57 GHz Acoustic Resonator with k2 of 7.3 % and Q of 56 in Thin-Film Lithium Niobate

This paper reports an acoustic resonator at 57 GHz with a high electromechanical coupling $(k^{2})$ of 7.3% and 3-dB quality factor (Q) of 56, collectively enabling a record-breaking Figure of merit $(\mathrm{F}\mathrm{o}\mathrm{M},\ Q\cdot k^{2})$ of 4.1, an order of magnitude higher than the state-of-the-art acoustic resonators. The device leverages the third-order antisymmetric (A3) Lamb mode in 110 nm 128°Y-cut lithium niobate (LiNbO
3
) piezoelectric thin film. A new film stack, namely transferred thin-film LiNbO
3
on sapphire substrate with an intermediate amorphous silicon (Si) layer, facilitates the record-breaking performance at millimeter-wave (mmWave). The acoustic resonator features a compact footprint of 0.006 $\mathrm{m}\mathrm{m}^{2}$. Acoustic design consideration is reported. The thickness of the $\mathrm{L}\mathrm{i}\mathrm{N}\mathrm{b}\mathrm{O}_{3}$ was measured with X-ray diffraction and verified with cross-sectional electron microscopy images. Triple-axis rocking curves confirm the $\mathrm{L}\mathrm{i}\mathrm{N}\mathrm{b}\mathrm{O}_{3}$ crystalline quality is higher than prior transferred $\mathrm{L}\mathrm{i}\mathrm{N}\mathrm{b}\mathrm{O}_{3}$ layers on Si. Upon further development, the reported platform can enable various signal processing functions at mmWave.