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Silicone Resins and Their Composites Intelligent Field Emission Arrays by Ching-yin Hong M.S., Materials Science and Engineering National Tsing Hua University, 1997 B.S., Chemical Engineering National Taiwan University, 1995 Submitted to the Department of Materials Science and Engineering in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy at the MASSACHUSETTS INSTITUTE OF TECHNOLOGY June 2003 © 2003 Massachusetts Institute of Technology. All rights reserved. Signature of Author ...……………………………………………………………………… Department of Materials Science and Engineering April 18, 2003 Certified by ………………………………………………………………………………... Akintunde I. Akinwande Professor of Electrical Engineering and Computer Science Thesis Supervisor Certified by ………………………………………………………………………………... Lionel C. Kimerling Professor of Materials Science and Engineering Thesis Supervisor Accepted by ……………………………………………………………………………….. Harry L. Tuller Professor of Ceramics and Electronic Materials Chair, Departmental Committee on Graduate Students 2 INTELLIGENT FIELD EMISSION ARRAYS by CHING-YIN HONG Submitted to the Department of Materials Science and Engineering on May 9, 2003 in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy in Materials Science and Engineering ABSTRACT Field emission arrays (FEAs) have been studied extensively as potential electron sources for a number of vacuum microelectronic device applications. For most applications, temporal current stability and spatial current uniformity are major concerns. Using the kinetic model of electron emission, field emission can be described as two sequential processes— the flux of electrons to the tip surface followed by the transmission of the electrons through the surface barrier. Either of these processes could be the determinant of the emission current. Unstable emission current is usually due to absorption/desorption of gas molecules on the tip surface (barrier height variation) and non-uniform emission is usually due to tip radius variation (barrier width change). These problems could be solved if the emission current is determined by the electron supply to the surface instead of the electron transmission through the surface barrier. In this thesis, we used the inversion layer of a MOSFET to control the electron supply. It results in additional benefits of low turn-on voltage and low voltage swing to turn the device on and off. A novel CMP-based process for fabricating integrated LD-MOSFET/FEA is presented. We obtained FEA devices with an extraction gate aperture of 1.3 m and emitter height of 1 m. We present a comprehensive study of field emitter arrays with or without MOSFET. The silicon field emitter shows turn-on voltage of ~24 V with field enhancement factor (bFN) of ~370. We demonstrated that the LD-MOSFET provides excellent control of emission current. The threshold voltage of the LD-MOSFET is ~ 0.5V. The integrated device can be switched ON and OFF using a MOSFET gate voltage swing of 0.5V. This results in an ON/OFF current ratio of 1000:1. The current fluctuation is significantly reduced when the MOSFET is integrated with the FEA device and the device is operated in the MOSFET control regime. The emission current of the integrated LD-MOSFET/FEA remains stable regardless the gas and vacuum condition. The saturation current level of the integrated devices in the MOSFET controlled region is also the same regardless the emitter array size or the FEA’s position on the wafer. We also present a comprehensive study of three-dimensional oxidation in silicon emitter tip formation. Stress plays an important role in the oxidation mechanism. A new sharp emitter tip formation mechanism is proposed: rather than a continuous oxidation process, 3 an emitter neck breaking stage occurs before the sharp emitter tip is formed. Stress from volume difference of silicon and silicon dioxide is the main cause for the emitter neck breaking. Initial formation of microcracks around the neck occurs at high temperature due to volume difference stress, oxide grows into the cracks right after crack formation, and a sharp emitter tip is then formed by further oxidation. Thesis advisor: Akintunde I. Akinwande Title: Professor of Electrical Engineering and Computer Science Co-advisor: Lionel C. Kimerling Title: Thomas Lord Professor of Materials Science and Engineering 4 Acknowledgements There are many people I want to thank. First my sincere thanks go out to my advisor Tayo Akinwande. Your guidance had a major impact on my professional development over the years. Your constant encouragement, support, and counseling enable me to stand by myself with confidence. You are not only my advisor but also one of my best friends I met at MIT. I would like to thank my co-advisor, Prof. Lionel Kimerling, and my thesis