This work demonstrates a group of shear horizontal (SH0) mode resonators and filters using lithium niobate (LiNbO₃) thin films on silicon carbide (SiC). The single-crystalline X-cut LiNbO₃ thin films on 4H-SiC substrates have been prepared by ion-slicing and wafer-bonding processes. The fabricated resonator has demonstrated a large effective electromechanical coupling ( k²) of 26.9% and a high-quality factor (Bode-Q) of 1228, hence resulting in a high figure of merit (FoM = k² · Bode-Q) of 330 at 2.28 GHz. Additionally, these fabricated resonators show scalable resonances from 1.61 to 3.05 GHz and impedance ratios between 53.2 and 74.7 dB. Filters based on demonstrated resonators have been demonstrated at 2.16 and 2.29 GHz with sharp roll-off and spurious-free responses over a wide frequency range. The filter with a center frequency of 2.29 GHz shows a 3-dB fractional bandwidth of 9.9%, an insertion loss of 1.38 dB, an out-of-band rejection of 41.6 dB, and a footprint of 0.75 mm². Besides, the fabricated filters also show a temperature coefficient of frequency of -48.2 ppm/°C and power handling of 25 dBm. Although the power handling is limited by arc discharge and migration-induced damage of the interdigital electrodes and some ripples in insertion loss and group delay responses are still present due to the transverse spurious modes, the demonstrations still show that acoustic devices on the LiNbO₃-on-SiC platform have great potential for radio-frequency applications.

In this work, we present first-order antisymmetric (A1) mode resonators in 128° Y-cut lithium niobate (LiNbO₃) thin films with electromechanical coupling coefficients (k²) as large as 46.4%, exceeding the state-of-the-art. The achievable k² of A1 in LiNbO₃ substrates of different orientations is first explored, showing X-axis direction in 128° Y-cut LiNbO₃ among the optimal combinations. Subsequently, A1 resonators with spurious mode mitigation are designed and fabricated. In addition to the large k², the implemented devices show a maximum quality factor (Q) of 598 at 3.2 GHz. Upon further optimization, the reported platform can potentially deliver a wideband acoustic-only filtering solution in 5G New Radio. [2020-0003]

Recent advancements in the field of piezoelectric micro-resonators have produced devices, such as lithium niobate laterally vibrating resonators, with very high electromechanical coupling factors (kₜ²) and respectable quality factors (Q). As a result, the records of the figure of merit (FoM) for radio-frequency MEMS resonators have been broken several times in the past five years. As exciting as these high FoMs are, they impose unique caveats in accurately extracting the electromechanical coupling often due to the presence of spurious modes. It is a less noted issue for micro-resonators with moderate kₜ², as spurious modes either are absent or do not significantly affect the common extraction technique based on identifying resonances and anti-resonances. This paper will first theoretically analyze how disregarding spurious modes can potentially lead to inaccurate extraction of kₜ² of the intended mode and then offer a framework that accounts for spurious modes and accurately extracts electromechanical coupling using a multi-resonance recursive fitting. [2018-0209]

This paper presents a short review of the microwave acoustics area, where exciting material innovations and performance advancements have been made in the past decade. The ever-growing demand for more sophisticated passive signal processing functions on-chip has fueled these developments. As a result, microwave acoustic devices have maintained performance leadership in mobile applications. By evaluating three fundamental parameters, namely electromechanical coupling ( k ² ), quality factors, and frequency scalability, of microwave acoustics, this paper aims to, extensively but not exhaustively, capture the rationales behind approaches achieving higher performance microsystems. Outlooks for different material systems and addressing their underlying challenges are also offered in hopes of establishing a balanced roadmap for future microwave acoustics development.

This work presents a new class of microelectromechanical system (MEMS) resonator toward 60 GHz for the fifth-generation (5G) wireless communications. The wide range of the operating frequencies is achieved by resorting to different orders of the antisymmetric Lamb wave modes in a 400-nm-thick Z-cut lithium niobate thin film. The resonance of 55 GHz demonstrated in this work marks the highest operating frequency for piezoelectric electromechanical devices. The fabricated device shows an extracted mechanical Q of 340 and an fx Q product of 1.87 x 10¹³ in a footprint of 2 x 10⁻³ mm². The performance has shown the strong potential of LiNbO₃ antisymmetric mode devices for front-end applications in 5G high-band.

This paper presents a new class of micro-electro-mechanical system (MEMS) C-band filters for 5G front-ends. The filter is comprised of resonators based on the first-order asymmetric Lamb wave (A1) mode in thin-film lithium niobate. Two filters have been demonstrated at 4.5 GHz with sharp roll-off, flat in-band group delay, and spurious-free response over a wide frequency range. The first design shows a 3-dB fractional bandwidth (FBW) of 10%, an insertion loss (IL) of 1.7 dB, an out-of-band (OoB) rejection of -13 dB, and a compact footprint of 0.36 mm ² , while the second design shows a 3-dB FBW of 8.5%, an IL of 2.7 dB, an OoB rejection of -25 dB, and a footprint of 0.9 mm ² . The demonstrations herein mark the largest FBW achieved for acoustic-only filters at 5G frequencies.