Multimode digital control of a suspended polysilicon microstructure

Digital control of a suspended 350 μm×380 μm×1.6 μm-thick surface-micromachined polysilicon plate is demonstrated in three degrees of freedom, with application to multimode accelerometers, vibratory rate gyroscopes, and actively positioned micromirrors. Plate displacement about the 2.2 μm nominal position above the substrate is measured with shielded capacitive sensors connected to CMOS buffer circuits fabricated adjacent to the microstructure. Four micromechanical sigma-delta loops are used to control eight electrostatic actuators that drive the plate vertically (z) and in out-of-plane rotation (θ and φ). Resonant frequencies are 2.7 kHz for the θ rotational mode and 3.7 kHz for both z and φ modes. The system is evaluated using a mixed mechanical/electromechanical/circuit simulation in SPICE. Closed-loop transient simulation of a 150-Hz square-wave position input signal is in good agreement with experimental results. Squeeze-film damping limits the plate slew rate to 0.83 mm/s in air. Position is controlled to within ±25 mm, being limited by quantization noise at the 50 kHz sampling rate