Coupled Shear SAW Resonator With High Electromechanical Coupling Coefficient of 34% Using X-Cut LiNbO₃-on-SiC Substrate

In this work, a coupled shear mode surface acoustic wave (CS-SAW) resonator has been proposed and demonstrated for the first time, which utilizes both <inline-formula> <tex-math notation="LaTeX">${e}_{{16}}$ </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">${e}_{{34}}$ </tex-math></inline-formula> for piezoelectric excitation and achieves exceedingly high electromechanical coupling coefficient (<inline-formula> <tex-math notation="LaTeX">${k}^{{2}}{)}$ </tex-math></inline-formula> of 34% at 5 GHz based on a LiNbO3-on-SiC (LNoSiC) substrate. By properly selecting the three-dimensional (3D) Euler angle (<inline-formula> <tex-math notation="LaTeX">$\alpha {)}$ </tex-math></inline-formula> and designing the thickness (<inline-formula> <tex-math notation="LaTeX">${h}_{\text {LN}}{)}$ </tex-math></inline-formula> to wavelength (<inline-formula> <tex-math notation="LaTeX">$\lambda {)}$ </tex-math></inline-formula> ratio of the LiNbO3 thin film, two piezoelectric coefficients (<inline-formula> <tex-math notation="LaTeX">${e}_{{16}}$ </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">${e}_{{34}}{)}$ </tex-math></inline-formula> can be coherently coupled together in one single vibration pattern, so that the electromechanical coupling <inline-formula> <tex-math notation="LaTeX">${k}^{{2}}$ </tex-math></inline-formula> add up. When the Euler angle approaches 0°, the CS-SAW resonator operating at 5 GHz exhibits a high <inline-formula> <tex-math notation="LaTeX">${k}^{{2}}$ </tex-math></inline-formula> of 34% and an excellent figure of merit (FoM = <inline-formula> <tex-math notation="LaTeX">${k}^{{2}} \cdot ~{Q}_{\text {max}}{)}$ </tex-math></inline-formula> of 221, which are both the highest among all reported high frequency SAW resonators near or above 5 GHz. This demonstration not only greatly enhances the bandwidth of SAW filters at high frequencies, but also opens up a groundbreaking way of designing new and more complex coupled modes for a variety of applications.