Stability of Pt/Al2O3-based electrode langasite SAW sensors with Al2O3 capping layer and yttria-stabilized zirconia sensing layer

Surface acoustic wave (SAW) devices have been shown recently to be an appropriate technology for harsh-environment sensor applications, such as those in the automotive, aerospace, power plant, and metallurgic industries. Usages include temperature, pressure, torque, strain, gas, liquid, biochemical, and mass sensors in structural health monitoring, industrial process monitoring, environment checking and control, and condition based maintenance. For harsh-environment high-temperature applications, battery-free wireless operation associated with sensor robustness, small profile, and mass fabrication capability make SAW sensors a very attractive solution. For high-temperature (< 800°C) gas sensor applications, the stability of SAW sensor platform including protective and sensing film layers must be guaranteed over time and under temperature cycling conditions. This paper reports on the performance and stability of langasite SAW resonator (SAWR) sensor platform using PtAl
2
O
3
-based electrode, with atomic layer deposited (ALD) Al
2
O
3
protective layer and yttria-stabilized zirconia (YSZ) gas sensing layer. The effect of 50nm thick Al
2
O
3
or YSZ layers on SAWR sensor resonant frequency and temperature sensitivity responses were investigated. Temperature sensitivities of SAWR sensors with and without ALD Al
2
O
3
protective layer have been compared with numerical predictions for a bare substrate. Measured temperature sensitivities of SAWR sensors with and without the YSZ sensing layers have been compared against each other. In addition, the stability of the SAWR sensor frequency responses have been monitored during multiple 110-hour temperature cycling (over 620 hours in total) from 300°C to 750°C. The measured frequency deviation at 750°C over six of these 110-hour cycling tests was 0.05%. The results obtained confirm that the presence of the ALD Al
2
O
3
protective layer or the YSZ sensing layer do not compromise the stability of high-temperature harsh-environment SAWR sensors, validating their use for the aforementioned applications.