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 (FoM, Q center dot 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 degrees Y-cut lithium niobate (LiNbO3) piezoelectric thin film. A new film stack, namely transferred thin-film LiNbO3 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 mm(2). Acoustic design consideration is reported. The thickness of the LiNbO3 was measured with X-ray diffraction and verified with cross-sectional electron microscopy images. Triple-axis rocking curves confirm the LiNbO3 crystalline quality is higher than prior transferred LiNbO3 layers on Si. Upon further development, the reported platform can enable various signal processing functions at mmWave.