off. Therefore, the residual power coupling only leads to a small ion acceleration due to an enlarged penetration depth of the radial component of the electrical field. The residual electrical field is not significantly damped and can penetrate further into the reactor volume, because no plasma shielding by a means of a critical plasma density is present. For p = 3 Pa, the acceleration of Ar + ions is still observable, whereas for p = 4 Pa the ion energy in the afterglow remains nearly constant. This phenomena can be explained by ion-neutral charge transfer collision leading to reduction of ion kinetic energy of ions for higher process pressures. The total number of ions present in the afterglow decays with two characteristic time constants. The first time constant τ corresponds well with the decay time of electron density as exemplarily shown in figure 3.8 for p =24Paandp = 4 Pa. This accordance of the decay time in the early afterglow is also experimentally observed by other groups []. For the late afterglow, a longer decay time of the total number of ions is revealed as shown in figure 3.8. The phenomena of a presence and slower decay of Ar + ions in the late afterglow can be explained by argon metastables reactions leading to production of Ar + ions due to two mechanisms as investigated and simulated by Wenig et al. [5, 52] for an inductively coupled plasma. On the one hand, superelastic collision of metastable states can reheat the electron gas and on the other hand, chemo-ionization processes can lead to production of Ar + ions [5]. Therefore, the observation of Ar + ions in the late afterglow, where electron

54 4 Chapter 3 Characterization of reactor setup normalized ne, count rate t/µs normalized ne, count rate t/µs (a) p =24Pa (b) p =4Pa Figure 3.8: Comparison of time dependent electron density behavior and ion flux of Ar + ions for p =24Paandp = 4 Pa at plasma conditions P = 5 W, Φ Ar = sccm, t on = 4 ms, t off = 4 ms: ( ) normalized electron density n e,( ) normalized count rate of Ar + ions as integral over all energies. impact ionization due to heated electrons does not play a role, can be described based on metastable states in the plasma. In argon, two metastable states are present described as Ar 4s 3 P 2 and Ar 4s 3 P [52]. For metastable states, superelastic collisions with slow electrons are an important loss mechanisms leading to the production of ground state argon atoms and fast electrons capable of ionizing further argon atoms by means of electron impact ionization. Furthermore, superelastic collisions from the upper to the lower metastable state significantly lead to a heating of electrons contributing to further ionization. Additionally, collisions of two metastables lead to the production of an argon ground state / ion pair and a fast electron as well due to chemo-ionization. Therefore, Ar + ions are observable in the late afterglow decaying with a further time constant τ 2 as revealed by time resolved mass spectrometry. The revealed time constants for various pressures are merged in table 3.. Figure 3.8 shows, that Ar + ions are observed up to t = 3 µs andt = µs after pulse switch-off for p =24Paandp = 4 Pa, respectively. p τ τ 2 24 Pa 72.3 µs 4.3 µs 3 Pa 9. µs 78.9 µs 4 Pa 94.3 µs µs Table 3.: Decay times for various pressures in the afterglow at plasma conditions P = 5 W, Φ Ar = sccm, t on = 4 ms, t off = 4 ms. The importance of the influence of argon metastable states can be proofed by means of the addition of e.g. oxygen as quenching gas. Metastable states are predominantly deexcited by argon oxygen collisions leading to production of ground state argon atoms. A good compilation of these rate constants for the quenching processes can be found in []. Therefore, an addition of oxygen to the plasma reduces the number of metastable argon and significantly influences the behavior in the late afterglow. Figure 3.9 shows the afterglow ion flux of Ar + of an argon and an argon:oxygen plasma with 2% oxygen at p =4Pa. For the pure argon plasma, Ar + ions are observable more than 7 µs after their vanishing in

55 3.3 Influence of reactor coating on plasma parameters 4 normalized count rate t/µs Figure 3.9: Time dependent ion flux of Ar + ions as integral over all energies at plasma conditions P = 5 W, p =4Pa,t on =4ms,t off = 4 ms: ( ) Ar plasma Φ Ar = sccm, ( )Ar:O 2 plasma Φ Ar : Φ O2 = sccm : 2 sccm. the argon:oxygen gas mixture due to reactions of metastables. These metastable states are quenched by oxygen and do not contribute to production of Ar + ions in the late afterglow. 3.3 Influence of reactor coating on plasma parameters For the plasma diagnostics of coating plasmas, the influence of the coating of the reactor surfaces, e.g. Plasmaline antenna and bottle cage has to be taken into account. Therefore, this section reveals the influence of coated reactor walls on the plasma and floating potential of a cw argon plasma. Figure 3. illustrates the floating potential U fl and plasma potential U pl depending on the number of applied optimized permeation barrier coating processes as merged in table 5.3. For a clean reactor chamber, U fl =3.4VandU pl =.Visrevealed for floating and plasma potential, respectively. For a rough estimation of the ion energy, it can be assumed, that the ions gain the energy of the potential difference between plasma and floating potential, which is U pl U fl =7.6 V for the obtained data. This value is in good agreement with the mean ion energy of ĒAr +(2 Pa) = 7.8 ev measured by means of mass resolved ion energy analysis (cp. section 3..2). A strong increase of floating and plasma potential is found depending on the number of applied coating processes in between the Langmuir probe measurements. Up to 25 applied processes, the potentials increase almost linearly with.95 V/process. The difference of both potentials remains constant as well as the determined plasma density and electron temperature. The drift in the potentials can be explained by a surface coating of the conduction reactor walls, especially the bottle cage, with an isolator (SiO x ). Therefore, the walls are charging and the plasma is not as grounded as compared to an uncoated conducting surface. The relative to ground measured potentials are shifted due to a voltage drop over a dielectric on top of the surface representing a kind of a capacitor. For a fixed current driven by the plasma, the voltage across the coating increases linear with the thickness of the dielectric and leads to the analyzed increase in floating and plasma potential. This phenomena of increasing potentials negatively influences the possibilities of plasma

56 42 Chapter 3 Characterization of reactor setup 4 Ufl,Upl / V number of coatings Figure 3.: Influence of a coating of the reactor chamber on ( ) plasma potential U pl and ( ) floating potential U fl at plasma conditions P = 5 W, p = 2Pa and Φ Ar = sccm as function of the number of applied optimized permeation barrier coating processes as merged in table 5.3. diagnostics based on electrical measurements, because the potentials of the plasma relatively change compared to the system ground. Therefore, e.g. high apparent ion energies are observed by means of mass spectrometry due to the grounded potentials inside the spectrometer. They shift with increasing coating thickness to higher energies. Thus, a determination of ion energy distribution functions of coating plasmas is not possible with the used setup. Additionally, probe measurements become challenging because for some applied voltages the surface charge leads to arcing depending on surface geometry, e.g. edges of the bottle cage. These arcing phenomena are observed for more than 25 applied coating procedures preventing further measurements by means of a Langmuir probe.

57 4. Plasma sterilization for aseptic filling of beverages The design of a plasma sterilization process according to today s regulations of food packaging associations is described in this chapter. Therefore, the classifications and definitions of aseptic filling processes are introduced in section 4.. For a fast and effective PET bottle sterilization, plasma parameters are revealed with respect to the results of fundamental research as recapitulated in section 4.2 and the discharge is characterized regarding these results. Finally, the sterilization capabilities are demonstrated for two relevant microorganisms as discussed in section Classification and definition of aseptic filling For the filling of beverages and pasty food, different methods are established depending on the requirements of the product to handle. In principle, two different kinds of products are distinguished:. acid foods or acidified food: food with a ph-value < 4.5 (e.g. soft drinks, fruit juices) 2. low-acid food: food with a ph-value 4.5 and a water activity a w >.85 (e.g. water, beer, milk) Regarding sterilization concerns, the first kind of products is not important due to the ph-value of the beverage, which does not provide an appropriate environment for spore germination [9, 3]. Additionally, salmonellae and staph are not able to grow [3]. In this environment, only mildews or yeast can proliferate leading to a visible spoiling. Thus, they represent no health risk for the customers. Therefore, food with a ph-value below 4.5 is no concern of regulations of FDA 2 [3]. VDMA 3 categorized these foods to be filled via hygienic filling machines of VDMA class IV [4] under lower requirements. Typically, the packaging materials are only partly decontaminated by means of UV or IR radiation, dry or wet heat or chemicals [9]. Contrarily, the low-acid foods and beverages require special treatment during filling applications. Conventional methods realize the filling of an unsterile product in an unsterile packaging, followed by a sterilization of the packed food, e.g. sterilization of filled cans by Water activity a w is a measure of free moisture in a food and is the quotient of the water vapor pressure of the substance divided by the vapor pressure of pure water at the same temperature [2] 2 FDA: The U.S. Food and Drug Administration is an agency of the United States Department of Health and Human Services and is responsible for the safety regulation of most types of foods and drugs. 3 VDMA: Verband Deutscher Maschinen- und Anlagenbauer e.v. is a non-profit organization representing the german machinery and industrial equipment manufacturers. The sector Food Processing and Packaging Machinery Association publishes recommendations and guidelines for food packaging applications. 43

58 44 Chapter 4 Plasma sterilization for aseptic filling of beverages heat. Modern packaging materials such as PET do not allow for a heat treatment. Therefore, beverages and pasty foods are typically filled into bottles or containers aseptically by means of an aseptic packaging machine according to VDMA class V [4] defined as follows. Definition of aseptic packaging machines Aseptic packaging machines are packaging machines in which a sterile product is packaged free from recontamination in a package which has been presterilized - usually within the packaging machine - or which has been formed and sterilized in the packaging machine [5]. Therefore, an aseptic packaging machine has to assure a presterilization of the packaging material and a prevention of recontamination of food by e.g. machine parts or filling tubes. 4.. Demands of aseptic filling For a realization of an aseptic process, the meaning of sterile has to be defined in terms of testable and traceable criterions by means of a microbiological validation. Therefore, the objective of microbiological validation of aseptic processes is to demonstrate that a sterilant or sterilization technology is capable of creating a condition of commercial sterility on equipment or surfaces or packaging materials used for aseptic packaging of low-acid foods [92]. Definition of commercial sterility According to [6], commercial sterility is defined: Commercial sterility of equipment and containers used for aseptic processing and packaging of food means the condition achieved by application of heat, chemical sterilant(s), or other appropriate treatment that renders the equipment and containers free of viable microorganisms having public health significance, as well as microorganisms of nonhealth significance, capable of reproducing in the food under normal nonrefrigerated conditions of storage and distribution. To provide evidence of the sterilization capability, challenge tests are performed to show the germ reduction of at least 4 decades for packaging material and the machine interior and 5 decades for the filler according to VDMA [5] for a suitable test germ. Table 4. lists the minimum requirements for aseptic packaging machines. Furthermore, on behalf of the FDA, a process authority has to test the sterilization capabilities for machine validation according to FDA standards. E.g. the FPA 4 performs tests as process authority defining a reduction of B. subtilis of 5 decades [3] and the NFL 5 also claims a 5 decade reduction of B. subtilis [92]. Two different kinds of challenge tests are described for a testing of sterilization capabilities [9, 9]:. count reduction test and 2. endpoint test. 4 FPA: The Food Products Association (formerly the National Food Processors Association, NFPA) is the principal scientific and technical U.S. trade association representing the food products industry. 5 NFL: The National Food Laboratory is an independent subsidiary of the Food Products Association (FPA).

59 4. Classification and definition of aseptic filling 45 part of interest count reduction rate packaging material logarithmic count reduction: 4 packaging machine interior logarithmic count reduction: 4 filler temperature/time-combination for sterilization steam: 2 C; 3 min. for products handling parts of the filler (or equivalent conditions) with other sterilization media: logarithmic count reduction: 5 Table 4.: Minimum requirements for aseptic packaging machines for suitable test germs [5] revealed by count reduction test or endpoint test according to [9] Both kinds are based on an artificial contamination of substrates or packaging materials with relevant microorganisms. The test substrates are treated by the sterilization process and the number of viable germs is determined before and after the sterilization process. Thus, a reduction rate can be determined. For the count reduction test, the reduction rate is determined for each sample placed at different positions in the aseptic machine, e.g. the packaging material, the inside of the sterile zone or the filling parts. Therefore, it allows for a system resolved determination of reduction capability. Contrary, for the endpoint test, contaminated packaging materials are applied to the system, sterilized and directly filled with a liquid culture medium. Afterwards, the number of unsterile packages is determined. Thus, the endpoint test considers the whole aseptic process including filling without a system resolved statement of capabilities. Typically, a number of at least packages are used for this kind of test with different base contaminations [9, 9]. For the test of sterilization capabilities of plasma sterilization in this thesis, the count reduction test is performed as described in section Established sterilization processes for aseptic packaging The sterilization processes of today s aseptic PET bottle filling machines are typically based on the usage of toxic chemicals responsible for spore reduction. The two main sterilization agents are hydrogen peroxide (H 2 O 2 ) and peracetic acid (C 2 H 4 O 3 ). Hydrogen peroxide For the decontamination of aseptic filling packaging materials, hydrogen peroxide is used in concentrations from 3% to 35% and sprayed in the package. The sterilization mechanisms of hydrogen peroxide is mainly oxidation based on decomposing and formation of active oxygen and hydroxide. Therefore, it allows for a sterilization of a wide range of microorganisms. A limiting effect of the capabilities of hydrogen peroxide are two enzymes of the germs, catalase and peroxidase. They are able to decompose hydrogen peroxide molecules to form water and oxygen molecules and therefore reduce the lethal effect. Due to a low sterilization capability of ambient temperature hydrogen peroxide, an activation by hot air (6 C T 25 C) has to be performed [7]. A uniform film of peroxide in the container and a proper activation are crucial factors of sterilization capabilities. Therefore, a mixture of peroxide and hot air instead of two separated processes are used to reduce the droplet size and decrease treatment time. An enlarged exposure time of hydrogen peroxide to packaging materials such as PET increases the difficulty to remove residues after

60 46 Chapter 4 Plasma sterilization for aseptic filling of beverages the sterilization process [7]. For a diminishment of residues, the packages are flushed by sterile air after the sterilization process to reduce the residual concentration of peroxide in the package. The residues represent a risk of possible interactions with filled beverages and for customers. The FDA defines a residual amount of hydrogen peroxide below.5ppm within a container [3, 7]. Peracetic acid Peracetic acid is an antimicrobial sterilant capable of effective usage at lower temperatures compared to hydrogen peroxide, e.g. ambient temperature. Due to its low ph-value and high oxidizing potential, the main sterilization mechanisms of peracetic acid is an oxidation of the components of the spores by means of active oxygen and hydroxide similar to hydrogen peroxide sterilization. An advantage of Peracetic acid is that it cannot be deactivated by catalase and peroxidase. Peracetic acid is used as a liquid by means of rinsing methods. Machines based on peracetic acid sterilization are available in the European and U.S. markets, but approval by FDA has still to be made. Disadvantages of established processes The established chemical methods for aseptic sterilization processes exhibit major problems due to the handling of liquid or gaseous toxic substances. The main disadvantages can be merged as follows: Surface coverage and homogeneous treatment: Sprayingofhydrogenperoxide in containers does not result in a cohesive film due to hydrophobic characteristic of PET. Therefore, only a coverage of 6% is reached for improved spraying techniques [7]. Assuming this inhomogeneous surface coverage, a homogeneous treatment is challenging. Residues: The residual concentration of sterilants has to be minimized according to regulations of FDA. The critical value of residual hydrogen peroxide below.5ppm according to FDA is challenging to prevent food modifications like decomposition or production of off-flavors. A proper flushing of packages after sterilization process is necessary increasing the treatment time. Additionally, a chemical interaction like solution or storage effects of hydrogen peroxide or peracetic acid by the container material can not be excluded leading to undefined influence on the packed food. Employee safety: The maximum allowable concentration (MAC) of hydrogen peroxide exposure is below ppm [3]. Therefore, a complex strategy has to be developed for the protection of employees. Therefore, alternative sterilization methods are necessary to be established based on new kinds of processes without the mentioned tremendous disadvantages of established sterilizers Selection of test spores and artificial contamination As previously described, the capabilities of a sterilization process have to be revealed by a reduction of relevant microorganisms. Therefore, test germs have to be chosen constituting a reproducible and high resistance against the main sterilization mechanism [9, 9, 5, 8].

61 4.2 Mechanisms of plasma sterilization 47 Table 4.2 lists the recommended test spores for various sterilization media. B. subtilis is an important spore for the evaluation of aseptic machines typically based on hydrogen peroxide or peracetic acid as sterilization medium. Today, B. subtilis is redefined by B. atrophaeus [9]. Therefore, in this thesis, B. atrophaeus (DSM , ATCC 7 589) shown in figure 4.(a) is used as a test spore. As revealed by fundamental research of plasma sterilization, VUV and UV radiation are the most important mechanisms for plasma sterilization of sprayed spores [3]. Therefore, additionally A. niger (DSM 957, ATCC 6275) shown in figure 4.(b) is used as test spore due to its known resistance against UV radiation (cp. table 4.2). (a) B. atrophaeus (DSM 2277) (b) A. niger (DSM 957) Figure 4.: SEM pictures of B. atrophaeus and A. niger [3] The test spores are sprayed on the test substrates to realize a homogeneous distribution of microorganisms on the sample. This spore distribution is known to be comparable to inartificial contamination of packaging materials [9]. 4.2 Mechanisms of plasma sterilization The mechanisms of plasma sterilization are a topic of ongoing research of various institutes due to usability of plasma based sterilization methods for many applications in the biological, medical and food packaging branch [, 2, 3, 4, 5]. In the European Union funded BIODECON project [33], main sterilization mechanisms are investigated concerning the influence on sterilization capabilities of low-pressure plasmas. Additionally, experiments are performed using particle and light sources to simulate the components of a plasma [6] and their impact on biological material. Two main processes are found to play a major role during the plasma sterilization of endospores: VUV and UV radiation on the one hand, and spore etching on the other hand. A plasma combines both mechanisms as an efficient sterilant. Compared to especially designed lamps for sterilization applications, plasmas have the advantage to allow for a homogeneous exposure of the germs in the plasma without shadowing effects due to the directional light of lamps. Both mechanisms of radiation and spore etching are discussed as follows: 6 Deutsche Sammlung von Mikroorganismen und Zellkulturen GmbH - German Collection of Microorganisms and Cell Cultures 7 American Type Culture Collection

62 48 Chapter 4 Plasma sterilization for aseptic filling of beverages medium test spore hydrogen peroxide B. subtilis B. subtilis B. subtilis B. subtilis B. subtilis or and heat B. globigii A. niger B. subtilis var. globigii hydrogen peroxide B. subtilis B. subtilis B. subtilis and uv radiation C. bifermentans B. subtilis peracetic acid B. subtilis B. subtilis A. niger superheated steam B. stearothermophilus B. stearothermophilus B. stearothermophilus A. niger B. stearothermophilus B. polymyxa dry heat B. stearothermophilus B. stearothermophilus B. stearothermophilus A. niger B. stearothermophilus wet heat B. stearothermophilus B. stearothermophilus A. niger B. stearothermophilus C. sporogenes C. sporogenes γ-radiation B. pumilus B. pumilus B. pumilus B. pumilus uv radiation A. niger A. niger reference [9] [] [92] [8] [9] comment VDMA class IV VDMA class V (aseptic filling) Table 4.2: Recommended test spores for various sterilization mediums (nowadays B. subtilis is replaced by B. atrophaeus [9]).

