18 GHz Y36 Lithium Niobate Ferroelectric Tunable Bulk Acoustic Wave Resonator

Lithium Niobate (LN) has been a piezoelectric material of interest for various types of acoustic resonators operating at GHz frequencies as it offers very high electromechanical coupling ($k_t^2$). However, very little is known about the incorporation of thin-film LN into Bulk Acoustic Wave (BAW) resonators above 7 GHz, and how its ferroelectric properties affect the resonance characteristics. In this work, we present the first Lithium Niobate Tunable Bulk Acoustic Wave (LN TBAW) resonator, which is a BAW type resonator that incorporates a 50 nm thick Y36 cut LN film as the piezoelectric film and has its resonance frequency response tuned through ferroelectric switching. The fabricated device achieves a $k_t^2$ up to 16.5% and a maximum Quality factor (Q) of 105.8 at 17.6 GHz in its "on" state, while its series and parallel resonances are suppressed when tuned to an "off" state. Ferroelectric switching to the "off" state is attained with a voltage less than 1.5 V. This novel demonstration opens a new path towards the implementation of TBAW into tunable Radio Frequency (RF) front-end filters.