  This paper reports the first high-performance acoustic filters toward
millimeter wave (mmWave) bands using transferred single-crystal thin film
lithium niobate (LiNbO3). By transferring LiNbO3 on the top of silicon (Si) and
sapphire (Al2O3) substrates with an intermediate amorphous Si (aSi) bonding and
sacrificial layer, we demonstrate compact acoustic filters with record-breaking
performance beyond 20 GHz. In the LN-aSi-Al2O3 platform, the third-order ladder
filter exhibits low insertion loss (IL) of 1.62 dB and 3-dB fractional
bandwidth (FBW) of 19.8% at 22.1 GHz, while in the LN-aSi-Si platform, the
filter shows low IL of 2.38 dB and FBW of 18.2% at 23.5 GHz. Material analysis
validates the great crystalline quality of the stacks. The high-resolution
x-ray diffraction (HRXRD) shows full width half maximum (FWHM) of 53 arcsec for
Al2O3 and 206 arcsec for Si, both remarkably low compared to piezoelectric thin
films of similar thickness. The reported results bring the state-of-the-art
(SoA) of compact acoustic filters to much higher frequencies, and highlight
transferred LiNbO3 as promising platforms for mmWave filters in future wireless
front ends.
