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

This letter presents a versatile design method for achieving precise frequency and bandwidth control of compact acoustic filters monolithically at millimeter wave (mmWave) in transferred thin-film lithium niobate (LiNbO₃). Prototypes are implemented with lateral field excited first-order antisymmetric (A1) mode bulk acoustic resonators (XBARs). The design leverages the in-plane anisotropy of the ${e} _{15}$ piezoelectric coefficient in 128° Y-cut LiNbO₃, enabling monolithic control of electromechanical coupling ( ${k} ^{2}$ ) by simply rotating the resonator layout. This allows for filters with customizable fractional bandwidths (FBWs). Additionally, fine-tuning of the center frequency ( ${f} _{c}$ ) 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 18.6GHz, achieving a low insertion loss (IL) of 1.84 dB, and a precise designed FBW of 9.5%. 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.