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Controlled Latching in Sequential Assembly of 3D MEMS
Keywords: 3D MEMS, Assembly, Latching
This paper reports the design, modeling and implementation of a controlled latching method for use in the assembly of three-dimensional MEMS structures from two-dimensional plates. Here individually-addressable polymer pads serve as glue to attach 2D plates together. When the plates are properly positioned, the pads are melted by resistive heating, adhere to the adjacent surface, and then solidify. The individual addressability permits selective latching of components in a sequential assembly process. This technique was demonstrated by assembling micro-scale supercapacitors. The devices were fabricated using a 15m thick SU-8 structural layer on top of a 0.6um-thick gold layer. The gold layer was patterned with rectangular electrodes, deformable hinges, resistive heaters and current loops for actuation by Lorentz forces. The SU-8 layer was patterned into rectangular frames for the electrodes (1000m x 700m) and elliptical holes for the adhesive pads. The photoresist (AZ4620) pads were patterned on top of the heaters, inside the holes. After a dry etching release step, the electrodes were folded on top of each other, bringing the corresponding photoresist pads together. A current was then driven through the heaters, causing the pads to melt and fuse together. Discontinuing the current allowed the fused pads to solidify, holding the two SU-8 frames together. Thermal modeling of the pads predicted that a minimum current of 23mA would be required to melt them, assuming a melting temperature of 180oC. The melting current was found, experimentally, to be 29mA with a standard deviation of 2mA. The latching strength of the adhesive pads was tested by applying Lorentz force to the current loops. A force of 6.8N, corresponding to the maximum current that the current loops could sustain without damage, was insufficient to separate the latched segments. This provides a lower bound on the attachment strength.
Nader S. Shaar, Graduate Research Assistant
Massachusetts Institute of Technology
Cambridge, MA

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