Optical and mechanical properties of cu al o thin films prepared by plasma enhance CVD

Optical and Mechanical Properties of Cu-Al-O Thin Films Prepared by Plasma-Enhanced CVD CHEN WEN (B E., Tsinghua Univ., China) A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF SCIENCE DEPARTMENT OF MATERIALS SCIENCE NATIONAL UNIVERSITY OF SINGAPORE 2004 Acknowledgements In the course of this work, many people have assisted me and offered their support I would like, first, to express my thanks to my supervisor, Hao Gong, for providing excellent supervision and guidance throughout the whole project His support and invaluable advice were greatly appreciated I would like to thank Yue Wang and Chin Hock Ong for many insightful discussions and technical assistances during my experiments and thesis writing My sincere thanks go to many people who contributed to the experimental part of this work: Wei Ji, Hendry Izaac Elim for the Z-scan measurement; Kaiyang Zeng for the nanoindentation test and Lei Huang for the contact angle measurement At last, I want to acknowledge the Department of Materials Science (NUS), for providing the environment in which I worked, and all the department staff, this project would not have been possible without their assistance I Contents Acknowledgements I Table of Contents II Summary VI List of Tables VII List of Figures VIII Introduction 1.1 Introduction of the Current Work 1.2 Thesis Outline Literature Review 2.1 Thin Film Technology II CONTENTS 2.2 Transparent Conductive Oxide (TCO) 2.3 Optical Properties of TCOs 12 2.4 Mechanical Properties of TCOs 14 References 16 Experimental Details 21 3.1 The PECVD Setup 22 3.1.1 Transportation system 22 3.1.2 Reaction system 24 3.2 Film Growth Process 27 3.3 Characterization Techniques 29 3.3.1 X-ray diffraction (XRD) 29 3.3.2 UV-visible spectrophotometer 31 3.3.3 Scanning electron microscopy (SEM) 33 3.3.4 Atomic force microscope (AFM) 35 3.3.5 Secondary ion mass spectrometry (SIMS) 36 3.3.6 Z-scan 38 3.3.7 Instrumented indentation 39 III CONTENTS 3.3.8 Other techniques 44 References 48 Optical Properties of Copper Aluminium Oxide Thin Films 50 4.1 Introduction 50 4.2 Optical Effects in Z-scan Characterization 52 4.3 Conclusion 66 References 67 Mechanical Properties of Copper Aluminium Oxide Thin Films 69 5.1 Introduction 69 5.2 Typical Mechanical Behavior 72 5.3 Effect of Different Growth Conditions 94 5.3.1 Effect of different substrate temperatures 94 5.3.2 Effect of different Cu/Al ratios 109 5.4 Discussion 115 5.5 Conclusion 125 References 127 IV CONTENTS Summary and Suggestion for Future Work 132 6.1 Summary 132 6.2 Suggestion for Future Work 137 V Summary Copper aluminium oxide thin films were prepared by plasma-enhanced chemical vapor deposition The optical and mechanical properties of the films were investigated by the Z-scan technique and nanoindentation Upon laser bombardment, the film experienced optical annealing and film transmittance increased At a laser intensity of 133 GW/cm2 , a transmittance change of 25% was achieved Such optical response of the film may be useful in optical processing Nanoindentation measurement showed that film strength decreased with substrate temperature and Cu/Al ratio The strongest film has a hardness of 12.1 GPa and an elastic modulus of 120.1 GPa The weakest film exhibits a hardness of 0.1 GPa and an elastic modulus of 19.0 GPa Structural and compositional analysis revealed that fine Al2 O3 grains contributed mostly to strengthen the films whereas particle size hardening also took some effect The study provided knowledge for making transparent conductive oxide devices with high durability and long lifetime VI List of Tables 2.1 Thin film applications 5.1 XRD peak list of the typical sample 82 5.2 XRD peak list of films prepared at different substrate temperatures 98 5.3 EDX results of films prepared at different substrate temperatures 102 5.4 Surface energies and surface roughness factors 107 5.5 EDX results of films prepared with different Cu/Al ratios 110 5.6 Grain sizes of films prepared with different Cu/Al ratios 112 VII List of Figures 3.1 Schematic diagram of the transportation system 23 3.2 Schematic diagram of the transportation tube 23 3.3 Schematic diagram of the reaction system 25 3.4 Schematic diagram of CVD precursors (a) Cu(acac)2 , (b) Al(acac)3 