19.3 GHz Acoustic Filter with High Close-in Rejection in Tri-layer Thin-Film Lithium Niobate

This paper introduces a tri-layer periodically poled piezoelectric lithium niobate (P3F LN) acoustic filter operating at 19.3 GHz, offering significant performance improvements for sub-6 GHz and emerging 6G frequency bands. The filter is designed to address the challenges of increased insertion loss (IL) and degraded out-of-band (OoB) rejection at higher frequencies, particularly in the FR3 range (7 to 24 GHz).

Key findings include:

1. High Performance: The tri-layer P3F LN filter achieves a low insertion loss (IL) of 2.2 dB, a 3-dB fractional bandwidth (FBW) of 8.5%, and an impressive 49.9 dB close-in rejection. These results demonstrate the potential for integration into FR3 diplexers, enhancing performance in high-frequency applications.

2. Design and Fabrication: The filter is constructed on a 310 nm trilayer Y-cut lithium niobate platform with a 1 µm-thick silicon sacrificial layer. The design uses a fifth-order ladder filter with a Π-type topology, facilitating simple interconnect routing and minimizing layout complexity at mmWave frequencies.

3. Enhanced Out-of-Band Rejection: The filter leverages the multi-mode behavior of the P3F stack, utilizing the adjacent modes introduced by layer mismatches during fabrication. These modes help achieve strong out-of-band rejection, crucial for diplexer applications.

4. Challenges and Future Work: While the filter demonstrates high performance, it also highlights the limitations of reduced rejection performance at frequencies far from the passband. Future improvements will focus on optimizing the P3F film stack to exploit additional modes and enhance overall rejection.

5. Applications: The novel tri-layer P3F LN acoustic filter holds promise for use in FR3 band filters, diplexers, and multiplexers for future 5G and 6G wireless communication systems.

In conclusion, the work demonstrates a significant advancement in acoustic filter design for high-frequency applications, offering a compact, high-rejection solution with potential for integration into next-generation communication systems.