23.8-GHz Acoustic Filter in Periodically Poled Piezoelectric Film Lithium Niobate With 1.52-dB IL and 19.4% FBW

This letter 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- 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 <inline-formula> <tex-math notation="LaTeX">$Y$ </tex-math></inline-formula>-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 (<inline-formula> <tex-math notation="LaTeX">$Q$ </tex-math></inline-formula>) and coupling coefficient (<inline-formula> <tex-math notation="LaTeX">$k^{2}$ </tex-math></inline-formula>) at millimeter-wave (mmWave). Also, the multilayer 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.