committee member, Prof. Sam Allen, for your valuable advice and guidance, especially in the field of materials science. It is fortunate for me to work with the members in my research group: Meng Ding, Leonard Dvorson, John Kymissis, Guobin Sha, Annie Wang, Liangyu Chen, and Yong- Woo Choi. Especially, I want to thank John Kymissis for all the valuable discussions and helps in the device characterization. I also want to thank my officemate Guobin Sha for the consultant in device physics and simulation. I have enjoyed working with you all. I would like to express my appreciation to Mr. Cheng-Yen Wen from Department of Applied Physics at Harvard University for his great help in TEM sample preparation and inspection. It is impossible to finish this thesis work without the contributions of the technical and support staff of the MIT Microsystems Technology Laboratories, especially Bernard Alamariu, Paul Tierney, and Joe Walsh, on the fabrication of the devices. I appreciate your training and helps using the equipment in my fabbing days in ICL. I would also like to thank my mother and father for their encouragement and support. I have achieved things in life because of you. Thank you for having the confidence in me to succeed. 5 Mostly I would like to thank my wonderful husband Hong-Ren Wang for his love over the years. You gave me the support, understanding, and also lots of brilliant discussions and suggestions from a materials scientist point of view. I am very lucky to have you as my partner in life and also in my profession. I would also like to acknowledge the financial support that made this project possible. This work was funded by DARPA/ONR Grant N00014-96-1-0802 and Creative Micro Tech supported by NIH Phase II SBIR Grant 2R44RR12265-02A1 and 5R44RR12265- 03. 6 Table of Contents Abstract ………………………………………………….…… 3 Acknowledgements ………………………………………………….…… 5 Table of Contents ………………………………………………….…… 7 List of Figures ………………………………………………….…… 9 List of Tables ………………………………………………….…… 19 1. Introduction ………………………………………………….…… 21 1.1 Field Emission Applications ………………………………….…… 22 1.2 Statement of the Problem ………………………………….…… 25 1.3 Objectives and Technical Approach ………………………….…… 26 1.4 Thesis Organization ………………………………….…… 28 2. Field Emission Theory ……..………………………………….…… 31 2.1 Field Emission ……..………………………………….…… 31 2.2 Sources of the Non-Uniform and Unstable Emission Current .……… 37 2.3 Theoretical Framework of MOSFET/Field Emission Device ….…… 45 2.4 Chapter Summary ……..………………………………….…… 65 3. Uniform and Sharp Silicon Field Emitters ………………………….…… 67 3.1 Fabrication of Sharp Silicon Emitters ………………………….…… 69 3.2 Structure Characterization ………………………………….…… 78 3.3 Three-Dimensional Thermal Oxidation of Silicon-Oxidation Sharpening 81 3.4 Chapter Summary ……..………………………………….…… 108 4. Device Design and Fabrication ………………………………….… 111 4.1 Device Design ……..…………………………………….… 111 4.2 Device Fabrication ……..…………………………………….… 118 4.3 Chapter Summary ……..……………………………………… 140 5. Field Emission Device Characterization and Analysis …………….. 141 5.1 Measurement Setup ……..……………………………………… 141 5.2 Device Characterization ……..……………………………………… 143 5.3 Chapter Summary ……..………………………………………. 149 6. Active Field Emission Device Characterization and Analysis ………… 181 6.1 MOSFET Characterization ...……………………………………… 181 7 6.1.1 Measurement Setup ...…..……………………………… 181 6.1.2 Device Characterization ...…..……………………………… 182 6.2 LD-MOSFET/FEA Characterization ..……………………………… 190 6.2.1 Measurement Setup ...…..……………………………… 190 6.2.2 Device Characterization ...…..……………………………… 191 6.3 Chapter Summary ..…..……………………………………… 221 7. Thesis Summary and Suggestions for Future Work ………………… 223 7.1 Thesis Summary ..…..……………………………………… 223 7.2 Main Contributions ..…..……………………………………… 225 7.3 Suggestions for Further Work ...…..……………………………… 225 Appendix ………….……..……………………………………… 227 A. Microsystems Technology Laboratories’ Fabrication Facilities ……… 227 B. Mask Sets for Silicon LD-MOSFET/FEA Devices ………..…… 228 C. Process Flow of the Fabrication of Silicon FEA/MOSFET Devices ……… 232 D. Silvaco Simulation Results for Three-Dimensional Oxidation ….……… 244 E. TEM Images of Oxidation Sharpening Experiments at Different Oxidation Temperatures and Time Duration ……………………………………… 248 F. Field Emission Ridges ……………………………………… 263 G. IV Characterization Results ……………………………………… 268 H. Sensitivity Analysis Derivation ……………………………………… 280 I. Langmuir Equation Derivation ……………………………………… 288 References ………….……..……………………………………….. 289 8 List of Figures Figure 1-1. Schematic of a typical FED. Figure 1-2. Structure of a TWT, in which the electron source is replaced by a FEA [1.14]. Figure 1-3. The operation of field emission devices integrating with a resistor. Figure 1-4. Integrating the resistive
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