  In this work, a 38.7 GHz acoustic wave ladder filter exhibiting insertion
loss (IL) of 5.63 dB and 3-dB fractional bandwidth (FBW) of 17.6% is
demonstrated, pushing the frequency limits of thin-film piezoelectric acoustic
filter technology. The filter achieves operating frequency up to 5G millimeter
wave (mmWave) frequency range 2 (FR2) bands, by thinning thin-film LiNbO3
resonators to sub-50 nm thickness. The high electromechanical coupling (k2) and
quality factor (Q) of first-order antisymmetric (A1) mode resonators in 128
Y-cut lithium niobate (LiNbO3) collectively enable the first acoustic filters
at mmWave. The key design consideration of electromagnetic (EM) resonances in
interdigitated transducers (IDT) is addressed and mitigated. These results
indicate that thin-film piezoelectric resonators could be pushed to 5G FR2
bands. Further performance enhancement and frequency scaling calls for better
resonator technologies and EM-acoustic filter co-design.
