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Thin film bulk acoustic wave resonators (FBARs) leveraging sputtered aluminum nitride (AlN) and scandium aluminum nitride (ScAlN) films, are a leading commercial solution for compact radio frequency (RF) filters in mobile devices. However, as 5G/6G bands extend beyond 6 GHz, achieving the required thinner piezoelectric film thicknesses below 500 nm presents a significant challenge to high-quality sputtering, resulting in a moderate quality factor (Q). Additionally, AlN/ScAlN platforms are limited by moderate electromechanical coupling (k2), restricting bandwidth. More recently, ultra-thin transferred single-crystal piezoelectric lithium niobate (LN) has enabled lateral field excited resonators (XBAR) at 10-30 GHz. While these devices boast a high Q and k2, they face challenges with low capacitance density, large footprint, and significant electromagnetic (EM) effects. On the other hand, thickness-field excited LN FBARs face challenges with bottom electrode integration. In this work, we implement a transferred LN on aluminum FBAR platform on sapphire wafers with an intermediate amorphous silicon layer without the need for a patterned bottom electrode. The resonators show first order symmetric mode (S1) at 10.5 GHz with a 3-dB series resonance Q of 38 and k2 of 14.1%, alongside third order symmetric mode (S3) at 27 GHz with a 3-dB series resonance Q of 22 and a high k2 of 11.3%. Further analysis shows that higher Q could be achieved by adjusting the low-loss piezoelectric to lossy metal volume ratio. 

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 e15 piezoelectric coefficient in 128° Y-cut LiNbO3, enabling monolithic control of electromechanical coupling ( k2 ) by simply rotating the resonator layout. This allows for filters with customizable fractional bandwidths (FBWs). Additionally, fine-tuning of the center frequency ( fc ) 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.