A high-sensitivity z-axis capacitive silicon microaccelerometer with a torsional suspension

This paper presents a new z-axis high-sensitivity silicon-micromachined capacitive accelerometer fabricated using a three-mask dissolved-wafer process (DWP). It employs capacitive sensing using overlap-area variations between comb electrodes and a torsional suspension system to provide high sensitivity without compromising bandwidth, full-scale range, or the pull-in voltage ceiling. Excellent electrical sensitivity is obtained by using high-aspect-ratio comb fingers with narrow air gaps of 2 /spl mu/m and a large overlap area of 12 /spl mu/m /spl times/300 /spl mu/m. Torsional suspension beams 150 /spl mu/m long with a cross-sectional area of 12 /spl mu/m /spl times/3 /spl mu/m are used to improve the mechanical gain. Simulations of the capacitance between sense fingers show a highly linear region over a wide 14-/spl mu/m tip deflection range. Accelerometers were fabricated and yielded sensitivities of 263-300 mV/g, a nonlinearity less than 0.2% over a range of -4 to +3 g, a full-scale range of -4 to +6 g, and pull-in voltages greater than 8 V. A 3 dB cutoff frequency of 35 Hz was measured in air. The calculated thermomechanical noise in the sensor is 0.28 mg over this bandwidth.