A surface micromachined electrostatic ultrasonic air transducer

摘要

Airborne ultrasound has many applications such as, ranging,nnondestructive evaluation, gas flow measurement, and acousticnmicroscopy. This paper investigates the generation and detection ofnultrasound in air at a few MHz. Conventional plane piston lead zirconiumntitanate (PZT) based transducers perform poorly for this application duento the lack of proper matching layer materials. Electrostatic, orncapacitive, transducers promise higher efficiency and broader bandwidthnperformance. The device structure in this work consists of a capacitornwhere one plate is a circular silicon nitride membrane coated with goldnand the other is a rigid silicon substrate. By applying a voltagenbetween the membrane and the silicon substrate, an electrostatic forcenis exerted on the membrane which sets it in motion, thus generating ansound wave in air. Presented here is an electrical equivalent circuitnmodel for electrostatic transducers which is based on the early work ofnMason (1942). The electrostatic transducers were designed andnconstructed for operation at 1.8 and 4.6 MHz. The transducers werenfabricated using standard micromachining techniques. An opticalninterferometer was used to measure the peak displacement of the 1.8 MHznelectrostatic transducer at 230 Å/V. A transmit-receive system wasnbuilt using two electrostatic transducers. The system had a signal tonnoise ratio of 34 dB at a transducer separation of 1 cm. Each transducernhad a 3-dB bandwidth of 20%, and a one-way insertion loss of 26 dB.nThere is excellent agreement between the measured device performance andntheoretical predictions