Surface Acoustic Wave (SAW) resonators have been widely used in wireless communication below 2 GHz. However, as wireless communication evolves into 5G and 6G, with the new frequency bands above 3 GHz and bandwidth exceeding 500 MHz, conventional SAW technology face serious bottlenecks in terms of high frequency (>3GHz), high quality factor (Q value), and high electromechanical coupling coefficient (k²).

The main limitation of traditional SAW technology is that it has been using single piezoelectric coefficient to achieve the conversion between electrical and mechanical energy. To solve the issue, a research team led by Prof. ZUO Chengjie from the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences (CAS) developed a Coupled Shear SAW (CS-SAW) resonator that utilizes two coupling coefficients of different directions (e₁₆ and e₃₄). Their work was published in IEEE Electron Device Letters on February 23^th.

Schematic diagram and scanning electron microscope (SEM) photos of the CS-SAW resonator design. (Image from Prof. ZUO’s team)

The team designed and prepared the CS-SAW resonator based on a LiNbO₃-on-SiC (LNoSiC) substrate. By selecting the proper three-dimensional (3D) Euler angle (α) and designing the thickness (h_LN) to wavelength (λ) ratio of the LiNbO₃ thin film, the horizontal and vertical electric fields simultaneously excite two piezoelectric coefficients (e₁₆ and e₃₄), making them coherently coupled in one single vibration pattern. Results showed that this CS-SAW resonator achieved an unprecedented high k² of 34% at 5 GHz and an excellent figure of merit (FoM) up to 221. Compared to all reported SAW resonators above 4 GHz in the recent ten years, the team’s CS-SAW resonator working at 5 GHz and 6 GHz have the highest FoM.

Summary of reported FoM of state-of-the-art SAW resonators from 4 GHz to 8 GHz in recent years. (Image from Prof. ZUO’s team)

This work explored the possibility of coupling two or more piezoelectric coefficients in single vibration pattern and designed a criterion for realizing such coupled shear modes, opening up a new research path for acoustics devices such as wideband filters, tunable resonators, highly sensitive sensors and so on.

Paper link: https://doi.org/10.1109/led.2024.3368426

(Written by ZHANG Yuhuan, edited by ZHANG Yihang, USTC News Center)