Large Coupling and Spurious-Free SH<sub>0</sub> Plate Acoustic Wave Resonators Using LiNbO<sub>3</sub> Thin Film
Shuxian Wu, Zonglin Wu, Hangyu Qian, Feihong Bao, Gongbin Tang, Feng Xu, Jie Zou
Abstract
With the rapid development of 5G communication technology, high-performance acoustic filters are urgently required to cope with the increasing number of frequency bands. Recently, an extensive attention has been paid to the advancement of lithium niobate (LiNbO3)-based thin-film acoustic resonators due to their large electromechanical coupling factor. In this article, a fundamental (0th) shear-horizontal plate acoustic wave (SH0-PAW) resonator based on a 30 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\circ } {Y}$ </tex-math></inline-formula> -cut LiNbO3/SiO2 thin film is proposed. Using the finite element method (FEM), the effective electromechanical coupling coefficient ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${k}^{{2}}_{\text {eff}}$ </tex-math></inline-formula> ) and phase velocity as the function of the rotation angle <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{^{^{}}}$ </tex-math></inline-formula> of the LiNbO3 film and the layer thicknesses are extracted. Meanwhile, the dispersion characteristic of the SH0 mode is analyzed. The fabricated resonators yield a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${k}^{{2}}_{\text {eff}}$ </tex-math></inline-formula> of 24.8%, a Bode- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${Q}_{\text {max}}$ </tex-math></inline-formula> of 688, and a figure of merit (FoM = <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${k}^{{2}}_{\text {eff}} \cdot \text {Bode-}{Q}_{\text {max}}$ </tex-math></inline-formula> ) of 170. Furthermore, a tilted interdigital-transducer (IDT) method is adopted to suppress the spurious modes of the proposed resonators. It has been experimentally verified that most of the transverse modes are suppressed, and the longitudinal modes are suppressed effectively. The proposed resonator provides an effective solution to design a wideband, low loss, and unrippled acoustic filter for communication systems.