Micro-Fabricated Shells for Mechanical and Fluidic Interconnects

Three micro-fabricated shell processes are developed in this work, each tailored to a specific application. The first is a two-wafer molding process used to fabricate polysilicon shell suspensions that are very light and stiff. The second is a single crystal silicon fusion bonded process designed to fabricated thicker shells to be used as micro-needles that can withstand large forces. The third is a sacrificial silicon process that can fabricate shells from any material that can be deposited onto silicon and can withstand the sacrificial silicon etch. The development of these three processes, along with their application to the fabrication of mechanical and fluidic interconnects, is described.
A molding process, in which polysilicon is deposited into a hollow mold formed by temporarily bonding two wafers together, was developed to fabricate mechanical interconnects. Using this process, micro-fabricated suspensions for hard disk drives were produced. These suspensions ranged in size from 1.5 to 2 cm in length, 0.5 to 1 cm in width, 50 to 200 umin depth, with 5 to 15 um wall thicknesses. Because of the advantages of a shell design, these suspensions have measured resonant frequencies up to 43% higher than those of commercial stainless steel suspensions with similar dimensions. In addition, finite element results show that different suspension designs will increase these resonant frequencies even further. Fabricated suspensions have stiffnesses around 1 N/m, can withstand bending forces up to 5 mN, and can deflect over 5 mmbefore fracturing.
A single crystal silicon fusion bonded shell process was developed to fabricate thick shell structures to be used as strong micro-needles. In this process, two silicon wafers are fusion bonded together to form the top and bottom of a closed, shell structure. Using this process, micro-needles that are 4 mm long, 200 um wide, with wall thicknesses ranging from 25 to 75 um have been fabricated that can withstand bending moments up to 1.56 mNm, over six times larger than any silicon micro-needles previously reported. In addition, by coating the inside and outside with a thin polymer layer, silicon micro-needles have been fabricated that can continue to pump fluid, and even be extracted from tissue, with the silicon layer fractured.
A sacrificial silicon process was also developed to fabricate micro-needles out of materials with superior mechanical properties. In this proccss, a solid silicon lancet is coated with a structural layer. The sacrificial silicon is then etched away to form a shell structure. Using this process, extremely compliant polymer micro-needles have been fabricated with measured bending stiffnesses around 60 N/m. In addition, due to the tough material properties of polymers, these micro-needles can withstand completely reversed deflections of over 180" without rupturing. By terminating the sacrificial silicon etch before completion, silicon and polymer hybrid needles have also been fabricated. These micro-needles have the compliance of a polymer shell, with tip sharpness of a silicon micro-needle.
Albert P. Pisano
Luke P. Lee
Publication date: 
January 30, 2001
Publication type: 
Ph.D. Dissertation
Stupar, P. A. (2001). Micro-fabricated Shells for Mechanical and Fluidic Interconnects. United States: University of California, Berkeley.

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