Low-noise real-time measurement of the position of movable structures in MEMS

A low-noise, low-perturbing electrostatic position measurement system for capacitive MEMS sensors is described.A small-amplitude, high-frequency voltage signal is applied between the movable and fixed plates of the sensing capacitance. The resulting current is modulated by the external forces applied to the structure. This high-frequency-shift makes it possible to filter low-noise contributes and minimize readout electrostatic perturbations on the device.Experimental results are reported for a comb-type MEMS test structure with quasi-static, sinusoidal, and impulsive forces. Resolution and long-term stability of nanometric values have been obtained.