63 4.3 Definition of plasma parameters 49 Radiation VUV and UV photons are known to constitute effective sterilization mechanisms due to their high penetration depth in to the cell and causing lesions of DNA strands. Approximately 8 DNA strand breaks are necessary to inactivate a cell [7]. Halfmann et al. [3, 8] reveal, that VUV and UV radiation are the main sterilization mechanisms of bacillus and fungal spores depending on the spore properties. They reveal, that photons of λ = 2..3 nm play a major role for plasma sterilization of B. atrophaeus spores on specimen holders in a monolayer structure [8]. In particular, the wavelength range of λ = nm is assumed to be very efficient for sterilization of B. atrophaeus spores. This result is in good agreement with research results of other groups using mercury lamps shining with λ 254 nm for high efficient sterilization of spores [9]. Additionally, it is shown, that B. atrophaeus and B. stearothermophilus show similar wavelength dependent sensitivities. For spores of A. niger, radiation below λ = 2 nm is found to be required for optimized plasma sterilization [3]. The radiation of photons λ 3 nm is found to be very capable of killing A. niger spores. Spore etching The etching of spores constitutes supporting mechanisms for the sterilization of multilayer spore structures besides the lethal effect of VUV / UV radiation. Impinging ions and chemical reactive species interact only with the outermost layers of the endospore cell [2] and the sterilization capability is low compared to radiation based processes. For a sterilization of stacked spores, an etching of the spore layers is required to reduce the shadowing effect of the incident photons. Halfmann [3] shows, that spore etching takes place on larger time scales compared to the sterilization of monolayer spores. Opretzka et al. [2] show, that the simultaneous impact of hydrogen atoms and low energy ions cause a perforation of the endosporic shell. Therefore, chemical sputtering is revealed to be the mechanism of etching the spore coats forming volatile components [2]. 4.3 Definition of plasma parameters Based on the results of basic research on the sterilization mechanisms as previously compiled, plasma parameters can be defined for a quick and efficient plasma sterilization of PET bottles. The gas mixture used for sterilization challenge tests is a mixture of nitrogen, oxygen and hydrogen. These gases are capable of efficiently generating radiation in the mentioned wavelength ranges [3] and used in this thesis for comparability reasons. By means of an optimization process using absolutely calibrated emission spectroscopy it is revealed, that a mixture of Φ H2 : Φ N2 : Φ O2 =.2 :. :.8 constitutes an optimum for high light emission intensities in the desired wavelength range for sterilization of B. atrophaeus and A. niger. Therefore, the optimized gas mixture of Φ H2 = 6 sccm, Φ N2 = 5 sccm and Φ O2 = 4 sccm is used at p = 3 Pa for the sterilization tests. The applied power is chosen to be P = 2 W (t on = t off = ms), to realize a homogeneous ignition, and as low heat impact to the substrates as possible under high emission conditions. 4.4 Characterization of sterilization plasma Figure 4.2(a) shows the electron density n e versus the radius of the system during the pulse. The characteristic shape of the electron density is revealed as previously shown for pure argon discharges (cp. figure 3.). A maximum of the density n e =2.8 7 m 3 close to the

64 5 Chapter 4 Plasma sterilization for aseptic filling of beverages 5 x 7 5 x7 4 4 ne / m ne / m r/mm (a) Radial electron density n e profile during pulse (t = 898 µs). 5 5 t/µs (b) Time resolved electron density n e at r = mm. Figure 4.2: Radial and temporal behavior of electron density n e for H 2 : N 2 : O 2 plasma at p =3Pa, P = W and ton = t off = ms. Plasmaline antenna is reveal decaying for an increasing radius. The time dependent behavior of the electron density at the position of the maximum density (r = 2 mm) is plotted in figure 4.2(b). An ignition phase of the plasma is observed for µs t 7 µs showing a drop at t = 25 µs andt = 55 µs, which can be explained by the power characteristics of the microwave generator as discussed in section 2... The plasma is stable for t>7 µs and a rather constant electron density is observed. Figure 4.3 shows the measured emission spectrum of the plasma for 2 nm 8 nm during the pulse. The characteristic emission of nitrogen-oxygen bands are observed during the on-phase of the plasma leading to high radiation intensities in the wavelength range λ = 2..3 nm. 4.5 Sterilization results 4.5. Sterilization of specimen holders The time dependent sterilization results of B. atrophaeus and A. niger are shown for contaminated, specimen holders in figure 4.4(a) and 4.4(b), respectively. A logarithmic reduction R as described by equation (2.9) of more than five orders of magnitude ( 5 CFU) is achieved for B. atrophaeus within 5 s. The time kinetic of A. niger presented in figure 4.4(b) reveals no survivors after 2 s treatment time. Therefore, the maximum reduction is 4 CFU due to the lower base contamination. The D-values of B. atrophaeus and A. niger can be estimated to be less than.9 min, which confirms the fast sterilization capability Sterilization of PET bottles Also for the treatment of three dimensional objects comparable sterilization results are revealed in a further challenge test. Figure 4.5 shows the sterilization results of PET bottles contaminated with B. atrophaeus. The bottles are homogeneously contaminated and treated

65 4.5 Sterilization results 5 Photons / (s nm m 3 ) 6 x2 NO N 2 N 2 + O H β H α O O λ/nm (a) Overview spectrum (2..8nm). Photons / (s nm m 3 ) 6 x NO N 2 N λ/nm (b) Detailed spectrum (2..4nm). Figure 4.3: Optical emission spectrum of N 2 :O 2 :H 2 plasma at p =3Pa, P = W and t on = t off =ms.

66 52 Chapter 4 Plasma sterilization for aseptic filling of beverages R t / s (a) Time dependent reduction of B. atrophaeus. R t / s (b) Time dependent reduction of A. niger. Figure 4.4: Time dependent reduction of B. atrophaeus and A. niger on specimen holders for H 2 :N 2 :O 2 plasma at p =3Pa, P = W and ton = t off = ms. The solid line represents an exponential fit to the data and the dotted lines show the base contamination. with the same plasma parameters as used for the treatment of specimen holders. Comparable to the results of the sterilization of specimen holders, a reduction of five orders of magnitude is achieved within four seconds, claiming a good three dimensional sterilization homogeneity of the low-pressure plasma. R t / s Figure 4.5: Time dependent reduction of B. atrophaeus inside PET bottles for H 2 : N 2 : O 2 plasma at p =3Pa, P = W and ton = t off = ms. The solid line represents an exponential fit to the data and the dotted line shows the base contamination. The treatment time required for a sterilization in accordance with regulations of FDA and VDMA are below five seconds, which are very low compared to the treatment times described in [3, 8]. Compared to the emission of the plasma used by Halfmann et al. [8] for sterilization of B. atrophaeus, the intensity of the ignited microwave plasma is more than one order of

67 4.6 Conclusion 53 magnitude higher for the considered microwave plasma setup. This explains the short treatment time and confirms the importance of radiation in the mentioned wavelength range for plasma sterilization of B. atrophaeus. 4.6 Conclusion The shown results reveal a fast and effective sterilization of B. atrophaeus and A. nigerspores by means of a microwave plasma developed for bottle-shaped geometries. The treatment time to achieve a logarithmic reduction of more than 5 decades of B. atrophaeus and 4 decades of A. niger is below five seconds, which is an adequate treatment time for industrial aseptic filling machines. The low-pressure plasma allows for a cold sterilization process without usage of toxic chemicals. Neither an increased temperature is necessary for an activation of the sterilant, e.g. activation of hydrogen peroxide by hot air (6 C T 25 C) [7], nor time consuming flushing of the bottles to diminish residues by sterile hot air [7] is required. Therefore, plasma based sterilization processes are additionally promising for new packaging materials like biodegradable Polylactides (PLA), which are less heat resistant than PET and gaining in importance for future environmentally compatible packaging applications. The results show, that a plasma process is capable of sterilizing thermolabile packaging materials in accordance with the requirements defined by FDA and VDMA to reveal the state of commercial sterility by means of challenge tests. Therefore, plasma sterilization for aseptic packaging of low-acid beverages and pasty food is a good alternative to established sterilization methods based on toxic sterilants.

68 54 Chapter 4 Plasma sterilization for aseptic filling of beverages

69 5. Permeation barrier coating This chapter describes the development of a silicon oxide permeation barrier coating of PET substrates for packaging applications. Section 5. describes the attributes of permeation and gives a mathematical description required for a determination of permeation constants of coatings and substrates. Section 5.2 deals with the plasma polymerization of hexamethyldisiloxane and gives insights into the plasma chemistry forming the basis of the deposition. It is followed by the development of a barrier coating system on PET substrates and a detailed analysis of the influence of process parameters during the barrier coating deposition, namely oxygen dilution, process pressure, microwave power and pulse conditions. The influence of their variation on the barrier property, the coating composition and the surface morphology of the deposited coating is analyzed and the neutral and ion composition of the plasma are investigated by means of mass spectrometry and optical emission spectroscopy. The permeation mechanisms of deposited coatings are examined in section 5.5 regarding coating defects. The last section of this chapter describes the approach of coating bottles by means of the developed optimized barrier coating system. 5. Mathematical description of permeation The oxygen permeation of packaging foils is typically tested by carrier gas method as described in section Therefore, the foil separates two chambers. One chamber is flushed with oxygen as test gas and the other one is flushed with a nitrogen:hydrogen mixture, which carries the permeated oxygen to an electrochemical detector. The permeation of oxygen through polymer foils is schematically shown in figure 5. and can be described in a four step process consisting of adsorption of oxygen molecules to the outer surface of the polymer, absorption of oxygen by the polymer at the surface, diffusion of oxygen through the polymer driven by concentration gradient and desorption of oxygen on the other side of the polymer. (a) Adsorption, (b) Absorption, (c) Diffusion, (d) Desorption. Figure 5.: Illustration of four steps of permeation. 55

70 56 Chapter 5 Permeation barrier coating For the simulation of oxygen permeation through polymers this complex behavior can be reduced to a two step process, because oxygen permeation through a polymeric membrane is a solubility/diffusion-controlled process [4]. Therefore, it can be mathematically described as a two step process consisting of a solution of the oxygen in the polymer and a diffusion through it as described in the following sections. Solution of oxygen in the polymer The first step is a combination of adsorption and absorption of oxygen at the surface of the polymer described by a solubility coefficient s of oxygen by the polymer. This dimensionless solubility coefficient s is defined as the ratio of concentration c s of gas dissolved in the polymer to the concentration c of molecules in the surrounding gas phase [2]: s = c s c. (5.) The concentration c of oxygen molecules at atmospheric pressure and % oxygen can be estimated to be c = N A = cm 3, (5.2) V mol where N A = mol and V mol = cm 3 denote Avogadro constant and the volume of one mol at standard conditions, respectively. Diffusion of oxygen through the Polymer The second step is the diffusion of oxygen molecules through the polymer described by Fick s first law c(x, t) J(x, t) = D, (5.3) x where J is the oxygen flux density depending on time t and a spatial coordinate x, which is perpendicular to the polymer film surface. The oxygen flux density J depends on the gradient of concentration c(x,t) and a diffusion constant D, which is assumed to be independent of the concentration. This is a reasonable assumption for thin polymers like PET, x if interactions of the permeating gas molecules with the polymer are small and can be neglected [5, 22]. Therefore Fick s second law, which describes the time dependency of the diffusion is given by c(x, t) = D 2 c(x, t) (5.4) t x 2 for the considered problem using the continuity equation for one dimension. Thus, the mathematical models are introduced to describe the permeation of oxygen through polymers based on a solution of the gas and a diffusion through the polymer. A solution can be determined for the oxygen flux density J(x, t) and the concentration c(x, t) as describe in the following sections. Steady state solution For the solution of Fick s laws it is necessary to define boundary conditions. As described, the test foils separate two chambers with different oxygen concentrations. On the one side pure oxygen is feed, which is dissolved by the polymer leading to an initial concentration of

71 5. Mathematical description of permeation x8 2 c = c s PET c =cm 3 c(x)/ cm x/ µm d PET Figure 5.2: Definition of boundary conditions to describe diffusion through d PET =23µm thick PET: c(x =,t)=c s and c(x = d PET,t)=cm 3 : ( ) steady state solution c stat (cp. equation. (5.6)) and ( ) initial condition (cp. equation (5.9)). c s = sc for diffusion through the polymer. The second chamber is flushed with nitrogen to carry all permeated oxygen to the detector. Hence the oxygen concentration in this chamber is approximately zero. Therefore the time independent boundary conditions according to figure 5.2 can be defined as c(x =,t)=c s and c(x = d PET, t) cm 3. (5.5) So, the steady state solution c stat (x) =c(x, t ) can be determined [5, 23] c(x, t) t ( = D 2 c(x, t)! = c x 2 stat (x) =c s x ). (5.6) d PET The boundary conditions and the steady state solution to describe the diffusion of oxygen through a polymer foil are shown in figure 5.2. The steady state oxygen flux J stat = J(x = d PET,t ) can be determined by first Fick s law (5.3) to J stat = D c s( x d PET ) = Dc s. (5.7) x d PET x=dpet This allows for the determination of the diffusion coefficient D of tested foils, if the thickness d PET of the polymer, the steady state flux J stat and the solubility coefficient s are known: Time dependent oxygen flux D = J statd PET c s. (5.8) For the evaluation of the time dependent oxygen flux J(x = d PET,t), which is detected by the oxygen sensor, Fick s second law (equation (5.4)) has to be solved for given boundary and initial conditions of the oxygen concentration. Therefore, in the literature an initial condition c(x, t = ) =, [23, 24] is assumed, which does not agree with the boundary condition at the position x =,whichisc(x =,t ) = c s. Therefore, for the solution of the time dependent problem, an initial condition is assumed, which describes an exponential

72 58 Chapter 5 Permeation barrier coating decay of oxygen concentration in the polymer foil at t = s. This initial condition is given by c(x, t =)=c s exp( k x) (5.9) and shown in figure 5.2. This initial condition is in agreement with boundary conditions (equation (5.5)) and is very reasonable for strongly decaying exponential function as shown in figure 5.2. For this initial condition (equation (5.9)) and boundary conditions (equation (5.5)) Fick s second law (equation (5.4)) can be analytically solved to determine the time and spatial dependent oxygen concentration c(x, t): ( c(x, t) =c s x ) [ ( ( ) 2 ( ) nπ + B n exp D t) ] nπ sin x. (5.) d PET d n= PET d PET The time dependent flux J(x = d PET,t), which is detected by the electrochemical oxygen sensor can afterwards be calculated using Fick s first law (equation (5.3)) leading to J(x = d PET,t)= Dc s d PET D [ ( ( nπ B n exp n= d PET ) 2 D t) ] nπ cos(nπ) d PET (5.) with B n = 2c s d PET nπ d ( PET ) 2 ( exp( k d PET)) cos(nπ) k 2 nπ + d PET d PET nπ. (5.2) Figure 5.3 shows the result of time dependent oxygen flux J(x = d PET,t)of23µm thick PET foils and the comparison with measurement. An excellent agreement of the measured and simulated time dependent oxygen flux is given. The solubility coefficient s of the PET foils is determined by a least square fit to the experimental data to be s =.9. The revealed value for the oxygen solubility in PET is in good agreement to values reported in literature [25]. =.86 9 cm2 s Additionally, the diffusion coefficient D can be calculated to be D PET according to equation (5.8). Besides the description of the solution of oxygen by means of a dimensionless solubility factor s as shown in equation (5.), the solubility S can be defined according to Henry s law by means of the concentration and the gas pressure p: Using this relation, equation (5.8) can be simplified to J stat = D S = c s p = sc p. (5.3) c s d PET = D sc d PET = DS p d PET = P p d PET, (5.4) where P = DS is the permeability coefficient. For the investigated PET substrate, the values of the solubility coefficient S PET and the permeability coefficient P PET are S PET = cm 3 bar and P PET =4.4 9 cm s bar, respectively.

