A SURFACE MICROMACHINED ELECTROSTATIC ULTRASONIC AIR TRANSDUCER

Airborne ultrasound has many applications such as, ranging, nondestruc tive evaluation, gas flow measurement, and acoustic microscopy. This p aper investigates the generation and detection of ultrasound in air at a few MHz. Conventional plane piston lead zirconium titanate (PZT) ba sed transducers perform poorly for this application due to the lack of proper matching layer materials. Electrostatic, or capacitive, transd ucers promise higher efficiency and broader bandwidth performance, The device structure in this work consists of a capacitor where one plate is a circular silicon nitride membrane coated with gold and the other is a rigid silicon substrate. By applying a voltage between the membr ane and the silicon substrate, an electrostatic force is exerted on th e membrane which sets it in motion, thus generating a sound wave in ai r. Presented here is an electrical equivalent circuit model for electr ostatic transducers which is based on the early work of Mason [1]. The electrostatic transducers were designed and constructed for operation at 1.8 and 4.6 MHz. The transducers were fabricated using standard mi cromachining techniques. An optical interferometer was used to measure the peak displacement of the 1.8 MHz electrostatic transducer at 230 Angstrom/V. A transmit-receive system was built using two electrostati c transducers. The system had a signal to noise ratio of 34 dB at a tr ansducer separation of 1 cm. Each transducer had a 3-dB bandwidth of 2 0%, and a one-way insertion loss of 26 dB. There is excellent agreemen t between the measured device performance and theoretical predictions.