Effects of Viscus Damping on MEMS Sound Sensors with Integrated Comb Finger Capacitors¹

The use of MEMS based sound sensors as miniature microphones operating in the audible frequency range is growing. In this work, MEMS sensors were designed to operate at their resonant frequency for applications in finding the direction of sound. The MEMS sensor consists of a 3x5 mm² cone shaped cantilever attached to the substrate at the narrow end. The thickness of the cantilever is about 25 μm. For electronic readout of the sensor response, an interdigitated comb finger capacitor was integrated at the far end. Two sensors were fabricated with 5 μm and 10 μm gaps between moving and fixed combs to assess how it affects sensor response. Measurements carried out using a laser vibrometer and electronic readout using comb finger capacitors found that the resonant frequency is at around 850 Hz. However, the peak widths and amplitudes were affected by the gap of the comb fingers indicating viscus damping when they interact with air. Finite element simulations, performed using COMSOL Multiphysics, found that damping is generated by both drag at the cantilever surface as well as movement of air between the comb fingers. In this presentation, measured and simulated responses of the two sensors will be addressed.