Atomic Layer Deposition on Biological Matter

Transcription

1 Atomic Layer Deposition on Biological Matter Dissertation Zur Erlangung des akademischen Grades Doktor der Ingenieurwissenschaften der Fakultät für Angewandte Wissenschaften der Albert-Ludwigs-Universität Freiburg im Breisgau vorgelegt von Seung-Mo Lee geboren in Chun-cheon, Südkorea Freiburg im Breisgau 2009

2 Dedicated to my lovely wife Ji-Sun, to my venerable father in heaven, to my mother, to my youger sister, and to my teacher Prof. Ulrich Gösele resting in peace To all those who taught me This dissertation is for you

3 If most of us are ashamed of shabby clothes and shoddy furniture, Let us be more ashamed of shabby ideas and shoddy philosophy -By Albert Einstein

4 Dekan: Prof. Dr. Hans Zappe 1. Referent: Prof. Dr. Ulrich Gösele 2. Referent: Prof. Dr. Margit Zacharias Vorsitzender der Prüfungskommission: Prof. Dr. Oliver Paul Beisitzender: Prof. Dr. Ulrich Egert Tag der Disputation: Mittwoch, Dez 16, 2009

5 Preface After invention of atomic layer deposition (ALD) by Dr. Tuomo Suntola and co-workers in 1974, the interest in ALD has strongly increased in the of 1990s and 2000s to satisfy the industrial need to scale down microelectronic devices. Thanks to increased scientific and technical interest, nowadays diverse manufacturers and institutions have performed various designs of ALD machinery together with the development of new ALD precursors. ALD can be defined as a film deposition technique which is based on the sequential use of self-terminating gas solid reactions. ALD is a subset of chemical vapor deposition (CVD) suitable for depositing inorganic layers with a thickness down to the level of a monolayer. ALD has the capability to coat complex shaped substrates with a conformal film of high quality. Due to these unique characteristics, ALD-grown materials have a wide range of applications, from catalysts to electroluminescent displays and microelectronics. ALD is recognized as one of the key technologies for the surface modification and functionalization of complex organic or inorganic nanostructures, such as nanowires, nanopores or nanotubes. Besides the excellent conformality of the ALD coating, some scientists have reported examples of inorganic nanostructures fabricated from complicated biological templates as one of the bottom up approaches for 3D nanofabrication. In this thesis, some examples will be introduced which illuminate novel applications of ALD using biological matter, such as dragline silks from Araneus spider and collagen membranes collected from chicken s eggshell matrices. It will be demonstrated that that metal oxide ALD coatings on these biological templates leads to conformally coated metal oxide films on the templates as well as to chemical/physical modifications of the inner protein structure of the silk and collagen involved. As a result, those modifications lead to an improvement of the mechanical properties. This modification process by ALD has been termed Multiple Pulsed Vapor Phase Infiltration or MPI in short. At present, the detailed mechanism associated with the chemical/physical modifications and the reasons causing those resulting property improvements are not clearly understood yet. It has, however, been deduced that this mechanical property improvement can be attributed to metal infiltration into the inner protein structure of the silk and the collagen, and this metal infiltration is related to the unique self-limiting film deposition mechanism of the ALD process which distinguishes it from other deposition techniques. On the other hand, aside from mechanical properties improvements, it is expected that, those modifications presumably could affect other physical properties, such as electrical, magnetic and optical properties. In this thesis, however, first and foremost preliminary results focusing on the improvement of mechanical properties are presented.

6 In order to give a general idea of the ALD process to the reader, in Chapter 1 the basic principle of ALD is explained using a metal oxide deposition, such as Al 2 O 3, TiO 2 and ZnO as an example. Further on, the fundamental differences between ALD and CVD are pointed out. Following the introduction of ALD, in Chapter 2, basic parameters from mechanics, required to describe and understand the mechanical deformation behaviour, are briefly described using a stress-strain curve plotted under uniaxial tensile test. The viscoelastic behaviour of materials is also introduced. In the Chapters 3, 4 and 5, ALD applications using biological matter are addressed. Firstly, in Chapter 3, an example to show the capability of ALD to conformally deposit materials (TiO 2 and ZnO) on complexshaped biological templates (macroporous collagen membranes) is presented. Moreover, some crystallographic growth features of TiO 2 and ZnO at various processing temperatures are also demonstrated. As a feasible application of those templated inorganic TiO 2 /ZnO structures, photocatalytic effects under UV illumination are presented. In Chapter 4, an example to illuminate a novel application of ALD, i.e., the chemical/physical modification of protein structure of spider dragline silk by the MPI process and the resulting mechanical property improvements are presented. In Chapter 5, similar to Chapter 4, using collagen which is a primary concern in tissue engineering, chemical/physical modification processes and subsequent improvements of the mechanical properties together with scientific validation of the modified collagen are discussed. Even though the mechanism related to the modification and subsequent improvement of the mechanical properties is not yet clear, in Chapter 4 and 5 the presumable models or mechanisms to explain mechanical deformation behaviour are proposed. Finally, in the Appendix some data figures which were not included in the main text are presented.

