The rapid development of the fifth-generation (5G) wireless system is driving strong demand for high-performance radio frequency filters. This work studies shear horizontal surface acoustic wave (SAW) devices using 15°-rotated Y-cut X-propagating (15°Y-X) LiNbO ₃ /SiO ₂ /SiC multilayered substrates. Single-crystalline 15°Y-X LiNbO ₃ films are bonded to SiO ₂ /SiC handling substrates by the smart cut technology. On the basis of accurate finite-element-method simulations, LiNbO ₃ /SiO ₂ /SiC wafer configurations are optimized to suppress spurious resonance due to Rayleigh-mode and transverse-mode responses, and one-port resonators with a clean spectrum, a high electromechanical coupling coefficient of 22.00%, and an admittance ratio (impedance ratio) over 65 dB are successfully implemented. Based on the characteristics of the resonators, high-performance filters with a center frequency of 1.28 GHz, a large 3-dB fractional bandwidth of 16.65%, and a low minimum insertion loss of 1.02 dB are successfully designed and fabricated. Furthermore, no ripples in the passband of the filters are observed. Additionally, the filters exhibit a temperature coefficient of center frequency of -63.8 ppm/°C and a large power durability of 33.2 dBm. This work confirms the high performances of the SAW devices using the 15°Y-X LiNbO ₃ /SiO ₂ /SiC multilayered substrate, and this type of SAW device exhibits a prospect of commercial applications in the 5G wireless system.

With the development of the radio frequency technique, the explosive growth of transmitted data in 5G era puts higher requirements on surface acoustic wave (SAW) filter bandwidth. In this work, 15°YX-LiNbO3(LN)/SiO2/Si multilayer structure was proposed, where both LN and SiO2 films possess uniform thickness and the interfaces between films are quite clear. With hierarchical cascading algorithm, the spurious resonance that resulted from Rayleigh mode was minimized through the modulation of Cu electrode thickness. One-port resonators measurement results confirm a high electromechanical coupling coefficient of shear horizontal mode ranging from 22.5% to 25.2%. Furthermore, a ladder-type filter with a center frequency of 1279 MHz and a large fractional bandwidth of 20.2% was successfully fabricated. Excellent bandpass filtering properties were achievable with minimum insertion loss of 0.8 dB and in-band fluctuation less than 0.9 dB. Multilayer structure SAW filters also exhibited a temperature coefficient of frequency of -57.7 ppm/°C and a power durability of 33 dBm, which are both significant improvements compared with that of devices built on bulk 15°YX-LN substrate. This work provides an effective solution for wideband and low-loss radio frequency filters in 5G communication systems, suitable for large-scale application and commercial promotion.

5G communication system has experienced a substantial expansion of the spectrum, which poses higher requirements to radio frequency (RF) filters in enhancing their operating frequencies and bandwidths. To this end, this work focused on solving the filtering scheme for challenging 5G n77 and n78 bands, and successfully implemented corresponding spurious-free surface acoustic wave (SAW) filters exploiting large-coupling shear horizontal (SH) modes based on X-cut LiNbO ₃ (LN)/SiC heterostructure. Here, we initially investigated the suppression methods for spurious modes theoretically and experimentally, and summarized an effective normalized LN thickness ( h _LN /λ) range of 0.15 to 0.30 for mitigating Rayleigh modes and higher order modes, as well as tilted interdigital transducers (IDT) by about 24° for eliminating transverse modes. Resonators with wavelengths (λ) from 0.95 to 2 μm were also fabricated, showing a scalable resonance from 2.48 to 4.21 GHz without any in-band ripple. Two filters completely meeting 5G n77 and n78 full bands were finally constructed, showing center frequencies ( f_c ) of 3763 MHz and 3560 MHz, 3-dB fractional bandwidths (FBW) of 24.8% and 15.6% and out-of-band ( OoB ) rejections of 18.7 dB and 28.1 dB, respectively. This work reveals that X-LN/SiC heterostructure is a promising underpinning material for SAW filters in 5G commercial applications.

