High temperature LGS SAW gas sensor

Abstract

Novel surface acoustic wave (SAW) devices using the recent langasite (LGS) family of crystals have been designed, fabricated, and tested for high temperature (up to 750 °C) gas sensor applications. The devices were fabricated using platinum (Pt) and palladium (Pd) thin film technology as electrodes and sensing films to withstand temperatures in excess of 1000 °C. Combinations of platinum and platinum/tungsten trioxide (Pt/WO 3) films have been used in the detection of C 2H 4 in N 2 (C 2H 4/N 2). Original all palladium metallic structures have been employed in the detection of H 2 in a N 2 carrier gas (H 2/N 2). SAW resonator structures using these films have been fabricated and continuously cycled from 250 to 750 °C for periods up to 3 weeks, with degradations in the |S 21| response up to 7 dB. Constantly held at 250 °C for 12 weeks, the maximum degradation in the |S 21| observed dropped to 3 dB. In order to perform the required gas testing, a high temperature, air sealed, stainless steel, dual chamber has been designed and fabricated to house these devices. The two-port SAW resonator structures fabricated with Pt, Pt/WO 3, and the original all-Pd films mentioned above have been tested with exposures to H 2/N 2 and C 2H 4/N 2 in the temperature range of 250–450 °C. Frequency variations up to 10 kHz for the 167 MHz high temperature SAW sensors were measured. The high temperature LGS SAW devices and experiments reported in this work show the capability of these devices to withstand prolonged exposure to temperatures ranging from 250 to 750 °C and to perform as high temperature gas sensors.