27 3.5 Schematic diagram of a double beam UV-visible spectrophotometer 32 3.6 Schematic diagram of SEM 34 3.7 Schematic diagram of a Z-Scan setup 38 3.8 Schematic diagram of an instrumented indentation system 41 3.9 Schematic diagram of a typical loading versus displacement curve 42 3.10 Schematic diagram of the indentation geometry at maximum load 43 3.11 Schematic diagram of a dynamic model for a nanoindenter 45 3.12 Schematic diagram of the contact angle 46 VIII LIST OF FIGURES 4.1 Normalized Z-scan data for the copper aluminium oxide thin film 53 4.2 Normalized Z-scan data at different laser intensities 55 4.3 Variation of β and n2 with laser intensity 57 4.4 Optical microscopy image of the film after laser bombardment 60 4.5 Transmittance of the as-deposited and annealed films 63 5.1 Nanoindentation curves of the typical sample 73 5.2 Hardness and elastic modulus of the typical sample 75 5.3 XRD pattern of the typical sample 81 5.4 SEM image of the typical sample 84 5.5 Depth profile of the typical sample by SIMS 89 5.6 AFM image of the typical sample 90 5.7 Growth rate versus inverse of substrate temperature of the films 95 5.8 Effect of substrate temperature on film mechanical properties 96 5.9 XRD patterns of films prepared at different substrate temperatures 98 5.10 XRD patterns of films before and after annealing 100 5.11 AFM images of films prepared at different substrate temperatures 104 5.12 Effect of substrate temperature on surface energy 109 IX 5.4 DISCUSSION intrinsic stress resisting dislocation motion in the deforming grain, (d/4r) represents the stress concentration arising from the pileup, and τ ∗ is the stress required to activate dislocation motion in the unfavorably oriented grain Rearrangement of Eq 5.17 leads to the Hall-Petch equation (Eq 5.6) which predicts that if the grain size is large, a greater stress concentration is developed in the adjacent grain, and thus the applied stress needed to activate flow in this grain is relatively low, and vice versa For boundaries other than grain boundary (e.g., cell boundary, particle boundary, subboundary, etc.), the Hall-Petch relation also works However, since these boundaries are not completely impenetrable by dislocations, their strengthening effect is not so strong as that of grain boundary As revealed by the XRD patterns, the grain sizes of the copper aluminium oxide films prepared at different growth conditions are all in the range of 20 nm– 30 nm Thus the grain size effect does not seem to account for the difference among the film strength On the other hand, AFM (Fig 5.11) and SEM (Fig 5.15) images suggest that the films are mostly composed of relatively large particles of several hundred nanometers in diameters, which may be regarded as aggregates of CuAlO2 and/or CuO grains These particle boundaries, though may be not as strong as the grain boundary, should be taken into consideration to reach a comprehensive understanding of the film strengthening mechanism In the case of 123 5.4 DISCUSSION films prepared at different substrate temperatures, it can be readily seen that film hardness increases inversely with the particle size This trend is not so distinct in films prepared with different Cu/Al ratios, but the film with the highest hardness also exhibits the smallest particle size However, due to the weak nature of the particle boundaries, it should be noted that the particle size effect might be just moderate compared with the Al2 O3 particle hardening effect As illustrated in Fig 5.11 (a), (b), and (c), the particle sizes of these films are nearly the same, yet the film hardness values decrease from 12.1 GPa to 6.9 GPa Such a large gap could not be explained by the particle size effect and may only be attributed to the different Al2 O3 volume fractions 124 5.5 CONCLUSION 5.5 Conclusion In this chapter, the mechanical properties of copper aluminium oxide thin films were characterized by the nanoindentation technique By employing the commonly used Oliver and Pharr’s method [6], the hardness and elastic moduli of the films were calculated The strongest film has hardness of 12.1 GPa and an elastic modulus of 120.1 GPa The weakest film, on the other hand, has hardness of only 0.1 GPa and an elastic modulus of 19.0 GPa It is believed that such a huge gap among the mechanical