Lattice XBAR Filters in Thin-Film Lithium Niobate

This work presents the demonstration of lattice filters based on laterally excited bulk acoustic resonators (XBARs). Two filter implementations, namely direct lattice and layout-balanced lattice topologies, are designed and fabricated in periodically poled piezoelectric film (P3F) thin-film lithium niobate (TFLN). By leveraging the strong electromechanical coupling of XBARs in P3F TFLN together with the inherently wideband nature of the lattice topology, 3-dB fractional bandwidths (FBWs) of 27.42\% and 39.11\% and low insertion losses (ILs) of 0.88 dB and 0.96 dB are achieved at approximately 20 GHz for the direct and layout-balanced lattice filters, respectively, under conjugate matching. Notably, all prototypes feature compact footprints smaller than 1.3 mm\textsuperscript{2}. These results highlight the potential of XBAR-based lattice architectures to enable low-loss, wideband acoustic filters for compact, high-performance RF front ends in next-generation wireless communication and sensing systems, while also identifying key challenges and directions for further optimization.

Frequency and Bandwidth Design of FR3-Band Acoustic Filters

This article presents an approach to control the operating frequency and fractional bandwidth (FBW) of miniature acoustic filters in thin-film lithium niobate (TFLN). More specifically, we used the first-order antisymmetric (A1) mode in lateral-field-excited bulk acoustic wave resonators (XBARs) to achieve efficient operation at 20.5 GHz. Our technique leverages the thickness-dependent resonance frequency of A1 XBARs, combined with the in-plane anisotropic properties of 128$^\circ$ Y-cut TFLN, to customize filter characteristics. The implemented three-element ladder filter prototype achieves an insertion loss (IL) of only 1.79 dB and a controlled 3-dB FBW of 8.58% at 20.5 GHz, with an out-of-band (OoB) rejection greater than 14.9 dB across the entire FR3 band, while featuring a compact footprint of 0.90 $\times$ 0.74 mm$^2$. Moreover, an eight-element filter prototype shows an IL of 3.80 dB, an FBW of 6.12% at 22.0 GHz, and a high OoB rejection of 22.97 dB, demonstrating the potential for expanding to higher-order filters. As frequency allocation requirements become more stringent in future FR3 bands, our technique showcases promising capability in enabling compact and monolithic filter banks toward next-generation acoustic filters for 6G and beyond.