Clear Barriers and High Volume Productivity Rainer Ludwig and Liz Josephson APPLIED MATERIALS ABSTRACT The volume of Transparent Barrier Coating used in commercial and industrial applications is growing. The target markets for these coatings require the delivery of stable, high quality coatings. Compared to all other vacuum web coating processes, EB-evaporation delivers the highest coating speed and productivity. This paper gives an update on the state of the art of technology for Transparent Barrier Coatings with EB-evaporation and shows recent results of our work regarding improvements of coating uniformity as well as an outlook for other applications. INTRODUCTION Optimum flexible packaging material should combine maximum protection of the content with minimum adverse effects to the environment. This can be achieved for example by the use of metallized films. By providing a gauzy Aluminum layer to the plastic film, the oxygen transmission rate ( OTR ) can be reduced by a factor of up to 100 and in addition a light protection is provided. Other new applications require also a low OTR or water vapor transmission rate ( WVTR ), but in addition transparency for visible light, microwavability, or the use of a metal detector for the packed food. This requirements can be fulfilled by replacing the aluminum layer of a metallized film by an oxide coating. Different oxides like Silicon oxide ( SiOx ), Aluminum oxide ( Al2O3 ), Magnesium oxide ( MgO ), or mixtures of different oxides can do the job. On the other hand, different vacuum coating processes can be used to produce oxide layers: With sputtering, all king of oxide layers can be produced at optimum quality level. But the coating speed is only in a low percentage range of a metallizing speed. Plasma Enhanced Chemical Vapor Deposition ( PECVD ) was proposed already by J. Felts in 1990 [ 1 ], but this process could not gain a wider acceptance. The most used process nowadays is evaporation, especially electron beam ( EB ) evaporation [2,3,4 ]. This process provides not only coating speeds similar, or even higher, than metallizing, but is also flexible for the use of different coatings and has a high robustness. This is of special importance for the production of transparence, not visible coatings. In the following sections, the principle of EB web coating, with main focus on how to ensure coating quality will be given. ELECTRON BEAM WEB COATING Fig. 1 shows the principle of inline layer control in an EB web coater, using feed back control from inline transparency measurement. Fig.1: closed loop control in EB web coater As in a Metallizer, a substrate roll is wound through a vacuum chamber from unwind reel to a rewind reel, passing a cooled coating drum. Below the coating drum, a crucible is located, filled with the coating material. The energy, needed for the evaporation of this material comes from an electron beam, provided by an electron beam gun. A scanning unit, directly installed in the electron beam gun, is moving the beam on the crucible surface. The power distribution on the crucible surface is controlled by the actual measured transparency value on the corresponding substrate surface area.
What is important for a stable Coating Quality? Layer Uniformity To be really sure about the layer thickness in an actual roll, all across its widths and length, it is a must to have an inline measurement system for a relevant layer property. For most transparent barrier layers the measurement of the optical transparency at low wave length can be used. An example for such a measurement unit is shown in Fig. 2 Fig. 2.: Inline layer measurement system Light from a light source outside of the vacuum is wave length selected ( wave length range 380 nm to 1000 nm ), is split up and transferred to multiple sensor heads across the substrate width. The substrate is moving in between the end of the glass fibers and the sensors. The system can also measure optical density up to OD 4. The measured values of the different heads can be used for a closed loop control with the beam scanning unit. This control is performed, using a special, patented software [ 5 ]. Uninterrupted Coating With high-voltage sources arcing always is an issue. Even with optimized EB guns, using separate pumped beam generator chambers and additional intermediate pumping between beam generator and process chamber, particles from the process may reach critical areas and cause arcing. Such arcs led to a shutdown of the high voltage power supply and can result in uncoated areas on the substrate in web running direction. Arcing cannot be avoided to 100 % and shutdown of power supply will occur. But uncoated areas on the substrate can be avoided, if the shut down of the power supply is short enough. Such fast switching was ensured in older type power supplies by using high power tubes. Nowadays modular, switching type power supplies are used, which also can minimize the voltage interruption time to be in the range of several hundred microseconds [ 6 ]. Such a power supply is shown in fig. 3. Fig. 3: High-Voltage Power Supply Pinholes Here with transparent barrier coating, there is a big advantage against metallizing. Even if pinholes are present, they are not visible! Usually pinholes are also not critical for the barrier function, in case the coated film is laminated afterwards. However, in case of printing directly on the coating, a invisible pinhole can become visible. For example a pinhole in a SiOx coating below a directly printed-on barcode can create problems.
