With the increase of operating frequency above 4 GHz, the required resonator size is reduced to hundreds of $\mu \text{m}^{{2}}$ , which is proportional to 1/ ${f}^{\,{2}}$ . In traditional, the size reduction of the resonator will induce its parallel resonant frequency decrease, thus resulting in the decrease of its effective coupling coefficient ( ${k}_{\,{t}}^{\,{2}}$ ). Besides the parallel resonant frequency, we found that the serial resonant frequency of resonators working above 3 GHz also varies with its area size. However, this effect is not taken into account in conventional circuit models, leading to the failure of high-performance filter design. To address this issue, a modified Mason model of bulk acoustic wave (BAW) resonators was proposed in this work. Using the modified model, a high-performance N79 band (4.8–4.96 GHz) filter based on Al0.904Sc $_{{0.096}}\text{N}$ piezoelectric film was designed and fabricated, which achieved a minimum insertion loss of −0.93 dB and a bandwidth of 240 MHz. The power capacity and wire-bond package of BAW filters were further studied. The whole process of the Mason model modification, filter design, and fabrication pave the road for high-frequency filter applications.

To meet the stringent requirements of 5G communication, we proposed a high-performance bulk acoustic wave (BAW) filter based on single crystal AlN piezoelectric films on a SiC substrate. The fabrication of the BAW filter is compatible with the GaN high electron mobility transistor (HEMT) process, enabling the implementation of the integration of the BAW device and high-performance monolithic microwave integrated circuit (MMIC). The single crystal AlN piezoelectric film with 650-nm thickness was epitaxially grown on the SiC substrate by Metal Organic Chemical Vapor Deposition (MOCVD). After wafer bonding and substrate removal, the single crystal AlN film with electrode layers was transferred to another SiC wafer to form an air gap type BAW. Testing results showed that the fabricated resonators have a maximum Q-factor up to 837 at 3.3 GHz resonant frequency and electromechanical coupling coefficient up to 7.2%. Ladder-type filters were developed to verify the capabilities of the BAW and process, which has a center frequency of 3.38 GHz with 160 MHz 3 dB bandwidth. The filter achieved a minimum 1.5 dB insertion loss and more than 31 dB out-of-band rejection. The high performance of the filters is attributed to the high crystallinity and low defects of epitaxial single crystal AlN films.

BAW (Bulk Acoustic Wave) filters, commonly implemented in ladder or lattice topologies, have been widely adopted as RF filters for high frequencies. However, the bandwidth of these filter topologies is limited by the effective electromechanical coupling coefficient of the resonators, which is decided by the piezoelectric material and is difficult to increase. In this study, a new topology is proposed to construct a N77 full band filter with high steep skirts which can coexist with N79 band by hybrid integrating BAW resonators with IPD (Integrated Passive Device) devices. The passband of the filter utilizes the left part of the band-stop filter to form large bandwidth and steep skirt. BAW resonators based on Al _0.8 Sc _0.2 N are connected to IPD circuit through flip-chip bonding. An N77-band 3.3-4.2 GHz filter has been designed and fabricated, demonstrating an average insertion loss of -2 dB, an out-of-band rejection of -28 dB, an average roll-off of 250 MHz and a compact effective area of only 1 mm × 0.85 mm.

Radio frequency (RF) filters for communication have been developed rapidly, driven by new communication standards and the dramatic expansion of wide-range applications. Although they are currently playing crucial roles in applications such as mobile communication, space-to-ground communication, and the Internet of Thing, there are significantly stringent and challenging requirements demanded for their rapid and successful applications. Compared with conventionally adopted low-temperature co-fired ceramics, integrated passive device filters, and dielectric filters, acoustic wave filters have been regarded as the competitive choice, mainly attributed to their wide bandwidth, small size, and low insertion loss. This paper reviews the advances and outlines future perspectives of high frequency acoustic wave devices for RF communication, focusing on several critical issues including bandwidth, roll-off, frequency, power-handling, insertion loss, out-of-band rejection, tunability, and size/package. It is focused mainly on the extreme performance breakthroughs of RF acoustic wave filter, e.g., how to achieve acoustic devices with operating frequency above 8 GHz, bandwidth around 1 GHz, and quality factor exceeding 2000. Various principles, strategies, and technologies for achieving the superior performance of super-high frequency RF filters are discussed, e.g., applying advanced materials such as scandium-doped AlN or single crystals of AlN and LiNbO3, creating new topology structures such as hybrid filters, and generating new types of vibration modes of acoustic waves.

Bulk acoustic wave (BAW) filters are crucial components in communications systems. Ladder and lattice types are two basic filter topologies used in unbalanced and balanced systems, respectively. Compared to ladder filters, lattice filters have advantages of larger bandwidth and better trade-off between out-of-band rejection and roll-off. However, the current lattice filters are restricted by their incompatible balanced terminals with other radio frequency devices. In this work, a hybrid integration of integrated passive device (IPD) and lattice-type bulk acoustic wave (BAW) filter with unbalanced terminals was proposed. The IPD network achieved phase flip and transformed the balanced terminal into unbalanced terminal of the lattice-type BAW filter. To realize a more compact integration of the BAW device and phase-flip network, for the first time we achieved Si microcap capacitor structure, which can also be adopted for 3D integration of other acoustic and electromagnetic devices. The fabricated filter achieved a minimum insertion loss of 1.39 dB and a 3-dB bandwidth of 145 MHz, a relative bandwidth fraction of 7.2%. Compared with the ladder type filter with same resonator number, the phase-flip lattice filter achieved ~30% improvement in bandwidth. This work brings new methodology for the topology designs of lattice filters and compact integration of capacitors and inductors with BAW devices. [2025-0086]

Lamb wave resonators (LWRs) based on lithium niobate on insulator (LNOI) substrates operating in an A1 mode at frequencies above 5 GHz typically exhibit high electromechanical coupling coefficients (K2) ranging from 20% to 30%. This makes them promising candidates to replace current bulk acoustic wave technology in next-generation broadband RF filters. However, the K2 of conventional LWRs still struggles to meet application requirements for bandwidths exceeding 1 GHz, such as WiFi-6E. This paper presents an analytical optimization approach for rapid and comprehensive scanning of the full 3D Euler space to find the maximum effective K2 of LWR devices. Such a full scan is difficult to achieve by the conventional time-consuming finite element method (FEM). The K2 results for a selected subset of Euler angles obtained through this analytical approach align with FEM simulation results. The optimized results show that LWRs on LNOI can achieve the highest K2 of 59.92% at Euler angles of (0, −151°, −60°), providing bandwidths exceeding 1 GHz at 5 GHz. To validate this optimization approach, LWRs with various orientations were fabricated on both 41° YX-cut and Z-cut LNOI wafers, with measured K2 values at different Euler angles agreeing well with both analytical and FEM results. The proposed K2 optimization method serves as a valuable guideline for selecting substrate cuts and device orientations in the design of ultra-broadband filters for next-generation telecommunications.