Surface acoustic wave sensors for high-temperature applications

Results of investigations of surface acoustic wave (SAW) devices working at high temperatures are presented in this paper. The considered substrate materials are lithium niobate (LiNbO/sub 3/), quartz and langasite (La/sub 3/Ga/sub 5/SiO/sub 14/). In addition, electrode materials and assembly, interconnect and packaging strategies are discussed or examined. Delay lines on LiNbO/sub 3/ had low SAW attenuation up to approx. 550/spl deg/C. The frequency response disappeared completely at about 800/spl deg/C. Delay lines on quartz had low SAW attenuation up to 500/spl deg/C, a temperature being significantly below the phase transition from /spl alpha/- to /spl beta/-quartz at 573/spl deg/C. Delay lines on La/sub 3/Ga/sub 5/SiO/sub 14/ withstand extreme temperatures: a SAW response was observed up to destruction of the packages used in these tests, however, resulted in a degradation of the device performance starting at about 600/spl deg/C. An identification (ID) tag on La/sub 3/Ga/sub 5/SiO/sub 14/, could be used as a wireless passive thermometer for measuring high temperatures in industrial process control applications.