73 5.2 Plasma polymerization of hexamethyldisiloxane J/ cm 3 m 2 day t/ s Figure 5.3: Time dependent oxygen flux J of 23 µm thick PET foil: ( ) simulation for s =.9, ( ) measurement. Characterization of laminate systems According to ideal laminate theory, the permeability coefficient P tot of a multi layer system with a thickness d tot can be determined depending on the thicknesses d i and coefficients P i of the single layers from d tot P tot = i d i P i. (5.5) Thus, the permeability coefficient of coatings can be calculated under the knowledge of the layer thicknesses and the substrate properties P PET and d PET. 5.2 Plasma polymerization of hexamethyldisiloxane For the deposition of silicon oxide coatings by means of a plasma process, various silicon sources are available. Gaseous sources e.g. silane (SiH 4 ) or liquid monomers such as hexamethyldisiloxane (HMDSO, Si 2 O(CH 3 ) 6 ) or tetraethoxysilane (TEOS, Si(C 2 H 5 O) 4 )are successfully used for plasma based SiO x deposition. The advantage of using silicon containing monomers is to be easier to handle compared to e.g. silane due to the pyrophorous property of silane in ambient air and to allow for high deposition rates [26]. Additionally, the sticking properties of radicals of silane and silane fragments are known to be very high leading to inhomogeneous coatings on unheated substrates. Therefore, for the investigations of this thesis, liquid HMDSO is used as silicon source and evaporated to allow for gaseous admixtures. It is characterized by a high silicon and low oxygen content and the opportunity to sensitively control the degree of retention or fragmentation of molecular structure of the monomer [26]. The chemical structure of hexamethyldisiloxane is illustrated in figure 5.4. It consists of asi O Si base encased by six methyl groups. During a plasma process, it is mainly fragmented due to electron impact dissociation and fragments are deposited as a solid on the surface. A hypothetical reaction describing the oxidation of hydrocarbon species to volatile compounds and a solid SiO 2 coating is given by [27, 28] as: Si 2 O(CH 3 ) 6 +6O 2SiO 2 +3CO 2 +3CO+4H 2 O+5H 2. (5.6)

74 6 Chapter 5 Permeation barrier coating CH 3 CH 3 CH 3 Si O Si CH 3 CH 3 CH 3 Figure 5.4: Chemical structure of HMDSO (Si 2 O (CH 3 ) 6 ). Monomer (HMDSO) fragmentation (e,o 2,O,...) volatile gas phase fragments gas phase reactions (oxidized) fragments lower mass fragments ions dissociation ionization powder agglomeration by e.g. negative ions surface oxidation oxidized carbon and hydrogen etching SiO x C y H z coating Substrate Figure 5.5: Schematic of plasma polymerization of HMDSO leading to deposition of SiO x C y H z like coating according to [28, 39, 26, 29, 3]. Generally, the polymerization of HMDSO can be described by various processes in the gas phase and close to the surface as shown in figure 5.5. In a first step, the monomer is activated leading to fragmentation, ionization and excitation reactions in the plasma. Typically, the monomer molecules undergo electron collisions in the plasma leading to ionization and dissociation of HMDSO and its fragments. Reactions with other gases, e.g. oxygen or added noble gases can lead to gas phase reactions producing oxidized compounds. Additionally, hydrocarbon chemistry is observable. The gas phase components are partly adsorbed on the substrate surface and experience ion bombardment. Furthermore, surface reactions, such as oxidation or etching take place. Therefore, the properties of the deposited coating, like composition and morphology and permeation barrier, are strongly influenced by the surface reactions leading to e.g. water or carbon oxide production [26, 29]. Various parameters, like gas composition, substrate temperature, electron density and temperature, ion density and ion energy influence the deposition process and need to be understood for the reproducible deposition of high quality SiO x coatings. Therefore, a detailed identification of the reaction steps of plasma polymerization of oxygen diluted HMDSO plasmas is one of the most important research tasks [29] Analysis of HMDSO vapor The residual gas analysis of HMDSO vapor by means of mass spectrometry leads to a complex system of fragments, which are created by electron impact ionization within the ionizer of the mass spectrometer. Figure 5.6 shows the mass spectrum of HMDSO vapor at an

75 5.2 Plasma polymerization of hexamethyldisiloxane 6 normalized spectrum ~ SiH + 3 SiCH + 5 Si2OC2H ++ SiC2H + 7 Si2OC4H ++ SiC3H Si2OCH + 3 Si2OC2H + 7 Si2OC3H + 9 Si2OC4H + Si2OC5H + 5 ~ Si2OC6H m/z Figure 5.6: Fragmentation pattern of HMDSO under electron impact ionization (E ei =7eV) normalized to the peak of Si 2 OC 5 H + 5 (m/z = 47) electron impact ionization energy of E ei = 7 ev. Hexamethyldisiloxane has a molecular mass of 62 amu, but the mass spectrum is dominated by the fragment Si 2 OC 5 H + 5 with a mass to charge ratio of m/z = 47 [8, 3, 32, 33, 34, 35]. This fragment is created by an elimination of one methyl group from the monomer molecule. In detail, the elimination of this methyl group is based on a breaking of a Si C bond (cp. table 5.2). Thus, the fragment ion Si 2 OC 5 H + 5 (m/z = 47) is created by a dissociative ionization of the HMDSO monomer molecule. Thereby, the dissociative ionization starts with an ionization of the monomer molecule to create Si 2 OC 6 H + 8 (m/z = 62), which dissociates by removal of a methyl radical CH 3 [33]. The ionization of HMDSO preferentially takes place close to the oxygen atom building (CH 3 ) 3 Si O + Si (CH 3 ) 3, because of the composition of the molecule. This can be explained by a higher amount of electrons close to the oxygen atom due to higher electronegativity of oxygen (cp. table 5.2), which are preferentially removed by electron impact leading to ionization [32]. The ionization of the oxygen atom leads to an increase of attraction of free electrons by the oxygen atom and to a cleavage of a Si C bond, because the bond energy (3.7 ev) is less than the energy of Si Obond(4.6eV). Thus, the HMDSO molecule dissociates by removal of a methyl radical [32]. The cross section for this dissociative ionization can be determined to be.7 5 cm 2 for electron impact ionization with an energy of E ei = 7 ev [3, 34]. Besides the main fragment of HMDSO under electron impact ionization at m/z = 47, there are two peaks related to isotopes of the same molecule at m/z = 48 and m/z = 49 due to silicon isotopes 29 Si (4.7%) and 3 Si (3.%). The intensities of these peaks show relative intensities of 6. and 8., which is in good agreement with [3]. The fragment ion Si 2 OC 5 H + 5 (m/z = 47) is a source for further fragmentation of HMDSO leading to fragments with smaller masses [34]. The following discussion focuses on the production of single charged ions Si 2 OC 4 H + (m/z = 3), SiC 3 H + 9 (m/z = 73), SiC 2 H + 7 (m/z = 59) and SiCH + 5 (m/z = 45). Additionally, the fragmentation paths of double charged ions Si 2 OC 4 H ++ 2 (m/z = 66) and Si 2 OC 2 H ++ 8 (m/z = 52) are considered. Figure 5.7 shows the fragmentation paths of the creation of single charged fragments.

76 62 Chapter 5 Permeation barrier coating m/z ion relative intensity [3] [32] [33] [8] [34] measurement 62 Si 2 OC 6 H Si 2 OC 5 H Si 2 OC 4 H Si 2 OC 4 H Si 2 OC 3 H Si 2 OC 3 H Si 2 OC 2 H Si 2 OC 2 H Si 2 OCH SiOC 2 H SiC 3 H Si 2 OH ,5 Si 2 OC 4 H Si 2 OC 4 H SiC 2 H SiOCH SiC 2 H Si 2 OC 2 H SiCH SiOH SiCH SiCH SiCH SiH C 2 H SiH Si CH Table 5.: Relative intensities of fragments of HMDSO under electron impact ionization at E ei = 7 ev normalized to Si 2 OC 5 H + 5 (m/z = 47): values reported from literature and measurement. bond bond energy element electronegativity Si O 4.6eV Si.7 Si C 3.7eV H 2.2 C H 4.5eV C 2.5 O 3.5 Table 5.2: Bond energies of HMDSO [36] and electronegativity of the elements by Allred and Rochow. The fragment ion SiC 3 H + 9 (m/z = 73) is produced by elimination of the stable molecule OSi(CH 3 ) 2 (m/z = 74) from the main fragment (m/z = 47). Based on further fragmentation of SiC 3 H + 9 (m/z = 73), the ion SiCH + 5 (m/z = 45) is created by elimination of C 2 H 4 (m/z = 28) under sufficient energy of electron impact ionization [34]. A second way of fragmentation of the main fragment shown in figure 5.7 is the formation of Si 2 OC 4 H + (m/z = 3), which is based on the elimination of a CH 4 (m/z = 6) mole-

77 5.2 Plasma polymerization of hexamethyldisiloxane 63 Si 2 OC 6 H 8 electron impact ionization Si 2 OC 6 H + 8, (m/z = 62) CH 3 (m/z = 5) dissociation Si 2 OC 5 H + 5, (m/z = 47) OSi(CH 3 ) 2 (m/z = 74) CH 4 (m/z = 6) SiC 3 H + 9, (m/z = 73) Si 2OC 4 H +, (m/z = 3) C 2 H 4 (m/z = 28) SiOC 2 H 4 (m/z = 72) SiCH + 5, (m/z = 45) SiC 2H + 7, (m/z = 59) Figure 5.7: Fragmentation paths of HMDSO according to [33, 34, 32]. cule [34]. The ion Si 2 OC 4 H + (m/z = 3) is afterwards fragmented into SiOC 2 H 4 (m/z = 72) and SiC 2 H + 7 (m/z = 59), which also represent fragments with significant intensities in the mass spectrum of HMDSO vapor. Besides the production of single charged fragments, significant parts of double charged ions are found (cp. figure 5.6) at (m/z=66) and (m/z=52), which belong to Si 2 OC 4 H ++ 2 and Si 2 OC 2 H ++ 8, respectively. The fragment Si 2 OC 4 H ++ 2 (m/z = 66) is a double charged ion, which is produced by elimination of two methyl groups from the HMDSO molecule and a double ionization. Both double charged ions are present in the spectrum for electron energies above E ei =2eVorE ei = 3 ev as reported by [3] and [3], respectively. The double charged ions differ in an elimination of a neutral molecule C 2 H 4. Besides this fragmentation path, a second fragmentation path occurs leading to the production of two single charged molecules SiOCH + 3 (m/z = 59) and SiC 3 H + 9 (m/z = 73) [33]: Si 2 OC 4 H ++ 2 (m/z = 66) SiOCH + 3 (m/z = 59) + SiC 3 H + 9 (m/z = 73). (5.7) Beyond the production of the mentioned molecules, numerous fragments with lower intensities and smaller masses are observable in the mass spectrum of HMDSO vapor shown in figure 5.6. In detail, the measured intensities and the correlation to the molecule configuration are listed in table 5.. They are in good agreement with fragmentation patterns reported by other groups [8, 3, 32, 33, 34]. Therefore, the good agreement of the measured fragmentation pattern with the results of other groups confirms the determination of the mass dependent transmission function (equation (2.9)) of the mass spectrometer described in section The correlation of the measured mass to charge ratios to a molecule composition, as listed in table 5., leads to different molecules revealed by different authors. Depending on the interpretation of the revealed data, different possible molecule compositions are published. Exemplarily, these can be found for m/z = 73, m/z = 59, m/z =45

78 64 Chapter 5 Permeation barrier coating and m/z = 29. A detailed investigation of the exact mass revealed by high resolution mass spectrometry would enable determination of the exact molecule composition, which could not be performed with the present mass spectrometer. Therefore, the measured intensities are combinations of the fragments with similar masses. Influence of electron impact energy on fragmentation pattern As shown in figure 5.6, an electron energy of E ei = 7 ev leads to numerous fragments of HMDSO using an electron impact ionization source as implemented in the mass spectrometer for residual gas analysis. For an investigation of lower mass fragments, less fragmentation is desired to prevent production of various dissociation products in the mass spectrometer. Therefore, for the investigation of the neutral gas composition within this thesis, an electron impact ionization energy of E ei =25eVisused. normalized spectra 5 ~ ~ m/z Figure 5.8: Influence of electron impact energy on fragmentation pattern of HMDSO normalized to the peak of Si 2 OC 5 H + 5 (m/z = 47) for important fragments: ( ) E ei =25eV,( ) E ei = 7 ev. (For peak assignment compare table 5.). Figure 5.8 shows the influence of reduced electron energy for important fragments of HMDSO. Only little production of SiC 3 H + 9 (m/z = 73) and marginal creation of fragments at m/z = 28, m/z = 7, m/z = 3 and m/z = 33 is observed. The double charged fragments Si 2 OC 4 H ++ 2 (m/z = 66) and Si 2 OC 2 H ++ 8 (m/z = 52) are not found due to higher appearance energies. Measurements reveal observable double charged fragments m/z = 66and m/z = 52 with relative intensities above.% for E ei 35 ev and E ei 4 ev, respectively, which is in agreement with appearance energies revealed by Seefeldt et al. [32] Analysis of ions Besides the investigation of the neutral gas composition of HMDSO containing plasmas, an analysis of the positive ions delivers insight into plasma chemistry during coating deposition. For the investigation of an ion spectrum by means of energy resolved mass spectrometry, a proper setting of the energy filter (cp. figure 2.5) has to be applied. As shown in section 3.3, a deposition of coatings on the bottle cage leads to shifts of floating potential U fl and

79 5.3 Pulsed coating deposition 65 plasma potential U pl with increasing layer thickness. Therefore, electrical measurements by means of Langmuir probes or energy resolved mass spectrometers are challenging. For mass spectrometer investigations, an ostensible shift of ion energies to higher ion energies is observed by increasing coating thicknesses due to fixed ground potentials of the mass spectrometer and shifting potentials of the plasma. Additionally, the ion energy distribution functions are slightly disturbed by increasing coating thicknesses. Therefore, the ion mass spectra of HMDSO plasmas are obtained by an iterative two step process consisting of () the determination of the mean ion energy of e.g. oxygen ions or HMDSO ions and applying this energy to the sector field energy analyzer for the next step. (2) A determination of a mass spectrum for ions of this mean energy. This procedure enables for a measurement of ions impinging on the substrate surface despite potential shifts of the plasma during deposition processes Fragment ambiguities of oxygen diluted HMDSO plasmas The addition of oxygen to HMDSO containing gas mixtures leads to ambiguities of peak assignments of the fragmentation pattern revealed by mass spectrometry. Especially, for carbon and oxygen containing molecules like CO (m/z = 28) and CO 2 (m/z = 44), an overlap to hydrocarbons exist. For example, a fragment detected at m/z = 28 can be described as CO or C 2 H 4 and a fragment at m/z = 44 can be composed as CO 2 or C 2 H 4 O. Magni et al. [3] investigate the ambiguities by means of complementary of mass spectrometry and in situ FTIR spectroscopy. They confirm, that the main contribution to m/z =28and m/z = 44 for high oxygen diluted plasmas is due to CO and CO 2, respectively, because combustion reactions of carbon containing molecules by oxygen shortcut the hydrocarbon chemistry [3]. Therefore, the peak assignment of these fragments during the following investigations is CO and CO 2, because high oxygen diluted plasmas are investigated. 5.3 Pulsed coating deposition In comparison to continuous wave plasmas, pulsed plasmas offer many advantages concerning interdependencies of plasma parameters like gas fluxes, process pressure and plasma power causing inhomogeneities of deposition. The homogeneity of the deposited layers is mainly influenced by the gas fluxes, the process pressure and pulse conditions of the plasma. Figure 5.9 shows the deposition rate inside a bottle along the z-axis (see figure 2.) depending on the time t off between the pulses. For the measurement of the coating thickness, an adhesive tape protects parts of the bottle surface during the coating process and permits the determination of the layer thickness by means of a stylus profiler. Therewith, the deposition rate can be determined along the z-axis by measuring the step height between coated and uncoated regions. An increase of t off leads to a considerable homogenization of the deposition rate. The residence time τ of a gas mixture inside the bottle has to be considered for an optimization of the deposition rate. It can be estimated by τ = pv Φ HMDSO + Φ O2, (5.8) where p denotes the process pressure, V the volume of the bottle and Φ HMDSO and Φ O2 the fluxes of HMDSO and oxygen, respectively. The residence time for the given example can be calculated to be τ =4ms. Fort off <τ(figure 5.9(a) and 5.9(b)) the deposition

80 66 Chapter 5 Permeation barrier coating deposition rate / (nm s - ) deposition rate / (nm s - ) z/mm (a) t off =ms. 5 z/mm (c) t off =4ms. deposition rate / (nm s - ) deposition rate / (nm s - ) z/mm (b) t off =2ms. 5 z/mm (d) t off =8ms. Figure 5.9: Deposition rate of SiO x coatings along z-axis (see figure 2.) for different inter-pulse durations t off for a HMDSO : O 2 plasma at Φ HMDSO = 4 sccm, Φ O2 = 4 sccm, p =3Pa,P = 5 W and t on = 4 ms. (The solid lines serve as a guide for the eye). is mainly observed close to the end of the Plasmaline antenna, where process gases flow in. For increasing t off the deposition rate becomes more homogeneous. Nevertheless, for t off =msandt off = 2 ms a depletion of the deposition rate is found along the z-axis. For t off = τ = 4 ms (figure 5.9(c)), no depletion of the deposition rate appears and a suitable level of homogenization is achieved. The mean deposition rate can be calculated to be 3.2nms for the given conditions. If the inter-pulse period is increased further (figure 5.9(d)), the mean deposition rate is reduced to.6nms for t off = 8ms and the influence on coating homogeneity is negligible. For an optimization of the coatings homogeneity, a complete gas exchange must occur within the inter-pulse period leading to a criterion for the value of t off depending on the total gas flux, the pressure and the volume of the bottle: t off τ. (5.9) If the inter-pulse period t off is chosen in compliance with equation (5.9), it is possible

