9.5–12 GHz solidly mounted bulk acoustic wave resonators utilizing overtone of thickness extension mode

Abstract

To realize HF bulk acoustic wave (BAW) devices, we have explored the application of integer multiples of overtones (3rd and 6th) of the thickness extension mode in solidly-mounted resonators (SM-BAWRs). When the average thickness of the acoustic films in the Bragg reflector of the SM-BAW device is wavelength (λ)/4, the fundamental mode is strongly excited, but overtone excitation is weak. However, it was found that 0.035λ–0.075λ acoustic layer thickness of a Bragg reflector was suitable for 3rd overtone excitation using a single LiNbO3 (LN) layer and 0.01λ–0.045λ for 6th overtone excitation using a polarization-flipped double LN layer. By using a 1 μm thick LN single thin layer, we successfully excited 3rd overtone at 9.5 GHz. Furthermore, overtones at 9.5 and 12 GHz were excited by bonding 1 μm and 0.9 μm thick LN plates with different polarities.