In addition to usual counteractive measures, like polished guiding rollers, with EB machines pinholes can be minimized with an optimized beam scanning program. However, the most influencing factor is the evaporation material itself. Electrostatic Charging of the Substrate Coating of plastic film with insulating layers at high speed is more critical than coating with a conductive metal layer. In electron beam web coaters we have an additional charging up by reflected electrons. For discharging of the substrate prior leaving the coating drum, an special plasma tool has to be used. Adhesion Adhesion of the coating to the substrate is a key issue for later on processing like laminating or retorting. Adhesion can be influenced by inline pretreatment. A detailed description is given elsewhere [ 7 ]. Depending on the requirements, in electron beam web coaters are used either DC driven Magnetron glow discharge tools as shown in fig. 4 or RF driven Hollow Anodes as shown in fig. 5 Fig. 4 : DC driven TreatMag The TreatMag [ 8 ] is used in all kind of web coaters, like, EB machines, sputter machines, capacitor web coaters and Metallizers. By running Ar-plasma it can be used for cleaning of the substrate surface. With reactive gases like oxygen or nitrogen, also a chemical modification of the substrate surface is possible. Fig. 5: Two Hollow Anodes in electron beam web coater Compared to the TreatMag, with a RF driven Hollow Anode [ 9 ], higher energetic particles can be provided to the substrate.
Substrate Overheating Like in Metallizers, the coating has to take place on a cooled drum. Due to the reflected electrons the substrate is charged up during coating with the effect of better thermal contact between substrate and drum. So thermal problems are lower compared to Metallizers. Transparent Barrier Coating The barrier properties, the most important layer function,cannot be directly measured. Even offline, only limited spot measurements, which are very time consuming, can be done. So at least an indirect method for inline indication of barrier properties should be helpful. For PET substrates, a correlation of barrier properties with the coating thickness is known. Fig.6 shows the dependence of OTR with coating thickness for different coatings. Fig. 6. OTR as function of layer thickness For all coatings ( Al, Al2O3, SiOx ) a stable barrier function starts at a thickness of about 20 nm. For Al, the OTR decreases further with increasing coating thickness. For Al2O3 and SiOx the OTR values stays constant with increasing thickness. That means, provided to have stable process parameters like vacuum, web tension, substrate temperature, layer composition, also the barrier properties should stay stable, in case a certain minimum coating thickness can be guaranteed. This is a good basis for quality insurance of the barrier properties. Of course, also the layer thickness cannot be measured directly inline, but there is a relationship again to the optical properties. This relationship is a direct one to OD in case of Al. In case of oxides a relationship to the transparency is given for constant stoichiometry. Fig. 7 shows the light transmission of 12 µm PET, uncoated and with different SiOx layers. Fig. 7. spectral transparency of SiOx layers In case all above marked items are handled with care ( the coating is uniform within the right thickness range, the coating is not interrupted and the substrate is not too much charged, the substrate was not overheated and the adhesion is good ) also the barrier value range should be stable. However, stable at what value is a question which is mainly influenced by the substrate itself. By applying the same coating to different substrates, the OTR value can vary in the case of 12 µm PET between more than 3 to well below 1 cm3/m2 day. For OPP the range is between 20 and several 100. In case of reactive evaporation of Al2O3, there is an additional knob which can be influenced by the machine supplier. It could be shown, that with plasma assisted evaporation, the range of barrier values is stabilized at the lower
