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 (LiNbO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub>). 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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${e} _{15}$</tex-math> </inline-formula> piezoelectric coefficient in 128° Y-cut LiNbO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub>, enabling monolithic control of electromechanical coupling (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${k} ^{2}$</tex-math> </inline-formula>) by simply rotating the resonator layout. This allows for filters with customizable fractional bandwidths (FBWs). Additionally, fine-tuning of the 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>) 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.