A low-voltage silicon condenser microphone for hearing instrument applications

Silicon microphones have been the subject of investigations since the early 1980s. Due to their poor performance, no silicon microphone for hearing instrument applications has been commercially available until today. Usually the sensitivity of the electromechanical transducer is too low. Thus the input-related noise of the following preamplifier stage becomes dominant and results in a high equivalent input-related noise. Here a silicon condenser microphone with the potential for hearing instrument applications will be presented. To get the best properties for the different mechanical parts, e.g., membrane and back plate, a dedicated process sequence has been developed. Therefore, circuitry and mechanical parts have to be produced separately and mounted later in a stacking process. The microphone has a 2×2 mm2, 0.4 µm-thick membrane and an air gap of 1.0 µm. Wafers with different membrane stress have been produced. The microphones have been acoustically, mechanically, and electrically characterized, partly on wafer-scale, and compared with an acoustical/electrical lumped element model. The sensitivity for the low-stress microphones using a 1.5 V power supply is about 7.0 mV/Pa and the input related A-weighted noise is 23 dB SPL, including the preamplifier. Due to a conservative layout, the parasitic capacitance is about 50%. An increase of 2–3 mV/Pa sensitivity and hence 3 dB SPL less noise can therefore be achieved by design optimization.