With the rapid commercialization of fifth generation (5G) technology in the world, the market demand for radio frequency (RF) filters continues to grow. Acoustic wave technology has been attracting great attention as one of the effective solutions for achieving high-performance RF filter operations while offering low cost and small device size. Compared with surface acoustic wave (SAW) resonators, bulk acoustic wave (BAW) resonators have more potential in fabricating high- quality RF filters because of their lower insertion loss and better selectivity in the middle and high frequency bands above 2.5 GHz. Here, we provide a comprehensive review about BAW resonator researches, including materials, structure designs, and characteristics. The basic principles and details of recently proposed BAW resonators are carefully investigated. The materials of poly-crystalline aluminum nitride (AlN), single crystal AlN, doped AlN, and electrode are also analyzed and compared. Common approaches to enhance the performance of BAW resonators, suppression of spurious mode, low temperature sensitivity, and tuning ability are introduced with discussions and suggestions for further improvement. Finally, by looking into the challenges of high frequency, wide bandwidth, miniaturization, and high power level, we provide clues to specific materials, structure designs, and RF integration technologies for BAW resonators.

Microelectromechanical (MEMS) piezoelectric accelerometers are diversely used in consumer electronics and handheld devices due to their low power consumption as well as simple reading circuit and good dynamic performance. In this paper, a tri-axial piezoelectric accelerometer with folded beams is presented. The four beam suspensions are located at two sides of the mass aligned with edges of the mass, and the thickness of the beams is the same as the thickness of the mass block. In order to realize the multi-axis detection, a total of 16 sensing elements are distributed at the end of the folded beams. The structural deformations, stress distribution, and output characteristics due to the acceleration in x-, y-, and z-axis directions are theoretically analyzed and simulated. The proposed accelerometer is fabricated by MEMS processes to form Mo/AlN/ScAlN/Mo piezoelectric stacks as the sensing layer. Experiments show that the charge sensitivity along the x-, y-, and z-axes could reach up to ~1.07 pC/g, ~0.66 pC/g, and ~3.35 pC/g. The new structure can provide inspiration for the design of tri-axial piezoelectric accelerometers with great sensitivity and linearity.

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

Laterally excited bulk acoustic resonators (XBARs) using interdigital electrodes are currently limited in potential 5G applications due to the accompanied spurious modes and small capacitance. This Letter presents an XBAR with scattering vias in electrodes, named SV-BAR. The non-parallel boundaries thoroughly disrupt the coherent formation of standing waves in the lateral direction, thereby suppressing the higher-order spurious modes. The duty factor of SV-BAR is optimized as 0.5 using COMSOL Multiphysics finite element analysis for the 50 Ω impedance matching. The SV-BAR is fabricated using a 390 nm Z-cut LiNbO3 on an insulator substrate and exhibits an impressive ratio of Zp/Zs near 64.5 dB, a Kt2 of 21.80% at 4.762 GHz, as well as a large value f · Bode_Qmax · Kt2 (3.43 × 1011). The measured temperature coefficient of frequency is −69.8 ppm/°C, which can be compensated using SiO2 thin films. The SV-BAR provides an effective solution for high-performance radio frequency filters for 5G communication.

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.