High frequency calibration of MEMS microphones using spherical N-waves

In the context of the scientific program SIMMIC supported by the French National Agency for Research (SIMI 9, ANR 2010 BLANC 0905 03), new wide band MEMS piezoresistive microphones have been designed and fabricated for weak shock wave measurements. The fabricated microphones have a high frequency resonance between 300 to 800 kHz depending on the membrane size. In order to characterize the frequency response of the fabricated sensors up to 1 MHz, new calibration methods based on an N-wave source were designed and tested. Short duration spherical N-waves can be generated by an electric spark source. To estimated a constant sensitivity coefficient, a known method is based on the estimation of the peak pressure from the lengthening of N-waves induced by non linear propagation. However, to obtain the sensitivity as a function of frequency, the output voltage must be compared to the incident pressure waveform, which must be accurately characterized. Taking advantage of recent works on the characterization of pressure N-waves generated by an electric spark source by means of optical methods, two calibration methods have been designed to obtain the frequency response. A method based on the comparison with pressure waveforms deduced from the analysis of schlieren images allowed to estimate the frequency response. A second method, based on a Mach-Zender optical interferometer, was found to be the best method to estimate the sensitivity of microphones up to 1 MHz. The methods were first tested by calibrating standard 1/8 inch condenser microphones. Then, frequency responses of different MEMS microphones prototypes were characterized to test different sensor designs. Results show that using a spark source and optical methods it is possible to calibrate sensors in the frequency range 10 kHz-1 MHz. The new calibration methods were used to improve the design of new high frequency MEMS pressure sensors.