57 GHz Acoustic Resonator with k<sup>2</sup> of 7.3 % and Q of 56 in Thin-Film Lithium Niobate
Jack Kramer, Sinwoo Cho, Michael E. Liao, Kenny Huynh, Omar Barrera, Lezli Matto, Mark S. Goorsky, Ruochen Lu
Abstract
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 <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> ) piezoelectric thin film. A new film stack, namely transferred thin-film LiNbO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> 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.