High coupling high overtone bulk acoustic wave resonator using (Ba,Sr)TiO3

This paper describes the realisation and characterization of High overtone Bulk Acoustic wave resonators (HBAR) by using (Ba,Sr)TiO₃ (BST) as active material. As BST is electrostrictive, it requires a dc voltage to exhibit an electromechanical coupling. Resonances variations versus the dc field are observed and analyzed by using an analytical model based on acoustic propagation. This study proves that BST can exhibit a high electromechanical coupling reaching 8.5% which is higher than Aluminium Nitride, the standard material for acoustic resonators in the GHz frequency range. Moreover, this simple HBAR test vehicle allows extracting the frequency variation experienced by BST. Due to non negligible second order effects as electrostriction, dielectric and stiffness non linearities, these tunable features are highly desirable for tunable filters. A final simulation shows that tunable passband filters could be obtained by using BST based resonators.