A surface micromachined accelerometer with integrated CMOS detection circuitry

Abstract

A surface micromachined, capacitive accelerometer described here integrates the mechanical sensing microstructures with CMOS detection circuits. The capacitive sensing structure consists of two polysilicon layers with the sensing and feedback electrodes underneath and the suspended plate as the proof mass. The sensing axis is perpendicular to the substrate. A full capacitive bridge is formed to translate a mechanical displacement signal into an electrical voltage signal. Electrostatic feedback is used to counteract the proof mass displacement due to acceleration. Interdigitated fingers are employed to generate a levitation force using the asymmetrical distribution of electrical fields. A unity gain buffer with low input capacitance is designed to actively drive the ground plane to minimize the parasitic capacitance. A S-D modulation technique is employed as the feedback control loop where the mechanical proof mass is used as the double integrator in the second-order S-D modulator. Further described is the accelerometer prototype fabrication using a typical double polysilicon surface micromachining process for microstructures and a 3 mm conventional CMOS process for the electronics. The total chip size is 2.5 mm×5 mm. The unity gain buffer has a gain of 0.9 kHz and 500 kHz bandwidth that is limited by the parasitic capacitance from the measurement setup. The gain of the variable gain amplifier (VGA) can vary from unity to 40 and is controlled externally. The accelerometer is first characterized in the open-loop, self-testing mode. The damping coefficient is measured to be 1.2×10-3 N/(m/s), which agrees with 1×10-3 N/(m/s) from theoretical analysis. The open-loop sensitivity of the accelerometer is 100 mV/g with a 100 mV driving voltage. The stiction of suspended microstructures is observed and various possible solutions are discussed.