end. Fig. 8 shows the patented arrangement of microwave tools in the evaporation chamber of an electron beam web coater for initiation a plasma in the evaporation cloud. Fig. 8: Microwave driven Plasma Source near Crucible Optical Coatings As an example for an optical coating, a multi layer of Al, Al2O3 and thin, semitransparent Al was produced in a TOPBEAM. Such, Fabry Perot type systems provide different colors based on interference effect as shown in Fig. 9. Fig.9 Fabry Perot type multi layer The thickness of the transparent middle layer is the determining factor for the color. By viewing the coating under different angles, the light has to pass different distances between the mirrors. This causes a color shift effect. Important for a defined color is a very precise thickness uniformity of the middle layer. This can be provided also by closed loop control of the EB, using a inline measurement system for spectral reflectance. Figure 10 shows samples of different Fabry Perot Type coatings, produced in a TOPBEAM. Fig. 10 Fabry Perot Type Coatings
CONCLUSIONS With EB web coating a proven and robust process for mass production of transparent barrier coatings is available. Depending different on requirements, also different coatings like SiOx, Al2O3 or even oxide mixtures can be produced. Based on improved inline layer thickness control possibilities, even optical multi layers for security, brand protection applications, which were in the past only subject of the much slower sputtering process can be produced. References 1. J. Felts, Transparent Gas Barrier Technologies,, 33 rd Ann. Techn. Conf. of the SVC, 1990 2. W. Lohwasser et. al. 2 Large Scale Electron Beam Web Coating, Not only for packaging, 43 rd Ann. Techn. Conf. of the SVC, 362, 2000 3. T. Ohta et. al, A Ceramic (SiO2-Al2O3 mixture) Coated Barrier Film by Electron Beam Evaporation, 43 rd Ann. Techn. Conf. of the SVC, 368, 2000 4. P. Seserko et. al., Transparent Barrier Coatings by electron Beam Evaporation An Update, 44 th Ann. Techn. Conf. of the SVC, 482, 2001 5. M. Bähr et. al., New Scan and control System (ESCOSYS TM ) for High Power Electron Beam Techniques, Proc. Of PSE, 1996 6. A. Thiede et. al., New Type of High Voltage Power Supplies for Electron Beam Web Coaters,46 th Ann. Techn. Conf. of the SVC, 149, 2003 7. R. Ludwig et. al., In Chamber Pretreatment for Vacuum Web Coaters, 48 th Ann. Techn. Conf. of the SVC, 2005 8. G. Loebig et. al., TreatMag, a New Tool for Inline Plasma Pre-treatment in Web Coaters for Packaging Applications, 41 st Ann. Techn. Conf. of the SVC, 502, 1998 9. M. Geisler et. al., RF Plasma Tool for Ion-Assisted Large-Scale and Sheet Processing, 44 th Ann. Techn. Conf. of the SVC, 482, 2001
2006 PLACE Conference September 17-21 Cincinnati, Ohio Clear Barrier and High Volume Quality Production Presented by: Rainer Ludwig Senior Manager Sales and Marketing APPLIED MATERIALS 1 Introduction Clear Barrier and High Volume Quality Production 2 Clear Barrier Optimum Flexible Packaging = Maximum Protection + Minimum Influence to Environment Metallizing Additional Requirements for the Packaging: Transparency for visible Light, Microwaveability, Use of Metal detector. Can be fulfilled with Oxide Coatings : Silicon oxide ( SiOx ), Aluminum oxide (Al2O3), Magnesium oxide( MgO ); Mixtures of Oxides 3 1
Oxide Coating Clear Barrier and High Volume Quality Production...how to produce an oxide coating? 4 Oxide Coating Sputtering can do it, but Coating Speed in the low Percentage Range of Metallization Plasma Enhanced Chemical Vapor Deposition proposed since 1990, but no Breakthrough Evaporation highest Productivity, mostly used, especially the most powerful Evaporation Process Electron Beam Evaporation 5 Electron Beam Web Coater Chamber Cross Section similar to Metallizer Most Difference is the Coating Source 6 2
Electron Beam Evaporation 7 EB Evaporation Robust gun with decoupled pumping Crucible with Evaporation Material HV-Power Supply ( 200 / 400 kw ) Beam Controller ESCOSYS 8 Winding Max. width: 2100 / 1100 mm Max. roll diameter: 1.2 / 1 m Coating Speed: up to 17 m/sec Reversible winding system optional 9 3
Coating Processes Direct Evaporation of Oxides SiOx, Al2O3, SiO2, MgO,... Reactive Evaporation Evaporation of Aluminum + Oxygen = Aluminum Oxide on the Substrate 10 Quality Clear Barrier and High Volume Quality Production 11 Layer Quality Layer Functions? How to document? How to produce? 12 4
