Capacitively coupled IDT for high temperature SAW devices

Harsh environment surface acoustic wave (SAW) sensors are being researched and developed jointly by the University of Maine (UMaine) and Environetix Technologies Corp. for wireless and wired sensor applications, such as those found in gas turbines and power plant combustors. One goal of this work is to extend the operational temperature range of SAW sensors above 1000°C, potentially up to near the melting point of piezoelectric langasite crystals at 1400°C. To achieve stable performance at 1000°C and above, UMaine has developed nanocomposite thin film electrode materials, such as PtRh/HfO₂, and protecting capping layers, such as SiAlON and Al₂O₃. However, these protective top layers, which aid in extending the life of the electrodes, are electrical insulators that prevent direct bonding to the electrodes. The UMaine team also found evidence of accelerated thin-film degradation close to the SAW interdigital transducer (IDT) bond pad welds at these extreme temperatures. This paper introduces a high temperature capacitive approach to electrically couple to the IDT, thus allowing electrical access to the SAW device. The capacitive coupling approach also avoids premature failure of the nanocomposite film caused by interdiffusion between the bond wires and the SAW IDT bond pads. The technique has been successfully implemented and SAW device operation at 1000°C has been achieved.