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Permanent Attachment of Silicon Structures via Joule Heat Induced Welding
Keywords: Silicon Welding, Gyroscope , MEMS Assembly
We introduce a new method for the permanent attachment of assembled silicon microstructures using the precise application of current for localized welding. The attachment technique for planar SOI structures was demonstrated using an assembly mechanism that allows the post-release adjustment of micromachined gaps. The concept utilizes a mechanical assembly technique that takes advantage of the motion reduction experienced by the individual beams of a folded suspension ultimately allowing sub-micron position control of released structures. The proposed mechanism was incorporated into the sense mode of a symmetrically decoupled MEMS tuning fork gyroscope in order to demonstrate its potential for the realization of high performance inertial sensors. Two types of mechanisms were implemented resulting in 7.8 and 8.8 % motion reduction allowing capacitive gaps of 7.5 μm to be adjusted to 1.67 μm; the gaps can be further reduced by alternate suspension designs and implemented on arbitrarily thick layers. After assembly, the mobile electrode structure was rigidly attached to the anchor by locally melting the interface via a controlled current. The weld is achieved by applying 15 V with a current limit of 3 mA across the latching mechanism resulting in the permanent attachment of the two structures. Thus, when the desired gap is achieved, the previously released anchor can be fixed to eliminate any potential spurious motion of the electrodes. Our results demonstrate feasibility of the approach, which can be extended beyond planar devices to include out of plane and die-level 3-D assembled structures that typically rely on epoxy bonding or solder assembly to maintain their shape.
Adam R. Schofield,
System Planning Corporation
Arlington, VA

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