The analysis and design of integrated capacitive displacement sensors

Capacitive displacement sensors are currently used in high-resolution non-contact displacement measurements such as wafer thickness and flatness. Currently these sensors are fabricated from discrete mechanical and electrical parts, with a triaxial cable connecting the sensor electrode at the mechanical sensor head to the sense electronics on a printed circuit board. By having an integrated front end located in close proximity to the mechanical sensor, we expect to achieve nanometer-order resolution with a precise digital representation of displacement as the output, through elimination of the triaxial cable. This is an improvement over the current sensors in terms of performance and cost. The main tasks of the thesis work are low-noise capacitance measurement circuitry design, high-resolution sigma-delta analog-to-digital converter design based on switched-capacitor techniques, and fabrication of a prototype of the design using a standard CMOS process. Thesis Supervisor: David L. Trumper Title: Rockwell Associate Professor of Mechanical Engineering Thesis Supervisor: Charles G. Sodini Title: Professor of Electrical Engineering and Computer Science Acknowledgments I would first like to thank Professor David Trumper and Professor Charles Sodini who gave me the opportunity to work on this research. Their knowledge and guidance have provided me with a learning experience, both technically and (more importantly) otherwise, which I consider far more rewarding and important than anything I have learnt over the previous years. This project would not be possible without the generous funding provided by the National Science Foundation and our industrial sponsor ADE Corporation. Many people have also contributed technically and provided assistance throughout the course of this research. Many thanks to Professor Harry Lee for providing technical guidance regarding the layout and circuit level design of the prototype chip when Charles was on sabbatical in Hong Kong. I would like to thank especially Michael Perrott, Don Hitko, Jeff Gealow, Iliana Fujimori, Ginger Wang, Dan McMahill, Pablo Acosta Serafini, Steven Decker, Jennifer Lloyd, Kush Gulati, Tracy Adams, Pradeep Subrahmanyan and Stephen Ludwick for their help in setting up my computer, getting me started with Cadence, and its associated technology files and various scripts, as well as providing me with helpful insights and advice on the prototype chip design and debug. Without their help I would not be able to make the fab deadline and to test the prototype chip. I would like to thank Maureen Lynch and Patricia Varley for handing the numerous administrative details of the research project. In addition, I would like to thank Chao Wang for being my friend and also my study partner. Without her support and encouragements, I would not be able to survive the demanding course-works and the doctoral qualifying examinations. I also want to acknowledge my roommate Yu Sui for providing many helps, big and small, to make my life easier at MIT. I want to say a big 'thank you' to Lynn Wood who helped me out in various ways being my international student host under the Host for International Students Program of the International Student Office. Furthermore I extend my sincere thanks to my brothers and sisters in the Hong Kong Student Bible Study Group. With them I have spent some of my most enjoyable and memorial moments at MIT. I would especially like to thank Winnie Choi, Ernest Yeh, Oliver Yip, Ada Cheung and Jacky Ho for their support in prayers when I needed them most. Finally I would like to thank my family members, in particular my parents and my brother whose support and encouragements are always there whenever I needed them. I would also like to thank God for being by my side always. I know I would not be able to be where I am now without Him. SaiBun WONG May 1998