An acoustic absorption film for SAW devices

Abstract

Surface acoustic wave (SAW) devices are used in several sensor applications, such as sensors for humidity, gas concentration, voltage and flow. The reflection of the surface acoustic waves at the edges of a SAW substrate causes interference with the main signal and degrades the characteristics of the device. Therefore, the edges of SAW substrates are often provided with wax or another material which absorbs the surface waves. SAW devices are being fabricated by sophisticated photolithographic processes, while the deposition techniques for the absorber material are mainly screen printing or manual painting. The development of new materials in IC technology gives opportunities for new acoustic absorption films. A very useful material is polyimide, a viscous organic material which maintains its absorptive acoustic properties after curing. Polyimide can be spin coated on the wafer and patterned by photolithographic techniques. Not only thin (0.2–1 μm) but also thick (up to 20 μm and more) polyimide layers can be fabricated, a feature which makes the material very attractive for use on SAW devices in the lower frequency range (80–200 MHz). The layers show good adhesion on the substrate and excellent absorbing properties. In this paper, the technology for fabricating polyimide acoustic absorption layers is described. The relationship between absorber configuration (length, thickness), frequency and the acoustic wave absorption is analysed. Experimental results of polyimide absorber films on STX quartz SAW devices with centre frequencies between 40 and 200 MHz are presented.