Structure with thin SiOx/SiNx bilayer and Al electrodes for high-frequency, large-coupling, and low-cost surface acoustic wave devices

Junyao Shen; Sulei Fu; Rongxuan Su; Huiping Xu; Weibiao Wang; Zengtian Lu; Qiong Feng; Fei Zeng; Cheng Song; Feng Pan

doi:10.1016/j.ultras.2021.106460

Structure with thin SiO_x/SiN_x bilayer and Al electrodes for high-frequency, large-coupling, and low-cost surface acoustic wave devices

Ultrasonics. 2021 Aug:115:106460. doi: 10.1016/j.ultras.2021.106460. Epub 2021 May 17.

Authors

Junyao Shen¹, Sulei Fu², Rongxuan Su¹, Huiping Xu¹, Weibiao Wang³, Zengtian Lu³, Qiong Feng³, Fei Zeng¹, Cheng Song¹, Feng Pan⁴

Affiliations

¹ Key Laboratory of Advanced Materials (MOE), School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China.
² Key Laboratory of Advanced Materials (MOE), School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China. Electronic address: fusulei@mail.tsinghua.edu.cn.
³ SHOULDER Electronics Limited, Wuxi 214124, Jiangsu, China.
⁴ Key Laboratory of Advanced Materials (MOE), School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China. Electronic address: panf@mail.tsinghua.edu.cn.

PMID: 34029835
DOI: 10.1016/j.ultras.2021.106460

Abstract

With the development of fifth-generation wireless systems, the Internet of Things, and health services, surface acoustic wave (SAW)-based filters and sensors have attracted considerable interest. This study presents a new structure for high-frequency, large-coupling, and low-cost SAW devices that helps implement high-frequency and wideband filters and enhances the sensitivity of sensors. The structure is based on 15°Y-X LiNbO₃, thin SiO_x/SiN_x bilayer overlay, and Al electrodes. Furthermore, a low-cost fabrication process for SAW devices based on this structure was designed. Simulation and experimental results show that the bilayer substantially weakens the leaky nature of shear-horizontal-type SAWs with a phase velocity higher than that of a slow-shear bulk wave in LiNbO₃. Thus, the limitation related to the velocity of 4029 m/s was overcome, and the phase velocity reached approximately 4500 m/s, which means an increase of 50% compared with that of conventional Cu/15°Y-X LiNbO₃ devices. Consequently, the frequency dramatically increases, and the quality of the SAW response is ensured. Simultaneously, a large electromechanical coupling factor close to 20% can be achieved, which is still suitable for wideband filters and sensors with high energy transduction coefficients. This new structure is expected to become a major candidate for SAW devices in the future.

Keywords: Filter; High frequency; Large coupling; Sensor; Surface acoustic wave.