A 6-GHz XBAR Filter Using Trapezoidal Electrodes Based on LiNbO<sub>3</sub> Thin Films
Min Zeng, Wenjuan Liu, Zhiwei Wen, Ronghui Wang, Yao Cai, Yan Liu, Shishang Guo, Chengliang Sun
Abstract
The development of fifth-generation (5G) and next-generation wireless communication systems has put a stringent demand on radio frequency (RF) filters. This work aims to overcome the issues of wideband and in-band ripples with filters comprised by the laterally-excited bulk acoustic wave resonator (XBAR) based on lithium-niobate-on-insulator (LNOI) substrates for 5G applications. The trapezoidal electrodes of XBARs have nonparallel boundaries, causing acoustic waves to scatter in LiNbO3 thin films, thereby suppressing spurious modes. The XBAR using trapezoidal electrodes (T-XBAR) is fabricated using a Z-cut LiNbO3 thin film. The series resonators were achieved by partially thinning the LiNbO3 film from 358 nm (the thickness of shunt resonators) to 323 nm using ion beam etching (IBE). The modified 2.5-order ladder-type filter was fabricated with four resonators connected in parallel to increase <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${C} _{0}$ </tex-math></inline-formula>, featuring a center frequency (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${f}_{c}$ </tex-math></inline-formula>) of 6.038 GHz, a 3-dB wideband of 671 MHz, and a minimal insertion loss of 3.836 dB. Besides, the filter exhibited a relatively low-temperature coefficient of frequency (TCF) of −43.03 ppm/°C. This research demonstrates a potential solution for high-frequency and wideband filters based on LNOI platforms meeting 5G/6G requirements.