Wireless surface acoustic wave sensors at 2.45 GHz based on AlN/Sapphire and operable up to 600 °C

This study reports significant progress in the field of high temperature wireless surface acoustic wave (SAW) sensors in the 2.45 GHz ISM band, using AlN/Sapphire based resonators. AlN thin films with a thickness of 0.6 µm, synthesized on (0001) sapphire substrates by Metal Organic Chemical Vapor Deposition (MOCVD), demonstrated high crystalline quality and nanometric surface roughness, essential properties for SAW applications. Initially, SAW devices operating in the 2.45 GHz ISM band were tested up to 500°C in both wired and wireless configurations using aluminium electrodes. Wireless interrogation was carried out successfully at a distance of 3 m up to 500°C, with a temperature coefficient of frequency (TCF) measured at −58 ppm/°C. Critically, when the NiAl alloy was used for the thin film electrodes, the operating temperature was extended to 600°C while maintaining a TCF of −43 ppm/°C and stable wireless performance at 3 m, with potential up to 10 m. These results exceed the state-of-the-art for high temperature SAW sensors in the wide 2.45 GHz ISM band. A long-duration annealing test has confirmed the self-passivation properties of the NiAl electrodes and the robustness of the NiAl/AlN/Sapphire structure, with a minimal degradation after 176 h at 500°C, thus demonstrating its potential for long term use in extreme environments.