Layer Quality Metallizing Clear Barrier Coating Thickness: OD, eye? Defects: Light Transmission? Adhesion: Tape Test? Layer visible Is there a coating at all? Clear Barrier: Detailed Roll Report, including all relevant Layer Functions is needed 13 Layer Functions Metallizing Clear Barrier Coating Barrier x x Light Protection x Content visible x Microwaveable x Metal free x Optical -, Electrical - and Barrier Properties 14 Layer Quality - Optical Properties Requirement: To be Transparent Inline measurement of optical Transparency 15 5
Layer Quality - Electrical Properties Requirement: Not conductive ( Always ) fulfilled for Transparent Layers 16 Layer Quality - Barrier Properties Direct Measurement: measure the Barrier Time consuming, no inline process Only spot check possible Indirect Inline Measurement utilizing other Layer Properties? Thickness Optical Properties 17 Barrier Properties OTR is a function of thickness ( shown on 12 µm PET ) But no direct inline measurement of thickness possible 18 6
Layer Quality: Optical and Barrier Properties Most Transparent Barrier Coatings have UV-Absorption Useable for Inline Measurement as indirect Measurement of thickness 19 Layer Quality How to Document? Optical Properties, Transparent Inline Measurement of Transparency Electrical Properties, not Conductive Inline Measurement of Transparency Both also valid also for uncoated Substrate Barrier Properties - Indirect Measurement: Is a Layer present in the right Thickness Range? Inline Measurement of Transparency in the UV Range 20 Quality How to Produce? Inline Measurement of Transparency in UV Inline Coating Control Other Quality related Items: Pinholes Layer Structure Electrostatic Charging Substrate Overheating Adhesion (Substrate )... 21 7
Layer Measurement with LMS-XL The LMS-XL is used for measurement of OD ( 0.00...4 OD ) or T. The measuring wavelength can be selected by a filter ( 350 nm...1000 nm ) Glass fibers from light source and measurement sensors Front view of base unit 22 Pinholes A Pinhole in a Transparent Coating is not visible! Not critical for Barrier in case of Laminating Can become visible in case of direct Printing Process dependent. Pinholes, mainly influenced by the Evaporation Material 23 Plasma Assisted Evaporation Microwave driven Plasma sources near Crucible Improved Barrier Range for reactive Process Al 2 O 3 24 8
Adhesion Adhesion is key issue for later processing like Laminating or Retorting Can be influenced with inline Plasma Pretreatment Different Tools are available: DC Magnetron RF Hollow Anode 25 ( Substrate ) Also important for Barrier is the Substrate Same Coating on different Type PET ( 12 µm ) OTR between < 1 and 4 Same Coating on different Type OPP OTR between < 20 and several 100 26 Electrostatic Charging With EB, additional Charging from reflected Electrons Charging is bad for winding Charging is good for Substrate Cooling Discharging of the Substrate prior leaving the Coating Drum 27 9
EB Web Coater Quality related Hardware Substrate charging and discharging Plasma Pre-Treatment of Substrate Plasma assisted Evaporation Inline Measurement Closed Loop Control 28 TOPBEAM 2100 TOPBEAM 1100 29 Top View TOPBEAM 2100 30 10
Further Applications for EB 1. Breakthrough for EB was ME-Tape 2. Breakthrough for EB was Transparent Barrier Based on improved Hardware / Software for highly Uniform Coatings, EB has started to take over certain applications from sputtering processes, like optical Multi - Layers 31 Color Shift Multi-Layer Top Layer (semi-transparent) Middle Layer (transparent) Base Layer (Mirror) 32 Layer Measurement with Optoplex Measurement of R (lambda ) with Multiple heads across web width Max./min values indicate the thickness Closed loop control with ESCOSYS 33 11
Security coatings Fabry Perot Filter: Al - Al2O3 - Al Different layer systems produced In TOPBEAM Advantage: Only one Evaporation material 34 CONCLUSIONS EB Web Coating offers high Process Flexibility and Coating speed EB Web Coaters for high Quality / high Volume Production are State of the Art in Japan and Europe since Years Based on recent Improvements, EB Web Coating is ready ( and already used ) for Applications with highest Requirements in Uniformity 35 Thank You PRESENTED BY Rainer Ludwig Senior Manager Sales and Marketing APPLIED MATERIALS Please remember to turn in your evaluation sheet... 36 12