Capacitive polysilicon resonator with MOS detection circuit

Abstract A resonator device is presented in which a vibrating polysilicon microbridge is combined with a detection circuit. The oscillations of the polysilicon beam are electrostatically excited and sensed. However, the capacitance changes for the vibration detection are extremely small (in the fF range). Thus, the oscillating element is integrated with a fixed polysilicon capacitor in a capacitive voltage divider whose output signal drives a NMOS inverter. SPICE simulation of the complete detection circuit yields an a.c. output signal of the order of mV for capacitance changes of the vibrating microbridge in the fF range. These values agree quite well with frequency response experiments where an a.c. output of several mV is observed in direct measurements. From gain/phase measurements, a resonance peak at about 120 kHz results for a polysilicon microbridge 300 μm long, 25 μm wide and 1 μm thick. The integration of a surface-micromachined capacitive polysilicon microbridge together with MOS circuitry for signal detection and amplification enables the realization of resonant sensors, which are well-known tools for accurate measurements.