properties of films prepared at different growth conditions originates from the different film microstructures XRD characterization suggests that the copper aluminium oxide films mainly consist of CuAlO2 and CuO grains In some films, fine Al2 O3 grains are also present According to the depth profile analysis by SIMS and surface morphological observation by AFM and SEM, it is proposed that the films are composed of a matrix of relatively large CuAlO2 and CuO particles with fine Al2 O3 grains scattered in the void and gaps These Al2 O3 grains play an important role in strengthening the film since films contain such grains possess hardness much higher than the theoretical value of CuAlO2 (2.2 GPa), on the other hand, films which are free of such grains exhibit hardness approaching the theoretical hardness of CuO (0.3 GPa) Other factors, such as particle size or stress state in the films may also influence the film strength, however, their contribution to the film strengthening 125 5.5 CONCLUSION does not seem to be as great as that of Al2 O3 grains Such a film structure has been previously observed in many other materials and it is proposed that the addition of nanometer-sized particles to a host matrix can be used to modify the properties of the matrix material or even achieve excellent properties which could not be expected from separate phase Thus it is possible that the existence of Al2 O3 nano-particles to copper aluminium oxide films may result in films with very high mechanical strength Such mechanical properties can be beneficial in a wide range of applications of TCOs, such as display panel, solar cell windows, transparent conducting electrode (TCE), etc., which necessitate long lifetimes and high durability 126 References [1] Zeng K Y., Zhu F R., Hu J Q., Shen L., Zhang K R., and Gong H., In- vestigation of Mechanical Properties of Transparent Conducting Oxide Thin Films, Thin Solid Films, 443, pp 60–65, 2003 [2] Chen J J., Gao Y., Zeng F., Li D M., and Pan F., Effect of Sputtering Oxygen Partial Pressures on Structure and Physical Properties of High Resistivity ZnO Films Applied Surface Science, 223, pp 318329, 2004 [3] Băuckle H., Westbrook J H., and Conrad H., The Science of Hardness Testing and Its Research Applications, ASTM, chapter 6, 1971 [4] Bhushan B., Handbook of Micro/Nanotribology, 2nd edition, CRC Press, chapter 5, 1999 [5] Tsui T Y., Ross C A., and 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investigation of 132 6.1 SUMMARY the optical and mechanical properties of copper aluminium oxide thin films was conducted in this work Cu-Al-O thin films were deposited using plasma-enhanced chemical vapor deposition, which is one of the most widely used techniques both in research and in semiconductor fabrication industry The optical properties of copper aluminium oxide films were studied by the Z-scan technique Under laser bombardment, the film exhibited increased transmittance at the focus point and produced signals similar to the optical nonlinearity At a laser intensity of 133 GW/cm2 , the largest transmittance change of 25% was achieved However, the signal was irreversible under different laser intensities; that is, the signal recorded in a low laser intensity could not be repeated after the film was exposed at a high intensity Careful microscopy observation revealed domes on the film surface with diameters close to the beam waist, which may be regarded as an evidence of grain growth or phase change in the areas bombarded by the laser beam These domes have higher transmittance than the rest parts of the film, thus produced a linear optical effect that mimicked the nonlinear optical signal Annealing the film in a furnace was then carried out to provide some qualitative information about the effect of annealing on film transmittance since direct measurement of the transmittance of the domes is difficult The results showed that a higher annealing temperature led to a higher transmittance, similar to the 133 6.1 SUMMARY case of a higher laser intensity in the Z-scan process Although the Z-scan measurements did not provide an evidence for nonlinear optical properties in the copper aluminium oxide thin film, the discovery of the surface modification may potentially be useful in optical data storage On the other hand, it was found that the