Toward Miniature High-Coupling Lithium Niobate Thin-Film Bulk Acoustic Wave Resonators at Millimeter Wave

Thin-film bulk acoustic wave resonators (FBARs), leveraging sputtered aluminum nitride (AlN) and scandium aluminum nitride (ScAlN) films, are a leading commercial solution for compact radio frequency (RF) filters in mobile devices. However, as 5G/6G bands extend beyond 6GHz, achieving the required operational bandwidth presents a significant challenge due to the moderate electromechanical coupling ($\textit{k}^{{2}}$) of AlN/ScAlN. More recently, transferred ultra-thin single-crystal piezoelectric lithium niobate (LN) has enabled lateral-field-excited bulk acoustic wave resonators (XBARs) at 10–30GHz. While these devices boast a high$\textit{k}^{{2}}$, they face challenges with low capacitance density, large footprint, and significant electromagnetic (EM) effects. On the other hand, thickness-field-excited (TFE) LN FBARs face challenges with bottom electrode integration. In this work, we demonstrate a high$\textit{k}^{{2}}$LN FBAR without pre-patterned bottom electrodes prior to the thin-film LN transfer. The resonators show the first-order symmetric (S1) mode at 10.5GHz with a 3-dB series resonance quality factor (${Q}_{s}$) of 38 and$\textit{k}^{{2}}$of 14.1%, alongside the third-order symmetric (S3) mode at 27GHz with a 3-dB (${Q}_{s}$) of 22 and a high$\textit{k}^{{2}}$of 11.3%. Further analysis shows that higher$Q$could be achieved by adjusting the low-loss piezoelectric to lossy metal volume ratio.