81 5.4 Development of barrier layer system on PET foils 67 to select gas fluxes and the process pressure independently and phenomena related to interdependencies are prevented. Each gas mixture undergoes only one pulse. Thereby, a homogeneous deposition rate and process chemistry are realized. The exhaust gases of the reaction are pumped during the inter-pulse period and fresh gas is provided until the next pulse is ignited. A homogeneous deposition rate and process chemistry can be realized by a consideration of equation (5.9) to determine the inter-pulse period of pulsed plasmas. It can be stated, that besides the description of pulsed plasmas by a pulse frequency and duty cycle, a definition of t on and t off is more reasonable. Because especially t off can be determined by a consideration of other process parameters and has a very significant influence on pulsed coating deposition. Therefore, this criterion is promising for upscaling of processes e.g. for treatment of different substrate sizes or reactor chamber volumes, which is a major topic of industrial plasma processes [37, 38]. 5.4 Development of barrier layer system on PET foils The aim of this work is to deposit transparent SiO x coatings on the inner surface of PET bottles to decrease the gas permeation. A barrier layer system has to be developed, which is capable to be deposited on PET substrates and reveal a good adhesion and barrier properties. For the development of the barrier system, PET foils are used instead of bottles due to a faster determination of the barrier property and an independency of problems due to complex geometry of bottles (cp. section 5.6). Therefore, the foils are mounted at the position of a bottle wall during the deposition process as described in section Adhesion promoting layer The deposition of SiO x coatings on pristine PET by means of low-pressure microwave plasma polymerization is found to be not applicable due to adhesion problems of the silicon oxide coating on PET. Even high barrier SiO x coatings as developed in this chapter are not capable of producing reasonable barriers on the substrate material. An investigation of the reasons for this behavior of reduced barrier property results from delamination of the barrier coating on PET substrates. µm 2µm Figure 5.: SEM pictures of nm thick SiO x barrier coatings deposited without adhesion promoting layer under optimized conditions (cp. section 5.4.6) on pristine PET. Figure 5. shows SEM pictures of silicon oxide coated PET foils. The layer thickness of the

82 68 Chapter 5 Permeation barrier coating SiO x coatings can be determined to be d = nm, which is above the critical thickness of SiO x coatings to act as a permeation barrier coating (cp. section 5.4.5). It can be deduced, that the silicon oxide like coating only exhibits poor adhesion to the PET substrate. Therefore, a delamination occurs leading to bad barrier properties of the coating. Figure 5. exemplary shows a delamination along coating cracks. The SiO x rolls up at these positions and the PET substrate material can be seen. Obviously, there is negligible adhesion between the coating and the substrate. By reason of this delamination, the barrier improvement of an inorganic SiO 2 coating on organic PET substrates by means of a microwave plasma with relative low ion energies is negligible. Plasma parameter influence on deposition of adhesion promoting layer For the realization of a transparent SiO x permeation barrier on PET, an adhesion promoting layer has to be deposited prior the deposition of an inorganic SiO x barrier coating. Therefore, this section describes the properties of the adhesion promoting layer and reveals plasma parameters for its deposition. For an investigation of the properties of an adhesion promoting layer, an optimized SiO x barrier coating is deposited on top of the adhesion promoting layer as it is developed in the following sections and recapitulated in section (table 5.3). In combination with a good adhesion promoting layer, this optimized barrier coating is capable of reducing the permeation flux of PET foils (J PET =68.2 ± 2.cm 3 m 2 day )to J =±.3cm 3 m 2 day. This value gives an estimation for typical deviations in the coatings reproducibility and measurement system (cp. section 5.4.6) and has to be taken into account during analysis of the revealed permeation results. The influences of various process parameters of a HMDSO : O 2 plasma during deposition of the adhesion promoting layer, namely duration of deposition, pulse duration and power, oxygen dilution and process pressure are investigated. Figure 5. show the influence of plasma parameters on the oxygen permeation flux. Each part of the figure shows the variation of one single parameter, while the other parameters are fixed at optimized values. Deposition time of adhesion promoting layer The influence of the deposition time t a.p. of the adhesion promoting layer shown in figure 5.(a) is very significant. A deposition of a good barrier coating on pristine PET (t a.p. = s) leads to an oxygen permeation of J =2cm 3 m 2 day due to coating delamination as shown in figure 5.. At least one second of deposition of an adhesion promoting layer realizes a good barrier improvement of the resulting layer system. Shorter treatment times can not be realized reliably by the current reactor setup. For longer deposition times t a.p., the resulting oxygen permeation remains constant and no further enhancement is achieved. For the barrier coating within this work, t a.p. was chosen to be t a.p. = 3 s, which ensures a sufficient coating time for the deposition of the adhesion promoting layer. The dependency of pulse duration t on of the HMDSO plasma on the barrier property is shown in figure 5.(b). It can be deduced, that the pulse duration t on has to be shorter than 2 ms for the deposition of adequate barriers. The pulse duration has a significant influence on the fragmentation as revealed in section by EMS measurements. For the deposition of good adhesion promoting layers, only a low fragmentation is desired to retain the structure of HMDSO.

83 5.4 Development of barrier layer system on PET foils J /cm 3 m 2 day J /cm 3 m 2 day t a.p. / s 5 5 t on / ms (a) Variation of deposition time t a.p. (b) Variation of pulse duration t on 2 3 J /cm 3 m 2 day.5.5 J /cm 3 m 2 day P/W p/pa (c) Variation of power P (d) Variation of process pressure p 3 3 J /cm 3 m 2 day J /cm 3 m 2 day Φ HMDSO / sccm Φ O2 / sccm (e) Variation of HMDSO flux Φ HMDSO (f) Variation of oxygen flux Φ O2 Figure 5.: Permeation rates J of barrier system for various plasma parameters during deposition of the adhesion promoting layer. Optimized conditions for the deposition of adhesion promoting layer are marked as ( ): Φ HMDSO = 5 sccm, Φ O2 = sccm, p =3Pa, P = 3 W, t on = ms, t off = 6 ms and t a.p. = 3 s. On top of the adhesion promoting layer, a barrier coating according to table 5.3 is deposited.

84 7 Chapter 5 Permeation barrier coating Microwave power The microwave power during pulse ignition has only minor influence on the deposition of adhesion promoting layers regarding barrier properties of the coating system. Figure 5.(c) shows the permeation rates of the coating system for various powers 75 W P 4 W. The revealed permeation rates differ in the range of the mean value and standard deviation (±.3cm 3 m 2 day ) of the coating and measurement process. For the deposition of the adhesion promoting system, a pulse power of P = 3 W is chosen to ensure a homogeneous plasma ignition along the z-axis. Process pressure The process pressure during the adhesion promoting layer deposition has a major influence on the barrier property as displayed in figure 5.(d). An optimized pressure is revealed to be 2 Pa p 3 Pa. For lower pressures, no homogeneous plasma ignition is observed leading to only partly coverage of the substrate by the adhesion promoting coatings. This result is in agreement with the pressure dependence of plasma elongation as investigated by Langmuir probe measurements in section 3... For pressures p>4 Pa, an increase of permeation is observed, too. Due to the increased process pressure, the deposition conditions are modified leading to less ion bombardment of the substrate as phenomenologically investigated in section Therefore, it can be stated, that a certain ion energy is required for the deposition of organic SiO x C y H z like films on PET surfaces. Gas composition The barrier property of the layer system strongly depends on the gas mixture of the plasma. Figure 5.(e) shows the result of different HMDSO fluxes 2 sccm Φ HMDSO 5 sccm. To realize the low gas fluxes in accordance to homogeneity criterion (equation (5.9)) and the realizable inter-pulse periods (cp. section 2.), argon was added to remain a total gas flux of Φ HMDSO + Φ Ar = 5 sccm. An oxygen addition to HMDSO plasma during deposition of the adhesion promoting layer significantly reduces the barrier property. Figure 5.(f) shows a strong increase of permeation rate by adding oxygen to the plasma. Concluding the results of parameter variations of deposition of the adhesion promoting layer, it is revealed that good adhesion promoting layers have an organic composition and therefore are deposited without any oxygen addition. A low fragmentation of the HMDSO molecules has to be achieved by short pulses to maintain a similar structure in the layer composition. Additionally, a high pulse power has to be chosen for homogeneity reasons. A deposition of an adhesion promoting layer alone reveals no barrier improvement, even if the layer is deposited for more than 4 s a permeation flux of J =67cm 3 m 2 day is determined, which is comparable to uncoated PET J PET. Composition of adhesion promoting layer Figure 5.2 shows the FTIR absorption spectrum of the adhesion promoting layer. The layer composition mainly consists of carbon hydrogen bonds and is SiO x C y H z like. A comparison to the structure of PET substrate material (cp. figure 2.4) reveals similarities in spectra regarding C O bonds at 26 cm and 72 cm and 72 cm. Furthermore, the spectrum of pristine PET and the organic adhesion promoting layer show major similarities regarding CH bonds at 296 cm, 4 cm, cm and 83 cm. Additionally,

85 5.4 Development of barrier layer system on PET foils 7 absorption / a.u Si-O-Si C-H Si-H C=O C-H C-O C-H Si-O-Si SiCH 3 C-H k/cm 2 5 Figure 5.2: FTIR absorption spectrum of adhesion promoting layer deposited at plasma conditions Φ HMDSO = 5 sccm, Φ O2 = sccm, p =3Pa,P = 3 W, t on = ms and t off = 6 ms. silicon containing molecules are observed at 23 cm (SiH), 8 cm (SiC) and as SiOSi at 3 cm and 8 cm. In contrast, the spectrum of the SiO x as shown in figure 2. is different from the organic bond composition of PET and the optimized adhesion promoting layer. It can be concluded, that the good adhesion of SiO x C y H z like coatings is due to carbon and hydrogen containing groups, whereas silicon oxide like compositions reveal only poor adhesion on PET. Mass spectroscopic investigation of plasma during deposition of adhesion promoting layer An analysis of neutral gas composition of the plasma used for deposition of the optimized adhesion promoting layer does not show reproducible measurable changes compared to the analysis of HMDSO vapor. Due to the short pulse duration (t on = ms) and the long interpulse duration (t off = 6 ms), the average deviation of the neutral gas phase does not allow for the measurement of a variation. Therefore, it can be stated, that the plasma conditions only lead to a minor fragmentation and a gas composition comparable to HMDSO vapor (cp. figure 5.8) is present. This result is confirmed by an analysis of the positive ion spectrum as shown in figure 5.3. The spectrum is dominated by Si 2 OC 5 H + 5 (m/z = 47) and its isotopes at m/z = 48 and m/z = 49. Furthermore, smaller amounts of the following ions are observed: Si 2 OC 6 H + 8 (m/z = 62), Si 2 OC 3 H + 9, (m/z = 7) and SiC 3 H + 9 (m/z = 73). Ions of lower mass fragments are not found in the spectrum of positive ions. Therefore, the surface is mainly hit by ions with a SiO x C y H z like structure resulting from low fragmentation of the monomer molecule. Optimized adhesion promoting layer For the deposition of good barrier coatings on PET surfaces by means of a microwave plasma, the substrate/coating interaction is a relevant topic for investigations. The deposition of SiO x like coatings on pristine PET leads to delamination as shown in figure 5. resulting

86 72 Chapter 5 Permeation barrier coating normalized spectra SiC3H + 9 Si2OC3H + 9 Si2OC4H + Si2OC4H + 3 Si2OC5H + 5 Si2OC6H m/z Figure 5.3: Ions spectrum during deposition of adhesion promoting layer at plasma conditions Φ HMDSO = 5 sccm, Φ O2 = sccm, p =3Pa,P = 3 W, t on = ms and t off = 6 ms. in poor barrier properties. As adhesion promoting layers, SiO x C y H z coatings are appropriate. They can be deposited under low fragmentation conditions of pure HMDSO plasmas (Φ HMDSO = 5 sccm) realized by means of short pulses (t on = ms). The pulse power has only minor influence on the coating properties. A threshold value for a proper homogeneous plasma ignition has to be exceeded. To ensure a homogeneous plasma treatment, the power is chosen to be P = 3 W. Furthermore, the process pressure has to be optimized realizing a certain ion bombardment of the substrate and preventing inhomogeneous plasma ignition for too low pressures. Therefore, a pressure of p = 3 Pa is revealed to cope these requirements. The deposited coating is characterized by FTIR spectroscopy to mainly consist of SiCH bonds and similarities to the structure of the PET composition are revealed.

87 5.4 Development of barrier layer system on PET foils Influence of oxygen dilution during barrier coating deposition On top of the previously developed adhesion promoting layer, a barrier coating is deposited, that is discussed in the following sections. The influence of main plasma parameters, namely oxygen dilution, pulse duration and power and process pressure are independently investigated, which is possible due to the revealed homogeneity criterion (equation (5.9)). The influence of oxygen admixture is very significant for plasma polymerization using HMDSO : O 2 plasmas and permits a control of coating properties ranging from organic layers containing carbon and hydrogen to inorganic, quartz-like ones. Hence the permeation barrier properties are governed by the amount of oxygen added to the plasma as constituted in equation (5.6). Permeation behavior 2 J/cm 3 m 2 day Φ O2 / sccm Figure 5.4: Permeation rates of 6 nm thick SiO x coatings for various oxygen fluxes 5 sccm Φ O2 6 sccm at plasma conditions Φ HMDSO = 4 sccm, p =3Pa,P = 5 W, t on = 4 ms and t off according to equation (5.9). Figure 5.4 shows the behavior of oxygen permeation through coated PET foils depending on the oxygen flux Φ O2 of the plasma. The thickness of the deposit is d =6 nm,whichis above the critical thickness for the deposition of barrier coatings (cp. section 5.4.5). The HMDSO flux is held constant at Φ HMDSO = 4 sccm for various oxygen fluxes and the layers are deposited at a process pressure of p = 3 Pa. The plasma inter-pulse period t off is adapted according to equation (5.9) to ensure a complete gas exchange between two pulses. The pulse power is P = 5 W. Increasing the oxygen dilution of the plasma, a strong reduction of oxygen permeation is observed [39, 4], leading to a residual permeation of J =cm 3 m 2 day for oxygen fluxes above Φ O2 4 sccm, which is a reasonable value for food packaging applications [4]. The permeability coefficient P SiOx is determined according to equation (5.5) to be P SiOx =.7 5 cm s bar. It is four orders of magnitude lower than the coefficient of the untreated PET substrate.

88 74 Chapter 5 Permeation barrier coating Coating composition atomic percentage Φ O2 / sccm Figure 5.5: Atomic concentration of O 2, Si and C in deposited layers for various oxygen fluxes 5 sccm Φ O2 4 sccm at plasma conditions Φ HMDSO = 4 sccm, p = 3Pa, P = 5 W, t on = 4 ms, t off according to equation (5.9) determined by EDX measurements: ( ) oxygen, ( ) silicon, ( ) carbon. The results of EDX measurements for various oxygen fluxes Φ O2 are shown in figure 5.5. A strong influence of Φ O2 on the layer composition is observed regarding carbon, silicon and oxygen content. The carbon content of the coatings deposited under low oxygen fluxes (Φ O2 = 4 sccm) is found to be 49%. This can be explained by insufficient oxidation of carbon containing groups of HMDSO during plasma polymerization. For increasing Φ O2,the carbon content vanishes and for Φ O2 2 sccm it is below the detection limit of the EDX setup. Additionally, the measurements reveal a stoichiometry of SiO.7, which is reached for high oxygen dilutions of the HMDSO : O 2 plasma. Besides the EDX analysis, the layer composition of deposited coatings is evaluated by means of FTIR spectroscopy to confirm the results of EDX measurements and consider hydrogen incorporated in the layer, which can not be detected by EDX measurements. The composition of the coatings deposited under various oxygen fluxes (figure 5.6) shows great differences for low and high oxygen dilutions of the plasma. For low oxygen dilutions an apparent amount of carbon in the layer is detected. The absorption peak at 26 cm can be attributed to symmetric deformation of CH 3 of methylsilyl (SiCH 3 ) groups in the deposit [78]. The position of this peak is known to be sensitive to the number of methyl groups bounded on a silicon atom [78] and a shift from 26 cm for HMDSO : O 2 =: to 278 cm for HMDSO : O 2 = : is observed as SiCH 3 vanishes. Additionally, a large absorption at 8 cm is present, which is correlated to stretching vibrations of SiC and rocking vibrations of CH 3 in Si(CH 3 ) x [78, 79]. At this wave number also absorption based on bending of SiO is present which is overlapped by SiC stretching vibrations at 8 cm [79, 8, 82]. Furthermore, absorptions correlated to stretching vibrations of CH in the molecule SiCH 3 at 2965 cm and SiH at 225 cm are existent in the coating [5, 86, 42]. The absorption of vibrations correlated to SiCH 3 groups vanish for higher oxygen dilutions, which can be related to an increased oxidation of carbon containing molecules during the layer deposition. For oxygen fluxes Φ O2 4 sccm no peaks are

89 5.4 Development of barrier layer system on PET foils 75 SiOSi SiCH SiOSi 3 CH CC SiCH 3 absorption / a.u. SiOH : :5 : :25 :5 : SiOH SiOSi k/cm 5 Figure 5.6: FTIR absorption spectra for various oxygen dilutions from HMDSO : O 2 =:to : at plasma conditions Φ HMDSO = 4 sccm, p =3Pa,P = 5 W, t on =4ms and t off according to equation (5.9). Peak position (SiCH3) / cm Φ O2 / sccm Figure 5.7: Peak position of SiCH 3 symmetric deformation vibrations for various oxygen fluxes Φ O2 at plasma conditions Φ HMDSO = 4 sccm, p =3Pa,P = 5 W, t on = 4 ms and t off according to equation (5.9). present, which are related to carbon containing groups. Hence the absorption at 8 cm can be attributed to pure bending vibrations of SiO and it can be stated that the layers are carbon free. The absorption at 3 cm is correlated to asymmetric stretching of SiOSi and shows the known high wave number shoulder, which is also related to asymmetric stretching of the same bond [43]. A shift of this absorption peak to higher wave numbers during the transition from organic to inorganic character of the layer is observed, which conforms to investigations of Walker et al. [78] and Pai et al. [8]. The maximum value of the observed peak position of SiOSi stretching vibrations is at 3 cm. It is assumed that the peak position at 7 cm, which is typically correlated to SiOSi stretching in pure SiO 2 [44, 45],

90 76 Chapter 5 Permeation barrier coating is not reached because of silanol (SiOH) bound hydrogen in the coating. SiOH stretching and bending vibrations are found at 33 cm and 93 cm, respectively. For a detailed analysis of the layer composition of the deposited coatings by means of FTIR spectroscopy, the absorbance of different peaks are analyzed. Therefore, the absorption peak of symmetric deformation vibrations of CH 3 of methylsilyl (SiCH 3 ) groups at 26 cm is analyzed, which is a sensitive representant of bounds correlated to carbon containing groups in the coating. The absorption peaks are analyzed by means of Gaussian profile fitting as exemplarily described in section and figure 2.. As a reference, the absorption correlated to the peak of asymmetric stretching vibrations of SiOSi at 3 cm is used. Therewith, a ratio r SiCH3 representing organic content of the layer can be calculated as r SiCH3 = SiCH3 (26 cm ) SiOSi(3 cm ). (5.2) Analogous, an analysis of absorption based on SiOH of the deposited coatings is applied. Therefore, stretching and bending vibrations of SiOH at 33 cm and 93 cm, respectively, are considered and the ratio r SiOH is determined by r SiOH = SiOH(93 cm, 33 cm ) SiOSi(3 cm ). (5.2) Figure 5.8 shows the ratios r SiCH3 and r SiOH. A change from organic coatings to inorganic, quartz-like is observed for increasing oxygen fluxes Φ O2 as it is revealed by EDX measurements as previously discussed. Additionally, carbon containing groups are found for Φ O2 < 4 sccm, which are not observed by EDX measurements due to the detection limit. The layers are carbon-free for Φ O2 4 sccm and consist of a residual amount of.5.6 rsioh rsich Φ O2 / sccm Figure 5.8: Ratios r SiCH3 and r SiOH for various oxygen fluxes Φ O2 :( ) ratior SiCH3 of absorption correlated to symmetric deformation vibration of CH 3 (26 cm ) and asymmetric stretching vibrations of SiOSi (3 cm ) according to equation (5.2), ( ) ratio r SiOH of absorption correlated to SiOH (93 cm and 33 cm ) and asymmetric stretching vibrations of SiOSi (3 cm ) according to equation (5.2).

