A simple out-of-plane capacitive MEMS accelerometer utilizing lateral and vertical electrodes for differential sensing

This paper presents an out-of-plane (z-axis) accelerometer, which incorporates the use of two different MEMS capacitive electrode structures in combination for implementing a linear closed-loop system. During the implementation, the complexity of the design and fabrication steps of the sensing element is kept at a minimum. The proposed accelerometer uses capacitive MEMS sensing element fabricated with a 4-mask process. This sensing element includes a comb finger type lateral electrode and a vertical parallel plate electrode placed beneath the proof mass. During the closed-loop operation, the proof mass is electrostatically pulled down towards the substrate so that the lateral comb electrode can be used as a varying overlap type capacitor. Thus, the bottom and the comb electrodes form a "hybrid" differential capacitor pair, which is suitable to use with a closed-loop, force rebalancing readout circuit such as in [1], and acceleration can be sensed linearly in z-axis with low zero-g offset.