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Reactive High Power Impulse Magnetron Sputtering of Zinc

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Reactive High Power Impulse Magnetron Sputtering of Zinc REACTIVE HIGH POWER IMPULSE MAGNETRON SPUTTERING OF ZINC OXIDE FOR THIN FILM TRANSISTOR APPLICATIONS Dissertation Submitted to The School of Engineering of the UNIVERSITY OF DAYTON In Partial Fulfillment of the Requirements for The Degree of Doctor of Philosophy in Materials Engineering By Amber Nicole Reed Dayton, Ohio May 2015 REACTIVE HIGH POWER IMPULSE MAGNETRON SPUTTERING OF ZINC OXIDE FOR THIN FILM TRANSISTOR APPLICATIONS Name: Reed, Amber Nicole APPROVED BY: _________________________ ___________________________ Dr. Andrey A. Voevodin Dr. Charles Browning Advisory Committee Chairman Committee Member Adjunct Professor Department Chair Department of Chemical and Department of Chemical and Materials Engineering Materials Engineering ____________________ Dr. Kevin Leedy Committee Member AFRL/RYDD _________________________ ___________________________ Dr. Christopher Muratore Dr. Terry Murray Committee Member Committee Member Professor Professor Department of Chemical and Department of Chemical and Materials Engineering Materials Engineering _________________________ ___________________________ John G. Weber, Ph.D. Eddy M. Rojas, Ph.D., M.A., P.E. Associate Dean Dean, School of Engineering School of Engineering ii © Copyright by Amber Nicole Reed All rights reserved 2015 iii ABSTRACT REACTIVE HIGH POWER IMPULSE MAGNETRON SPUTTERING OF ZINC OXIDE FOR THIN FILM TRANSISTOR APPLICATIONS Name: Reed, Amber Nicole University of Dayton Advisor: Dr. Andrey A. Voevodin Zinc oxide (ZnO) is an emerging thin film transistor (TFT) material for transparent flexible displays and sensor technologies, where low temperature synthesis of highly crystallographically ordered films over large areas is critically needed. This study maps plasma assisted synthesis characteristics, establishes polycrystalline ZnO growth mechanisms and demonstrates for the first time low-temperature and scalable deposition of semiconducting grade ZnO channels for TFT applications using reactive high power impulse magnetron sputtering (HiPIMS). Plasma parameters, including target currents, ion species and their energies were measured at the substrate surface location with mass spectroscopy as a function of pressure and applied voltage during HiPIMS of Zn and ZnO targets in O2/Ar. The results were correlated to film microstructure development investigated with x-ray diffraction, atomic force microscopy, scanning electron microscopy and transmission electron microscopy which helped establish film nucleation iv and growth mechanisms. Competition for nucleation by (100), (101) and (002) oriented crystallites was identified at the early stages of film growth, which can result in a layer of mixed crystal orientation at the substrate interface, a microstructural feature that is detrimental to TFT performance due to increased charge carrier scattering in back-gated TFT devices. The study revealed that nucleation of both (100) and (101) orientations can be suppressed by increasing the plasma density while decreasing ion energy. After the initial nucleation layer, the microstructure evolves to strongly textured with the (002) crystal plane oriented parallel to the substrate surface. The degree of (002) alignment was pressure-dependent with lower deposition pressures resulting in films with (002) o alignment less than 3.3 , a trend attributed to less energy attenuation of the low energy (2- + + + 6 eV) Ar , O , and O2 ions observed with mass spectrometry measurements. At + + + pressures of 7 mTorr and lower, a second population of ionized gas (Ar , O , and O2 ) species with energies up to 50 eV appeared. The presence of higher energy ions corresponded with a bimodal distribution of ZnO grain sizes, confirming that high energy bombardment has significant implications on microstructural uniformity during large area growth. Based on the established correlations between process parameters, plasma characteristics, film structure and growth mechanisms, optimum deposition conditions for (002) oriented nanocrystalline ZnO synthesis at 150 ⁰C were identified and demonstrated for both silicon oxide wafers of up to 4 inch diameter and on flexible polymer (Kapton) substrates. The feasibility of the low temperature processing of ZnO films for TFT applications was verified by preliminary tests with back-gated device prototypes. Directions of future research are outlined to further develop this low temperature growth method and apply results of this study for ZnO applications in semiconductor devices. v Dedicated to my dad. vi ACKNOWLEDGEMENTS I would like to thank C. Muratore, A.A. Voevodin and P.J. Shamberger for their guidance. I would also like to thank everyone who provided moral and technical