The first low voltage, low noise differential silicon microphone, technology development and measurement results

The first differential silicon microphone is presented. This capacitive working device consists of two backplates with a membrane in between. Due to the balanced arrangement the air gap can be minimized. Thus, a higher electrical field and sensitivity can be achieved for low voltages. A dedicated process sequence has been developed in order to get the optimum mechanical and electrical properties for all structural layers. Furthermore, a sandwich structure has been developed to achieve a reproducible, very sensitive microphone membrane with a thickness of only 0.5 μm and a stress of 45 MPa. The total sensitivity for a bias of 1.5 V was measured to be 13 mV/Pa and the A-weighted equivalent input noise was measured to be 22.5 dB SPLA. This noise level does not correspond to the simulations where only 21.0 dB SPLA have been predicted. Modeling of the membrane using distributed resistors shows that the lumped element resistor used for the membrane resistance has been underestimated and thus, the noise level. The upper limit of the dynamic range has been determined to be 118 dB SPL and the total harmonic distortion at 80 dB SPL is below 0.26%.