modification threshold of the copper aluminium oxide thin film by laser beam is quite low and caution must be taken in examining this material by laser In the current study, the modification threshold was found to be 29 GW/cm2 for a Ti-sapphire laser with 100 fs pulse duration operated at 1kHz repetition rate Thus, a lower laser intensity or smaller pulse duration and repetition rate should be adopted to avoid or weaken the accumulated thermal effect on the film surface when examining the film by laser beam The mechanical properties of copper aluminium oxide thin films were characterized by the nanoindentation technique with continuous stiffness mode By employing the commonly used Oliver and Pharrs method for nanoindentation, the hardness and elastic moduli of the films were calculated The strongest film, prepared at a substrate temperature of 500 ◦ C and a Cu/Al ratio of in the precursors, has a hardness of 12.1 GPa and an elastic modulus of 120.1 GPa The weakest film, prepared at a substrate temperature of 600 ◦ C and a Cu/Al ratio of in the precursors, has a hardness of only 0.1 GPa and an elastic modulus of 19.0 GPa It was also found that the film strength decreased with the substrate temperature 134 6.1 SUMMARY and the Cu/Al ratio According to a model based on the concept of bulk modulus, the theoretical hardness values of CuAlO2 and CuO were calculated to be 2.2 GPa and 0.3 GPa, respectively Since both the nanoindentation load-displacement curves and SIMS depth profile measurement indicated that the films were quite homogenous throughout the film thickness, it was believed that the gaps among the mechanical properties of films prepared at different growth conditions actually originated from different film microstructures Moreover, several strengthening mechanisms were believed to account for the difference between the theoretical hardness and the experimental hardness XRD characterization revealed that the copper aluminium oxide films in the current study mainly consisted of CuAlO2 and CuO grains In some relatively harder films, a peak related to Al2 O3 (024) grains was found Surface morphological observations by AFM and SEM, combing with XRD, suggested that the films were composed of a matrix of relatively large CuAlO2 and CuO particles with fine Al2 O3 grains scattered in the void and gaps Because of their very high hardness and relatively small size, the Al2 O3 grains were believed to play an important role in strengthening the film by the particle hardening mechanism It can be easily observed that films contain such grains possess hardness values much higher than the theoretical value of CuAlO2 ; on the other hand, films which not contain such grains exhibit hardness values approaching the theoretical hardness of CuO 135 6.1 SUMMARY Another factor that needed to be considered is the particle size effect It was observed that the films were mostly composed of particles with diameters larger than 100 nm These particles may be regarded as aggregates of CuAlO2 and CuO grains Although the particle boundaries are not so strong as the grain boundaries to resist plastic deformation, they may still take some effect in strengthening the films Other factors, such as film density or stress state may also influence the film strength; however, their contributions to the film strengthening not seem to be significant 136 6.2 SUGGESTION FOR FUTURE WORK 6.2 Suggestion for Future Work In the current work, it was found that films with high mechanical strength did not have good conductivity and all the films contained several phases Future work is needed to optimize the growth conditions to achieve single phase films with both high conductivity and high mechanical strength 137 ... TCO films and most of them were focused on n-type TCOs Report on mechanical properties of p-type TCOs is rare, if any In particular, no report on the mechanical properties of copper aluminium oxide... some information on transparent conductive oxide (TCO) materials, especially on Cu- Al- O Sections 2.3 and 2.4 introduce the contemporary research on the optical and mechanical properties of TCO... results of optical and mechanical properties of Cu- Al- O films are discussed in the following two chapters, namely chapter and chapter In the final chapter, an overall summary of the current work and