Optimization of Induced Voltage From CMOS-Compatible MEMS Electrodynamic Microphone With Coaxial Planar Inductances

This paper presents an improved design of the electrodynamic CMOS-MEMS microphone. The studied microphone is based on two integrated concentric planar inductors, a fixed outer one and an inner one attached on a flexible vibrating diaphragm. First, the magnetic field, generated by the outer inductor, was evaluated. Unlike conventional electrodynamic microphones based on Faraday's law, the induced voltage was found to be proportional to the product of the diaphragm displacement and its velocity. Even though the induced output voltage is in the range of a few microvolts, and its frequency is doubled compared to that of the incident acoustic wave. This result pushed us to adjust and improve the basic principle of this kind of microphone by trying to generate a vertical downward shift of the microphone diaphragm while maintaining the COMS compatibility. The Lorentz force appears to be a possible alternative to the wafer bonding, as it will be automatically generated when we bias the inner inductor by a dc current. First, this paper shows that the induced voltage will be linear for a vertical fluctuation magnitude lower than 10 μm with an induced voltage in the range of several microvolts. Using the developed vertical offset technique, we show that induced voltages can be increased by a factor of 10 and the dynamic response of the electrodynamic microsensor will be appropriate for a use as a microphone.