Frequency and Bandwidth Design of Millimeter Wave Thin-Film Lithium Niobate Acoustic Filters

This paper presents a versatile design method for precise frequency and bandwidth control of compact acoustic filters monolithically at millimeter-wave (mmWave) in thin-film lithium niobate(LN). Prototypes are implemented with first-order antisymmetric (A1) mode acoustic resonators. The design leverages the in-plane anisotropy of the e15 piezoelectric coefficient in 128° Y-cut LiNbO3, enabling control of electromechanical coupling by rotating the resonator layout. Additionally, fine-tuning of the center frequency is achieved through selective trimming of the film for series and shunt resonators, enabling a single design to be scaled across frequencies with enhanced fabrication tolerance. To validate the approach, we designed and fabricated a filter centered at 18.6 GHz, achieving a low insertion loss of 2 dB, and a precise designed fractional bandwidth of 9.6%. This platform shows a significant promise for enabling a monolithic filter bank with precise band selection, paving the way for the next generation of mmWave acoustic filters.