High-Q Millimeter-Wave Acoustic Resonators in Thin-Film Lithium Niobate Using Higher-Order Antisymmetric Modes

This letter presents miniature millimeter wave (mmWave, above 30 GHz) acoustic resonators based on a single-layer thin-film lithium niobate (LN) platform. More specifically, we present high performance third-order antisymmetric (A3) mode laterally excited bulk acoustic resonators (XBAR). Compared to prior demonstrations, the proposed platform features a compact footprint due to a smaller lateral wavelength and aperture. We showcase an A3 mode device operating at 39.8 GHz with a high extracted electromechanical coupling (k2) of 4%, a high 3-dB series resonance quality factor (Q_s) of 97, and a high 3-dB anti-resonance quality factor (Q_p) of 342, leading to a figure of merit (FoM=k^2*Q_p) of 13.7 with a footprint of 32x44 micron^2. To demonstrate frequency scalability, the piezoelectric film thickness is varied while keeping the device layout. As a result, we present a multitude of high-performance devices covering a wide frequency range of 30-50 GHz, validating the proposed XBAR design at mmWave.