The simulation, implementation, and experimental study of ultraonic sensors made with micromachining techniques on silicon wafers is discussed. The theoretical model of the sensor is based on a lumped-element equivalent circuit and takes intoconsideration the customary components of a condenser microphone including stiffness and friction of the air gap. The si,ulation results show the effect of the frictional damping on the tension of the membrane and by the air gap stiffness, respectively, and separated by a resistive drop-off region. Depending on the amount of resistive damping, either of the restoring forces can control the resonance frequency. An implementation of the microphone consists of a membrane chip with a Si_3N_4 membrane and a backplate chip with holes or slits bonded to the membrane chip. Experimental results for the sensitivity and frequency response of a two-chip silicon microphone are presented. The microphone shows an almost constant sensitivity of about 1.6mV/Pa in the frequency range up to 180 kHz. finally, the micromechanical processing steps of a new single-chip, ultrasonic sensor not yet implemented are discussed.
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