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Laser Silicon Welding of Folded MEMS IMU Structures for Structural Rigidity Enhancement
Keywords: Silicon Welding, Folded MEMS, Inertial Measurement Unit (IMU)
A major challenge with developing a chip-level IMU is combining high-performance sensors to detect motion along independent axes with minimal cross-axis sensitivity and crosstalk. Folded MEMS IMU structures have previously been developed in both cubic and pyramidic forms with accelerometers and gyroscopes located on each sidewall. A comparison is made between epoxy bonding and a novel laser silicon fusion process on pyramidic IMU devices. Demonstration of silicon welding has recently been accomplished using resistive heating, indicating feasibility of silicon fusion is possible. However resistive welding of silicon for the application of folded MEMS structures is challenging due to the large size of the sidewall latches compared to the features previously welded by joule heating. Laser fusion has thus been explored using a 12.5 W copper vapor laser focused onto the sidewall edges with a beam size of ~150 μm resulting in a power density of ~700 W/mm². Temperature at the beam location rises above the 1414°C melting point of silicon and creates a permanent bond after rapid cooling. Comparing the epoxy-bonding method used for prior structures, mechanical properties of folded MEMS pyramids bonded with both methods were analyzed. Vibration testing was performed from 20–20000 Hz with 6 g of maximum acceleration and shock testing was done showing survivability of up to 262 g of acceleration with a pulse width of 0.6 seconds. Structural response is detected with a laser vibrometer to measure the velocity of the top portion of the IMU pyramid while under test. A vibratory frequency response analysis of the epoxy-bonded structures indicates modes at 970 Hz, 3.80 kHz, 5.42 kHz, and 8.02 kHz. Performing the same test on an identical structure with fused silicon sidewalls shows that frequencies of all modes are minimized or shifted above 10 kHz, which is adequate for practical IMU applications.
Montgomery Chittenden Rivers, Ph.D. Candidate
University of California, Irvine
Irvine, CA

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