7 Table of Contents CHAPTER GENERAL ASPECTS OF ATOMIC LAYER DEPOSITION AND IT S APPLICATIONS INTRODUCTION PRINCIPLE AND CHARACTERISTIC FEATURES OF THE ALD PROCESS INVESTIGATED ALD MATERIALS REACTION MECHANISMS OF METAL OXIDES BY ALD COMPARISON OF ALD AND CVD MULTIPLE PULSED VAPOR PHASE INFILTRATION CHAPTER BASIC PARAMETERS IN MECHANICS STRESS AND STRAIN STRESS - STRAIN CURVE STRAIN ENERGY, BREAKING ENERGY AND TOUGHNESS STIFF MATERIALS, STRONG MATERIALS AND TOUGH MATERIALS ELASTIC DEFORMATION AND PLASTIC DEFORMATION CHAPTER METAL OXIDE DEPOSITION ON BIOTEMPLATE: MACROPOROUS PHOTOCATALYTIC TiO 2 OR ZnO MEMBRANES TEMPLATED FROM CHICKEN S EGGSHELL MATRICES BACKGROUND Historical background of photocatalysis Principle of photocatalysis and applications Crystal structure of TiO 2 and ZnO Escherichia coli (E. coli) bacteria INTRODUCTION EXPERIMENTAL Preparation of the inner shell membrane from a chicken s egg TiO 2 /ZnO atomic layer deposition on ISM Characterization Microbiology Photocatalytic experiments with ISM/TiO 2 and ISM/ZnO Tensile test of native ISM, ISM/ZnO/100 and ISM/TiO2/275 membranes RESULTS AND DISCUSSION Film quality, crystallographic features and bactericidal efficiency Mechanical flexibility and thermal stability CONCLUSION CHAPTER METAL INFILTRATION INTO SPIDER DRAGLINE SILK BACKGROUND Overview of spiders and mechanical properties of spider silk Chemical structure and macroscopic model for spider silk Models for the description of dragline silk s mechanical properties Function of metals in biological tissues EXPERIMENTAL Silk collection Multiple pulsed vapor phase infiltration process Tensile test TEM and EDX analysis Solid state nuclear magnetic resonance (NMR) spectroscopy Wide angle X-ray scattering (WAXS) RESULTS AND DISCUSSION... 67

8 4.3.1 Variation of mechanical properties under diverse conditions Scientific validation of the MPI process Model system for the metal infiltration mechanism Model system for mechanical property improvements of silk CONCLUSION CHAPTER METAL INFILTRATION INTO COLLAGEN COLLAGEN AND TISSUE ENGINEERING STRUCTURE OF COLLAGEN COLLAGEN OF A CHICKEN S EGGSHELL MEMBRANE BIOMINERALIZATION OF COLLAGEN ARCHITECTURES EXPERIMENTAL Preparation of the collagen membrane (CM) from a chicken s eggshell matrix MPI process Tensile tests Cross section sample preparation by focused ion beam (FIB) SEM, TEM and EDX Raman spectroscopy Wide angle X-ray scattering Small angle X-ray scattering RESULTS AND DISCUSSION Mechanical deformation behaviour Metal infiltration into collagen Chemical analysis via Raman shift Structural analysis via x-ray scattering Biomineralization versus metal infiltration CONCLUSION SUMMARY REFERENCES AND NOTES ACKNOWLEDGEMENT APPENDIX A1. FIGURES A2. TABLES A3. REFERENCES AND NOTES CURRICULUM VITAE...139

9 Chapter 1 1 Chapter 1 General Aspects of Atomic Layer Deposition and It s Applications With the rapid development of semiconducting devices, scientists have made an effort to develop a method to process extremely thin high-k dielectric layers with high conformality. The special self-limiting growth mechanism of Atomic Layer Deposition (ALD) facilitates the film thickness and compositional control at the atomic level, as well as the deposition on large and complicated structures. On one hand, theses unique features make ALD a promising thin film deposition method for the next generation of micro- and nanoelectronics. On the other hand, recently, ALD has been applied in other fields, such as photovolatics, sensing, proactive coatings, nanostructuring by template approach, optoelectronics, piezoelectronics, chemical surface modification of diverse materials, micro- and nanoelectromechanical systems etc. ALD is also regarded as one of the innovative tools for the development of nanotechnology. In this chapter, general features of ALD will be briefly introduced together with some examples of applications. In the last part, one of the possible new applications of ALD induced by the separation of reactants, i.e. Multiple Pulsed Vapor Phase Infiltration (MPI), which could be widely applied to biomaterials, will be introduced. 1.1 Introduction In 1974, Dr. Tuomo Suntola and co-workers introduced a new thin film deposition method which was able to improve the quality of ZnS films used in thin-film electroluminiescent flat pannel dispalys. The first display with ZnS films deposited by the new method was lit in the display board in Helsinki Airport in Since then, thanks to the new process,

10 2 ALD General more than 2 million electroluminescent displays have been produced (about m 2 (20 ha)) [1]. Nowadays we call the new deposition method Atomic Layer Deposition (ALD). Until late 1980s ALD was applied to produce compound semiconductors and efforts to make III-V compunds were lasted. However, due to the chemical instability of group III alkyl compounds and group V hydrides, only little progress was achieved with ALD, as compared to molecular beam epitaxy (MBE) or metal-organic vapor phase epitaxy (MOVPE) [2]. Meanwhile, in the middle of 1990s, with the increased interest in silicon based microelectronics devices, the large takeoff of ALD began. Miniaturizing the dimension of the devices and inceasing aspect ratios in intergrated circuits required a new thin film deposition method with high controllability of film thickness and chemical film compositions on the atomic scale. Consequently, ALD came into focus as a potential candidate to facilitate those requirements. Several comprehensive reviews [3,4] cover sucessfully deposited ALD materials and corresponding processing conditions in detail. Another review descirbes the appliation of ALD in nanoscience [5]. In the following the general outline of ALD focusing on charateristic features and principles is presented. From the review articles [3-5], some contents relevant to the main topic of this thesis are extracted and presented accordingly. In the last part, a new application of the ALD process, named Multiple Pusled Vapor Phase Infiltration (MPI) [6], is briefly introduced. 1.2 Principle and characteristic features of the ALD process ALD is a technique based on sequential surface chemistry that deposits highly conformal thin-films of materials onto substrates of varying compositions. ALD is chemically similar to Chemical Vapor Deposition (CVD), except that the ALD approach splits the CVD reaction into two half-reactions, keeping the precursor materials seperate during the reaction. Such separation leads to a self-limiting mechanism and thus a control of the film growth on the atomic/molecular level. Unlike the CVD process, the ALD process is performed in a cyclic manner. Generally, one growth cycle consists of the following four steps (in case of ALD with two reactants): Step A: Injection and exposure of the first reactant (precursor A) Step B: Purge and evacuation to remove the surplus reactant and the byproducts from the vapor phase reaction Step C: Injection and exposure of the second reactant (precursor B) Step D: Purge and evacuation to remove the surplus reactant and the byproducts from vapor phase reaction

