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

This work presents an all-round optimized LiNbO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub>(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°<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">YX</i> LN/AT-Quartz and 32°<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">YX</i> LN/<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$60^{\circ }Y 90^{\circ }X$</tex-math> </inline-formula> 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°<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">YX</i> (AT-cut), <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$60^{\circ }Y 90^{\circ }X$</tex-math> </inline-formula>, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$69^{\circ }Y 90^{\circ }X$</tex-math> </inline-formula>, and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$Y - Z$</tex-math> </inline-formula> Quartz, along with the prevalent LN/SiO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>/Si structure for a comparison. Besides an unprecedented performance metrics in terms of electromechanical coupling factor (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$K^{2}$</tex-math> </inline-formula>) up to 20%, Bode-<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$Q_{\max }$</tex-math> </inline-formula> exceeding 1000, and an ultralow temperature coefficient of frequency (TCF<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> _a </i>) of −1.45 ppm/°C, the implemented resonators on LN/AT-Quartz and LN/<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$60^{\circ }Y 90^{\circ }X$</tex-math> </inline-formula> 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<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${}_{\min }$</tex-math> </inline-formula>) of 0.81 dB, validating the effectiveness of the proposed scheme.