A area variable capacitive microaccelerometer with force-balancing electrodes

A surface micromachined accelerometer which senses a inertial motion with an area variation is developed. The grid-type planar mass of 7 /spl mu/m thick polysilicon is supported by four thin beams and suspended above a Si substrate with a 1.5 /spl mu/m air gap. The motion sensing electrodes are formed on the substrate. The accelerometer is designed as an interdigital rib structure that has a differential capacitor arrangement. The movable electrodes are mounted on the mass and the pairs of the stationary electrodes are patterned on the substrate. In the accelerometer that has comb-type movable electrodes, the mechanical stress and the electrical pulling effects between a movable electrodes and the fixed electrodes occur. However this grid-type structure can have a large area variation in a small area relatively without stress and pulling, high sensitivity can be achieved. In order to improve the dynamic range and a linearity, a pair of comb shape force-balancing electrodes are implemented on both sides of the mass. The force-balancing electrodes are made of the same layer as the mass and anchored on a Si substrate. When acceleration is applied in the lateral direction, the difference of capacitance results from the area variation between the two capacitors and is measured using a charge amplifier. The accelerometer has a sensitivity of 95 mV/g and a bandwidth of DC/spl sim/1 kHz. A resolution of 3 mg and a non-linearity of 0.1%(F.S) is achieved for a measurement range of /spl plusmn/5 g.