11 Chapter 1 3 Figure 1.1 A schematic representation of the basic principle of the ALD process. This diagram shows a metal oxide, i.e. MO 2 (M can be Ti, Zr or Hf etc.) deposition by ALD. For this deposition, a metal containing precursor ML 4 (L: ligand) as the metal source and H 2 O as the oxygen source are used. Sub-step 1 (chemisorption and saturation of precursor A), Sub-step 2-3 (purge), Sub-step 4 (chemisorption and saturation of precursor B) and Sub-step 5-6 (purge) in the figure correspond to Step A, Step B, Step C and Step D, respectively (for details, see the text). In Sub-step 1, the surface is exposed to a ML 4 vapor pulse. The precursor is chosen in a way that it reacts quickly with the reactive surface sites, forming a stable saturated chemisorbed layer. Once saturation is achieved, the purging of ML 4 vapor begins. Sub-step 2 presents the situation at the beginning of the purge, while Sub-step 3 shows a completed ML 4 vapor purge. The same exposure purge sequence is repeated for H 2 O vapor in Sub-step 4 (pulse), Sub-step 5 (purge onset) and Sub-step 6 (completed purge), completing a full ALD growth cycle. In Sub-step 4, the surface bound ligands receive a proton from H 2 O and leave the surface as volatile byproduct HL, being replaced with an OH group. The recreated OH terminated surface is now available for the next ALD growth cycle. These steps are repeated with each cycle adding a sub-monolayer quantity of material to the surface until a thin MO 2 film is formed. Sub-step 3 and Sub-step 6 emphasize the purge process of each unreacted precursor and the reaction byproduct. In a case where Sub-steps 1 and 4 or 5 overlap, the self-limitation is lost and CVD growth takes place in addition to ALD.

12 4 ALD General Figure 1.1 schematically illustrates one ALD reaction cycle. Each reaction cycle adds a given amount of material to the used substrate surface. In order to grow (or deposit) a material layer (for example, metal oxide, MO 2 in the case of Figure 1.1), ALD reaction cycles are repeated as many times as required for the desired film thickness. Accordingly, with the number of ALD cycles, one can precisely control the thickness of the film. One cycle time can be also adjusted from sub seconds to few minutes, depending on (1) the objective of the process; (2) the chemical charateristics of precursors being used; (3) the structure of the substrate and the deposition temperature; (4) the reactivity of the precursor with the substrate. The cycle time (for instance, exposure time and purge time) is dependent on the reciprocal reactivities between two precursors, the spontaneity of the layer formation reaction as well as the geometric features of the used substrate. Normally, when the geometry of the used substrate is flat or rather simple, with short cycle times one can obtain high quality films. Whereas, in the case of substrates with complex geometries (e.g. porous alumina or diverse biological templates), longer cycling times are required to assure complete and uniform coverage of the template. The growth rate (deposited film thickness per cycle) is likely to be dependent on the size of the used precursor molecule, because the steric hinderance between large precursor molecules limits the number of molecules being able to adsorb on the substrate. With small molecules as precursors, monolayer growth can be achieved [7]. Apart from the precursor molecule size, chemical properties of the substrate itself (for instance, surface energy of the used substrate) or the intermediate reactions between precursors and byproducts during processing can also affect the layer growth. a b Figure 1.2 Schematic ALD process window. a, Conceptual illustration of an ALD process window with respect to the processing temperature. In order to obtain suitable chemisorption of precursors onto a substrate surface via chemical bonding, the temperature should be lower than the precursor decomposition temperature to assure stable chemisorption. On the other side, the temperature should be higher than the lower limit to prevent precursor condensation or incomplete reaction. As an example, figure b shows ALD temperature windows for diverse metal oxides depending on the precursor pairs being used (Source:

13 Chapter 1 5 In an ideal case, the injected precursor molecules chemisorb or react with surface functional groups saturatively, limiting further adsorption. The self-limiting growth mechanism brings unique following advantages to ALD [7]: 1. Film thickness depends only on the number of reaction cycles, which allows precise and simple film thickness control. 2. Unlike CVD, there is less need of reactant flux homegeniety, which gives large area deposition capability, guarantees highly conformal deposition and faciliates solid state precursors. 3. Unlike CVD, seperated injection/exposure prevents vapor phase reactions before deposition on the substrate, which allows for the use of highly reactive precursors and gives enough time for each reaction step to be finished. This also allows for films to be deposited at relatively low temperature. On the other hand, each chemical reaction cycle of ALD requires a certain activation energy to allow the used precursors to react with each other. Most of the ALD systems introduce the required activation energy by heating (usually called thermal ALD). Some systems use reactive gases like plasmas (Plasma Enhanced ALD, or PEALD) or UV irradiation. Nevertheless, the ALD process window (Figure 1.2) is often wide, which also makes the deposition process insensitive to small changes in temperature and precursor flow rate, and allows for the deposition of nanolaminates in a continous process. 1.3 Investigated ALD materials ALD can be used to deposit a large variety of materials including oxides, nitrides, sulfides and metals. The number of new ALD processes is steadily increasing. Figure 1.3 illustrates the materials grown by ALD until February 2005 in a visual form using a periodic table. The variety of deposited materials includes oxides, nitrides, sulphides, II-V and III-V compounds and elements including both metals and non-metals etc. Table 1.1 lists the frequently used precursor combinations in two reactants system ALD.

