Abstract
We present the design and characterization of a large-displacement thermal actuator fabricated in a conventional TSMC 0.35μm 2P4M CMOS process. The thermally-driven microstructure contains multi-layers of metal, silicon dioxide, and polysilicon, and is fabricated by two dry etching steps after completion of CMOS. To avoid thermal drift caused by change of ambient temperature, we adopt a capacitive sensing scheme that makes use of vertically sensed comb finger electrodes with a nominal sensing capacitance at 11 fF. The microactuator is characterized by static and dynamic measurements, with a measured out-of-plane motion up to 24 μm at 17 mW, a thermal time constant at 0.24 ms, and a mechanical resonant frequency at 25.2 kHz, By external measurement, we understand the relationship of supply power, ambient temperature and actuating displacement. We integrate the actuator and the capacitive sensing circuit within one chip. The measured results show that the device’s sensitivity is 4020 (V/m), and the measured minimum input-referred noise voltage of the sensing pre-amplifier is 5.9 μV/rt.Hz, corresponding to a minimum input-referred noise displacement of 0.15 nm. Because the open-loop operation can’t handle external disturbance, so we design a closed-loop control system to promote the device’s stability.