Despite the dominance of bulk acoustic wave (BAW) filters in the high-frequency market due to their superior performance and compatible integration process, the advent of the 5G era brings up new challenges to meet the ever-growing demands on high-frequency and large bandwidth. Al_1-xSc_xN piezoelectric films with high Sc concentration are particularly desirable to achieve an increased electromechanical coupling (K_t²) for BAW resonators and also a larger bandwidth for filters. In this paper, we designed and fabricated the Al_1-xSc_xN-based BAW resonators with Sc concentrations as high as 30%. The symmetry of the resonance region, border frame structure and thickness ratio of the piezoelectric stack are thoroughly examined for lateral modes suppression and resonant performance optimization. Benefiting from the 30% Sc doping, the fabricated BAW resonators demonstrate a large effective electromechanical coupling (K_eff²) of 17.8% at 4.75 GHz parallel resonant frequency. Moreover, the temperature coefficient of frequency (TCF) of the device is obtained as −22.9 ppm/°C, indicating reasonable temperature stability. Our results show that BAW resonators based on highly doped Al_1-xSc_xN piezoelectric film have great potential for high-frequency and large bandwidth applications.

With the booming development of the 5G market in recent years, super-high frequency (SHF) resonators will play an increasingly critical role in 5G and future communication systems. Facing the growing market demand for miniaturized, high-bandwidth, and low insertion loss filters, the design of SHF resonators and filters with a high effective electromechanical coupling coefficient (K²_eff) and quality factor, low insertion loss, high passband flatness, strong out-of-band rejection, and high power handling capacity has placed high demands on piezoelectric material preparation, process optimization, and resonator design. The polarity-inverted Al(Sc)N multilayer substrate has become one of the key solutions for SHF resonators. This review provides a comprehensive overview of the recent advances in SHF Al(Sc)N bulk acoustic wave (BAW) resonators. It systematically discusses the device design methodologies, structural configurations, and material synthesis techniques for high-quality Al(Sc)N thin films. Particular emphasis is placed on the underlying mechanisms and engineering strategies for polarity control in Al(Sc)N-based periodically poled multilayer structures. The progress in periodically poled piezoelectric film (P3F) BAW resonators is also examined, with special attention to their ability to significantly boost the operating frequency of BAW devices without reducing the thickness of the piezoelectric layer, while maintaining a high K²_eff. Finally, the review outlines current challenges and future directions for achieving a higher quality factor (Q), improved frequency scalability, and greater integration compatibility in SHF acoustic devices, paving the way for next-generation radio frequency (RF) front-end technologies in 5G/6G and beyond.

This study reports the design, fabrication, and performance of Ku-band solidly mounted resonator–bulk acoustic wave (SMR-BAW) devices engineered for high-frequency operation and enhanced thermal stability. By implementing a temperature-compensated Bragg reflector (BR) and a periodically poled aluminum nitride (AlN) piezoelectric layer, we achieved improved thermal stability in the Ku-band bulk acoustic wave (BAW) resonator. The fabricated resonators exhibit a series resonance frequency ( ${f}_{s}$ ) of 14.17 GHz, a parallel resonance frequency ( ${f}_{p}$ ) of 14.36 GHz, and a maximum quality factor ( ${Q}_{\max }$ ) of 258. The temperature coefficient of frequency (TCF) for the second-order thickness-extension (TE2) mode are measured as −28.62 ppm/°C for ${f}_{s}$ and −28.99 ppm/°C for ${f}_{s}$ and ${f}_{p}$ , respectively, demonstrating a marked improvement over state-of-the-art acoustic resonators in the same frequency band. Experimental results closely match simulation designs, validating the effectiveness of materials selection and structural design. These findings demonstrate an effective pathway for thermally stable, high-frequency acoustic devices for advanced wireless communications and high-precision frequency control systems.

Bulk acoustic wave (BAW) resonators with periodically poled piezoelectric films (P3F) are highly promising for super high frequency (SHF) in 5G and the emerging 6G radio frequency (RF) applications. Despite their potential, the dispersion relationship of P3F Al(Sc)N-based BAW resonators has not been systematically explored, which remains one of the most crucial device physics for understanding and engineering device resonant characteristics. This work presents a comprehensive investigation into the dispersion behaviors of BAW resonators fabricated with periodically poled AlN/AlN and AlN/Al_0.7Sc_0.3N stacks designed for Ku-band operation. Our analysis reveals the Type I dispersive nature of the second thickness-extension (TE2) mode. Furthermore, we establish a direct correlation between this dispersion and the presence of spurious resonances, providing crucial insight into their origin. To further optimize performance, we fabricated resonators with a frame structure, which demonstrated a significantly enhanced quality factor. Moreover, the major intrinsic acoustic dissipation mechanisms of P3F Al(Sc)N-based BAW acoustic devices are systematically investigated. Taken together, our findings on dispersion, spurious modes, and dissipation mechanisms provide a foundational understanding of periodically poled Al(Sc)N BAW resonators, which points to a pathway toward optimizing next-generation acoustic devices for high-frequency applications.

This work reports the growth of high-crystallinity Al_0.8Sc_0.2N piezoelectric film on an 8-inch wafer, exhibiting a full-width at half-maximum (FWHM) of 0.32° in the (002) X-ray diffraction (XRD) rocking curve. Leveraging a cavity-embedded wafer bonding and device fabrication process, Al_0.8Sc_0.2N film bulk acoustic wave resonators (FBARs) were successfully realized with a resonant frequency at 5.149 GHz. The hero device exhibits a raw-measured maximum quality factor ( $Q_{max}$ ) of 1387, an electromechanical coupling coefficient ( $K_{eff}^{2}$ ) of 15.97%, and a figure of merit ( $\textit {FOM} =Q_{max}\cdot K_{eff}^{2}$ ) of 222. These results highlight the critical potential of heteroepitaxial AlScN BAW technology for next-generation 5G/6G and Wi-Fi communication applications.