Coherent Porous Silicon Wick for a MEMS Loop Heat Pipe

Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. To copy otherwise, to republish, to post on servers or to redistribute to lists, requires prior specific permission. Acknowledgement I would like to thank my research adviser Prof. Albert P. Pisano for his guidance and support. My thanks are due to Matthew W. Chan for his help with the coherent porous silicon experiments. I also appreciate the help of members of the Berkeley Mechanical Engineering and Design (BMAD) laboratory, staff of the Berkeley Sensor and Actuator Center (BSAC), and staff of the Berkeley microfabrication laboratory. I am most grateful to my family for their constant help and support. The electronics industry desires compact high-heat-flux thermal management solutions for keeping device junction temperatures within the safe operating limit. Loop heat pipes (LHPs) are passive phase change-based thermal transport systems that can satisfy these demands, provided they can be miniaturized to fit inside compact devices and components. This can be achieved by fabricating LHPs on silicon and Pyrex wafers using MEMS microfabrication techniques. Although most of the LHP components can be easily implemented on planar substrates, the wicking structure is a major stumbling block in the successful implementation of these MEMS loop heat pipes. This report presents the design and fabrication aspects of a coherent porous silicon (CPS)-based micro-columnated wicking structure for a micro-columnated loop 2 heat pipe (µCLHP). The micro-columnated wick has a vertically-wicking dual-scale topology, with the primary wick fabricated out of CPS and the secondary wick etched onto a capping wafer. This design allows for enhanced device heat flux carrying capacities due to the ability to independently optimize the capillary pumping pressures and thin-film evaporation characteristics of the wick. CPS is obtained by illuminated electrochemical etching of silicon. The theory and experimental methods used to obtain CPS are explained, an experimental etching setup is designed, and preliminary etching results are reported. A detailed fabrication process flow for the µCLHP device is outlined, and the potential issues that could arise during its implementation are discussed.