support. In particular, I would like to thank P.J. Shamberger and R.D. Naguy for the AFM imaging, J.J. Hu for the TEM analysis, A.J. Safriet and J.E. Bultman for design and fabrication work on many of the systems used in this study, J.E. Bultman for providing the SolidWorks schematics of the deposition chamber, and EQP, J.L. Jespersen and J. Grant for assistance with the analysis of the XPS data. I’d also like to thank K.D. Leedy and G.H. Jessen for providing the PLD-grown ZnO films and technical discussion on TFT devices. I’d also like to thank J.G. Jones, M.N. Pike, S.D. Apt, A.M. Campo, C.N. Hunter, M. E. McConney, N.R. Glavin, A.R. Waite, and M.H. Check and the whole Advanced Nanoscale Electronic Materials and Processing research team for various other help and support. I am profoundly grateful for the support of my family. Additionally, I am thankful for my dogs, Icky, Ooey, Ollie and Lucy, who could always make me smile. vii TABLE OF CONTENTS ABSTRACT ……………………………………………………………………………..iv DEDICATION……………………………………………………………………………vi ACKNOWLEDGEMENTS ……………………………………………………………..vii LIST OF FIGURES ……………………………………………………………………...xi LIST OF TABLES ……………………………………………………………………xxiii LIST OF SYMBOLS/ABBREVIATIONS …………………………………...…….....xxiv CHAPTER I: INTRODUCTION …………………………………………….…………...1 CHAPTER II: BACKGROUND………………………………………………………….4 2.1 Zinc Oxide as a Thin Film Transistor Channel……………………………..…4 2.2 Film Nucleation and Growth………………………………………………...11 2.3 Magnetron Sputtering and the Sputtering Discharge………………………...16 2.4 High Power Impulse Magnetron Sputtering…………………………………23 2.4.1 Plasma Dynamics during the HiPIMS Process…………………....24 2.4.2 Film Growth during HiPIMS…………………………...…………39 CHAPTER III: RESEARCH OBJECTIVES………………...…………………………..45 CHAPTER IV: EXPERIMENTAL METHODS……………………………...…………48 4.1 The Deposition System…………………………………..…………………..48 4.2 Process Characterization and Plasma Diagnostics……..…………………….50 4.2.1 Sputtering Yield Calculations……………………………………...50 viii 4.2.2 Energy-Resolved Mass Spectroscopy……………………………...52 4.2.3 Target Current Measurements………………………...……………54 4.3 Substrate Selection and Preparation……………………………..…………...54 4.4 Zinc Oxide Deposition…………………………………………..…………...55 4.5 Microstructural Characterization……………………………..……………...59 4.5.1 Thickness Measurements…………………...……………………...59 4.5.2 X-Ray Diffraction Analysis………………………………………..59 4.5.3 Electron Microscopy………………………………………...……..61 4.5.4 Atomic Force Microscopy………...……………………………….61 4.6 Electrical Characterization………..………………………………………….62 4.6.1 Device Fabrication…………………………………………...…….62 4.6.2 Preliminary Back-Gated TFT Device Testing ………...……….….63 CHAPTER V: RESULTS AND DISCUSSION…………………………………………64 5.1 Process and Plasma Characterization…………………..…………………….64 5.1.1 Reactive HiPIMS from a Zinc Target……………………...………64 5.1.2 Reactive HiPIMS from a Zinc Oxide Target………...…………….77 5.2 Film Growth and Microstructure…..………………………………………...89 5.2.1 Film Growth from a Zinc Target………………………..…………89 5.2.2 Film Growth from a Zinc Oxide Target ………...………………....93 5.2.3 Discussion of Film Growth Mechanisms and Plasma Conditions..104 5.2.4 Summary for Film Growth Study………………………………...107 5.3 Effect of Spatial Inhomogeneity of the Depositing Flux on Film Microstructure...............................................................................................110 ix 5.4 Electrical Characterization of Zinc Oxide TFT Channels………………….112 CHAPTER VI: CONCLUSIONS……...……………………………………………….115 CHAPTER VII: FUTURE WORK…………………………………….……………….119 REFERENCES………………………………………………………………………....121 APPENDICES………………………………………………………………………….134 A. Supplemental Ion Energy Distribution Data………………………………..134 B. Supplemental Microstructural Data………………………………………...136 C. Supplemental TFT Device Characterization Data………………………....145 D. X-Ray Photoelectron Spectroscopy………………………………………...156 x LIST OF FIGURES 2.1 Schematic drawing of (a) ZnO FET on Si substrate for DC characterization and (b) of microwave ZnO FET on PECVD SiO2/GaAs substrate. (c) Micrograph of the microwave transistor. Reproduced with permission from [15]. ................................ 5 2.2 Cross-sectional TEM of a PLD-deposited ZnO film over the TFT gate. Reproduced with permission from [16]. (top left). Schematic diagram of TFT with a ZnO channel (bottom left). Close up of the TFT channel showing the depletion zones at the grain boundaries in the off-state (top right) and the collapse of the depletion zones under the gate field in the on-state (bottom right). ........................................... 6 2.3 Summary of FWHM of (002) rocking curve XRD peaks at different substrate temperatures for ZnO films deposited with pulsed DC magnetron sputtering [52], PLD[34-36], and RF sputtering [46,47]. .................................................................... 8 2.4 AFM micrograph of surface of ZnO layer deposited using (a)
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