MEMS-based high-frequency vibration sensors

We present the development of two vibration sensors, one designed for out-of-plane sensing and one designed for in-plane sensing. Both sensors, fabricated on a 5 mm square MEMS (microelectromechanical system) chip, are variable capacitors with one electrode of the capacitor suspended by springs and the other electrode fixed to the chip. When a bias voltage is placed across the capacitor and the sensor is excited, the spring-suspended structure vibrates with respect to the fixed structure and induces a time-varying signal. Both sensors have resonant frequencies near 200 kHz. The out-of-plane sensor employs an open grill design rather than square etch holes to reduce the effects of squeeze film damping and increase sensitivity. The in-plane sensor uses a finger-type design which attempts to take advantage of the relatively high damping effect of air in the out-of-plane direction in order to suppress unwanted out-of-plane motion. Finite element simulations estimating the resonant frequency are presented and compared to characterization measurements of the resonant frequency. Characterization measurements of the sensitivity of each device are also presented. In addition, we report on a laboratory experiment in which we measure the response of each sensor to steady-state excitation