Maskless Laser-Write Lithography of a-Si:H TFT Passive Pixel Sensor for Hemispherical Imager by Geonwook Yoo A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy (Electrical Engineering) in The University of Michigan 2011 Doctoral Committee: Professor Jerzy Kanicki, Chair Professor Peter F. Green Professor Kensall D. Wise Associate Professor Jamie D. Phillips © Geonwook Yoo 2011 Acknowledgements I sincerely appreciate Prof. Jerzy Kanicki, my research advisor and doctoral committee chair, for his support and guidance throughout my graduate study. It was a great and precious opportunity for me to work with him on this exciting project. I am deeply grateful for my doctoral committee members, Prof. Kensall D. Wise, Prof. Peter F. Green, and Prof. Jamie D. Phillips for their help in completing my thesis. I also truly thank Daniela Radtke at Fraunhofer Institute for Applied Optics and Precision Engineering in Germany, Tae-Kyung Won at AKT America, Inc., Sandro Tedde at Siemens AG TC, and Werner Salewski at Heidelberg Instrument. A large portion of the success of this work is attributed to their collaboration and advice. My special thanks belong to Daniela for her help on laser-write lithography. I express my gratitude to all LNF staffs, especially to Brian VanDerElzen. I thank my current and previous group members: Gwanghyeon Baek, Dr. Hojin Lee, Dr. Alex Kuo, Dr. Tze-ching Fung, Dr. Charlene Chen, and Dennis Feng. It was their support and friendship that helped me to get through the course of graduate study. In addition I am very grateful to all my seniors and friends both in U.S and Korea for their belief and support. I would like to thank DARPA/MTO HARDI program (Dr. D. Shenoy), Samsung scholarship program, and EECS department for their financial supports. The views, opinions, and/or findings contained in this paper are those of the authors and should not be interpreted as representing the official views or policies, either expressed or implied, of the Defense Advanced Research Projects Agency or the Department of Defense (approved for public release, distribution unlimited) ii Lastly and most importantly, much gratitude from the bottom of my heart is due to my family in Seoul, Korea for their support and love. iii Table of Contents Acknowledgements…………………………………………………………ii List of Tables……………………………………………………………..viii List of Figures……………………………………………………………...ix Chapter 1. Introduction……………………………………………………….……1 1.1 Motivation/Overview ........................................................................... 1 1.2 Organization of Dissertation ................................................................ 6 2. Advanced a-Si:H Thin-Film Transistor Structures for Sensor Application……………………………………………………………..….10 2.1 Introduction ........................................................................................ 10 2.2 Fabrication of Fork and Hexagonal a-Si:H TFT ................................ 11 2.3 Electrical Properties and Instability of Fork a-Si:H TFT ................... 13 2.3.1 Experimental results .................................................................... 13 2.3.2 Discussion of fork a-Si:H TFT geometry effect ......................... 15 2.3.3 Fork TFT parameters extraction .................................................. 23 2.3.4 Electrical instability of fork a-Si:H TFT ..................................... 26 2.4 Electrical Instability of Hexagonal a-Si:H TFT ................................. 27 2.4.1 Experiments ................................................................................. 27 2.4.2 Experimental results and discussion ........................................... 30 2.5 Conclusion.......................................................................................... 32 iv 3. Maskless Laser-Write Lithography…………………………………35 3.1 Introduction ........................................................................................ 35 3.2 Maskless a-Si:H TFT Processing using a Standard LWL on a FlatSurface ......................................................................................... 36 3.2.1 Fabrication of a-Si:H TFTs ......................................................... 36 3.2.2 Experimental results and discussions .......................................... 41 3.2.3 Summary ..................................................................................... 45 3.3 Modified LWL System for TFT Processing on a Non-Planar Surface .. ............................................................................................................ 45 3.3.1 Laser source ................................................................................. 47 3.3.2 Substrate table ............................................................................. 47 3.3.3 Optical and metrology system ..................................................... 47 3.3.4 Data preparation/conversion and exposure ................................. 48 3.3.5 Subfield and stitching .................................................................. 51 3.3.6 Structuring accuracy of LWL on a curved surface ..................... 52 3.3.7 Advanced LWL system for device/circuit fabrication processing on a curved surface with FOV = 120 ° ........................................ 55 3.4 Conclusion.......................................................................................... 57 4. Maskless Processing of a-Si:H TFT on a Curved Surface…………60 4.1 Introduction ........................................................................................ 60 4.2 Fabrication of a-Si:H TFTs on a Curved Surface using Maskless LWL ................................................................................................... 61 4.2.1 Customized curved glass substrate .............................................. 61 4.2.2 Thin-film deposition and etching on curved surfaces ................. 63 4.2.3 Spin-coat of photo-resist on curved surface ................................ 64 4.2.4 Level-to-level alignment on a curved surface ............................. 68 v 4.2.5 Concentric exposure strategy ...................................................... 69 4.2.6 Maskless LWL TFT fabrication process sequence on a curved surface .......................................................................................... 71 4.3 Electrical Properties of a-Si:H TFT on a Curved Surface ................. 73 4.3.1 Experimental results and discussion ........................................... 73 4.4 Conclusion.......................................................................................... 77 5. Electrical Instability of the a-Si:H TFTs on a Curved Surface……………………………………………………………….…….81 5.1 Introduction ........................................................................................ 81 5.2 Experiments........................................................................................ 82 5.3 Results and Discussions ..................................................................... 86 5.4 Conclusion.......................................................................................... 94 6. Maskless processing of a-Si:H TFT Passive Pixel Sensors on a Curved Surface……………………………………………………………96 6.1 Introduction ........................................................................................ 96 6.2 Passive Pixel Sensor and its Operation .............................................. 97 6.3 Fabrication of a-Si:H TFT PPS on a Curved Surface using LWL ... 102 6.4 Experimental and Testing Set-up ..................................................... 103 6.5 Experimental Results and Discussion .............................................. 106 6.5.1 Electrical characteristics and instability of the switching TFT . 106 6.5.2 Transient response and dynamic range of fabricated PPS ........ 110 6.5.3 Comparison between experimental and simulated results ........ 111 6.6 Conclusion........................................................................................ 113 7. 50 µm pixel pitch 128 x 128 a-Si:H TFT PPS Imager………...…..116 vi 7.1 Introduction ...................................................................................... 116 7.2 Fabrication of 50 µm pixel pitch 128 x 128 a-Si:H TFT PPS array 117 7.3 Electrical characteristics of the fabricated switch a-Si:H TFTs....... 123 7.4 Organic photodiode structure and its expected opto-electronic properties .......................................................................................... 126 7.5 Integration of the organic photodiode with the a-Si:H TFT PPS array . ........................................................................................................ 1288 7.6 Expected opto-electronic properties of the fabricated 128 x 128 PPS imager ............................................................................................... 129 7.7 Conclusion........................................................................................ 131 8. Conclusions and Future Work……………………………………..134 8.1 Conclusions ...................................................................................... 134 8.2 Recommendations for future work .................................................. 136 Appendix-List of Publications…………………..………………………138 vii List of Tables Table 2.1 Extracted parameters of fork TFT; case (1) where source is applied on U- shaped electrode and case (2) where source is applied on rod-shaped electrode. Geometrical factor a) Weff0/L b) fg1 and c) fg2 is used to extract