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Investigation of Eutectic and Solid State Wafer Bonding of Silicon with Gold

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Investigation of Eutectic and Solid State Wafer Bonding of Silicon with Gold Investigation of Eutectic and Solid State Wafer Bonding of Silicon with Gold by Maryam Abouie A thesis submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Materials Engineering Department of Chemical and Materials Engineering University of Alberta © Maryam Abouie, 2014 ABSTRACT Permanent wafer bonding technology is a very important process for different types of applications such as MEMS (microelectromechanical systems), LED (light-emitting diode) devices, advanced packaging, 3D stack and SOI (silicon on isolator) substrate applications. Depending on the type of application, several bonding technologies exist. The MEMS market road map suggests the market for MEMS will be close to $21B in 2017. The driving force for development of different wafer bonding processes is the large applications that need to be addressed. The wafer bonding market generated almost 5 million 8-inch wafers bonded for BSI (back-illuminated sensor), CIS (CMOS image sensor) and MEMS device applications and it is expected to reach 16 million 8-inch bonded wafers in 2019. The growing demand is mainly driven by the miniaturization required for 3D Stack TSV (through- silicon via) applications. Among different wafer bonding techniques, eutectic bonding is more appealing due to its good hermeticity, ability to create electrical connection, good mechanical strength and low temperature processing. In this study, Au-Si eutectic bonding was applied due to its high bonding strength, excellent hermeticity and good tolerance to surface topology prior to bonding. Au/a-Si wafer bonding was introduced in order to reduce the risk of possible damage to active devices in Au/c-Si wafer bonding due to formation of large craters in c-Si. Solid state ii bonding was introduced in order to reduce the process temperature, which is desirable in the manufacturing industry. Bonding parameters were optimized as much as possible. The microstructure of bonded pairs was imaged using SEM (scanning electron microscopy) and TEM (transmission electron microscopy) for both Au/a-Si and Au/c-Si samples and for both eutectic and solid state methods. In-situ TEM imaging of the Au/a-Si diffusion couple during annealing was performed in an attempt to understand and explain the reason for the layer exchange mechanism in Au/a-Si wafer bonding. The diffusion coefficient for Au in a-Si was also calculated using in-situ TEM results. The mechanical strength of bonded wafers was measured using shear testing on eutectic and solid state bonded Au/a-Si and Au/c-Si samples. KEYWORDS: Au, a-Si, c-Si, wafer bonding, eutectic bonding, solid state bonding iii PREFACE This thesis is an original work by Maryam Abouie carried out at University of Alberta under the supervision of Professor D.G. Ivey and Professor Q. Liu. Part of Chapter 4 of this thesis has been published as Maryam Abouie, Qi Liu, Douglas G. Ivey, “Eutectic and solid-state wafer bonding of silicon with gold”, Materials Science and Engineering: B, vol. 177, issue 20, December 2012, 1748-1758. I was responsible for the data collection and analysis as well as the paper composition. Douglas G. Ivey and Qi Liu assisted with paper composition and edits. iv ACKNOWLEDGMENTS I would like to thank Dr. D. G. Ivey for his guidance and supervision throughout my study at university of Alberta. I appreciate his dedication to his students, research and teaching. Special appreciation is extended to Dr. Q. Liu for his helpful advice and assistance during the course of my thesis. I greatly acknowledge Shihong Xu for SEM analysis, Anqiang He for XPS analysis, Shiraz Merali for XRD analysis, Martin Kupsta for FIB sample preparation and Peng Li for TEM analysis. Great thanks to Glen Fitzpatrick and everyone at Micralyne for providing samples and their cooperation and support. I also would like to acknowledge ACAMP for providing access to their lab equipment to do SAM and shear testing. Special thanks to Dr. M. T. McDermott at the U of A for access to Raman spectroscopy. The Natural Sciences and Engineering Research Council (NSERC) of Canada is greatly acknowledged for funding this research. Finally, I want to thank my husband for all of the encouraging words and support during the completion of this thesis and throughout my studies. Special thanks go to my mom and dad for their endless support and belief in me in everything I do. v TABLE OF CONTENTS ABSTRACT ...........................................................................................................................ii PREFACE ............................................................................................................................ iv ACKNOWLEDGMENTS ........................................................................................................ v Chapter 1 - Introduction ................................................................................................... 1 Chapter 2 - Literature Review ........................................................................................... 5 2.1. Introduction to Micro Electro Mechanical Systems ............................................... 5 2.2. MEMS packaging .................................................................................................... 5 2.3. Wafer bonding ....................................................................................................... 7 2.3.1. Bonding requirements and procedure............................................................ 8 2.3.2. Direct bonding ................................................................................................. 9 2.3.3. Intermediate layer bonding .......................................................................... 14 2.4. Au-Sn eutectic bonding ........................................................................................ 19 2.5. Au-Ge eutectic system ......................................................................................... 19 2.6. Au-Si eutectic bonding ......................................................................................... 19 2.6.1. Principles of Au-Si eutectic bonding ............................................................. 20 2.6.2. Amorphous Si (a-Si) versus crystalline Si (c-Si) ............................................. 21 2.6.3. Metastable phases ........................................................................................ 25 2.6.4. Metal induced crystallization ........................................................................ 26 2.6.5. Influence of metal thickness ......................................................................... 27 2.6.6. Au diffusion in Si ............................................................................................ 30 2.6.7. Si diffusion in Au ............................................................................................ 33 2.6.8. Stress in Si layer ............................................................................................ 33 Chapter 3 - Experimental Methods ................................................................................. 36 3.1. Diffusion couple experiments .............................................................................. 36 3.2. Small samples bonding experiments ................................................................... 37 3.3. Full wafer bonding experiments .......................................................................... 48 vi 3.4. In-situ TEM experiments ...................................................................................... 49 Chapter 4 - Diffusion Couple Results .............................................................................. 51 4.1. a-Si diffusion couple ............................................................................................. 51 4.2. c-Si diffusion couples............................................................................................ 56 4.3. Summary .............................................................................................................. 65 Chapter 5 - Small Sample Bonding Results ..................................................................... 67 5.1. Au-Si eutectic bonding ......................................................................................... 67 5.1.1. a-Si ................................................................................................................. 67 5.1.2. c-Si ................................................................................................................. 93 5.2. Au-Si solid state bonding .................................................................................... 102 5.2.1. a-Si ............................................................................................................... 102 5.2.2. c-Si ............................................................................................................... 106 5.3. Evaluation of bond strength .............................................................................. 109 5.3.1. a-Si ............................................................................................................... 109 5.3.2. c-Si ............................................................................................................... 111 5.4. Summary ............................................................................................................ 111 Chapter 6 - Full Wafer Bonding Results ........................................................................ 112
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