  This work reports an acoustic filter at 23.5 GHz with a low insertion loss
(IL) of 2.38 dB and a 3-dB fractional bandwidth (FBW) of 18.2%, significantly
surpassing the state-of-the-art. The device leverages electrically coupled
acoustic resonators in 100 nm 128{\deg} Y-cut lithium niobate (LiNbO3)
piezoelectric thin film, operating in the first-order antisymmetric (A1) mode.
A new film stack, namely transferred thin-film LiNbO3 on silicon (Si) substrate
with an intermediate amorphous silicon (a-Si) layer, facilitates the
record-breaking performance at millimeter-wave (mmWave). The filter features a
compact footprint of 0.56 mm2. In this letter, acoustic and EM consideration,
along with material characterization with X-ray diffraction and verified with
cross-sectional electron microscopy are reported. Upon further development, the
reported filter platform can enable various front-end signal-processing
functions at mmWave.
