First investigations on stoichiometric lithium niobate as piezoelectric substrate for high-temperature surface acoustic waves applications

Surface acoustic waves (SAW) technology is very promising to achieve high-temperature wireless sensors. However, there is currently a need for piezoelectric substrates with a high electromechanical coupling coefficient (K-2 > 1%), able to operate under harsh environments, especially in the intermediate temperature range (300-600 degrees C). None of the conventional SAW substrate can face this challenge. In particular congruent lithium niobate, whose K-2 can exceed 5%, shows serious limitations from 300 degrees C, mainly related to Li vacancies. Recent studies have demonstrated the potential of stoichiometric lithium niobate (s-LN) for high-temperature bulk acoustic waves applications. In this paper, we investigate this piezoelectric material for high-temperature SAW applications. In particular, we examine carefully the potential structural and chemical changes that s-LN surface can undergo during a high-temperature exposure. Finally, SAW resonators based on s-LN substrates are in situ characterized up to 600 degrees C.