Anisotropy-Matched LN/Quartz Heterostructure With Inherent Spurious Mitigation for Wideband SAW Devices

This work presents an all-round optimized LiNbO₃(LN)/Quartz platform that intrinsically overcomes the long-standing spurious-mode barrier limiting wideband applications for lithium-niobate-on-insulator (LNOI), while simultaneously delivering excellent thermal stability and low-loss response in shear horizontal surface-acoustic-wave (SH-SAW) devices. Through a systematic investigation into the slowness landscape of various LN and Quartz orientation pairs, a design framework for anisotropy-matching in heterostructures was established toward the intrinsic suppression of spurious modes, and 32°YX LN/AT-Quartz and 32°YX LN/ $60^{\circ }Y 90^{\circ }X$ Quartz platforms emerged as optimal configurations due to their effective modulation of surface velocity profiles. To verify the simulation results, resonators were prepared on four LN/Quartz platforms with various Quartz orientations, including the 36°YX (AT-cut), $60^{\circ }Y 90^{\circ }X$ , $69^{\circ }Y 90^{\circ }X$ , and $Y - Z$ Quartz, along with the prevalent LN/SiO₂/Si structure for a comparison. Besides an unprecedented performance metrics in terms of electromechanical coupling factor ( $K^{2}$ ) up to 20%, Bode- $Q_{\max }$ exceeding 1000, and an ultralow temperature coefficient of frequency (TCF _a ) of −1.45 ppm/°C, the implemented resonators on LN/AT-Quartz and LN/ $60^{\circ }Y 90^{\circ }X$ Quartz demonstrated superior spectral purity with inherent suppression of transverse modes, higher order modes, and Rayleigh modes concurrently throughout a wide spectral span without additional countermeasures. Synchronously implemented surface-acoustic-wave (SAW) filters under standard interdigitated transducer (IDT) layouts further exhibited both flat in-band response and Out-of-Band (OoB) rejection beyond 30 dB, with a typical design offering a 3-dB fractional bandwidth (FBW) of 14.2% and minimum insertion loss (IL ${}_{\min }$ ) of 0.81 dB, validating the effectiveness of the proposed scheme.