Thin films and techniques for SAW sensor operation above 1000°C

High temperature (300°C to 1400°C) wireless sensors have applications in energy exploration and generation, harsh environment industrial processing, and aerospace engineering. Existing technology developed at the University of Maine allows the fabrication of surface acoustic wave (SAW) langasite (LGS) sensors with Pt-Rh/ZrO₂ electrodes that can deliver long-term stable operation up to 850°C. Since LGS remains piezoelectric up to its melting point of ~1400°C, it is desirable to extend the current SAW sensor temperature range of operation. In addition, it is desirable to diminish the SAW interdigital transducer (IDT) electrode dimensions to increase the wireless frequency of operation towards the GHz range. In this work, new thin film electrode materials have been investigated to allow the operation of SAW one-port resonators up to 1000°C and beyond. In particular, alternative Pt/Al₂O₃ and Pt-Rh/HfO₂ thin film electrode compositions are presented, which yield operation of SAW resonator sensors up to 1100°C. In addition to a previously used capping layer, an interfacial layer has been added between the LGS and the electrodes to delay any interdiffusion between the materials and extend the temperature and/or time of sensor performance. Finally, it is also reported in this work that exposure of untreated SAW device electrodes with 120 nm thick and 2μm wide Pt-Rh/ZrO₂ co-deposited IDT fingers to temperatures above 850°C can create long platinum-rich nano-whiskers. These structures short-circuit the SAW interdigital (IDT) fingers, rendering the device unusable. The short-circuit problem was solved by the use of multilayered electrode structures and the used of the capping layer.