AlN and LGS Surface Acoustic Wave Resonators for Passive Wireless High-Temperature Sensing

Surface acoustic wave (SAW) resonators demonstrate significant potential in passive wireless high-temperature sensing applications. However, their performance and reliability under extreme temperatures are fundamentally constrained by the thermal stability of the piezoelectric substrate. This paper presents the design, characterization, and sensing device of single-port SAW resonators based on aluminum nitride (AlN) and langasite (LGS) substrates. High-temperature experiments were conducted in air atmosphere from 25°C to 750°C using a muffle furnace, enabling real-time monitoring of the resonant frequency evolution. The AlN-based resonator, benefiting from a high phase velocity, achieved a resonant frequency of 2.1 GHz and exhibited superior temperature sensitivity, making it advantageous for developing compact, highly sensitive wireless sensors. However, it showed irreversible sensing characteristic drift during thermal cycling, limiting its stable operating temperature to below 600°C. In contrast, the LGS-based resonator maintains exceptional stability and repeatability over multiple thermal cycles up to 750°C. Furthermore, we successfully demonstrate a passive wireless sensing system using the LGS resonator, which enables precise temperature detection by monitoring shifts in the resonant frequency.