14 6 ALD General Figure 1.3 Overview of materials grown by ALD until Feb This table is reproduced and edited from the table in Puurunen s review [3]. Growth of pure elements as well as compounds with oxygen, nitrogen, sulphur, selenium, tellurium and other compounds grouped together are indicated through shadings of different types at different positions. For details, see Puurunen s article [3]. The majority of the ALD processes investigated to date rely on thermal ALD. In thermal ALD processes the reactants (precursors) have their intrinsic reactivity towards the other reactants and the overall kinetics is highly dependent on the deposition temperature. The main reasons which limit the deposition of certain materials with thermally activated ALD are the decomposition of precursors before reaching the substrate or too low a reactivity between the reactants. The deposition temperature may be further limited by the substrate, which may be a temperature-sensitive material (such as biomaterials) or device structures. Low deposition temperatures may also limit the film quality if the film forming reactions are slow or incomplete. For example, slow desorption of reaction byproducts may result in increased amounts of impurities in the films. Some processes, however, demonstrated aggressive enough half-reactions and produced high quality films even at low temperatures [8]. Others suffered from excessively long cycle times and showed high impurity contents [9]. Even though diverse types of ALD processes except the thermal ALD have been developed in order to resolve those limitations, a fundamental development of precursors, which overcome those limitations, is highly desired (information included in Table 1.2).

15 Chapter 1 7 Table 1.1 ALD materials together with corresponding precursor combinations reported in literature and list of possible applications of ALD materials. a TEMAH: Tetrakisethyl Film Precursors [3] Applications methylaminohafnium, o High-κ dielectric [10] Hf[N(C 2 H 5 )(CH 3 )] 4. AlCl 3 / H 2 O or O 3 o OLED Passivation [11] Al 2 O 3 AlBr 3 / H 2 O b o Anti-reflection and optical filters [12-14] Ti(OEt)4 : Titaniumethoxide, Al(CH) 3 / H 2 O or O 3 o Wear and corrosion inhibiting layers [15] Ti(OC 2 H 5 ) 4 HfO 2 TiO 2 HfCl 4 / H 2 O TEMAH a / H 2 O TiCl 4 / H 2 O Ti(OEt) 4 b / H2 O Ti(O i Pr) 4 c o High-κ dielectric [16-19] o High-κ dielectric [19,20] o Photocatalysis [21] o UV blocking layer [22] o Photonic crystals [23] SiO 2 SiCl 4, / H 2 O o Dielectric [18,19,24] ZrO 2 ZnO SnO 2 Ta 2 O 5 La 2 O 3 ZnS ZrCl 4 / H 2 O Zr(O t Bu) 4 d / H2 O ZrI 4 / H 2 O 2 e ZnEt 2 / H2 O f Zn(OAc) 2 / H2 O SnCl 4 / H 2 O SnI 4 / O 2 TaCl 5 / H 2 O Ta(OEt) 5 g / H2 O La(thd) 3 h / O3 La[N(SiMe 3 ) 2 ] 3 i / H2 O ZnCl 2 / H 2 S o High-κ dielectric [18,19] o Photocatalysis o Wear and corrosion inhibiting layers o Piezoelectric layers o UV blocking layer [22] o Photocatalysis [21] o Photonic crystals [24] o Optical applications (solar cells, integrated optics, optical coatings, laser) [25] o o Anti-reflection and optical filters Sensors (gas sensors, ph sensors) [26,27] o Anti-reflection and optical filters [28] o Sensors (gas sensors, ph sensors) [28] o High-κ dielectric [28] o High-κ dielectric [29,30] o Piezoelectric layers o Optoelectronic applications [31,32] WS 2 WF 6 / H 2 S o Solid Lubricant layers [33] o Biomedical coatings( biocompatible Zr 3 N 4 Zr(NMe 2 ) j 4 / NH 3 materials for in-vivo medical devices and instruments) Ta 2 N TaCl 5 / H 2 O 5 Ta(OEt) 5 / H 2 O o Photonic crystals [34] TaN Ta(N t Bu)(NEt 2 ) k / NH 3 o Diffusion barrier for Cu metallization [35] c Ti(O i Pr) 4 : Titaniumisopropoxide, Ti[OCH(CH 3 ) 2 ] 4 d Zr(O t Bu) 4 : Tetrakisethyl methylaminozirconium, Zr[N(CH 3 )(C 2 H 5 )] 4 e ZnEt2 : Diethylzinc, Zn(C 2 H 5 ) 2 f Zn(OAc)2 : Zincacetatedihydrate, Zn(O 2 CCH 3 ) 2 (H 2 O) 2 g Ta(OEt)5 :Tantalumthoxide, Ta(OC 2 H 5 ) 5 i La[N(SiMe3 ) 2 ] 3 : Lanthanumtris[bis(trimethylsil yl)amide], C 18 H 54 LaN 3 Si 6 j Zr(NMe2 ) 4 : zirconium(tetra)dimethylamid e, Zr[N(CH 3 ) 2 ] 4 AlN AlMe 3 / NH 3 AlCl 3 / NH 3 TiN TiCl 4 or Til 4 / H 2 WN WF 6 / NH 3 CuCl / H 2 Cu(thd) 2 / H 2 Cu l Cu(acac) 2 / H2 Cu(hfac) 2 x H 2 O m / Ch 3 OH MoF 6 / H 2 Mo MoCl 5 / H 2 Mo(Co) 6 / H 2 Ni(acac) 2, 2 step process Ni NiO by O 3 reduced afterwards by H 2 Ta TaCl 5 / H 2 W WF 6 / B 2 H 6 or Si 2 H 6 Ti TiCl 4 / H 2 o Piezoelectric layers o Diffusion barrier for Cu metallization [36] o Conductive gate electrodes o Biomedical coatings (biocompatible materials for in-vivo medical devices and instruments) o Diffusion barrier for Cu metallization [36] o Optical application [37] h La(thd)3,where thd is 2,2,6,6,-tetramethyl-3,5- heptanedione, (CH 3 ) 3 CCOCH 2 COC(CH 3 ) 3 k Ta(NtBu)(NEt2 ) 3 : Tris(diethylamido)(tertbutylimido)tantalum, (CH 3 ) 3 CNTa[N(C 2 H 5 ) 2 ] 3, where N t Bu is called tertbutylimido group. l Cu(acac)2 :Copper(II) acetylacetonate, (C 5 H 7 O 2 ) 2 Cu m Cu(hfac)2 H 2 O: Copper(II) hexafluoroacetylacetonate hydrate, C 10 H 2 CuF 12 O 4. H 2 O

