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Keywords: MEMS, Copper, XeF2
Many types of MEMS devices have been fabricated through electro- or electroless-plating of metals, such as Ni. Typically, this is accomplished using LIGA or LIGA-like processes where the metal is plated into a mold fabricated through patterning a photoimageable polymeric layer. After plating, the polymeric mold is removed though chemical etching or ashing, leaving the freestanding metal structure. Through the advent of TSV technology, techniques have been developed for plating Cu into DRIE etched vias in Si wafers. This technology has matured to the point where arrays of defect free plated Cu vias can be realized across the surface of the Si wafer. TSVs generally require small diameter Cu posts extending through the depth of the Si wafer between a bond pad on each surface. However, this technology can be modified to filling other types of DRIE etched features with plated Cu to realize complex 3D Cu microstructures in and on the Si wafer. Furthermore, the Si die containing these Cu structures can be eutectically bonded to other surfaces, such as pads on printed circuit boards. Then the Cu structure can be released from the bulk Si though exposing it to XeF2 gas in a low vacuum. This gas chemically converts solid Si to gaseous SiF2, but is nonreactive with most other materials, including Cu, SiO2 and E-glass. After the bulk Si has been removed through this process, the free standing Cu microstructure remains, attached to the substrate to which it was eutectically bonded. A variety of MEMS devices can be realized with this technique, such as antennas, inductors, air gap capacitors, sensors and electrostatic actuators.
Charles Ellis, Director
Auburn University
Auburn, AL

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