MEMS microphones with narrow sensitivity distribution

This paper presents a method for calibrating MEMS microphones after the fabrication process. This allows compensating process tolerances and packaging stress of the capacitive membrane-backplate system. A programmable MEMS microphone with variable bias voltage and variable gain was developed and fabricated. The sensitivity of this microphone can be adjusted within a range of 11 dB. The adjustability was used to trim the sensitivity towards a specified value and to reduce the variation of the sensitivity. In mass production a tight sensitivity of ±1 dB could be reached, for a specified sensitivity of −38 dBV/Pa @ 1 kHz. In addition the microphone signal to noise ratio was increased to values above 66 dB(A) by choosing high bias voltages and matching gain calibration. The relationship between bias supply voltage and the non-linear dynamics of a double backplate sensor is explained. The influence of programming on the sensitivity, noise, SNR, THD and frequency response was investigated.