Layered LiNbO3/AT-Quartz Wideband Devices With Inherent Transverse Mode Suppression

This letter reports on a novel layered structure with inherent spurious transverse suppression for shear-horizontal surface acoustic wave (SH-SAW) wideband devices. The platform integrates a giant electromechanical coupling factor ( $k_{\text {eff}}^{2}$ ) lithium niobate (LN) thin film with a commercially available AT-quartz substrate characterized by strong concave shear horizontal slowness, inherently suppressing transverse modes through slowness curve manipulation. We compared the proposed LN/AT-quartz platform with prevalent LN/SiO2/Si structure through 3-D finite-element analyses and device measurements, theoretically and experimentally verifying the superior capability of AT-quartz for transverse mode suppression. Besides large $k_{\text {eff}}^{2}$ over 20%, maximum quality factor ( $Q_{\max }$ ) exceeding 800, and spurious-free responses up to 6 GHz achieved in fabricated LN/AT-quartz resonators, transverse modes were inherently mitigated in LN thin-film layered surface acoustic wave (SAW) devices for the first time. The fabricated synchronous gigahertz filter shows a 3-dB fractional bandwidth (FBW) of 10.5%, a minimum insertion loss (ILmin) of 0.36 dB, and flat passband with transverse modes well inherently suppressed utilizing standard interdigital transducer (IDT) layout.