Since the invention of interdigital transducer (IDT) and surface acoustic wave (SAW) technology in 1965, surface acoustic wave resonators have been widely used in medium and low frequency wireless communication below 2 GHz. With the development of wireless communication entering 5G and 6G, the new frequency bands defined by standards are all above 3 GHz and the bandwidth is above 500 MHz. This has caused traditional SAW technology to encounter development bottlenecks in high frequency, high quality factor, high electromechanical coupling coefficient, and other aspects. The main limitation is that traditional SAW technology uses a single piezoelectric coefficient to achieve the mutual conversion of electrical and mechanical energy.

The research team led by Professor Zuo Chengjie from the School of Microelectronics at the University of Science and Technology of China has proposed and implemented a new type of coupled shear mode surface acoustic wave device (X-SAW) for the first time in the world. By coupling two different directions of shear piezoelectric coefficients, a high electromechanical coupling coefficient of up to 34% and a quality factor of up to 650 have been achieved at a high frequency of 5 GHz. On February 23rd, the research findings were published in the IEEE Electronic Device Letters, titled Coupled Shear SAW Resonator with High Electrical Coupling Coefficient of 34% using X-cut LiNbO3 on SiC Substrate. The reviewer evaluated this as an "extraordinary achievement".

This study designed and fabricated a high-frequency, high electromechanical coupling coefficient, and high-quality coupled shear mode surface acoustic wave (X-SAW) resonator on an X-cutLiNbO3 on SiC substrate. By selecting the appropriate Euler angle and designing the ratio of the thickness of lithium niobate thin film to the wavelength of the interdigital electrode, the horizontal and thickness electric fields simultaneously excite two shear piezoelectric coefficients, and make them work together in a mechanical vibration mode, thereby significantly improving the electromechanical coupling coefficient.

The X-SAW resonator implemented in this work operates at 5GHz, with a high electromechanical coupling coefficient of 34% and a corresponding resonator optimal value (FoM) of 221. Compared with SAW resonators above 4 GHz reported in the past decade, the FoM values of the two X-SAW resonators operating at 5GHz and 6GHz are both the highest in the world. This study discovered the possibility of coupling two or more different shear piezoelectric coefficients in the same vibration mode, and developed design criteria for implementing this coupled shear mode through theoretical analysis, opening up a new research path for the field of acoustic devices. It is expected to open up new degrees of freedom in areas such as broadband filters, broadband tunable oscillators, and high sensitivity sensors, and will have an impact on related industries.

The research work was supported by the National Natural Science Foundation of China.

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