91 5.4 Development of barrier layer system on PET foils 77 hydrogen incorporated as silanol (SiOH). In summary, a strong correlation of the layer composition and the barrier properties of the coating can be derived from the above mentioned behavior. Organic deposits only represent a minor barrier and the permeation is similar to the permeation of the uncoated PET foils. By increasing oxygen flux, a strong reduction of the permeation is observed combined with a reduction of carbon in the layer. For oxygen fluxes Φ O2 4 sccm, the layers become quartz-like and carbon free. Hydrogen is found as silanol incorporated in the coating. A barrier improvement by a factor of at least 65 is reached by a 6 nm thick SiO x coating. Surface morphology The surface morphology of deposited coatings for various oxygen dilutions is shown in figure 5.9. For higher oxygen additions, a surface smoothing is observed. For low oxygen dilutions, the roughness of the layers is revealed to be R a =.6nmaccordingtoequation (2.8) and the layers are SiO x C y H z like as previously determined. With increasing oxygen admixture, the roughness is minimized to R a =.5 nm, when the coatings are SiO x 7.2nm div 7.2nm div 5 nm div 5 nm div 5 nm div 5 nm div (a) Φ O2 = 5 sccm, R a =.6nm. (b) Φ O2 = 3 sccm, R a =.5nm. 7.2nm div 5 nm div 5 nm div (c) Φ O2 = 6 sccm, R a =.5nm. Figure 5.9: Surface morphology of SiO x coatings for various oxygen fluxes Φ O2 and revealed coating roughnesses R a at plasma conditions Φ HMDSO = 4 sccm, P = 5 W, p = 3 Pa, t on = 4 ms and t off according to equation (5.9).

92 78 Chapter 5 Permeation barrier coating like. A similar behavior is observed by other research groups [7, 46]. Erlat et al. [7] correlate the barrier property and the surface morphology to show that good barrier coatings are as smooth as possible. This result is in good agreement with the observations within this thesis, but seems to be an induced phenomena due to relevant changes in surface composition. They investigate the influence of oxygen admixtures during the deposition of SiO x coatings by organic monomers and ascribe the surface smoothing to oxygen etching of carbon from the surface, which is in agreement to our observations of variations in composition and roughness. Neutral gas composition Plasmas containing organic molecules, such as HMDSO are used for a wide range of applications, but an analysis of the chemical reactions taking place in the gas phase and as surface processes are not well understood. Additionally, only basic correlations describing a macroscopic view on the deposition process are published due to a poor knowledge of dissociation process of such molecules [87]. Therefore, the analysis of the neutral and ion mass spectra within this thesis describes the behavior of significant ions and neutrals for parameter variation. Due to the homogeneity criterion (cp. equation 5.9), the influence of a parameter variation is separately investigated. Figure 5.2 exemplarily shows the neutral gas composition of the plasma for three different oxygen fluxes normalized to the peak of maximum intensity. As expected, a strong influence of the oxygen addition to the plasma leads to a shift from a HMDSO dominated neutral gas composition to an oxygen dominated. The behavior of significant neutrals is shown in figure 5.2. The shown data represent the partial pressure p (m/z) of a certain molecule to charge ratio m/z 65 divided by the total pressure p denoted as p (m/z) : p (m/z) = p (m/z) 65 i= p i = p (m/z) p. (5.22) As revealed in equation (2.8), the count rate C (m/z) of the mass spectrometer at a certain mass (m/z) is proportional to the partial pressure within the reactor chamber. Therefore, equation (5.22) can be modified to be p (m/z) = p (m/z) 65 i= p i = C (m/z) 65 i= C. (5.23) i Figure 5.2 shows the partial pressures p (m/z) normalized to its maximum for the performed oxygen variation of significant fragments. For all shown molecules, an asymptotic characteristic is observed reaching nearly constant values for Φ O2 4 sccm. This observation is in good agreement with the previous determined permeation behavior and coating composition. Two kinds of fragments can be distinguished showing different characteristics for increasing oxygen dilution. The amount of HMDSO in the gas phase, represented by the fragment Si 2 OC 5 H + 5 (m/z = 47) and SiOCH + (m/z = 45) are strongly decreasing due to the reduced partial pressure of HMDSO. Their trend is similar to relative behavior of HMDSO partial pressure p HMDSO compared to the oxygen partial pressure p O2. Also a strong reduction of carbon containing HMDSO fragments like CH + 3 (m/z = 5), CO + (m/z = 28) is observed, whereas H 2 O + (m/z = 8) and OH + (m/z = 7) only slightly decrease. Contrarily, an

93 5.4 Development of barrier layer system on PET foils 79 normalized count rate / a.u. normalized count rate / a.u. normalized count rate / a.u H + 2 H2O + O + 2 CO + 2 Si2OC4H + Si2OC4H m/z H + 2 H2O + O + 2 CO + 2 (a) Φ O2 = 2 sccm Si2OC4H + Si2OC4H m/z H + 2 H2O + O + 2 CO + 2 (b) Φ O2 = sccm Si2OC4H + Si2OC4H m/z (c) Φ O2 = 4 sccm Figure 5.2: Normalized mass spectra of neutrals for various oxygen fluxes Φ O2 at plasma conditions Φ HMDSO = 4 sccm, p =3Pa,P = 5 W, t on = 4 ms and t off according to equation (5.9) and E ei = 25 ev electron impact ionization energy. Si2OC5H + 5 Si2OC5H + 5 Si2OC5H + 5 Si2OC6H + 8 Si2OC6H + 8 Si2OC6H + 8

94 8 Chapter 5 Permeation barrier coating normalized p(m/z) normalized p(m/z) Φ O2 / sccm (a) ( ) Si 2 OC 5 H + 5 (m/z = 47), ( ) SiOCH + (m/z = 45), ( ) CO + 2 (m/z = 44), ( ) CO + (m/z = 28) Φ O2 / sccm (b) ( ) O + 2 (m/z = 32), ( ) H 2 O + (m/z = 8), ( ) OH + (m/z = 7), ( ) O + (m/z = 6), (+) CH + 3 (m/z = 5) Figure 5.2: Relative behavior of significant neutrals for various oxygen fluxes Φ O2 at plasma conditions Φ HMDSO = 4 sccm, p =3Pa,P = 5 W, t on = 4 ms and t off according to equation (5.9) and E ei = 25 ev electron impact ionization energy. increase of O + 2 (m/z = 32) and O + (m/z = 6) is observed due to the increase of oxygen partial pressure. Furthermore, the fraction of CO + 2 (m/z = 44) remains nearly constant over the wide range of oxygen flux variation 2 sccm Φ O2 8 sccm. Magni et al. [3] observe a comparable behavior of CO 2 by means of mass spectrometry and in situ FTIR spectroscopy for various oxygen dilutions of HMDSO plasmas. They state, that the consumption of complex fragments of HMDSO, such as SiO x C y H z, is mainly due to their diffusion to the reactor and substrate surfaces and contribution to the layer growth [3]. As shown in figure 5.2, the behavior of Si 2 OC 5 H + 5 (m/z = 47), SiOCH + (m/z = 45), CO + (m/z = 28) and CH + 3 (m/z = 5) follow the trend of HMDSO partial pressure, which means, that the source of these carbon containing molecules is based on dissociation of HMDSO. Exceptionally, CO 2 remains constant. Therefore, besides the fragments of HMDSO, an additional source of carbon must exist, which is stated to be most probably the reactor surfaces [3]. It can be concluded, that the deposition of SiO x like coatings is not based on the deposition of e.g. SiO x radicals, but can mainly be described as the deposition of SiO x C y H z like species on the surface and the removal of carbon by means of oxygen. This removal forms mainly CO 2, which is present in the gas phase as detected by means of mass spectrometry. Besides the exceptional behavior of CO 2,alsoH 2 O(m/z = 8) and OH (m/z = 7) do not show a strong dependence on partial pressure of HMDSO. Therefore, the production of H 2 O and OH can to a certain extend also be ascribed to the consumption of hydrogen from the surfaces. Analysis of ions The positive ion spectra for various oxygen dilutions is shown in figure For low oxygen dilutions, mainly ionized fragments of HMDSO are accelerated towards the substrate sur-

95 5.4 Development of barrier layer system on PET foils 8 normalized count rate / a.u. normalized count rate / a.u. normalized count rate / a.u O + H3O + O + 2 SiCH + 5 SiC3H m/z (a) Φ O2 = 2 sccm m/z O + H3O + O + H3O + O + 2 O + 2 SiCH + 5 SiCH + 5 SiC3H + 9 (b) Φ O2 = sccm SiC3H m/z (c) Φ O2 = 4 sccm Figure 5.22: Normalized mass spectra of positive ions for various oxygen fluxes Φ O2 at plasma conditions Φ HMDSO = 4 sccm, p =3Pa,P = 5 W, t on = 4 ms and t off according to equation (5.9). Si2OC2H + 7 Si2OC2H + 7 Si2OC2H + 7 Si2OC4H + 3 Si2OC4H + 3 Si2OC4H + 3 Si2OC5H + 5 Si2OC5H + 5 Si2OC5H + 5 Si2OC6H + 8 Si2OC6H + 8 Si2OC6H + 8

96 82 Chapter 5 Permeation barrier coating face and detected by the mass spectrometer. A structure comparable to the fragmentation pattern of HMDSO is revealed represented by fragments of the configuration as listed in table 5. for fragmentation of HMDSO vapor under electron impact. Therefore, it can be stated, that the dissociation of the HMDSO molecule by electron impact is the dominant process in HMDSO containing plasmas [3]. Additionally, fragments are observed based on reactions of fragments with oxygen, such as H 3 O + (m/z=9) and H 2 O (m/z=8). For the lowest oxygen dilution Φ O2 = 2 sccm, ions of masses 6 m/z are observed based on fragments of HMDSO. The presence of these ions vanishes for higher oxygen dilutions. Ions of m/z 62 as discussed by [47] as product of ion and neutral reactions are not observed by our measurement setup due to the transmission function of the mass spectrometer (cp. figure 2.7). normalized count rate / a.u Φ O2 / sccm (a) ( ) HMDSO + m/z = 62, ( ) Si 2 OC 5 H + 5 m/z = 47, ( ) Si 2 OC 4 H + 3 m/z = 33, ( ) Si 2 OC 3 H ++ m/z = 9 normalized count rate / a.u (c) ( ) O + 2 m/z = 32, ( ) H 2 O + m/z = 8, ( ) OH + m/z = 7, ( ) O + m/z = Φ O2 / sccm normalized count rate / a.u Φ O2 / sccm (b) ( ) SiO 2 C 3 H + 9 m/z = 7, ( ) SiC 2 H + 7 m/z = 59, ( ) SiCH + 5 m/z = 45, ( ) C 2 H 5 m/z = 29, (+) CO + m/z =28 normalized count rate / a.u (d) ( ) C 2 H + 3 m/z = 27, ( ) C 2 H + 2 m/z = 26, ( ) H 3 O + m/z = 9, ( ) CH + 3 m/z = Φ O2 / sccm Figure 5.23: Relative behavior of significant ions for various oxygen fluxes Φ O2 at plasma conditions Φ HMDSO = 4 sccm, p =3Pa,P = 5 W, t on = 4 ms and t off according to equation (5.9).

97 5.4 Development of barrier layer system on PET foils 83 Similar to the observation of the neutral gas phase, an asymptotic characteristic is observed for the significant ions bombarding the substrate surface as shown in figure 5.23 for oxygen fluxes Φ O2 4 sccm. For lower oxygen fluxes, gradients in the composition of the ion spectra are revealed representing strongly changing deposition conditions. The amount of higher mass fragment ions impinging on the surface as shown in figure 5.23(a) are reduced by about 5% of the initial value. Furthermore, the number of ions related to dissociation along the HMDSO fragmentation paths is reduced reaching steady stated for Φ O2 4 sccm (figure 5.23(b)). Contrarily, the fraction of O + 2 (m/z = 32), O + (m/z = 6) and OH + (m/z = 7) is increasing and playing a major role in the spectra for Φ O2 4 sccm as shown in figure 5.22 and 5.23(c) leading to atomic and molecular oxygen bombardment of the substrate surface. Surface reactions of oxygen and deposited hydrocarbons are known to be important mechanisms for the deposition of SiO 2 like coatings as previously described for the neutral gas composition. Concluding the behavior of neutrals and ions for various oxygen dilutions, the deposition of SiO x coatings is found to be schematically describable as HMDSO + e Si 2 OC 5 H + 5 +CH 3 +2e (5.24) Si 2 OC 5 H e SiO x C y H (+) z (g)+2e (5.25) SiO x C y H (+) z (g) SiO x C y H z (s) (5.26) SiO x C y H z (s)+(o +, O + 2, O, O 2 ) SiO x (s)+co 2 +H 2 O + OH (5.27) where (s) and(g) indicate the solid reaction products deposited on the substrate and chamber walls and gaseous products representing exhausts of the process, respectively. Equation (5.24) describes the fragmentation of HMDSO as dissociative ionization by electron impact ionization leading to production of Si 2 OC 5 H + 5 (m/z = 47) and CH 3. This process is comparable to the fragmentation by means of an electron impact ionization source used for neutral analysis within a mass spectrometer, which is well investigated [32, 33, 34, 48]. Further fragmentation of Si 2 OC 5 H + 5 (m/z = 47) as constituted in equation (5.25) leads to numerous ions and molecules SiO x C y H (+) z (g) being transported towards the substrate and chamber walls, respectively. A SiO x C y H z like coating is deposited on the surfaces as observed by means of FTIR spectroscopy for low oxygen dilutions. Depending on the oxygen admixture, removal of carbon and hydrogen takes place at the substrate surface during layer growth as schematically expressed by equation (5.27). Molecular oxygen, atomic oxygen and ions of both as revealed in figures 5.2 and 5.23 are dominantly present for higher oxygen dilutions leading to oxidation of SiO x C y H z (s) and shifts in stoichiometry to form SiO x coatings for these process conditions. Additionally, the oxidation process of SiO x C y H z (s) coatings as surface reactions leads to a surface smoothing as shown by AFM analysis (cp. figure 5.9). These surface reactions mainly lead to CO 2,H 2 O and OH production as found in the gas phase and previously described. Emission spectra Beside the analysis of the spectra by means of mass spectrometry, an investigation by optical emission spectroscopy supports the revealed results. Therefore, the light emission of the plasma in the range 2 nm λ 8 nm is investigated as shown in figure 5.24 for parameters generating good barrier properties. The oxygen flux is fixed at Φ O2 = 4 sccm.

