面向毫米波的微型高机电耦合系数铌酸锂薄膜体声波谐振器

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 6 GHz, 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 10–30 GHz. 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 10.5 GHz 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 27 GHz 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.