An integrated silicon capacitive microphone with frequency-modulated digital output

In this paper a new integrated capacitive silicon microphone with frequency-modulated digital output is described. The FM output can be processed directly by a digital system without the need of analog-to-digital conversion, which is normally required with capacitive microphones in digital systems. The integrated microphone comprises a polyimide structure fabricated directly on a silicon substrate, on which the oscillator has already been completed using a standard CMOS process. This is possible since the polyimide microphone process is fully IC-compatible. The capacitance of the microphone modulates the output frequency of the oscillator, and from experiments a sensitivity of 234 Hz/Pa is measured at a carrier frequency of 263 kHz. The frequency response is flat (±1 dB) within the measured range of 100 Hz to 15 kHz. The A-weighted noise is 4.5 Hz for a bandwidth of 15 kHz, yielding an equivalent noise level of the microphone of 60 dB(A) re. 20 μPa. The measurements have shown to be in good agreement with the theoretical models.