BULK AND SURFACE ACOUSTIC WAVE SENSORS

Sensors involving bulk and surface acoustic wave (SAW) interactions are investigated. In particular, the design and fabrication of bulk ultrasonic sensors employing piezoelectric films on thin metal sheets and SAW-based force and vapor sensors are described.; Piezoelectric zinc oxide films typically 10 microns thick are deposited in a planar magnetron radio-frequency (RF) sputtering system onto 50 micron thick stainless steel shim stock to make ultrasonic receiving transducers for nondestructive evaluation (NDE) and acoustic emission (AE) applications. Theoretical analysis of these transducers predicts a broad, smooth frequency response, in agreement with calculations based on the Mason equivalent circuit. A measured frequency response that is flat from 3 MHz to 22 MHz is obtained.; Some useful modifications resulting from connecting the transducer to an active semiconductor device are considered. If an active element such as a dual-gate DMOS transistor is used, amplitude control and mixing can be accomplished conveniently in the transducer itself. One can also make an array with alterable directionality or a large aperture transducer which responds to acoustic power incident upon it regardless of the phase of the arrival at the individual transducing elements.; Important aspects in the design of single-mode SAW oscillators, such as mode-selection, bulk-mode suppression, and temperature compensation, are considered. Experimental results on SAW oscillator force sensors fabricated on LiNbO(,3) and ZnO-on-glass are given.; An integrated silicon-based vapor sensor is realized with a ZnO-on-Si SAW oscillator containing an absorptive polymer located on a thinned membrane region of the device. If the membrane thickness is comparable to or smaller than a SAW wavelength, wave energy is present at both membrane surfaces. Thus the SAW transducer and associated amplifiers can be protected by an enclosure, leaving only the polymer exposed to the ambient atmosphere. Experiments on a 30 MHz, ZnO-on-Si thin-membrane temperature-compensated SAW oscillator for vapor sensing are described.