Tunable Ba_0.5Sr_0.5TiO₃ film bulk acoustic resonators using SiO₂/Mo Bragg reflectors

Tunable and switchable Ba_0.5Sr_0.5TiO₃ film bulk acoustic resonators (FBARs) based on SiO₂/Mo Bragg reflectors are explored, which can withstand high temperature for the deposition of Ba_xSr_1-xTiO₃ (BST) films at 800 ℃. The dc bias-dependent resonance may be attributed to the piezoelectricity of the BST film induced by an electrostrictive effect. The series resonant frequency is strongly dc bias-dependent and shifts downwards with dc bias increasing, while the parallel resonant frequency is only weakly dc bias-dependent and slightly shifts upwards at low dc bias (< 45 V) while downwards at higher dc bias. The calculated relative tunability of shifts at series resonance frequency is around -2.3% and the electromechanical coupling coefficient is up to approximately 8.09% at 60-V dc bias, which can be comparable to AlN FBARs. This suggests that a high-quality tunable BST FBAR device can be achieved through the use of molybdenum (Mo) as the high acoustic impedance layer in a Bragg reflector, which not only provides excellent acoustic isolation from the substrate, but also improves the crystallinity of BST films withstanding higher deposition temperature.