98 84 Chapter 5 Permeation barrier coating The spectra are recorded using the trigger signal of the generator to ensure a measurement during the on-phase of the microwave generator. 3 x 22 O Photons / (s nm m 3 ) Si OH H γ O H β λ/nm 5 x 2 (a) Overview spectrum (2..8nm). Photons / (s nm m 3 ) Si OH H γ O H β λ/nm (b) Detailed spectrum (2..5nm). Figure 5.24: Absolute calibrated optical emission spectrum of deposition plasma at Φ HMDSO = 4 sccm, Φ O2 = 4 sccm, p =3Pa,P = 5 W, t on = 4 ms and t off =4ms. The spectrum is dominated by emission from oxygen at λ = nm. Additionally, atomic oxygen related emission is observed at λ = 436.8nm, λ = 532.9nm, λ = 65.8 nm. Emission based on hydrogen atoms is also found with remarkable intensities at λ = 434.nm (H γ ), λ = 486.nm (H β )andλ = 656.2nm (H α ). Various emission lines based on silicon atoms are observed below λ = 3 nm, namely λ = 22.4nm, λ = 22.nm, λ = 22.6nm, λ = 243.5nm, λ = 25.6nm, λ = 25.4nm, λ = 25.6nm, λ = 25.9nm, λ = 252.4nm, λ = 252.8nm, λ = 263.nm, λ = 288.nm and λ = 29.4nm. Besides the atomic line emission, also molecule related emission is present in the spectra. Emission of CO is related to λ = 229.3nm, λ = 24.4nm and λ = 248. nm. Furthermore, at λ = 29.nmandλ = 237. nm, emission from CO + ions is revealed. OH molecule bands

99 5.4 Development of barrier layer system on PET foils 85 are found at λ = 283 nm and λ = 36 nm and CH at λ = 43.4 nm [49]. normalized emission / a.u Φ O2 / sccm Figure 5.25: Relative intensities of optical emission related to significant ions and molecules for various oxygen fluxes Φ O2 at plasma conditions Φ HMDSO = 4 sccm, p =3Pa,P = 5 W, t on = 4 ms and t off according to equation (5.9): ( ) CH(λ = 43.4nm), ( ) CO + (λ = 29 nm), ( ) CO(λ = 24.4nm), ( ) OH(λ = 36 nm), (+) O (λ = 777.3nm), ( ) H β (λ = 486 nm). The relative intensities of significant species representing the integrals over the emission peaks are plotted in figure 5.25 for various oxygen fluxes sccm Φ O2 8 sccm. A good correlation of the behavior revealed by mass spectrometry and optical emission spectroscopy is deduced. The characteristics of the neutral particle emission of CO (λ = 24.4nm), OH (λ = 36 nm), CH (λ = 43.4nm) and O (λ = nm) compare well to the results of mass spectrometry as shown in figure 5.2. The emission of CO (λ = 24.4 nm) decreases with decreasing partial pressure of HMDSO as emission from oxygen O (λ = 777.3nm) is concurrently increasing. The known saturation characteristic for Φ O2 4 sccm is also present in the investigated optical emission. Hydrogen related emission, exemplarily considered as H β (λ = 486. nm) also strongly decreases as the partial pressure of HMDSO is reduced. The behavior of emission from OH molecule bands at λ = 36 nm shows nearly a linear decrease for Φ O2 5 sccm comparable to the drop of OH (m/z = 7) in the neutral gas composition shown in figure 5.2(b). This investigation confirms the schematic reaction equation (5.27) leading to the production of OH due to surface processes. Additionally, the molecule band at λ = 29. related to CO + (m/z = 28) agrees well with the ion investigations as presented in figure 5.23(b). The proposed model of silicon oxide deposition by means of HMDSO as described by equation (5.24) to (5.27) is confirmed using optical emission spectroscopy. A good agreement of the investigated composition of neutral and ion spectra as well as the optical emission spectra and the barrier properties is revealed. A saturation of the plasma composition regarding ions and neutrals leads to constant coating properties for oxygen admixtures Φ O2 4 sccm as shown for layer composition and barrier properties. Insights into the deposition process of SiO x barrier coatings are found. It consists of the deposition of SiO x C y H z like fragments on the substrate surface followed by an oxygen etching of carbonated and hydrogenated molecules leading to the production of CO 2,OHandH 2 Oas reaction products.

100 86 Chapter 5 Permeation barrier coating Influence of pulse power and pulse duration during barrier coating deposition Permeation behavior for various pulse powers 2 J/cm 3 m 2 day P/W Figure 5.26: Permeation rates of 6 nm thick SiO x coatings for various pulse powers 6 W P 3 W at plasma conditions Φ HMDSO = 4 sccm, Φ O2 = 4 sccm, p =3Pa, t on = 4 ms and t off =4ms. The pulse power has an influence on the barrier performance and the composition of the deposited barrier besides the influence of oxygen dilution of the pulsed HMDSO : O 2 plasma. Figure 5.26 shows the dependence of the oxygen permeation on various pulse powers. The ratio of HMDSO : O 2 for the given power variation is : and the process pressure is p = 3 Pa. With increasing power the permeation drops continuously and for pulse powers P 5 W an asymptotic behavior leading to a permeation of J =cm 3 m 2 day is achieved. absorption / a.u. SiOH P = 75 W P = 5 W P = 2 W P = 25 W SiOSi SiCH 3 SiOH P = 3 W k/cm 5 Figure 5.27: FTIR absorption spectra for various pulse powers 75 W P 3 W at plasma conditions Φ HMDSO = 4 sccm, Φ O2 = 4 sccm, p =3Pa,t on = 4 ms and t off = 4 ms.

101 5.4 Development of barrier layer system on PET foils 87 Coating composition for various pulse powers The FTIR spectra of the coatings deposited under various pulse power conditions (figure 5.27) reveal a reduction of carbon related vibrations with increasing power. For pulse powers P 5 W carbon can be detected leading to symmetric deformation vibrations of SiCH 3 at 278 cm. For higher power levels the amount of carbon vanishes and the layers become quartz-like [7, 8]. Similar to the observations of the oxygen variation, a particular absorption band of silanol at 33 cm and 93 cm is present, which shows no definite tendency depending on plasma power. For the variation of pulse power a good correlation of the layer composition and the barrier properties as described for the oxygen variation is also confirmed. The transition from organic to inorganic coatings is mapped by the transition from low to high barrier coatings. Investigation of surface morphology for various pulse powers Comparable to the observations of the changes in surface morphology for alterations of the layer composition, a reduction of the roughness R a is observed for increasing powers. Figure 5.28 points out a reduction of R a =.3 nm for P = 6 W to R a =.6 nm for P = 2 W. An increase of applied microwave power leads to a homogenization of the discharge a rise in HMDSO fragmentation as it is discussed in the next paragraphs considering the mass spectroscopy results. For higher powers, an increase of CO 2 is observed as oxidation products. It is formed as a result of surface oxidation reactions as previously discussed for oxygen addition and leads to a smoothing of the surface. For low fragmentation and density conditions, the higher mass fragments of HMDSO are deposited on the surface leading to rougher SiO x C y H z like films. Permeation behavior for various pulse durations Besides the variation of pulse power a variation of the pulse duration t on leads to different layer properties of the plasma polymerized SiO x coatings. Figure 5.29 shows the depen- 8.5nm div 8.5nm div 5 nm div 5 nm div 5 nm div 5 nm div (a) P = 6 W, R a =.3nm (b) P = 2 W, R a =.6nm Figure 5.28: Surface morphology of SiO x coatings for various pulse powers P and revealed coating roughnesses R a at plasma conditions Φ HMDSO = 4 sccm, Φ O2 = 4 sccm, p =3Pa, t on = 4 ms and t off =4ms

102 88 Chapter 5 Permeation barrier coating 2 J/cm 3 m 2 day t on / ms Figure 5.29: Permeation rates of 6 nm thick SiO x coatings for various pulse durations ms t on ms at plasma conditions Φ HMDSO = 4 sccm, Φ O2 = 4 sccm, p =3Pa, P = 5 W and t off = 4 ms. dency of the oxygen permeation rate on various pulse durations. The pulse power is held constant at P = 5 W for all cases, which leads to an increase of mean power by enlarging t on. Short pulses (t on < 3 ms) cause no significant reduction of permeation and the permeation fluxes of the coated PET foils are similar to the value of uncoated PET. The best barrier action is achieved for t on 4 ms. For longer pulse durations the thermal heat load is significantly increased and the treatment of thermolabile substrates becomes difficult. Coating composition for various pulse durations absorption / a.u SiOH 3 t =2ms on t =3ms on t =4ms on t =5ms on t =8ms on 25 k/cm 2 SiOSi SiCH 5 SiOH Figure 5.3: FTIR absorption spectra for various pulse durations ms t on 8 ms at plasma conditions Φ HMDSO = 4 sccm, Φ O2 = 4 sccm, p =3Pa,P = 5 W and t off = 4 ms. Increasing pulse duration or pulse power lead to similar results concerning the coating composition. The spectra are shown in figure 5.3 and demonstrate for increasing pulse durations a similar characteristic as for increasing power conditions. A reduction of carbon

103 5.4 Development of barrier layer system on PET foils 89 is observed for pulse durations ms t on 3 ms. For pulses t on 4ms the layers are quartz-like and no vibrations correlated to carbon containing molecules are present in the barrier coating. A reduction of hydrogen content is found for long pulses (t on = 8 ms), which is correlated to the observations of an increased heat load to the substrates. Neutral gas composition for various pulse powers The behavior of the neutral gas composition for various powers is shown in figure 5.3. Additionally, details for significant neutrals are shown in figure A transient behavior is revealed for 6 W P 2 W. The amount of the HMDSO fragment Si 2 OC 5 H + 5 (m/z = 47) is reduced by 5% due to increasing fragmentation. The fragment CH + 3 (m/z = 5) shows a similar behavior, which is explained by the first step of fragmentation of HMDSO leading to the production of Si 2 OC 5 H + 5 (m/z = 47) and CH + 3 (m/z = 5) as described by equation (5.24). An increase of oxidized products, such as SiOCH + (m/z = 45), CO + 2 (m/z = 44), CO + (m/z=28), H 2 O + (m/z=8) and OH + (m/z = 7) is observed. The amount of O + 2 (m/z = 32) and O + (m/z = 6) in the gas phase slightly decreases by increasing power. This effect is due to a deposition of oxygen in the layer as revealed by FTIR spectroscopy and oxidizing carbonated species as described by equation (5.27) leading to production of the aforementioned oxygen containing fragments, especially CO 2,H 2 O and OH. The asymptotic characteristic of all species can be explained by a more or less constant electron density for increasing powers as revealed by Langmuir probe measurements for argon plasmas in section 3..3 (cp. figure 3.4). A power increase P 2 W does not influence the fragmentation characteristics due to stagnation in electron density. Neutral gas composition for various pulse durations Besides the variation of the amplitude of applied microwave power, also the pulse duration is an important parameter for influencing the layer properties. The neutral gas analysis for various pulse durations ms t on ms is plotted in figure 5.33 and in detail in figure Comparably to the investigations of increasing pulse power, an analogous behavior of the considered species for enlarged pulse durations is observed. Contrarily to the observation for various power amplitudes, the characteristics of the neutral gas composition for increasing pulse durations do not show an asymptotic behavior due to ongoing fragmentation by means of electron impact and etching reactions on the surface for longer pulses [5]. Therefore, a nearly linear behavior is deduced for t on 4ms. Within the first four milliseconds, a stronger dependency on pulse duration is revealed which is assumed to be due to the ignition and starting fragmentation of a renewed gas mixture after inter-pulse period. Investigation of fragmentation rates The degree of fragmentation of HMDSO can be analyzed to get insight into the plasma process. For HMDSO containing plasmas, the degree of fragmentation f can be defined as the normalized fraction of the count rates of Si 2 OC 5 H + plasma on 5 (m/z = 47) with plasma C (47)

104 9 Chapter 5 Permeation barrier coating normalized count rate / a.u. normalized count rate / a.u. normalized count rate / a.u H2O + O + 2 CO m/z (a) P = 6 W m/z H2O + H2O + O + 2 O + 2 CO + 2 CO + 2 (b) P = 5 W m/z (c) P = 35 W Figure 5.3: Normalized mass spectra of neutrals for various pulse powers P at plasma conditions Φ HMDSO = 4 sccm, Φ O2 = 4 sccm, p =3Pa,t on = 4 ms and t off =4msand E ei = 25 ev electron impact ionization energy. Si2OC5H + 5 Si2OC5H + 5 Si2OC5H + 5 Si2OC6H + 8 Si2OC6H + 8 Si2OC6H + 8

105 5.4 Development of barrier layer system on PET foils 9 normalized p(m/z) normalized p(m/z) P/W (a) ( ) Si 2 OC 5 H + 5 m/z = 47, ( ) SiOCH + m/z = 45, ( ) CO + 2 m/z = 44, ( ) CO + m/z = P/W (b) ( ) O + 2 m/z = 32, ( ) H 2 O + m/z = 8, ( ) OH + m/z = 7, ( ) O + m/z = 6, (+) CH + 3 m/z =5 Figure 5.32: Relative behavior of significant neutrals for various pulse powers P at plasma conditions Φ HMDSO = 4 sccm, Φ O2 = 4 sccm, p =3Pa,t on = 4 ms and t off =4msand E ei = 25 ev electron impact ionization energy. plasma off and without plasma C(47) [5, 52, 53]: f = Cplasma on (47) plasma off C (47). (5.28) For the variation of power conditions, a strong influence on the degree of fragmentation is observed as shown in figure The degree of fragmentation shows an asymptotic behavior for pulse powers P 2 W and reaches a value of f =.58. The observations agree well with the investigations of the characteristics of electron density n e for increasing powers. As shown in figure 3.4, a saturation of n e is observed due to shielding effects, which are explained in section The HMDSO molecule is mainly fragmented due to electron impacts, which leads to a good correlation of electron density n e and the fragmentation rate for a variation of pulse power. For an increase in pulse duration, the degree of fragmentation f rises due to enlarged reaction times leading to the shown asymptotic increase of fragmentation for longer pulses. Analysis of ions for various pulse powers The investigation of significant ions during a variation of pulse power reveals significant changes in the composition of the ion spectra. Figure 5.36 shows the ion spectra for pulse powers 5 W P 4 W and the characteristics of significant ions is additionally shown in figure A strong reduction of higher mass fragment ions is observed. As deduced from figure 5.37(a) and 5.37(b), the amount of HMDSO + (m/z = 62), Si 2 OC 5 H + 5 (m/z = 47), Si 2 OC 4 H + 3 (m/z = 33), Si 2 OC 3 H ++ (m/z = 9), SiO 2 C 3 H + 9 (m/z = 7), SiC 2 H + 7 (m/z = 59), and SiCH + 5 (m/z = 45), representing the ionized fragments of HMDSO strongly decrease. Comparable to the investigations of the neutral spectra and the degree of fragmentation f, the ion spectra exhibit a saturation for powers P 2 W.

106 92 Chapter 5 Permeation barrier coating normalized count rate / a.u. normalized count rate / a.u. normalized count rate / a.u H2O + O + 2 CO m/z (a) t on =ms m/z H2O + H2O + O + 2 O + 2 CO + 2 CO + 2 (b) t on =2ms m/z (c) t on =6ms Figure 5.33: Normalized mass spectra of neutrals for various pulse durations t on at plasma conditions Φ HMDSO = 4 sccm, Φ O2 = 4 sccm, P = 5 W p =3Paandt off =4ms and E ei = 25 ev electron impact ionization energy. Si2OC5H + 5 Si2OC5H + 5 Si2OC5H + 5 Si2OC6H + 8 Si2OC6H + 8 Si2OC6H + 8

107 5.4 Development of barrier layer system on PET foils 93 normalized p(m/z) normalized p(m/z) t on / ms (a) ( ) Si 2 OC 5 H + 5 m/z = 47, ( ) SiOCH + m/z = 45, ( ) CO + 2 m/z = 44, ( ) CO + m/z = t on / ms (b) ( ) O + 2 m/z = 32, ( ) H 2 O + m/z = 8, ( ) OH + m/z = 7, ( ) O + m/z = 6, (+) CH + 3 m/z =5 Figure 5.34: Relative behavior of significant neutrals for various pulse durations t on at plasma conditions Φ HMDSO = 4 sccm, Φ O2 = 4 sccm, p =3Pa,P = 5 W and t off = 4 ms and E ei = 25 ev electron impact ionization energy. fragmentation rate fragmentation rate P/W (a) a t on / ms (b) a Figure 5.35: Fragmentation rates of HMDSO for variation of power and pulse duration

108 94 Chapter 5 Permeation barrier coating normalized count rate / a.u. normalized count rate / a.u. normalized count rate / a.u O + H3O + O + 2 SiCH + 5 SiC3H m/z (a) P = 6 W m/z O + H3O + O + H3O + O + 2 O + 2 SiCH + 5 SiCH + 5 SiC3H + 9 (b) P = 5 W SiC3H m/z (c) P = 35 W Figure 5.36: Normalized mass spectra of positive ions for various pulse powers P at plasma conditions Φ HMDSO = 4 sccm, Φ O2 = 4 sccm, p =3Pa,t on = 4 ms and t off = 4 ms. Si2OC2H + 7 Si2OC2H + 7 Si2OC2H + 7 Si2OC4H + 3 Si2OC4H + 3 Si2OC4H + 3 Si2OC5H + 5 Si2OC5H + 5 Si2OC5H + 5 Si2OC6H + 8 Si2OC6H + 8 Si2OC6H + 8

109 5.4 Development of barrier layer system on PET foils 95 normalized count rate / a.u P/W (a) ( ) HMDSO + m/z = 62, ( ) Si 2 OC 5 H + 5 m/z = 47, ( ) Si 2 OC 4 H + 3 m/z = 33, ( ) Si 2 OC 3 H ++ m/z = 9 normalized count rate / a.u P/W (c) ( ) O + 2 m/z = 32, ( ) H 2 O + m/z = 8, ( ) OH + m/z = 7, ( ) O + m/z =6 normalized count rate / a.u P/W (b) ( ) SiO 2 C 3 H + 9 m/z = 7, ( ) SiC 2 H + 7 m/z = 59, ( ) SiCH + 5 m/z = 45, ( ) C 2 H 5 m/z = 29, (+) CO + m/z =28 normalized count rate / a.u P/W (d) ( ) C 2 H + 3 m/z = 27, ( ) C 2 H + 2 m/z = 26, ( ) H 3 O + m/z = 9, ( ) CH + 3 m/z =5 Figure 5.37: Relative behavior of significant ions for various pulse powers P at plasma conditions Φ HMDSO = 4 sccm, Φ O2 = 4 sccm, p =3Pa,t on = 4 ms and t off = 4 ms.

