The development of MEMS acoustic resonators meets the increasing demand for in situ detection with a higher performance and smaller size. In this paper, a lithium niobate film-based S1 mode Lamb wave resonator (HF-LWR) for high-sensitivity gravimetric biosensing is proposed. The fabricated resonators, based on a 400-nm X-cut lithium niobate film, showed a resonance frequency over 8 GHz. Moreover, a PMMA layer was used as the mass-sensing layer, to study the performance of the biosensors based on HF-LWRs. Through optimizing the thickness of the lithium niobate film and the electrode configuration, the mass sensitivity of the biosensor could reach up to 74,000 Hz/(ng/cm2), and the maximum value of figure of merit (FOM) was 5.52 × 107, which shows great potential for pushing the performance boundaries of gravimetric-sensitive acoustic biosensors.

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The arrival of the 5G era has promoted the need for filters of different bandwidths. Thin-film bulk acoustic resonators have become the mainstream product for applications due to their excellent performance. The Keff2 of the FBAR greatly influences the bandwidth of the filter. In this paper, we designed an AlN-based adjustable Keff2 FBAR by designing parallel capacitors around the active area of the resonator. The parallel capacitance is introduced through the support column structure, which is compatible with conventional FBAR processes. The effects of different support column widths on Keff2 were verified by finite element simulation and experimental fabrication. The measured results show that the designed FBAR with support columns can achieve a Keff2 value that is 25.9% adjustable.

The laterally-excited bulk acoustic resonators (XBARs) have been gaining attention for their ability to fabricate super high frequency (>5 GHz) and wide fractional bandwidth filters recently. However, the limited cognition about the resonant characteristics of XBARs remains an obstacle for its commercial application. In this paper, the influence of electrode pitch (P) on resonant frequency (fres ), quality factor (Q) and effective electromechanical coupling coefficient (K2 eff ) in X-cut LiNbO3 membrane-based resonators are investigated for both the XBAR mode and first-order asymmetry mode (A1 ). The XBAR mode with fres above 8 GHz was excited at a metallization ratio (η) less than 0.5 when P was within 5 μm to 10 μm, as well as η less than 0.6 when P was within 15 μm to 20 μm. Meanwhile, we revealed that increasing P had a robust effect on the fres of XBAR mode but did produce an improvement on its K2 eff and Q. In contrast, both fres and Q of the A1 mode gradually declined with increasing P. In addition, we have proposed a ratio (C' 0 /Cnum ) as a function of η, varying with P, to achieve a more accurate prediction of capacitance behavior in resonators. Overall, this research has demonstrated the sophisticated resonant and capacitive properties of X-cut membrane-based resonators for the first time, which paved the way for the fabrication of high-performance filters.

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