Modeling and Characterization of a Pull-in Free MEMS Microphone

In this study, we examine the feasibility of designing a MEMS microphone employing a levitation based electrode configuration. This electrode scheme enables capacitive MEMS sensors that could work for large bias voltages without pull-in failure. Our experiments and simulations indicate that it is possible to create robust sensors properly working at high DC voltages, which is not feasible for most of the conventional parallel plate electrode-based-microscale devices. In addition, the use of larger bias voltages will improve signal-to-noise ratios in MEMS sensors because it increases the signal relative to the noise in read-out circuits. This study presents the design, fabrication, and testing of a capacitive microphone, which is made of approximately 2 μm thick highly-doped polysilicon as a diaphragm. It has approximately 1 mm
²
surface area and incorporates interdigitated sensing electrodes on three of its sides. Right underneath these moving electrodes, there are fixed fingers being held at the same voltage potential as the moving electrodes and separated from them with a 2 μm thick air gap. The electronic output is obtained using a charge amplifier. Measured results obtained on three different microphone chips using bias voltages up to 200 volts indicate that pull-in failure is completely avoided. The sensitivity of this initial design was measured to be 16.1 mV/Pa at 200 V bias voltage, and the bandwidth was from 100 Hz to 4.9 kHz.