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 200mm 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 V/Pa when using 100 mm 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.

Equivalent electrical circuits for electroacoustic MEMS design: a review

“…Although mainly capacitive, a few microphones from the literature use an electrodynamic transduction and an electrical equivalent circuit. In [30][31][32], a MEMS microphone based on the electrodynamic transduction, using a coil with a steady current instead of a permanent magnet is designed. The mechanical circuit is represented by the apparent stiffness and the effective mass and the mechanical damping is neglected.…”

“…S parameters are related to the power transferred and reflected by the device, using a termination impedance Z 0 = 50 Ω. S 21 coefficients for the closed and open switch cases are derived in equations ( 29) and (30), where Z S1 and Z S2 are the impedance of the first and second filter respectively, written as (30) and where Z Ls = jωL s and Y Ls = Z −1 Ls . Parameters S 21,o and S 21,c are calculated and depicted in figure 27.…”