16 8 ALD General Table 1.2 Requirements for ALD precursors. The listed information in this table is summarized from references [2,3,7]. 1 Adequate volatility Requirement 2 Reasonable thermal stability Explanation Necessary for efficient transportation of the precursor to the ALD reactor at a rough limit of 0.1 Torr and at the deposition temperature. Thermal-decomposition can destroy the selflimiting film growth. 3 Aggressiveness, Appropriate reactivity Surface reaction should ensure fast completion. Short cycle times lead to high productivity. There should be no gas phase reaction. 4 No etching of the films or substrates No competing reaction pathways. Etching prevents the film growth. 5 No dissolution into the film or substrate Dissolution would destroy the self-limiting film growth. 6 Non-reactive volatile by-products Necessary to avoid corrosion. 7 Sufficient purity, Cost effectiveness, Easy handling and synthesis, Non-toxicity, Environmental friendliness 1.4 Reaction mechanisms of metal oxides by ALD a b c Figure 1.4 Chemical structure of TIP, TMA and DEZ. a, TIP. b, TMA. c, DEZ. Up to now, the basic principle of an ALD process and investigated ALD materials were briefly introduced. In the following, focusing on metal oxides deposited by a binary ALD reaction, more detailed reaction mechanisms of ALD film formation will be described, since the present work was performed with such ALD processes (in particular with Al 2 O 3, TiO 2 and ZnO). The oxides Al 2 O 3, TiO 2 and ZnO were deposited by trimethylaluminum (TMA)/water, titaniumisopropoxide (TIP)/water and diethylzinc (DEZ)/water precursor pairs, respectively. TMA, TIP and DEZ (Figure 1.4) were used as metal source and water was used as oxygen source. It is generally well accepted that, during ALD metal oxide growth, hydroxyl groups play an important role as intermediate species remaining on the surface of the deposited film after the water exposure [38-47]. During the subsequent

17 Chapter 1 9 exposure of metal containing precursors, the hydroxyl groups react with the incoming metal compounds, thereby becoming anchored to the surface as described by the following reaction. x( OH) + ML n (g) ( O ) x ML n-x + xhl(g) (R1) And ( O ) x ML n-x further react with water molecules during the next process step ( O ) x ML n-x + (n-x)h 2 O(g) ( O ) x M( OH) n-x + (n-x)hl(g) (R2), where blue ( )/red ( ) bar denote bonds between substrate surface and L (ligand) / M (metal) and L, respectively. On the other hand, not all of the hydroxyl groups ( OH) formed in (R2) do necessarily remain as such on the surface but some of them may react with other free water. 2( OH) ( O)+ H 2 O(g) (R3) This dehydroxylation increases with temperature causing a gradual decrease of the surface hydroxyl group density [40,42,43,45,47]. The amount of metal precursor anchored to the surface, and thereby the ALD growth rate, is determined either by the steric hinderance between the ( O ) x ML n-x surface species or by the density of the hydroxyl groups. Therefore, under conditions with extensive dehydroxylation, the hydroxyl group density may become a limiting factor with respect to the film growth rate. Hydroxyl groups may form on the surface also by rehydroxylation which is a reaction essentially reverse to the (R3). M O + H 2 O(g) M( OH) 2 (R4) O M O M + H 2 O(g) HO M O M OH (R5) The resulting hydroxyl groups are bound to the same (R4) or adjacent surface metal ions. Based on the simplified reaction scheme from (R1)-(R5), it is known that Al 2 O 3, ZnO and TiO 2 film are deposited as follows: (i) Al 2 O 3 deposition from the reaction of TMA/H 2 O [9,40,42,48,49]. A binary reaction for Al 2 O 3 chemical vapor deposition, 2Al(CH 3 ) 3 +3H 2 O Al 2 O 3 + 6CH 4 is separated into two half-reactions:

18 10 ALD General a, Al-OH* + Al(CH 3 ) 3 Al-O-Al-(CH 3 ) 2 * + CH 4 ; b, Al-O-Al-(CH 3 ) 2 * + H 2 O Al-O-Al-(CH 3 )OH* + CH 4, where the asterisks designate the surface species. TMA and H 2 O reactants were employed alternatively in an ABAB..binary reaction sequence to deposit Al 2 O 3 films. (ii) ZnO deposition from the reaction of DEZ/H 2 O [50-54]. Similar to the deposition of Al 2 O 3 from TMA/H 2 O, ZnO ALD is also based on a ZnO CVD process as follows Zn(CH 2 CH 3 ) 2 + H 2 O ZnO + 2C 2 H 6 ZnO ALD is proposed to occur by speration of this binary reaction into two half-reactions: a, Zn-OH* +Zn-(CH 2 CH 3 ) 2 Zn-O-Zn-(CH 2 CH 3 )* + C 2 H 6 ; b, Zn-O-Zn-(CH 2 CH 3 )* + H 2 O Zn-O-Zn-OH* + C 2 H 6, where the asterisks designate a surface species. Alternating ABAB..reaction sequence is repeated to deposit ZnO films. (iii) TiO 2 deposition from reaction of TIP/H 2 O [55-58]. Unlike Al 2 O 3 and ZnO, the TiO 2 deposition mechanism is rather complicated. The behaviour shows strong dependency on the processing temperature and is highly affected by decomposition of TIP. It is suggested that, below a temperature of 250 C, TiO 2 grows via the two following reactions ( denotes bonds between substrate surface) [58]: 2( OH) + Ti[OCH(CH 3 ) 2 ] 4 ( O ) 2 Ti[OCH(CH 3 ) 2 ] 2 + 2(CH 3 ) 2 CHOH (R6) ( O ) 2 Ti[OCH(CH 3 ) 2 ] 2 + 2H 2 O ( O ) 2 Ti( OH) 2 + 2(CH 3 ) 2 CHOH (R7) In the reaction, half of the ligands are released during the Ti[OCH(CH 3 ) 2 ] 4 pulse anchoring on the surface hydroxyl groups (R6). The H 2 O pulse hydrolyzes the rest of the ligands and converts the surface back to being hydroxyl group terminated (R7). At higher temperatures, surface dehydroxylation becomes more intense and thus there are less OH groups at the surface after the H 2 O pulse. Therefore, the mechanism is changing. Now, only a single isopropoxide ligand is released during the titanium precursor pulse (R8), the remainder of the ligands is released during the H 2 O pulse and the surface again becomes OH terminated (R9) as follows: ( OH) + Ti(OCH(CH 3 ) 2 ) 4 O Ti[OCH(CH 3 ) 2 ] 3 + (CH 3 ) 2 CHOH(g) (R8)

