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Lattice XBAR Filters in Thin-Film Lithium Niobate

Taran Anusorn, Byeongjin Kim, Ian Anderson, Ziqian Yao, Ruochen Lu

Abstract

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.

Lattice XBAR Filters in Thin-Film Lithium Niobate

Abstract

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.
Paper Structure (7 sections, 5 equations, 5 figures)

This paper contains 7 sections, 5 equations, 5 figures.

Table of Contents

  1. Introduction
  2. XBARs in P3F TFLN
  3. Lattice Acoustic Filter Design
  4. Measured Filter Results and Discussions
  5. Direct Lattice Filter
  6. Layout-Balanced Filter
  7. Conclusion

Figures (5)

  • Figure 1: Survey of (a) IL and (b) FBW in acoustic filters above 10 GHz.
  • Figure 2: (a) Exploded view and (b) top view of an XBAR implemented in bi-layer P3F 128$^{\circ}$Y-cut TFLN, indicating key dimensional parameters (not to scale). (c) Simulated frequency responses of XBAR unit cells with a fixed bottom TFLN thickness of $t_1 = 110$ nm and varying top-layer thicknesses $t_2$; the corresponding stress mode patterns ($T_{xz}$) are shown adjacent to their associated antiresonances. (d) mBVD model for acoustic resonators exhibiting multiple vibrational modes.
  • Figure 3: (a) Two-port lattice network. Proposed planar-interconnect implementations: (b) direct lattice and (c) layout-balanced lattice configurations. (d) Simulated 50-$\Omega$ filter responses based on the mBVD circuit model, with a summary of the design parameters. Dotted $S_{21}$ curves illustrate the impact of parasitic A1 and A3 modes on the filter responses.
  • Figure 4: (a) Fabricated direct lattice filter and its standalone resonators: (b) A and (c) B, indicating the number $N_e$ and length $l_e$ of the IDEs. Measured (d) transmission and (e) reflection responses of the prototype filter. (f) Measured admittance of the resonators along with the extracted mBVD model parameters, considering only the S2 mode; the influence of each mode is highlighted.
  • Figure 5: (a) Fabricated layout-balanced lattice filter and its standalone resonators: (b) A1, (c) A2, and (d) B, indicating the number $N_e$ and length $l_e$ of the IDEs. Measured (e) transmission and (f) reflection responses of the prototype filter. (g) Measured admittance of the resonators along with the extracted mBVD model parameters, considering only the S2 mode; the influence of each mode is highlighted.