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A 5 x 5 array of 800 μm x 800 μm micro-mirrors was designed and fabricated in CMOS-MEMS. Electrothermal bimorphs with embedded high resistivity polysilicon resistors were used as open loop actuators. The length of the actuators was set to 430 μm to achieve a maximum mirror mechanical rotation angle of ±45°, based on previously measured bimorph radii of curvature. Mirror deflection angles up to -23° and +15° were demonstrated at up to 15 V DC on individual array elements. Resonant modes were observed at 63 Hz, 183 Hz and 286 Hz compared with 51 Hz, 192 Hz and 263 Hz by simulation. Characterization of a full array of mirrors was not performed due to the mechanical fracture of the actuators under manual handling. Subsequent designs will address mechanical robustness and fill factor by decoupling the design of the actuator and the mirror.
Modal analysis of the design was performed using the finite element method. Thermal conductances of structural elements were determined using finite element analysis and then simulated as lumped parameter elements in equivalent thermal circuits using Spectre. DC and AC simulations were performed to assess the time constants and cross-talk of the actuators.
Post-CMOS processing of the mirrors consisted of direct write backside lithography, backside DRIE-Si etch with optical endpoint to target mirror thickness, frontside oxide etch and front-side silicon release etch. The structural dimensions resulted in a mechanically weak chip structure after silicon release etch. To compensate for this weakness a post-CMOS process compatible chip carrier was designed and fabricated. The mirror-array chip was mounted in the chip carrier prior to front-side silicon release etch and secured using SU8 epoxy. The chip remained in the carrier for final packaging and
test.
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