19 Chapter 1 11 O Ti[OCH(CH 3 ) 2 ] 3 + 2H 2 O ( O ) 2 TiOH + 3(CH 3 ) 2 CHOH(g) (R9) 1.5 Comparison of ALD and CVD As already mentioned above, ALD is a special case of CVD. Even though ALD uses a similar chemistry to CVD, the difference between them is large. CVD involes a chemical reaction which transforms vapor phase precursor molecules into solids, depositing as thin films or powder on the surface of a substrate. As illustrated in Figure 1.5, in CVD vaporized precursors with a constant pressure (Figure 1.6) are simulataneously delivered into a reactor with a carrier gas. The precursor molecules diffuse inside the reactor to the vicinity of a substrate surface. An adsorption of the diffused molecules on surface occurs, followed by a reaction yielding solid reaction products. Since the substrate temperature is critical and can influence the type of the reaction, the reactions are activated and maintained by heat, plasma, photons, electrons, ions or a combination thereof. Vapor phase reaction products are also formed and are removed from the reaction chamber. Figure 1.5 Schematic illustration of a CVD process. For details, see the text. In contrast, in ALD the precursors are not mixed and are introduced into the reactor sequentially. Thanks to the self-limitation as a consequence of the pulsed deposition scheme, the thickness control is performed as a function of cycles, whereas, in CVD the thickness is controlled by the processing time (e.g. nm/min). A common feature of CVD and ALD is that all surfaces exposed to the precursor vapor are coated. This means that films of uniform thickness can be produced on 3D

20 12 ALD General substrates. Basically, CVD is a gas phase reaction which can cause a particle deposition. In a CVD process, the life time of precursors is not long enough for precursor molecules to be transported and diffused on the complicated 3D substrates. Therefore, CVD is rather a line-of-sight deposition method and shadowing effects lead to non-uniformity of the films along a 3D substrate. As a result, one can expect better uniformity with ALD than with CVD. By adjusting the cycle time, in particular, exposure time, the film uniformity and conformality on complicated substrates can be maximized (Figure 1.7). Including the differences mentioned above, the other differences and special features of ALD and CVD are summarized in Table 1.3. a b Figure 1.6 Schematic partial pressure profiles of ALD and CVD during a process. In the case of ALD (figure a), since the film is deposited based on exposure/purge of each precursor, the partial pressure profile has a form of a square wave function. In contrast, in classical CVD (figure b) the precursors are introduced into a reactor at the same time, the pressure profile of each precursor is kept constant. In figure a, E and P denotes exposure and purge, respectively. The pressure profile of CVD (figure b) shown here is only valid for classical CVD, but not for pulsed CVD. a b Figure 1.7 Difference of coating behaviour of ALD and CVD. In ALD (figure a), thanks to the selflimiting reaction mechanism, an extremely uniform film can be deposited. In CVD (figure b), one can expect a very uniform film but practically the film is less uniform than ALD film. Since CVD is rather a line-of-sight deposition method, shadowing effects lead to non-uniformity of the films along a 3D sample.

21 Chapter 1 13 Table 1.3 Comparison of ALD and CVD. This data is summarized after extracting information in diverse books, research articles and internet web pages [2-5,7]. Criteria ALD CVD Uniformity Control o o o Å range Controlled by counting the number of reaction cycles Ensured by the saturation mechanism o o o 10Å range, Controlled by process control and monitoring, time Requires uniform flux of reactant and uniform temperature Film Quality o o o Excellent stoichiometry Low pinhole count Stress control possible o o o Excellent stoichiometry Low pinhole count Stress control possible Conformality 100% step coverage in 60:1 aspect ratio 100% step coverage in 10:1 aspect ratio Cleanliness No particles due to separated half reaction Particles due to gas phase reactions Vacuum Requirement Medium Medium Process Window Precursor Deposition reaction Contamination < 1% dependency on 10% process parameter changes o Highly reactive precursors o Precursors must not decompose at process temperatures Surface reaction 5 ~ 30 wt % (C, O). But with PE-ALD contamination can be minimized (< 1 wt %). Strong dependency on process parameter changes o Less active precursors o Precursors can decompose at process temperature Surface reaction + Gas phase reaction < 1 wt % 1.6 Multiple pulsed vapor phase infiltration ALD has been developed for the controlled-deposition of various kinds of thin films (such as oxides, nitrides, elemental compounds etc.) with control on the atomic or molecular level. Up to now, the mainstream of ALD research has focused on expanding the variety of materials which can be deposited in a very controlled way. To this end, scientists in the field of ALD have tried to add additional systems (such as plasma) to a conventional ALD setup to activate precursors properly or they have made an effort to develop new precursors to be easily applied to a conventional thermal ALD system. On the other hand, most of the materials have been deposited mainly on solid state substrates which are normally used in the field of microelectronics devices. As the interest in nanomaterials (such as nanopores, -wires, -tubes and laminates) increased, recently some new aspects of ALD were introduced to be applied to general nanofabrication or nanotechnology. There were reports on some examples of structuring using nano-sized organic materials or biomaterials based on template approaches performed by low temperature ALD processes

