CDMA and FSCW surface acoustic wave temperature sensors for wireless operation at high temperatures

This paper reports on the successful operation of surface acoustic wave (SAW) devices at high temperature, including wireless measurements of SAW devices up to 750°C. Two types of SAW devices and corresponding systems were investigated: (1) 15-bit code-division multiple-access (CDMA) SAW tags, interrogated using a device-specific direct sequence spread spectrum (DSSS) binary phase shift key (BPSK) code; and (2) multi-track reflective delay lines, interrogated using a frequency-stepped continuous-wave (FSCW) radar signal. The SAW devices used in this work were fabricated on langasite (LGS, La₃Ga₅SiO₁₄) using Pt/Rh/ZrO₂ electrodes, which can withstand temperatures up to 1000°C. A FEM/BEM simulation package using both the Pt/Rh/ZrO₂ thin film and LGS properties was used to simulate the interdigital transducers (IDTs) used in the fabricated structures. The two classes of SAW sensors were first directly wired and characterized at high temperature. The FSCW system was then used to test the wireless operation of LGS SAW devices up to 750°C.