Durability of TiAl based surface acoustic wave devices for sensing at intermediate high temperatures

Abstract

TiAl based surface acoustic wave (SAW) devices, which offer a promising cheap and easy to handle wireless sensor solution for intermediate high temperatures up to 600 °C, were prepared and investigated with respect to their durability. To obtain the devices, Ti/Al multilayers were deposited on high-temperature stable piezoelectric catangasite (CTGS) substrates and structured as electrodes via the lift-off technique. AlNO cover layers and barrier layers at the substrate site served as an oxidation protection. The devices were characterized regarding their electrical behavior by ex-situ measurements of their frequency characteristics after heat treatments up to 600 °C in air. In addition, long-term in situ measurements up to 570 °C were performed to analyze a possible drift of the resonant frequency in dependence on the temperature and time. Scanning electron microscopy of the surfaces of the devices and scanning transmission electron microscopy of cross sections of TiAl interdigital transducer electrode fingers and the contact pads were conducted to check the morphology of the electrode metallization and to reveal if degradation or oxidation processes occurred during the heat treatments. The results demonstrated a sufficient high-temperature stability of the TiAl based devices after a first conditioning of system. A linear dependence of the resonant frequency on the temperature of about −37 ppm/K was observed. In summary, the suitability of TiAl based SAW sensors for long-term application at intermediate temperatures was proven.