Harsh Environment SAW Resonators on Thin Film Lithium Niobate Substrate for RF Wireless Sensing Applications

In this work, harsh environment wideband and low-loss shear horizontal surface acoustic wave (SH-SAW) resonators are demonstrated in the GHz frequency range for integrated wireless sensing applications. Owing to their excellent electrical and chemical properties in high temperature extreme conditions, platinum thin film electrodes were utilized to achieve the acoustic resonators on X-cut lithium niobate on insulator (LNOI) piezoelectric substrate. SH-SAW devices with wavelengths of 1.2, 1.4, and 1.6 μm were implemented realizing high resonance frequencies that address the need for reduced-size antennas in sensor systems with integrated wireless readout. The fabricated resonators exhibit high electromechanical coupling $\left( {k_t^2 > 39.9\% } \right)$ and maximum quality factors (Q<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</inf>) ranging from 1325 at 1.65 GHz to 1120 at 2.22 GHz, resulting in a figure of merit $({\text{FoM}} = \left. {{\text{k}}_t^2 \times {\text{Q}}} \right)$ up to 528. The resonators’ temperature coefficient of frequency (TCF) was measured up to -77.4 ppm/°C at λ = 1.2 μm, demonstrating the potential for ultra-sensitive and compact passive wireless sensors.