サブミクロンギャップ静電容量型 MEMS 加速度センサアレイの熱機械ノイズ評価

抄録

The thermomechanical noise of a capacitive MEMS accelerometer array was evaluated by measuring the impedance around the resonant frequency. The unit accelerometer of the array was 80 μm square, designed as a scale-down of a conventional single-axis capacitive accelerometer. Since the size effect shows the capacitance sensitivity per unit volume increases by the proportional downsizing, we made a 10-by-10 array of the unit accelerometer. The accelerometer was fabricated from silicon-on-insulator wafer of 5-μm in device layer thickness. We used electron beam lithography and deep reactive ion etching to pattern 300-nm-wide gaps. We proved that the capacitance sensitivity of the array is 0.99 fF/g, which is as same as that of a single accelerometer of the same device area. By measuring the electrical impedance around the resonant frequency, the damping coefficient was estimated using an electrical equivalent circuit model. The thermomechanical noise of the accelerometer can be reduced below 10 μg/√Hz by the vacuum encapsulation at 100 Pa, which is low enough for instrumentation applications.