Generation of surface acoustic waves for general sensing applications

Abstract

The technology of surface acoustic waves (SAW) is widely used in a number of applications, such as detection of surface cracks or flaws, touch-panels, and SAW filters. The propagation of SAWs on the surface of solid materials has been used to detect hidden cracks or other surface discontinuities in a medium. They are also readily attenuated when a soft object touches the surface of medium. For a piezoelectric material, SAWs can be generated using a transducer of interdigitated electrodes fabricated on the surface [Herbert Matthews]. However, most materials are non-piezoelectric. A method to produce SAWs on a non-piezoelectric material is to convert from bulk acoustic waves using the equivalence of SnelPs law in Optics. To satisfy the Snell's law, longitudinal bulk wave propagating in the acoustic prism must be slower than the SAWs on the non-piezoelectric material. This posts a challenge because the SAW velocity is approximately 60% of longitudinal wave velocity in the same material. It is very hard to find a material in which the longitudinal wave velocity is slow and the propagation loss is acceptable. Lucite is identified as meeting the low longitudinal wave velocity requirement. Acoustic right-triangle prisms with angles satisfying the Snell's law are made of Lucite. One right-angle face is bonded with a piezoelectric transducer and the hypotenuse face is bonded to a substrate such as soda-lime glass. Another transducer-prism assembly is also bonded to the glass to detect SAWs. The total loss from the transmitter to the receiver is measured. The SAW velocity is also measured and compared with calculated value. Besides Lucite, we search for other acoustic prism materials that have better CTE match and better acoustic impedance match to both the glass and the transducer. Bismuth and liquid crystal polymer (LCP) are possible candidates.