An impedance spectroscopic study of MEMS microphones

In this paper we present a study of the mechanical properties of MEMS microphones using impedance spectroscopy. An ideal microphone model was used to derive the equation of motion of the diaphragm and the electrical impedance of the microphone as a function of frequency. Both measurements and model show that the phase of the impedance is sensitive to the changes in the mechanical parameters of the microphone. The model predicts the presence of a peak near the mechanical resonance in the phase vs. frequency graph with the following properties: increasing the bias increases the peak height; increasing the diaphragm stiffness shifts the peak position to a higher-frequency; and decreasing the relative damping widens the peak width. This behavior is confirmed with actual measurements of the impedance. From the close correlation between measurement and model we deduce the actual damping of the diaphragm to be approximately 1 × 10^-4 kg/s.