Fabrication of high temperature surface acoustic wave devices for sensor applications

Surface acoustic devices have been shown to be suitable not only for signal processing but also for sensor applications. In this paper high temperature surface acoustic wave devices based on gallium orthophosphate have been fabricated, using a lift-off technique and tested for high frequency applications at temperatures up to 600 ºC. The measured S-parameter (S11) has been used to study the mass loading effect of the platinum electrodes and turnover temperature of GaPO4 with a 5? cut. The analysis of these results shows that the mass loading effect can be used to predict the desired resonant frequency of the SAW devices. Also two different adhesion layers for Pt metallisation were studied. Our results show that Zirconium is a more suitable under layer than Titanium

High-temperature 434 MHz surface acoustic wave devices based on GaPO₄

Research into surface acoustic wave (SAW) devices began in the early 1970s and led to the development of high performance, small size and high reproducability devices. Much research has now been done on the application of such devices to consumer electronics, process monitoring and communication systems. The use of novel materials such as gallium phosphate (GaPO4), extends the operating temperature of the elements. SAW devices based on this material operating at 434 MHz up to 800C, can be used for passive wireless sensor applications. Interdigital transducer (IDT) devices with Platinum/ Zirconium metallization and 1.4 micron finger-gap ratio of 1:1 have been fabricated using direct-write e-beam lithography and a lift-off process. The performance and long term stability of these devices has been studied, and the results are reported in this paper