22 14 ALD General [5]. A common concern of most of those ALD researchers has concentrated on the resulting ALD film and the functionality of the deposited film itself. Figure 1.8. Schematic of the difference between a conventional ALD process and the MPI process. Conventionally, by multiple pulses of ALD precursors such as TMA (green arrows) and water (pink arrows), thin films (such as alumina, illustrated by the red shell) are deposited on rigid materials (such as metals) without chemical modification of the bulk. In contrast, in the case of soft materials such as biomaterials or polymers, ALD provides a chemical modification of the bulk in addition to the thin film deposition (MPI process). As introduced above, unlike to the case of other deposition methods, an ALD film is deposited by multiple pulses of two or three precursors (binary or ternary reaction) depending on the required film. For each precursor the exposure/purge step is repeated. Since most organic materials or biomaterials are sensitive to the highly reactive metal containing precursors, one could anticipate side effects caused by ALD. Few years ago, some groups reported that, in a case of a polymer substrate, the ALD film growth is differing to the growth on other solid substrates [59]. The ALD process changed the chemical structures of the polymer [11]. More recently even more convincing results, supporting the fact that ALD can modify physical/chemical properties of soft materials (such as biomaterials), were reported [6]. It was proven that metals can infiltrate biomaterials by the alternating exposure/purge step of multiple pulses of vapor phased

23 Chapter 1 15 precursors occurring during the ALD process (Figure 1.8), named as Multiple Pulsed Vapor Phase Infiltration (MPI). In this thesis, one example of ALD on a biotemplate (Chapter 3) and two examples of MPI processes to biomaterials are presented (Chapter 4 and 5).

24 16 ALD General

25 Chapter 2 17 Chapter 2 Basic Parameters in Mechanics Mechanics is one of the well-established branches of physics. Its main focus is to investigate the behaviour of physical bodies when subjected to forces or displacements, and the subsequent effects of the bodies on their environment. The discipline has its roots in several ancient civilizations successively deveploping by learning from experience. During the early modern period, scientists such as Galileo, Kepler, and especially Newton, established the so called classical mechanics. In this chapter, several basic parameters required for understanding the mechanical behaviour of diverse physical bodies under the influence of outside force environmental conditions will briefly be introduced. Most of the contents described in this chapter are based on classical text books dealing with mechanics of materials [60-65]. 2.1 Stress and strain When a solid body with a given volume is subjected to an external force (in particular, tensile force), the material will typically elongate in the direction of the applied force. The relative elongation is called strain (often denoted by ε). Usually, this elongation leads to a contraction of the material in the direction perpendicular to the applied force, by a relative amount νε, where the coefficient ν is called Poisson ratio. For an isotropic piece of material, the relative increase of the volume during uniaxial stretching is 1-2ν which means that the Poisson ratio has an upper limit of ν 0.5, because the specimen volume is not expected to shrink under the influence of tensile forces. The load divided by the

26 18 Basic Terminologies in Mechanics surface area A is called stress (units: Pascal [Pa]). Generally, stresses and strains are not just uniaxial and need to be described by a tensor. a b c Figure 2.1 Tensile stress and shear stress. When the cubic piece of material (figure a) is subjected to tensile stress along the y-direction only, its length L is increased by Lε. The relative elongation ε is called (tensile) strain. In most cases, the dimension of the cube subjected to tensile load will contract perpendicularly to the load direction (figure b). The ratio ν of the contraction in the z-direction (or x- direction) relative to the elongation in y-direction is called the Poisson ratio. When the load is tangential to the top surface, shear deformation occurs (figure c). The shear is measured by the parameter γ, which (for small deformations) corresponds to the tilting angle of the cube edge initially parallel to the y-deformation. 2.2 Stress - strain curve A. Engineering stress-strain curve. Perhaps the most common test of a material s mechanical response is the tensile test, in which one end of a rod or wire specimen is clamped in a loading frame and the other subjected to a controlled displacement δ = (L-L 0 ) (see Figure 2.2). The engineering measures of stress and strain, denoted in this module as σ e and ε e respectively, are determined from the measured load and deflection using the original specimen cross-sectional area A 0 and length L 0 as σ e = F/A 0 and ε e = δ/l 0, respectively. In the low strain part of the curve, many materials obey Hooke s law to a reasonable approximation, so that stress is proportional to strain with the constant of proportionality being the modulus of elasticity or Young s modulus, denoted by E (= ε e /σ e ) σ e = Eε e (2.1)

27 Chapter 2 19 Figure 2.2 Schematic drawing of a tensile test and a stress-strain curve of a ductile material. By pulling on a specimen, the material s reponse to forces being applied in tension is determined. When the stress is plotted against the strain, a stress-strain curve is obtained. For the explanation, the stressstrain curve of a ductile material is exemplified. The nature of the curve varies from material to material. The stress strain behaviour of typical materials is illustrated in terms of the engineering stress (σ e ) and the engineering strain (ε e ) where the stress and strain are calculated based on the original dimensions of the specimen. The stress value calculated from instantaneous values of the specimen dimension is called true stress and the corresponding stressstrain curve is called true stress (σ t )-strain (ε t ) curve. C1: true stress (σ t )-strain (ε t ) curve; C2: engineering stress (σ e )-strain (ε e ) curve; R1: elastic deformation region (reversible); R2: strain hardening region (permanent deformation); R3: necking region (permanent deformation); P1: yielding point (σ y,ε y ), limit of the elastic region; P2: ultimate tensile strength (UTS), onset point of necking; P3: fracture point (σ f,ε f ); P4: proportional limit; E: Young s modulus, stiffness or modulus of elasticity. As strain is increased, many materials eventually deviate from this linear proportionality, the point of departure being termed the proportional limit. This nonlinearity is usually associated with stress-induced plastic flow in the specimen. Here the material is undergoing a rearrangement of its internal molecular or microscopic structure, in which atoms are moved to new equilibrium positions. This plasticity requires a mechanism for molecular mobility, which inside crystalline materials can arise from dislocation motion. Materials lacking this mobility, for instance by having internal microstructures that block dislocation motion, are usually brittle rather than ductile. The stress-strain curve for brittle materials is typically linear over the full range of strain, eventually terminating in fracture without significant plastic region. The stress needed to increase the strain beyond the proportional limit in a ductile material continues to rise beyond the proportional limit; the material requires an ever-increasing stress to continue straining, a mechanism termed strain hardening. These microstructural rearrangements associated with plastic flow are usually not reversible, even if the load is removed, so the

