18-GHz Bi-Layer A3-Mode Acoustic Resonators with Near-Zero TCF and High Electromechanical Coupling

In this paper, a novel bi-layer Lamb wave resonator (LWR) has been proposed and demonstrated for the first time, with near-zero temperature coefficient of frequency (TCF) and high electromechanical coupling coefficient (k²). The suspended resonator features a bi-layer composite structure of silicon oxide (SiO₂)/128° Y-cut lithium niobate (LN). By carefully designing the thickness of the SiO₂ layer on LN and optimizing the electrode dimensions, the TCF and k² of the A3-mode LWRs are optimized through 2D FEM simulations. The fabricated bi-layer A3-mode resonator achieves a high resonant frequency of 18 GHz with a high k² of 11.0% and a Qmax of 285. At the same time, a near-zero TCF of +0.145ppm/°C has been achieved. More importantly, this work discovers a new path designing suspended thin-film resonator structures to achieve high frequency, low TCF, high k², and good mechanical stability all at the same time, which is illuminating for LWR designs at various frequencies as needed by satellite and 5G/6G wireless communications.