110 96 Chapter 5 Permeation barrier coating The relative amount of molecular O + 2 (m/z = 32) and atomic oxygen O + (m/z = 6) ions increases by a factor of more than five for a power rise from P = 5 W to P = 2 W. Also an increase of oxidized hydrogen ions as H 2 O + (m/z = 8) and OH + (m/z = 7) is revealed. Contrary to the saturation characteristic, they tend to decrease for P 25 W, which is assumed to be due to an increased heat load for high power conditions. Additionally, an increase of CO + (m/z = 28) is found, which can be explained as product of dissociative electron impact ionization of CO 2 in the neutral gas composition. CO 2 is rising for increasing power conditions as shown in figure 5.32(a). Analysis of ions for various pulse durations The behavior of ions of the oxygen diluted HMDSO plasma for increasing pulse durations t on show similar characteristics as revealed for rising powers. Exemplarily, figure 5.38 contains the ion spectra for an increase of t on. The characteristics of significant ions as shown in figure 5.39 reveal a asymptotic behavior of all considered ions for pulse durations t on 4ms. The higher mass fragments of HMDSO nearly vanish for pulses t on 4 ms, whereas dominantly molecular and atomic oxygen containing ions are produced. Similar to the observations of the power variation, the amount of CO + increases (cp. figure 5.39(b)) due to ionization of rising amount of CO 2 molecules (cp. figure 5.34(a)) mainly produced as surface oxidation product as schematically constituted in equation (5.27). Concluding the results of a power variation by means of an increase of the amplitude or the pulse duration of oxygen diluted HMDSO plasmas, a good correlation to the characteristics of the electron density is revealed. An increase of power leads to an increased fragmentation of HMDSO molecules and a shift of neutral gas composition to lower mass fragments. Concurrently, the amount of higher mass ions is strongly reduced. An asymptotic behavior is observed for pulse powers and pulse durations of P 2 W and t on 4 ms, respectively. The layer composition and permeation behavior are in good agreement with the observations of mass spectrometry. Constant behavior of these properties are revealed for the mentioned threshold values of pulse power P and duration t on. Considering the behavior of ions and neutrals for a variation of the pulse duration, the fragmentation of HMDSO shows an increasing behavior for longer pulses, whereas the ion spectra reveal constant behavior for t on 4 ms. Additionally, the layer composition and the barrier property are also constant for t on 4 ms. Therefore, it can be assumed, that ions play a major role during the deposition of SiO x like coatings, which will be also considered in section regarding ion energies. Emission spectra A time resolved analysis of the behavior of significant species reveals insights into the sequence of pulsed SiO x coating deposition. Figure 5.4 shows the relative optical intensities during a microwave power pulse as function of time t. The acquisition time is t = µs for the shown behavior determining the temporal resolution of the spectra. For all species, a transient characteristic is observed for t µs due to an ignition phase of the plasma. At the beginning of the ignition, emission from oxygen is observed firstly due to the high oxygen dilution of the plasma with Φ O2 = 4 sccm and Φ HMDSO = 4 sccm. The oxygen emission λ = nm increases following the behavior of electron density depending

111 5.4 Development of barrier layer system on PET foils 97 normalized count rate / a.u. normalized count rate / a.u. normalized count rate / a.u O + H3O + O + 2 SiCH + 5 SiC3H m/z (a) t on =ms m/z O + H3O + O + H3O + O + 2 O + 2 SiCH + 5 SiCH + 5 SiC3H + 9 (b) t on =2ms SiC3H m/z (c) t on =6ms Figure 5.38: Normalized mass spectra of positive ions for various pulse durations t on at plasma conditions Φ HMDSO = 4 sccm, Φ O2 = 4 sccm, p =3Pa,P = 5 W and t off = 4 ms. Si2OC2H + 7 Si2OC2H + 7 Si2OC2H + 7 Si2OC4H + 3 Si2OC4H + 3 Si2OC4H + 3 Si2OC5H + 5 Si2OC5H + 5 Si2OC5H + 5 Si2OC6H + 8 Si2OC6H + 8 Si2OC6H + 8

112 98 Chapter 5 Permeation barrier coating normalized count rate / a.u t on / ms (a) ( ) HMDSO + m/z = 62, ( ) Si 2 OC 5 H + 5 m/z = 47, ( ) Si 2 OC 4 H + 3 m/z = 33, ( ) Si 2 OC 3 H ++ m/z = 9 normalized count rate / a.u t on / ms (c) ( ) O + 2 m/z = 32, ( ) H 2 O + m/z = 8, ( ) OH + m/z = 7, ( ) O + m/z =6 normalized count rate / a.u t on / ms (b) ( ) SiO 2 C 3 H + 9 m/z = 7, ( ) SiC 2 H + 7 m/z = 59, ( ) SiCH + 5 m/z = 45, ( ) C 2 H 5 m/z = 29, (+) CO + m/z =28 normalized count rate / a.u t on / ms (d) ( ) C 2 H + 3 m/z = 27, ( ) C 2 H + 2 m/z = 26, ( ) H 3 O + m/z = 9, ( ) CH + 3 m/z =5 Figure 5.39: Relative behavior of significant ions for various pulse durations t on at plasma conditions Φ HMDSO = 4 sccm, Φ O2 = 4 sccm, p =3Pa,P = 5 W and t off = 4 ms.

113 5.4 Development of barrier layer system on PET foils 99 normalized emission / a.u t/µs (a) ( ) OH(λ = 36 nm), ( ) O(λ = 777.3nm), (+) H β (λ = 486 nm) normalized emission / a.u t/µs (b) ( ) CH(λ = 43.4nm), ( ) CO(λ = 24.4nm), Figure 5.4: Time resolved relative optical emission during microwave power pulse at plasma conditions Φ HMDSO = 4 sccm, Φ O2 = 4 sccm, p =3Pa,P = 5 W, t on =4ms and t off =4ms. on the generator characteristic as shown in figure 3.5 and saturates for t 4 µs. The second observed emission representing hydrogen is H β due to excitation of hydrogen based on fragmentation of HMDSO. Also this emission roughly saturates for t 4 µs. The emission characteristic of CH and CO reveal insight into the deposition process. Observable emission from CH (λ = 43.4 nm) is found for t µs due to the first step of fragmentation forming CH 3 according to equation (5.24). The emission drops due to the generator characteristic and rises again due to ongoing fragmentation of HMDSO. After t = 7 µs, the emission from CH remains constant at a very low level at about % of the peak value. Therefore, it can be stated, that the initial peak of CH emission is due to the fragmentation of HMDSO in the gas phase of the plasma, that takes about 5 µs for the investigated gas composition. For t 7 µs, the fragmentation of HMDSO as carbon source is assumed to become minor important and also carbon etched from the surface contributes to the residual CH emission. Therefore, a time dependent two step process of SiO x coating deposition is revealed from the measurements. It consists of a fragmentation of HMDSO leading to the production of hydrocarbons and oxidation reactions in the gas phase of these species. After t = 8 µs, the regime changes to an oxygen dominated plasma with negligible emission from CH and CO. As revealed by mass spectrometry, OH is one of the surface reaction products as well as CO 2. The amount of OH emission at λ = 36 nm remains constant for t 8 µs leading to a second process consisting of surface etching of previously deposited SiO x C y H z like fragments Influence of process pressure during barrier coating deposition Permeation behavior To show the influence of process pressure, the permeation rates of 6 nm SiO x coatings are presented in figure 5.4 for various pressures 2 Pa p 6 Pa. An increase of oxygen per-

114 Chapter 5 Permeation barrier coating 2 J/cm 3 m 2 day p/pa Figure 5.4: Permeation rates of 6 nm thick SiO x coatings for various process pressures 2 Pa p 6 Pa at plasma conditions Φ HMDSO = 4 sccm, Φ O2 = 4 sccm, P = 5 W, t on = 4 ms and t off according to equation (5.9). meation is found for pressures p>3 Pa and for process pressures p 5 Pa the permeation is similar to uncoated PET. An analogous behavior is observed by Grüniger et al. [4] and Erlat et al. [7]. Coating composition SiOSi SiOH absorption / a.u. SiOH p=2pa p=3pa p=4pa SiOSi p=5pa k/cm 5 Figure 5.42: FTIR absorption spectra for various process pressures 2 Pa p 5 Pa at plasma conditions Φ HMDSO = 4 sccm, Φ O2 = 4 sccm, P = 5 W, t on = 4 ms and t off according to equation (5.9). The FTIR analysis of the coatings deposited under different process pressures shows only slight differences concerning carbon content of the layer (figure 5.42). For increasing pressures a slight increase of SiCH 3 deformation vibrations peak intensity at 278 cm is observed.

115 5.4 Development of barrier layer system on PET foils 2.5 rsioh.5.4 rss p/pa Figure 5.43: Ratios r SiOH and r ss for various process pressures p: ( ) ratior SiOH of absorption correlated to SiOH (93 cm and 33 cm ) and asymmetric stretching vibrations of SiOSi (3 cm ) according to equation (5.2) and ( ) ratior ss of absorption of side shoulder and asymmetric stretching vibrations of SiOSi (3 cm ) according to equation (5.29). A significant rise of SiOH bounds in the deposited layer is observed as revealed by spectra fitting as described in section Silanol bound hydrogen is known to have a high influence on permeation behavior [54, 84, 55]. Additionally, a detailed investigation of the characteristic side shoulder of the dominant SiOSi vibration (3 cm ) leads to further insight in the coating porosity. The side shoulder representing SiO asymmetric stretching of SiO at 3 cm can be related to coating porosity as revealed by observations of other groups [56, 57, 58]. Therefore, an investigation of the fraction r ss of the side shoulder defined as r ss = SiO(3 cm ) SiOSi(3 cm ) (5.29) reveals a continuous increase of coating porosity as plotted in figure Thus, an increase in permeation can be explained by rising porosity and less coating densification for higher process pressures. Surface morphology A reduction of process pressure leads to a significant densification of the layers by an increase of ion energy, which is revealed by a reduction of surface roughness and porosity. Examinations of the morphology of the SiO x coatings deposited under various pressures show an increasing surface roughness for higher process pressures [7]. The AFM analysis of the deposited coatings shows a significant influence of the process pressure on the surface morphology. For low-pressures, a smooth surface is obtained with R a =.6 nm, whereas for increasing pressures, a strong increase of roughness is observed. Additionally, coarse structures are found supporting the previously shown increase in porosity. The process pressure strongly influences the ion bombardment on the surface influencing the coating densification and morphology as shown in figure Again a correlation of surface roughness and barrier properties is confirmed as published in [7].

116 2 Chapter 5 Permeation barrier coating 2.6nm div 2.6nm div 5 nm div 5 nm div 5 nm div 5 nm div (a) p =3Pa,R a =.6nm (b) p =4Pa,R a =.7nm 2.6nm div 5 nm div 5 nm div (c) p =6Pa,R a =.3nm Figure 5.44: Surface morphology of SiO x coatings for various process pressures p and revealed coating roughnesses R a at plasma conditions Φ HMDSO = 4 sccm, Φ O2 = 4 sccm, P = 5 W, t on = 4 ms and t off according to equation (5.9) The changes in the layer porosity and barrier properties can be explained by the influence of changes in ion energy during the coating deposition. As revealed for argon plasmas, a pressure variation strongly modifies the ion energy distribution function as shown in figure 3.3 and figure 3.6 for continuous and pulsed plasmas, respectively. For increasing pressures, a strong reduction of ion energy is observed leading to less ion bombardment and less densification of the deposited coating. Therefore, it can be stated, that for the deposition of good barrier coatings, a certain ion bombardment is required. A measurement of the ion energy distribution function during the deposition of barrier coatings is not realizable with the present setup due to the shift in potentials as explained in section 3.3 leading to a ostensible shift in ion energies to higher values for increasing coating of reactor walls Influence of layer thickness on barrier properties Plasma polymerized SiO x coatings need to exceed a critical thickness d c to act as a permeation barrier. Figure 5.45 shows the oxygen permeation for increasing coating thicknesses. A critical thickness of d c 5 nm can be deduced for the considered coating. This value is in good agreement with reviewed observations of Chatham [] and Leterrier [4] for plasma

117 5.5 Investigation of permeation mechanisms 3 2 J/cm 3 m 2 day d/nm Figure 5.45: Permeation rates of SiO x coatings for various coating thicknesses nm d 29 nm at plasma conditions Φ HMDSO = 4 sccm, Φ O2 = 4 sccm, p =3Pa,P = 5 W, t on = 4 ms and t off =4ms. polymerized SiO x barrier coatings Optimized plasma parameters for deposition of barrier layer system Recapitulating the previously described results of the parameter variations and the coating analysis, an optimized SiO x barrier coating can be deposited by means of a pulsed HMDSO : O 2 plasma under the conditions listed in table 5.3. An organic adhesion promoting system has to be deposited prior to the barrier coating. SiO x C y H z like coatings promote good adhesion of inorganic, quartz-like barrier coatings on PET substrates as intermediate layer between an organic PET substrate and an inorganic SiO x barrier coating. The developed barrier layer system allows for a barrier improvement of more than 65 on PET foils. P t on t off p Φ HMDSO Φ O2 Adhesion promoting layer 3 W ms 6 ms 3 Pa 5 sccm sccm Barrier coating 5 W 4ms 4 ms 3 Pa 4 sccm 4 sccm Table 5.3: Optimized plasma parameters for deposition of barrier layer system. An investigation of the deviations of the coating process and measurement system for coated foils reveals the mean permeation rate and the standard deviation to be J =. ±.3cm 3 m 2 day, which is an indicator for good reproducibility of the coating and measurement process. Therefore, 2 foils are coated under the conditions listed in table 5.3 and the oxygen permeation rate is determined. 5.5 Investigation of permeation mechanisms In section it was deduced, that permeation barrier coating need to exceed a critical thickness to act as a permeation barrier, but a further increase of coating thickness does not improve the barrier properties. Therefore, the permeation mechanisms may not only be due to a permeation through homogeneous and defect free layer. This section investigates

118 4 Chapter 5 Permeation barrier coating these phenomena regarding coating defects and permeation mechanisms. For an analysis of the permeation behavior, the temperature dependence of oxygen permeation is investigated and the activation energy of permeation is calculated according to activated rate theory [6, 8, 59]. Therefore, the permeation J(T )[molm 2 s ]ofnoninteracting gases (e.g. O 2 ) can be described for glassy polymers below their glass transition temperature by simplified Arrhenius behaviour J(T )=J exp ( Ep RT ), (5.3) where E p [kjmol ] denotes the activation energy of permeation, J [mol m 2 s ]isaconstant permeation flux unique to the system, R is the gas constant [J mol K ]andt [K] is the absolute temperature. -7 ln(j / (mol m s )) T - /K - x -3 Figure 5.46: Arrhenius plot of oxygen permeation for 293 K T 323 K: uncoated PET ( ), single sided coating for various oxygen fluxes Φ O2 = 5 sccm ( ), Φ O2 = sccm ( ), Φ O2 = 2 sccm ( ), Φ O2 = 4 sccm ( ), Φ O2 = 6 sccm ( ) and double side coating Φ O2 = 5 sccm ( ), Φ O2 = 4 sccm ( ) at plasma conditions Φ HMDSO = 4 sccm, p =3Pa,P = 5 W, t on =4ms,t off according to equation (5.9). Figure 5.46 shows the Arrhenius plot for uncoated PET and SiO x coated PET under various oxygen fluxes Φ O2 in the temperature range 2..5 C. The exponential dependency of oxygen permeation described by equation (5.3) is confirmed by linear fits in the logarithmic plot. Thus, the activation energy of oxygen permeation E p can be derived from the slope of the fits revealing a value of E p =3.4kJmol for untreated PET, which is comparable to values reported in literature [3, 3, 6, 8, 4]. An increasing flux of oxygen of the HMDSO : O 2 plasma leads to a significant increase of activation energy of permeation as shown in figure The activation energy is increased by more than 2 kj mol by changing the layer composition from organic to quartz-like. For inorganic SiO x coatings, a maximum activation energy of E p =53.7kJmol is observed, which is below the activation energy of oxygen permeation for vitreous silica E p > 8 kj mol [6, 8, 4, 2]. Therefore, the permeation through the silicon oxide material is not the only mechanism of permeation. Additionally, permeation through coatings defects has to be taken into account reducing barrier function of SiO x coatings and leading

119 5.5 Investigation of permeation mechanisms E p / (kj/mol) Φ O2 / sccm Figure 5.47: Activation energy E p of oxygen permeation for various oxygen fluxes Φ O2 of HMDSO : O 2 plasma ( ) at plasma conditions Φ HMDSO = 4 sccm, p =3Pa,P = 5 W, t on = 4 ms, t off according to equation (5.9) and uncoated PET ( ). (The solid line serves as a guide for the eye.) to a residual permeation of J =±.3cm 3 m 2 day Analysis of coating defects µm 2µm (a) (b) Figure 5.48: SEM pictures of SiO x coatings on PET after 5 hours etching in CCP oxygen plasma Typically, coating defects can not be visualized by means of optical and even electron microscopy, because of their high transparency and the thinness of the coatings []. For an investigation of coating defects, a method is described by da Silva Sobrinho et al. [] based on atomic oxygen etching of the polymer beneath the silicon oxide coating. This method allows for a detection of defects and determination of their density within the coating on the PET substrate. For an investigation of coating defects, coated PET films are treated for 5 hours by an oxygen plasma in a capacitively coupled plasma (CCP) reactor setup [6] to etch the substrate polymer, which is masked by the SiO x coating.

120 6 Chapter 5 Permeation barrier coating SEM micrograph analysis of the etched coated polymer reveal circular etch structures as shown in figure 5.48, which can be explained by isotropic undercutting of the SiO x layer at the position of coating defects. The defect density of the coatings is determined to be 3 mm 2. The same analysis is performed with SiO x C y H z like coatings revealing a comparable defect density. The presence of coating defects, as visualized by means of oxygen plasma etching, reveals rather homogeneous distributed defects within the deposited coating over the whole surface area as exemplarily shown in figure Different reasons for defect formation can be assumed to play a role: (a) Coating defects can occur depending on the surface and deposition conditions during SiO x film deposition. The deposition of the barrier films takes place at nearly room temperature. Therefore, the surface mobility of active species hitting the substrate is limited, which can influences the formation of coating defects. (b) Dust particles formed in the plasma can contribute to the development of coating defects due to their deposition on the substrate during the inter-pulse period. They can shield parts of the substrate during the deposition process and lead to formation of defects. (c) Small amounts of polymer additives in the PET substrate, e.g. flexibilizers, colorants, slip agents or brighteners can represent locally modified surface properties during the coating deposition leading to defect formation Discussion of permeation mechanisms For an analysis of permeation mechanisms, PET films are additionally coated on both sides to get more insight in oxygen permeation behavior. Therefore, 6 nm thick SiO x C y H z like and SiO x coatings are deposited according to the parameters mentioned in section with oxygen flows of Φ O2 = 4 sccm and Φ O2 = 4 sccm, respectively. Figure 5.49 visualizes the one and two side coated PET films and gives the values of residual permeation J and apparent activation energy E p. In principle, three different types of permeation mechanisms can be visualized for one and two side coated substrates:. permeation through bulk material of coating 2. permeation via coating defects 3. permeation via coating defect through one coating and via bulk material through other coating An analysis of the SiO x C y H z like and SiO x deposited coatings reveals insights into the permeation mechanisms. For the single layer structure (figure 5.49(a) and 5.49(b)), the permeation drops strongly for changing the material composition from SiO x C y H z like coatings to quartz like SiO x coatings. Additionally, the apparent activation energy is increased by more than 2 kj mol. A two side coating (figure 5.49(c) and 5.49(d)) reveals no significant changes for the SiO x C y H z coating on both sides of the PET film compared to single side coating shown in figure 5.49(a). The residual permeation and the apparent activation energy remain nearly constant. Taking into account, that the defect density of SiO x C y H z like coatings is comparable to SiO 2 coatings as confirmed by etching experiments, it can be concluded, that for SiO x C y H z like coatings the permeation is dominated by permeation through the bulk material of the coating (path ()).

