Aluminum Scandium Nitride as Piezoelectric Material for SAW/BAW Hybrid Resonators with Large Figure of Merit

Nowadays, no less than 21 sensors are used to monitor the health conditions of aircraft engines. However, the actual state of technology do not allow the measurements on moving parts in harsh environment (i.e. rotor shaft and blades). Thanks to its straightforward remote capabilities and the exceptional properties of Aluminum Scandium Nitride (AlScN), acoustic resonators based on AlScN thin films are considered the best choice for such application. Performances of SAW/BAW hybrid resonators based on AlN, AlSc(0.17)N and AlSc(0.40)N were investigated by finite element method in COMSOL Multiphysics. After noticing the excitation of SAW in AlN/AlScN resonator involves only d33, a series of experiments has been conducted to determine the relation between the design and performances of hybrid resonators. In contradiction with the predictions of previous works, the ratio thickness-over-pitch which maximize kt2 seems to be coupled to the etching depth. Achieving simultaneously a high quality factor (Q ≈ 2000 ) and a high electromechanical coupling factor (kt2 = 23.48 [%]), SAW/BAW hybrid resonators based on AlSc(0.40)N thin film reaches the highest Figure of Merit ever recorded for an AlN/AlScN based acoustic resonators: kt2 · Q = 494.