In this letter, highly (0002) oriented ZnO piezoelectric films were deposited on high acoustic velocity SiC substrates. Surface acoustic wave (SAW) devices on ZnO/SiC layered structure operating in the fundamental mode with high frequency to the 7-GHz range were successfully fabricated for the first time. The comprehensive experimental and theoretical investigations about phase velocities Vp, the electromechanical coupling coefficients K ² , and quality factors Q of one-port SAW resonators have been studied considering various ZnO normalized thicknesses h/λ. The resonators with large K2 and high Q are implemented over 5-7 GHz demonstrating K ² of 0.47%-2.83% and Q of 146-549. Specifically, a high K ² of 2.83% and a large Q of 549 are simultaneously achievable for the resonator at 5.19 GHz. Finally, with the high Vp of ZnO/SiC structure being up to 6800 m/s at h/λ = 0.14, a 6.8-GHz SAW filter was achieved. Our work shows that the ZnO/SiC structure is of great potential for high-frequency SAW devices application.

LiNbO3(LN)/SiO2/Si multilayered structure was recently reported as a new platform for achieving wideband radio frequency (RF) filters. However, the in-band ripples in filters resulting from the spurious Rayleigh mode, leads to deteriorated performance, and thus a wide Rayleigh elimination window (REW) is highly desired for realizing spurious-free wideband surface acoustic wave (SAW) filters with a wide design space and good process tolerance. Here, we investigated the spurious mode suppression on the LN/SiO2/Si platform theoretically and experimentally through modulating the cut angle (θ) of LN. The K2 dispersion characteristics of the main mode (shear horizontal wave) and spurious mode (Rayleigh wave) on LN/SiO2/Si substrates were systematically analyzed by finite element method (FEM), along with bulk LN for comparison. It is found that the REW is wider on LN/SiO2/Si than bulk LN, as Rayleigh wave can be totally eliminated with Cu electrode normalized thickness (hCu/λ) ranging from 0.1 to 0.19 when θ is between 19° and 22° on LN/SiO2/Si platform, in contrast to the quite narrow REW on bulk LN restricted to some specific hCu/λ. To verify the simulation results, resonators were prepared on 15°YX-LN/SiO2/Si, 20°YX-LN/SiO2/Si, bulk 15°YX-LN and bulk 20°YX-LN. Additionally, the typical spurious-free wideband SAW filter with hCu = 200 nm based on the 20°YX-LN/SiO2/Si platform demonstrates high performance with a center frequency (fc) of 1.27 GHz, a minimum insertion loss (ILmin) of 0.7 dB and a 3-dB fractional bandwidth (FBW) of ~20.1%. This work provides a workable solution in fabricating spurious-free wideband and low-loss SAW filters for 5G applications.

The fast development of the fifth-generation (5G) wireless systems and substantial growth of data usage have imposed stringent requirements for high-frequency and wideband radio frequency devices. Here, it is reported on a longitudinal leaky surface acoustic wave (LLSAW) mode acoustic resonator with a large electromechanical coupling factor ($k_{\mathrm{t}}^2$ ), high operating frequency, and efficient spurious suppression. Through systematical finite element method simulations, available design spaces such as supporting substrate, propagation angle, and lithium niobate (LN) thickness have been fully investigated with the aim of stimulating the intended LLSAW and suppressing spurious modes concurrently. Optimization results reveal that the LLSAW mode wave propagating in X-35°Y LN/SiC piezoelectric-on-insulator (POI) bilayer structure possesses a large $k_{\mathrm{t}}^2$ without significant interference from other spurious modes. To verify the theoretical analyses, LLSAW resonators were fabricated and exhibited a near spurious-free response with the operating frequency over 6 GHz, and $k_{\mathrm{t}}^2$ as large as 22.7%. This work demonstrates a high-performance LLSAW resonator on the POI platform with a simple prototype as well as potentially providing a high-frequency filtering solution for 5G applications in the 6-GHz spectrum.