28 20 Basic Terminologies in Mechanics proportional limit is often the same as or at least close to the materials elastic limit. Elasticity is the property of complete and immediate recovery from an imposed displacement on release of the load, and the elastic limit is the value of stress at which the material experiences a permanent residual strain that is not lost on unloading. The residual strain induced by a given stress can be determined by drawing an unloading line from the highest point reached on the stress-strain curve back to the strain axis, drawn with a slope equal to that of the initial elastic loading line. This is done because the material unloads elastically, with no required force driving the molecular structure back to its original position. A closely related term is the yield stress, denoted σ Y in these modules; this marks the stress needed to induce plastic deformation of the specimen. Since it is often difficult to pinpoint the exact stress at which plastic deformation begins, the yield stress is often taken to be the stress needed to induce a specified amount of permanent strain, typically 0.2%. For some materials (e.g., metals and plastics), the departure from the linear elastic region cannot be easily identified. Therefore, an offset method to determine the yield strength of the material tested is allowed. These methods are discussed in ASTM E8 (metals) and D638 (plastics). An offset is specified as a percentage of strain (for metals, usually 0.2% and sometimes for plastics a value of 2% is used). The construction used to find this offset yield stress is shown in Figure 2.2 in which a line of slope E is drawn from the strain axis at ε e = 0.2%; this is the unloading line that would result in the specified permanent strain. The stress at the point of intersection with the σ e -ε e curve (P1) is the offset yield stress. The rate of strain hardening diminishes up to a point labeled ultimate tensile strength (UTS). Beyond that point, the material appears to soften, so that each increment of additional strain requires a smaller stress. The apparent change from strain hardening to strain softening is an artifact of the plotting procedure, however, as is the maximum observed in the curve at the UTS. Beyond the yield point, molecular flow causes a substantial reduction in the specimen cross-sectional area A 0, so the true stress (σ t = F/A actually borne by the material is larger than the engineering stress computed from the original cross-sectional area (σ e = F/A 0 ). The load must equal the true stress times the actual area (F = σ t A). As long as strain hardening can increase σ t enough to compensate for the reduced area A, the load and therefore the engineering stress will continue to rise as the strain increases. Eventually, the decrease in area due to flow becomes larger than the increase in true stress due to strain hardening, so the load begins to fall. This is a geometrical effect. If the true stress rather than the engineering stress were plotted, no maximum in the curve would be observed. At the UTS the differential of the load F is zero, giving an analytical relation between the true stress and the area at necking as follows:

29 Chapter 2 21 df da = d A df d A da - dσ t ( σ t ) = 0 = σ t + σ t = (2.2) A σ t The last expression states that the load and the engineering stress will reach a maximum as a function of strain when the fractional decrease in area becomes equal to the fractional increase in true stress. Figure 2.3 Necking in a tensile specimen. Under the uniaxial tensile load the aluminum specimen starts to deform and reaches necking. Finally cup and cone shaped fracture occurs (source: [65]. Even though the UTS is perhaps the materials property most commonly reported in tensile tests, it is not a direct measure of the material due to the influence of geometry as discussed above, and should be used with care. The yield stress σ Y is usually preferred to the UTS in designing ductile metals, although the UTS is a valid design criterion for brittle materials which do not exhibit these flow-induced reductions in cross-sectional area. The true stress is not quite uniform throughout the specimen, and there will always be some location-perhaps a nick or some other defect at the surface - where the local stress has a maximum. Once the maximum in the engineering curve has been reached, the localized flow at this site cannot be compensated by further strain hardening, so this area is further reduced. This increases the local stress even more, which further accelerates the flow. This localized and increasing flow soon leads to a neck in the gage length of the

30 22 Basic Terminologies in Mechanics specimen such as the one seen in Figure 2.3. Until the neck forms, the deformation is essentially uniform throughout the specimen, but after necking all subsequent deformation takes place in the neck. The neck becomes smaller and smaller with the local true stress increasing all the time, until the specimen fails. This will be the failure mode for most ductile metals. As the neck shrinks, the non-uniform geometry there alters the uniaxial stress state to a complex one, involving shear components as well as normal stresses. The specimen often fails finally with a cup and cone geometry as seen in Figure 2.3, in which the outer region fails in shear and the interior in tension. When the specimen fractures, the engineering strain at break (fracture strain, ε f ) will include the deformation in both the necked and the unnecked region. Since the true strain in the neck is larger than that in the unnecked material, the value of ε f will depend on the fraction of the gage length that has necked. Therefore, ε f is a function of the specimen geometry as well as the material, and thus is only a crude measure of material ductility. A. True stress-strain curve. As discussed above, the engineering stress-strain curve must be interpreted with care beyond the elastic limit, since the dimension of the specimen s cross section experiences a substantial change from its original value. Using the true stress σ t = F/A rather than the engineering stress σ e = F/A 0 can give a more direct measure of the material s response in the plastic flow range. A measure of strain often used in conjunction with the true stress takes the increment of strain to be the incremental increase in displacement dl divided by the current length L as follows L dl 1 L dε t = ε t = dl = In L (2.3) L L L 0 0 This is named as the true or logarithmic strain. During yield and the plastic-flow regime following yield, the material flows with negligible change in volume; increases in length are offset by decreases in cross-sectional area. Prior to necking, when the strain is still uniform along the specimen length, this volume (V) constraint can be written: A L = constant dv = 0 AL = A0 L0 = A L V 0 0 (2.4) The ratio L/L 0 is defined as extension ratio (λ). Using these relations, the relation between true and engineering measures of tensile strain can be developed as follows