121 5.5 Investigation of permeation mechanisms 7 () (2) () (2) SiO x C y H z PET SiO x PET (a) SiO x C y H z on PET: J =68.2cm 3 m 2 day, E p =3.5 kj mol () (3) (3) (2) (b) SiO x on PET: J =cm 3 m 2 day, E p =53.7 kj mol () (2) () (2) SiO x C y H z PET SiO x PET SiO x C y H z SiO x (c) SiO x C y H z on both sides of PET: J =62.6cm 3 m 2 day, E p =3.2 kj mol (d) SiO x on both sides of PET: J =.6 cm 3 m 2 day, E p =65.6 kj mol Figure 5.49: Visualization of different permeation mechanisms for SiO x C y H z and SiO x coatings deposited on one or both sides of 23 µm thick PET substrates: () permeation through bulk material of coating, (2) permeation via coating defects, (3) permeation via coating defect through one coating and via bulk material through other coating In contrast, for SiO x coating, a double side coating enhances the barrier property and the apparent activation energy is increased further by more than kj mol leading to E p =65.6kJmol. The barrier is improved by more than compared to pristine PET by coating both substrate surfaces. This can be explained by enlarged oxygen permeation path along the coating system as shown in figure 5.49(d) path (2) due to high permeation barrier of SiO x coatings. Such a coating defect permeation depends on the positions of the defects on both sides and thereby, oxygen permeation paths of double side coatings are enlarged. Therefore, the main permeation mechanism of residual oxygen flux of SiO x like coatings is found to be permeation via coating defects. If coating defect permeation is assumed to be the most important mechanism for SiO x coatings above their critical thickness, a rough estimation of the radius of the defects of a single side coated PET substrate can be made. Therefore, it is assumed, that permeation is only possible via uncoated areas of the surface. For the permeation measurement, a test area of cm 2 is used. If the permeation is only possible via uncoated areas represented as coating defects, an area A residual of permeation can be defined responsible for residual

122 8 Chapter 5 Permeation barrier coating 2µm µm (a) Small coating defect: A 5 3 µm 2 (b) Large coating defect: A 7.5 µm 2 Figure 5.5: Detailed SEM picture of different coating defects after 5 h CCP atomic oxygen etching permeation: A residual = J residual cm 2 = 46, 6 3 cm 2. (5.3) J PET Using the determined defect density of 3 mm 2,3 6 defects are present in a cm 2 coating. The mean radius r of a coating defect can be estimated to be Aresidual r = =.2 µm. (5.32) 3 6 π The calculated mean defect radius r =.2 µm is within deviations of values reported by [3, 4] and in good agreement with values revealed by [62, 63] for plasma polymerized SiO x barrier coatings. The influence of the temperature on the determination of the defect size reveals a deviation of r = ±.2 µm, which is not significant for a reasonable estimation of the mean defect size by means of the permeation rates. This result revealed by oxygen etching of SiO x coatings confirms the assumption, that the permeation of microwave plasma polymerized quartz like coatings is dominated by coating defect permeation. Therefore, a mean permeation area per defect can be determined to be Ā 5 µm2. Figure 5.5 gives two examples for typical defects visualized by SEM investigations with different defect areas. The defect shown in figure 5.5(b) is a representant for large coating defects and has a defect area of A 7.5 µm 2. Additionally, smaller coating defects are found in the etched SiO x coatings characterized by defect areas of A 5 3 µm 2 as displayed in figure 5.5(a). 5.6 Coating of bottles In the previous sections, an optimized barrier layer system consisting of an adhesion promoting layer and a barrier coating is developed on PET foils as substrate material. An adaption of the optimized barrier layer system for the coating of complete PET bottles shows difficulties due to changed deposition conditions inside the bottle during the coating process. Especially, variations of the process pressure inside a bottle compared to the controlled pressure in the reactor vessel are observed due to a reduced pump diameter. The diameter of a typical bottle neck is d bn = 22 mm and the diameter of the Plasmaline antenna is d PL = 2 mm. Therefore, an effective pump area of A pump = 267 mm 2 is present for the bottle evacuation.

123 5.6 Coating of bottles 9 O2-permeation / cm 3 pck day A pump / mm 2 Figure 5.5: Permeation rates of coated PET bottles for various lengths of the Plasmaline l PL as function of pump area A pump : ( ) l PL = 34 mm, ( ) l PL = 38 mm, ( ) l PL = 46 mm, (x) l PL = 58 mm. The plasma parameters are listed in table 5.3. The process pressure during barrier coating deposition strongly influences the barrier properties of SiO x coatings due to changes in porosity and densification as revealed in section Figure 5.5 shows the oxygen permeation of coated bottles for various pump diameters and lengths of Plasmalines. The modification of the pump diameter is realized by cutting bottles at various bottle heights to create diameters from d = 46 mm, which is larger than a typical bottle neck, to d = 49 mm, representing a cup like geometry. All bottle geometries are coated using the optimized barrier system as merged in table 5.3. A strong influence of the pump diameter on the permeation rate is observed for all lengths of Plamsmalines. The permeation is reduced as the diameter is extended. An almost linear drop of permeation is revealed for the longest Plasmaline. The shorter ones show a nearly constant behavior of permeation with a small reduction for the largest pump diameter. The results confirm two previously obtained results regarding Plasma extension along the Plasmaline and influence of process pressure on the coating quality. The length of the Plasmaline is correlated to the Plasma extension inside the bottle as illustrated for the electron density in figure 3.2. A strong drop in density is observed at the end of the Plasmaline leading to poor deposition conditions in the areas not covered by the Plasmaline. Therefore, shorter Plasmalines do not allow for a homogeneous plasma ignition inside the bottle and areas remain uncoated or the required coating quality is not achieved. The influence of the process pressure is significant for the deposition of high barrier SiO x coatings as illustrated in figure 5.4. The pressure has to be as low as possible for good barrier coatings. Due to a reduced pumping area for standard bottle neck diameters, the permeation rates of coatings deposited in complete bottles are comparable to uncoated bottles. An increase in pumping area lead to a reduced pressure inside the bottle and the barrier property is enhanced. For the longest Plasmaline and the largest pumping diameter, a reduction of a factor of approximately 5 is achieved, which is not sufficient for packaging applications. An analysis of the coatings quality on the bottle wall and the bottom reveals more insight into the limiting factors.

124 Chapter 5 Permeation barrier coating Influence of bottom of bottle Besides the influence of the process pressure and the length of the Plasmalines, the complex base geometry of a PET bottle influences the permeation of the package. Figure 5.52 shows the permeation rate of the bottle walls of the bottles considered in figure 5.5. A similar behavior of the permeation through the walls depending on the pumping diameter and the length of the Plasmaline is observed. Contrarily to the results of barrier improvement of a factor of 5 for complete packages, the barrier properties of the bottle walls is increased by a factor of more than 3 regarding oxygen permeation. This is a reasonable value for food and beverage packaging application. Therefore, it can be concluded, that the permeation barrier system for PET foils can be successfully adapted to the coating of the PET bottles, but the complex bottom geometry and the process pressure up to now avoid a proper coating of three dimensional packages. O2-permeation / cm 3 m 2 day A pump / mm 2 Figure 5.52: Permeation rates of bottle walls as function of pump area A pump :( ) l PL = 38 mm, ( ) l PL = 46 mm, (x) l PL = 58 mm. The plasma parameters are listed in table 5.3. Simulation of pressure inside the bottle For a detailed investigation of the behavior of the process pressure inside a PET bottle, numerical simulations by means of a finite element method are performed. Figure 5.53 shows the results of the pressure simulation within a PET bottle for two process gas flows. The results are based on a numerical solution of the incompressible Navier-Stokes equation by means of Comsol Multiphysics [64]. As boundary conditions, the pressure at the area of pumping diameter is fixed at p = 3 Pa comparable to experimental conditions for the used gas flows. Figure 5.53 shows a strong increase of the pressure inside the bottle for increasing gas flows. Besides the increase of the mean pressure for increasing gas fluxes, especially an increase of pressure close to the bottle base is observed due to the gas flow. The process gases leave the Plasmaline with a velocity perpendicular to the bottom of the bottle. At the bottle base, the gas is reflected and accelerated to the direction of the bottle neck due to the pumping system. For the simulation, the bottom of the bottle is simplified, but the significant tendency of a pressure increase close to the bottom of the bottle is revealed. This behavior influences the coating quality of bottles and cup shape geometries leading to a reduced barrier property of the bottom of the package related to the barrier improvement of the bottle walls. Close to the complex bottom geometry, two challenges are present. On

125 5.6 Coating of bottles (a) 8 Pa 5 Pa 2 Pa 9 Pa 6 Pa 3 Pa (b) 8 Pa 5 Pa 2 Pa 9 Pa 6 Pa 3 Pa Figure 5.53: Simulated pressures in PET bottles for Φ = 2 sccm. - only a half bottle simulated the one hand an increased pressure close to the bottom leads to the deposition of low barrier coatings as revealed in figure 5.5. On the other hand a minor plasma density is observed related to the length of the Plasmaline antenna influencing the deposit quality as illustrated for bottle walls in figure The results of the pressure simulation confirm the measured permeation data and allow for an evaluation of the weaknesses during the coating of three dimensional packages. Beside the main problem of high pressures inside the bottles, a further concern is the complex bottle geometry leading to a reduced barrier improvement compared to the bottle walls. Possible future approaches The pressure during the deposition of SiO x permeation barrier coatings is a crucial parameter. It strongly influences the ion bombardment leading to changes in layer porosity and deterioration of the barrier properties. A reduction of the total gas flow to reduce the pressure inside the bottle does not lead to appropriate deposition conditions due to depletion of HMDSO for required gas flows and increased treatment times due to the homogeneity criterion (cp. equation (5.9)). The problem of monomer depletion is observed for the investigated setup for Φ HMDSO sccm as experimentally analyzed by coating of foils and cup shaped geometries. A comparable effect of monomer depletion is observed by other groups [22, 7] leading to insufficient barrier properties of deposited coatings. Therefore, only a tuning of the ion energy can be a future approach for the deposition of high barrier coatings in PET bottles by means of a Plasmaline setup. A modulation of the ion energy distribution function by means of a substrate bias as performed by other groups considering flat substrates, such as webs, constitutes a solution of the problem of less ion energies. Sonnenfeld et al. [8, 9] analyze the significant influence of a substrate bias on the barrier property of SiO x coatings on PET. An additional substrate rf bias reduces the oxygen permeation by two orders of magnitude compared to an ordinary microwave layer deposition. Additionally, Vautrin-Ul et al. [65] investigate the composition and density of SiO x coatings deposited by oxygen diluted HMDSO plasmas and observe a strong reduction of porosity for optimized bias conditions. Comparably, Benissad

126 2 Chapter 5 Permeation barrier coating et al. [66] show a reduction of silanol groups of deposited layers by tuning the biasing parameters indicating modifications in porosity. Also da Silva Sobrinho et al. [2] demonstrate an enhancement of the barrier properties by an rf modulated substrate bias in combination with a microwave discharge for the treatment of webs. Therefore, a biased bottle cage can be a solution of the previously described problems of coating of three dimensional objects with small pumping diameters and complex base structures. 5.7 Conclusion The pulsed deposition of SiO x barrier coatings on PET substrates allows for the homogeneous treatment of three dimensional packaging materials with regard to the homogeneity criterion described by equation (5.9). For the deposition of high barrier SiO x coatings on pristine PET substrates, an adhesion promoting layer has to be deposited prior to the deposition of the barrier coating. It is characterized as SiO x C y H z and deposited under low fragmentation conditions of the HMDSO monomer without oxygen addition. The barrier coating is deposited on top of the adhesion promoting layer. It is carbon free and SiO x like with a residual amount of SiOH incorporated in the coating. Optimized deposition conditions for the barrier layer are described as highly oxygen diluted and the monomer is strongly fragmented. The presence of ion bombardment is essential for the deposition of high barrier coatings. The coating mechanisms of SiO x,asanalyzedbymeansofmass spectrometry and optical emission spectroscopy, can be described as the deposition of larger fragments of HMDSO built under electron impact ionization on the surface of the substrate followed by an oxygen etching of these species to form mainly CO 2,H 2 O and OH. The permeation mechanisms of plasma polymerized silicon oxide coatings are characterized and a shift from bulk dominated permeation of SiO x C y H z like coatings to defect dominated permeation of SiO x coatings is revealed. The residual permeation of ±.3cm 3 m 2 day is due to permeation via coating defects as investigated by means of capacitive plasma oxygen etching of deposited coatings. It is a reasonable value for food packaging applications and very good for microwave plasma deposited SiO x barrier coatings. An adaption of the explored optimized barrier coating system to coat PET bottles is challenging due to an increased process pressure and complex bottom geometries. However, a coating of cup shaped geometries reveal a barrier improvement of the side walls by a factor of more than thirty regarding oxygen permeation, which is a feasible value for industrial beverage filling application and shows the capabilities of the developed coating process.

127 Summary The development of a combined plasma sterilization and permeation barrier coating process is the focus of the presented thesis. A plasma reactor system based on a surface wave exited microwave plasma is developed for the plasma ignition within three dimensional packaging materials for sterilization and coating processes. The system is characterized to investigate the properties of a low-pressure microwave plasma based on a Plasmaline antenna regarding time and spatial resolved electron density and electron temperature profiles of a noble gas plasma. Additionally, mass spectrometry investigations are performed for the determination of ion energy distribution functions due to their importance of ion energies on surface treatments. The behavior of revealed plasma parameters is explained by the properties of electrical field propagation within the surface wave excited plasma and two main regions are found: () a plasma heating layer surrounding the Plasmaline antenna is responsible for high energetic electrons and the production of a high density plasma, whereas the shielding properties of the plasma lead to (2) an low density-type plasma close to the bottle cage. The plasma parameters of this relevant region for surface processing can only be partly controlled due to the shielding effects of the plasma. In the field of plasma sterilization, a plasma process is developed based on results of investigating the plasma sterilization mechanisms as revealed in the European BIODECON project [33]. Microbiological challenge tests give proof of the sterilization capabilities of the optimized plasma process. A reduction of B. atrophaeus and A. niger according to the requirements of FDA and VDMA are achieved within five seconds. Therefore, a plasma sterilization process is promising for industrial processes and constitutes manifold advantages compared to chemical sterilants. Additionally, the plasma sterilization process is characterized by means of Langmuir probe measurements and by absolutely calibrated optical emission spectroscopy. For the permeation barrier coating of PET substrates, a silicon oxide permeation barrier coating system based on an oxygen diluted hexamethyldisiloxane plasmas is investigated. To achieve homogeneous treatment of three dimensional substrates, a pulsed plasma process is developed for SiO x film deposition. A criterion correlating the pulse conditions and the residence time of the process gases in the packaging is deduced allowing for a process scaleup. The influence of all relevant plasma conditions, namely gas composition and flow rates, process pressure and pulsed microwave power properties are investigated in detail. Their influence on barrier properties, surface morphology and composition of the coatings are investigated explicitly. It can be stated, that good permeation barrier coatings have to exceed a critical thickness of 6 nm and an adhesion promoting layer has to be deposited prior to the deposition of the barrier coating. The organic adhesion promoting layer is SiO x C y H z like, whereas the barrier coatings are quartz like SiO x. The deposited barrier coatings reduce the oxygen permeation of more than a factor of 65 if they are deposited on one side of the substrate and of more than for two-side coatings. The permeability parameter P 3

128 4 Summary of the deposited coating is four orders of magnitude lower than the coefficient of pristine PET. Besides the coating analysis, the neutral gas composition and the ion bombardment of the surface are considered by means of mass spectrometry. It is revealed, that the deposition process of SiO x films is based on the deposition of SiO x C y H z like HMDSO fragments on the surface produced by electron impact dissociation and oxygen etching mainly producing CO 2,OHandH 2 O as oxidation products. Furthermore, the permeation mechanisms of deposited films are analyzed revealing defect permeation as a dominating mechanism for the observed residual permeation. The defects are visualized by means of capacitive coupled oxygen etching of deposited barrier layers. The barrier coating system is developed based on the coating of PET foils. Adapting obtained results to PET bottles, problems due to challenging process conditions inside the bottles are discussed. However, promising start parameters for three dimensional coatings are revealed and on bottle walls a barrier improvement of more than 3 is observed for adequate pump diameters of the bottle. Concluding, plasma parameters for the sterilization and permeation barrier coating of PET substrates by means of a low-pressure surface wave excited plasma are determined and the processes are characterized regarding the sterilization capabilities and the coatings properties. The determined plasma parameters are relevant for an industrial application of a combined plasma sterilization and barrier coating process. The revealed homogeneity criterion allows for a generalization of pulsed plasma processes and for an independent investigation of process parameter influences on deposited coatings and the process chemistry as performed in this thesis.

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