A MEMS-based shifted membrane electrodynamic microsensor for microphone applications

Abstract

In this paper we present a multidisciplinary modeling of a MEMS-based electrodynamic microsensor, when an additional vertical offset is defined, aiming acoustic applications field. The principle is based on the use of two planar inductors, fixed outer and suspended inner. When a DC current is made to flow through the outer inductor, a magnetic field is produced within the suspended inner one, located on a membrane top. In our modeling, the magnetic field curve, as a function of the vertical fluctuation magnitude, shows that the radial component was maximum and stationary for a specific vertical location. We demonstrate in this paper that the dynamic response of the electrodynamic microsensor was very appropriate for acting as a microphone when the membrane is shifted to a certain vertical position, which represents an improvement of the microsensor's basic design. Thus, a proposed technological method to ensure this offset of the inner inductor, by using wafer bonding method, is discussed. On this basis, the mechanical and electrical modeling for the new microphone design was performed using both analytic and Finite Element Method. Firstly, the resonance frequency was set around 1.6 kHz, in the middle of the acoustic band (20 Hz – 20 kHz), then the optimal location of the inner average spiral was evaluated to be around 200µm away from the diaphragm edge. The overall dynamic sensitivity was evaluated by coupling the lumped elements from different domains interfering during the microphone function. Dynamic sensitivity was found to be 6.3 μV/Pa when using 100 µm for both gap and vertical offset. In conclusion, a bandwidth of 37.6 Hz to 26.5 kHz has been found which is wider compared to some conventional microphones.