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 ${e}_{{16}}$ and ${e}_{{34}}$ for piezoelectric excitation and achieves exceedingly high electromechanical coupling coefficient ( ${k}^{{2}}{)}$ of 34% at 5 GHz based on a LiNbO3-on-SiC (LNoSiC) substrate. By properly selecting the three-dimensional (3D) Euler angle ( $\alpha {)}$ and designing the thickness ( ${h}_{\text {LN}}{)}$ to wavelength ( $\lambda {)}$ ratio of the LiNbO3 thin film, two piezoelectric coefficients ( ${e}_{{16}}$ and ${e}_{{34}}{)}$ can be coherently coupled together in one single vibration pattern, so that the electromechanical coupling ${k}^{{2}}$ add up. When the Euler angle approaches 0°, the CS-SAW resonator operating at 5 GHz exhibits a high ${k}^{{2}}$ of 34% and an excellent figure of merit (FoM = ${k}^{{2}} \cdot ~{Q}_{\text {max}}{)}$ 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.

As wireless communication evolves into millimeter wave (mmWave) frequencies, it becomes extremely challenging to realize compact and integrated radio frequency (RF) front-end components with wide bandwidth and high energy efficiency. Here we propose a new fin-mounted acoustic resonator design based on nanosheet (thickness < 100 nm) Z-cut single-crystal lithium niobate (LiNbO3) thin film for large-bandwidth filtering applications in the 5G NR Frequency Range 2 (FR2) band. The fabricated fin-mounted acoustic resonator operating at 25.03 GHz exhibits a high electromechanical coupling coefficient ( ${k}^{{2}}{)}$ of 16% and quality factor ( ${Q}_{p}{)}$ of 132 at parallel resonant frequency (without any de-embedding), which achieves the highest Figure of Merit (FoM = ${k}^{{2}} \times {Q}{)}$ in the 5G FR2 band among all reported resonators to date. This demonstration provides a new solution to the internal stress problem of nanosheet piezoelectric thin film devices and opens up more possibilities in enabling piezoelectric nanostructures for filtering, sensing, and timing applications.

Recent advancements in mobile communication have escalated the demand for faster data rates, requiring higher carrier frequencies and compact, high-performance, and low-cost radio frequency (RF) filters. Micro-acoustic resonators offer significant advantages in mobile phone filtering due to their low loss and compact size. Addressing the need for low-cost filter solutions for higher frequencies, this study presents a silicon-substrate-based surface acoustic wave (SAW) technology platform to enable high-performance resonators and filters for 6G X-band (7–12 GHz) centimeter-wave (cmWave) wireless communications. Based on a silicon (Si) substrate, we propose a novel design scheme to excite shear vertical surface acoustic waves (SV-SAW) on a 128°Y LiNbO₃/SiO₂/poly-Si/Si layer stack and realize high-frequency resonators above 6 GHz with high-performance: electromechanical coupling coefficient (k²) of 7.6% ~ 8.9% and high-quality factor (Q) ranging from 193–679. The synthesized filters based on those high-performance resonators show low insertion loss (1.47 to 2.20 dB) and 3-dB bandwidth from 308 to 373 MHz. Especially, the demonstrated filter with a center frequency (f_c) at 8.63 GHz exhibits a low insertion loss of only 1.5 dB, which is the best when compared to all other LiNbO₃ acoustic filters in this frequency range, 3-dB bandwidth of 373 MHz, and decent out-of-band rejection across the entire 1–15 GHz range. These results mark a significant step forward for the microwave acoustics field and pave the way for enabling solidly-mounted, low-cost, and miniature-size SAW filters for emerging 6G cmWave wireless communications.