Enhanced Performance of ZnO/SiO₂/Al₂O₃ Surface Acoustic Wave Devices with Embedded Electrodes

With the advent of the 5G era, surface acoustic wave (SAW) devices with a larger bandwidth and better temperature stability are strongly required, meanwhile the dimensions of devices are continuously scaling down. In this work, a new layout of ZnO/SiO2/Al2O3 SAW devices with embedded electrodes was developed, and with the help of the finite element method (FEM), the propagation characteristics were simulated. Through adopting embedded electrodes, a large electromechanical coupling coefficient (K2) of 6.6% for the Rayleigh mode can be achieved (5 times larger than that of the conventional ZnO/Al2O3 structure), feasible for wideband SAW devices, and a low acoustic velocity (Vp) of 2960 m/s is exhibited simultaneously, which benefits the miniaturization of SAW devices. The dramatic enhancement of K2 is mainly attributed to the more efficient excitation of SAW in piezoelectric films. Furthermore, a SiO2 overlay is added on the top of the structure to gain an excellent zero temperature coefficient of frequency (TCF). Experimentally, we successfully fabricated SAW one-port resonators based on the proposed structure and good characteristics of high K2, low Vp, and small TCF as simulated were confirmed. Our results show that the proposed structure provides a viable route to design SAW devices with a large bandwidth, small size, and robust temperature compensation for practical use.