LiNbO3 Film Bulk Acoustic Resonator for n79 band
Résumé
Background, Motivation and Objective
Thin film Bulk Acoustic Wave (BAW) resonators based on lithium niobate (LiNbO3) exploiting a longitudinal polarization have been proposed as promising candidates for wide bandwidth filters in the 3 - 6 GHz range [1, 2]. As a matter of example, 450 nm-thick Y+36°-cut LiNbO3 Film Bulk Acoustic Resonator (FBAR) operating at 4.1 GHz displays an electromechanical coupling factor of 15.4% and a quality factor at antiresonance close to 390 [2]. However, the quality factor is in all cases limited up to 400 and no solutions based on BAW devices with specifications consistent with n79 band requirements (fr = 4.7 GHz, kt2 ≥ 25%, Q ≥ 500) are up to now reported.
[1] A. Reinhardt et al., IFCS/EFTF 2021, [2] M. Bousquet et al., IUS 2021.
Statement of Contribution/Methods
We have designed and fabricated LiNbO3 Film Bulk Acoustic Resonators (FBAR) with specifications in terms of operation frequency, electromechanical coupling coefficient and quality factor compatible with the n79 band. The respective thicknesses of the electrodes and piezoelectric films were chosen to maximize the electromechanical coupling coefficient while ensuring a sufficient energy trapping in the resonators.
Results/Discussion
4.7 GHz Film Bulk Acoustic Resonators (FBAR) based on Y+36°-cut LiNbO3 films have been fabricated using a layer transfer process (4-inch) including patterned bottom electrodes and a sacrificial layer cavity. Thanks to the longitudinal polarization and to an energy trapping effect, a single resonance at 4.7 GHz with an effective electromechanical coupling factor (kt2) of 24.2 % are obtained. Moreover the quality factor at antiresonance (Qa) is close to 574, leading to Q×f and Q× kt2 products of 2.7 × 1012 Hz and 139, respectively. These figure-of-merits are the highest reported up to date for LiNbO3 FBARs, highlighting the potentialities of LiNbO3 BAW devices for n79 band.