  This paper reports the first piezoelectric acoustic filter in periodically
poled piezoelectric film (P3F) lithium niobate (LiNbO3) at 23.8 GHz with low
insertion loss (IL) of 1.52 dB and 3-dB fractional bandwidth (FBW) of 19.4%.
The filter features a compact footprint of 0.64 mm2. The third-order ladder
filter is implemented with electrically coupled resonators in 150 nm bi-layer
P3F 128 rotated Y-cut LiNbO3 thin film, operating in second-order symmetric
(S2) Lamb mode. The record-breaking performance is enabled by the P3F LiNbO3
platform, where piezoelectric thin films of alternating orientations are
transferred subsequently, facilitating efficient higher-order Lamb mode
operation with simultaneously high quality factor (Q) and coupling coefficient
(k2) at millimeter-wave (mmWave). Also, the multi-layer P3F stack promises
smaller footprints and better nonlinearity than single-layer counterparts,
thanks to the higher capacitance density and lower thermal resistance. Upon
further development, the reported P3F LiNbO3 platform is promising for compact
filters at mmWave.
