The first-order antisymmetric (A1) mode lamb wave resonator (LWR) based on Z-cut LiNbO₃ thin films has attracted significant attention and is widely believed to be a candidate for next-generation reconfigurable filters with high frequency and large bandwidth (BW). However, it is challenging for traditional interdigitated electrodes (IDTs) based LWR filters to meet the requirement of a clean frequency spectrum response and enough out-of-band (OoB) rejection. To solve the problem, we propose LWRs with checker-shaped IDTs for the design of filters that meet the Wi-Fi 6E standard. By taking advantage of checker-shaped IDTs with unparalleled boundaries, the fabricated 6-GHz resonators successfully suppress higher-order A1 spurious modes, demonstrating a spurious-free impedance response and a high figure-of-merit (FOM) up to 104. Based on the demonstrated checker-shaped electrode design, the filter features a center frequency (f₀) of more than 6 GHz, a 3 dB BW exceeding 620 MHz, and an excellent OoB rejection >25 dB, consistent with the acoustic-electric-electromagnetic (EM) multi-physics simulations. Furthermore, through the capacitance-inductance matching network technology, the filter’s voltage standing wave ratio (VSWR) is successfully reduced below 2, showing an excellent 50 Ω impedance matching. This study lays a foundation for ultra-high-frequency and ultra-wideband filters for the Wi-Fi 6/6E application.

The demand for filters with a large fractional bandwidth, up to 5%, still puts a great challenge to the typical aluminum nitride (AlN) thin films-based bulk acoustic wave (BAW) filters in an acoustic-only approach. This paper presents the design, fabrication, and characterization of film bulk acoustic wave resonator (FBAR) and filters using aluminum scandium nitride (Al $_{0.8}$ Sc $_{0.2}$ N) thin films. The fundamental thickness-mode resonance of the FBAR is measured at 4.235 GHz with an excellent effective electromechanical coupling factor ( $K_{eff}^{2})$ of 14%. The filters employ a modified structure with additional $\vphantom{^{\int}}$ molybdenum (Mo) layers formed on the connection strips between each adjacent resonator. The proposed filtering topologies including ladder topology, lattice topology, and ladder-lattice topology are constructed. Results show that the modified connection strips can decrease the magnitude impedance at series resonant frequency and improve the insertion loss (IL) of the filters. The fabricated ladder filter demonstrates a high center frequency of 4.25GHz, low IL of 1.804 dB, wide bandwidth of 189 MHz, and out-of-band rejection of 30 dB, holding a great potential for the 5G wideband applications. 2022-0201

Spurious modes (SMs) suppression and electromechanical coupling coefficient ( ${k}_{t}^{{2}}$ ) improvement in Lamb wave resonators (LWRs) have always been the research hotspot. In this work, a 2.5 GHz spurious-free S1 mode AlN/ScAlN-based LWR with trapezoidal electrodes is proposed. The AlN/Sc0.096AlN composite film is prepared by Physical Vapor Deposition (PVD) to improve the ${k}_{t}^{{2}}$ without numerous abnormal grains. Measured results show that the SMs are effectively suppressed over a wide frequency range (1.5–3.5 GHz). Relying on the using of AlN/Sc0.096AlN composite film and the elimination of SMs, the ${k}_{t}^{{2}}$ of the S1 mode has been increased to more than twice and the ${f}\times {Q}\times {k}_{t}^{{2}}$ of the device can reach up to ${1}.{08}\times {10}^{{11}}$ Hz.

The effective electromechanical coupling coefficient ( ${K}_{eff}^{2}{)}$ of resonators is crucial for application in radio communication such as fabricating bandpass filters. In this letter, we present a $0^{\text {th}}$ symmetry ( ${S}_{0}{)}$ mode Lamé mode resonator (LMR) operating at 2.563 GHz with a high ${K}_{\vphantom {D_{j}}{eff}}^{2}$ of 7.83%. A 20% Scandium-doped Aluminum Nitride (Sc _0.2 Al _0.8 N)/AlN composite piezoelectric film is used as the piezoelectric layer. The ratio of piezoelectric layer thickness ( ${t}_{\vphantom {D_{j}}{pie}}{)}$ and electrode pitch ( ${P}_{ele}{)}$ is optimized by finite element analysis (FEA) method and the highest experimental ${K}_{eff}^{2}$ is obtained at ${t}_{pie}/{P}_{ele} =0.65$ . The proposed LMR with Sc doping and structural optimization shows fascinating prospects for constructing broadband filters. [2021-0113]

In this paper, 600nm Z-cut lithium niobate (LiNbO ₃ ) thin film based third-order antisymmetric (A3) mode resonators with different geometries has been investigated. Through a series of experiments, it has been found for the first time that effective electromechanical coupling coefficient (K _eff ² ) is influenced by the gap between electrodes (Ge), and K _eff ² decreases with the increase of Ge when electrode pitch (P) higher than 5μm. Finally, a A3 mode resonator with a high 2 Keff of 6.72% is obtained. This K _eff ² is the highest reported value in similar acoustic resonators working about 9 GHz and will be beneficial to the construction of X-band filters.

The thin film bulk acoustic resonator (FBAR) has emerged as a promising choice for liquid sensors because of its high frequency and sensitivity. To investigate the potential of FBAR devices working as the liquid sensors, we study the operating law of FBAR in liquid environments and explore the different loading effects of liquid on the shear mode and longitudinal mode. By analyzing the device and liquid interactions, we modify the Mason model of FBAR in the liquid environment. Subsequently, the influence of the piezoelectric film with different tilt angles and liquids on the characteristics of FBAR is discussed. We also prepared Sc_0.2Al_0.8N film-based FBAR to confirm the influence of different liquid environments on the resonant performances. The results show that the frequency drift of FBAR in the shear mode is related to density and viscosity of liquid, and the frequency drift of FBAR in the longitudinal mode is related to bulk modulus and density of liquid. The resonant frequency of FBAR in the shear mode is more sensitive with glycerol solution than that of FBAR in the longitudinal mode. This work can provide a research basis for the application of FBAR liquid sensors.