LOW-PRESSURE ACOUSTIC SENSORS FOR AIRBORNE SOUND WITH PIEZORESISTIVE MONOCRYSTALLINE SILICON AND ELECTROCHEMICALLY ETCHED DIAPHRAGMS

In this paper experimental results and theoretical considerations of an acoustic silicon sensor for airborne sound, which is based on boron-implanted monocrystalline piezoresistors located on an electrochemically etched silicon diaphragm, are presented. The bandwidth of the sensor is nearly 20 kHz (full audio bandwidth), a maximum sensitivity of 80 mu V Pa-1 (bias voltage, 8 V) was achieved and a lowest equivalent noise level of about 61 dB(A) was measured. The membrane thickness is about 1.3 mu m; the membrane area amounts to 1 mm(2). Furthermore, a good reproducibility and linearity could be obtained. The weak sensitivity can mainly be explained by a residual inherent tensile stress in the silicon-silicon dioxide-silicon nitride layer system of the diaphragm and by the large geometric dimensions of the resistors.