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University of Cincinnati UNIVERSITY OF CINCINNATI Date:___________________ I, _________________________________________________________, hereby submit this work as part of the requirements for the degree of: in: It is entitled: This work and its defense approved by: Chair: _______________________________ _______________________________ _______________________________ _______________________________ _______________________________ Coherent Porous Silicon Technology for Micro Loop Heat Pipes and Chromatography A dissertation submitted to the Division of Research and Advanced Studies of the University of Cincinnati in partial fulfillment of the requirement for the degree of Doctor of Philosophy (Ph.D.) In the Department of Electrical and Computer Engineering & Computer Science 2006 By Srinivas Parimi M.S., University of Cincinnati, Cincinnati OH, 2003 B.Tech., Nagarjuna University, AP India, 1999 Committee Chair: Thurman H. Henderson Co-Chair: Frank M. Gerner To my parents… ii Abstract In this work coherent porous silicon (CPS) is used as a base technology to develop micro Loop Heat Pipes (LHP) and multi-turn micro chromatograph. The issues with silicon passivation in a photon pumped electrochemical cell are discussed and innovative solutions are presented. The challenges faced in micropatterning CPS, such as stress development around the boundaries, material selection, electrolyte selection and process development are described. The micro LHP developed in this lab provides a planar surface for microelectronic chip cooling. Several generations of these devices were built with improvements in design and optimization of heat transfer. Recently 60W/cm2 of heat flux was removed using our current micro LHP. Many steady state models were developed in this work to understand the heat delivery and to optimize the same in a micro LHP. Microfabrication of individual components and packaging issues involved are described. The automation of the micro LHP test setup, as well as the test results are also shown. Using the patterned CPS wick and the new top-cap configuration, 20 W/cm2 of heat flux was transferred from the evaporator the condenser of the micro LHP, however the author believes that a near order of magnitude improvement is yet possible. A micro chromatograph was earlier developed in this lab using micro channels in (110) silicon. In the present modeling studies, replacing the micro channels with a CPS wick, utilizing the same above technology, showed (by modeling studies) drastic improvement in the efficiency of separation of species in a chromatographic device. By utilizing the pore walls of the CPS wick as a separating surface; all three spatial dimensions can be utilized. This enhanced the packing density and increased the number iii of plates per unit length of the chromatograph. The multi-turn CPS chromatograph utilizes some unique packaging techniques, namely damascene process. The challenges faced in packaging to seal the device, supply with electrodes for detection and providing input and output ports are discussed. A thorough model was developed to analyze and compare the efficiency of this device with the previous device developed in this lab using micro channels. iv v Acknowledgements I wish to express my sincere gratitude to many people whose support and patience helped me to complete this work. First and foremost, I would like to thank my advisor and mentor, Dr. H. Thurman Henderson, who has been my inspiration. He gave me precious guidance and advice all through my stay at UC. He has broadened my horizon in many different ways. His patience enormously helped me to settle down and successfully complete this work. Without his support and enthusiasm, I would have never been able to complete this dissertation. I would also like to acknowledge Dr. Frank Gerner, for helping me to understand the fundamentals of micro scale heat transfer and Dr. Neville Pinto made the understanding of chemical separations and chromatography easier for me. I would also like to express my gratitude to Jeff Simkins and Ron Flenniken. These two have been the most helpful persons at the MEMS center. There are many colleagues and friends who either directly or indirectly helped in completing my dissertation. Praveen Medis and Ahmed Shuja need to be specially mentioned as wonderful colleagues to work with and also good friends. I would also take this opportunity to thank my parents and my sister who helped me in many ways. I also thank, Yamini for her warm friendship and enormous patience to keep me focused. In numerous occasions, they have provided me with enough encouragement, which I desperately needed to continue. vi I would also thank NASA Glenn research center, Ohio super computing, NSF, NASA TEES program and UGS (University Grants Scholarship) for providing the necessary funding. vii Table of Contents Abstract..............................................................................................................................iii Acknowledgements............................................................................................................ vi Table of Contents.............................................................................................................viii List of Figures................................................................................................................... xii List of Tables ................................................................................................................... xix Chapter 1............................................................................................................................. 1 Micropatterning Coherent Porous Silicon .......................................................................... 1 1.1 Introduction............................................................................................................... 1 1.2 Review of literature................................................................................................... 2 1.3 Formation of CPS ..................................................................................................... 3 1.4 Chemistry of CPS etching......................................................................................... 9 1.5 CPS at the University of Cincinnati........................................................................ 11 1.6 CPS etch setup ........................................................................................................ 14 1.6.1 Etch catcher modification ................................................................................ 16 1.6.2 Flow, level and temperature control ................................................................ 18 1.7 Microfabrication of CPS......................................................................................... 25 1.8 Micropatterning of CPS .......................................................................................... 27 1.8.1 Two pieces rig.................................................................................................. 28 1.8.2 Four pieces rig.................................................................................................. 29 1.8.3 Micropatterning of HF and UV masks............................................................. 31 1.9 Materials investigated for HF passivation .............................................................. 32 1.9.1 Silicon nitride................................................................................................... 33 1.9.2 Low stress silicon nitride film.......................................................................... 34 1.9.3 Polysilicon........................................................................................................ 36 1.9.4 Low stress Si3N4 + gold ................................................................................... 39 1.9.5 SU-8 ................................................................................................................. 40 1.9.6 Microwave plasma enhanced CVD (MW-PECVD) diamond......................... 40 1.9.7 Low-stress Si3N4 + highly p-type doped polysilicon ....................................... 41 1.10 References............................................................................................................. 42 Chapter 2........................................................................................................................... 44 Introduction to Loop Heat Pipes (LHP)............................................................................ 44 2.1 Background............................................................................................................. 44 2.2 Electronic device package with cooling system ..................................................... 45 2.2.1 Thermal interface materials ............................................................................. 46 2.3 Trends in microelectronic cooling technologies..................................................... 47 2.3.1 Heat sinks......................................................................................................... 47 2.3.2 Liquid cooling.................................................................................................. 49 2.3.3 Two phase cooling ........................................................................................... 52 2.4 Micro loop heat pipes using coherent porous silicon (CPS) as a primary wick ..... 55 2.5 Summary of earlier modeling effort at UC............................................................
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