Junyao Shen

Wideband devices are increasingly needed with the development of wireless communication. To simplify the structures of wideband SAW devices and further explore the bandwidth limit, this work researches on ultra-wideband SAW devices using simplified bilayer structures. $X$ -cut LiNbO₃/Si structure is proposed and resonators with different propagation directions are simulated. $X$ - $175^{\circ }Y$ LiNbO₃/Si is the optimized design when strengthening the main mode and suppressing the spurious mode are well balanced. SAW resonators with different thicknesses of LiNbO₃ and Cu electrodes are theoretically investigated, and resonators with different wavelengths are fabricated. Results show that the electromechanical coupling factor of the resonators can reach about 29% and the admittance ratio can be larger than 70dB. Meanwhile, the spurious mode is sufficiently suppressed. Ultra-wideband and low-loss SAW filters are designed and fabricated. An ultra large bandwidth of 31.92% is achieved. Atotally flat passband and a minimum insertion loss of 0.99dB prove that the filters have high performances but not only a large bandwidth. This work further extends the bandwidth limit of SAW devices and reveals the application possibility of simplified bilayer structure $X$ - $175^{\circ }Y$ LiNbO₃/Si, effectively and economically developing the technology of wideband SAW devices. [2025-0244]

High-performance wideband surface acoustic wave (SAW) devices are essential components for wireless communication systems in the new age, and spurious modes and temperature stability are two important properties for wideband devices. Nevertheless, preparing wideband spurious-free SAW devices with low temperature drift is still a big challenge. To achieve four strengths including large bandwidth, low temperature drift, suppressed Rayleigh mode and suppressed transverse modes in the meantime, this work proposes and theoretically investigates SAW devices based on three structures including SiO2/Cu/Y-X LiNbO3/69°Y-90°X quartz, Cu/Y-X LiNbO3/69°Y-90°X quartz, and Al/41°Y-X LiNbO3/69°Y-90°X quartz. The characteristics of these devices are revealed and some optimized designs based on these structures are provided. Wideband SAW devices with nearly-zero temperature coefficients of frequency and suppressed Rayleigh mode can be achieved when SiO2/Cu/Y-X LiNbO3/69°Y-90°X quartz structure is applied. Transverse modes are weakened by quartz and can be more effectively suppressed if transverse mode suppression designs of interdigital transducers are applied, so the four strengths can be achieved meanwhile. The devices based on the other two structures can have at least two strengths as well as some unique properties such as ultra-large electromechanical coupling factor of 34.20% and suppressed transverse mode generation, which are suitable for different applications. This work provides more possible structures and designs for the engineers in filter design, and promotes the development of high-performance wideband SAW devices.

This article presents an investigation into the use of nanoscale phononic crystals (PnCs) as reflectors for surface acoustic wave (SAW) resonators, with a focus on pillar-based PnCs. Finite element analysis was employed to simulate the phononic dispersion characteristics and to study the effects of the pillar shape, material and geometric dimensions on achievable acoustic bandgap. To validate our concept, we fabricated SAW resonators and filters incorporating the proposed pillar-based PnC reflectors. The PnC-based reflector shows promising performance, even with smaller number of PnC arrays. In this regard, with a PnC array reflector consisting of 20 lattice periods, the SAW resonator exhibits a maximum bode-Q of about 1600, which can be considered to be a reasonably high value for SAW resonators on bulk 42° Y-X lithium tantalate (42° Y-X LiTaO₃) substrate. Furthermore, we implemented SAW filters using pillar-based PnC reflectors, resulting in a minimum insertion loss of less than 3 dB and out-of-band attenuation exceeding 35 dB. The authors believe that there is still a long way to go in making it fit for mass production, especially due to issues related with the accuracy of fabrication. But, upon its successful implementation, this approach of using PnCs as SAW reflectors could lead to reducing the foot-print of SAW devices, particularly for SAW-based sensors and filters.

This paper proposes a double metal dots interdigital transducer (IDT) configuration for suppression of spurious responses in temperature compensated surface acoustic wave (SAW) resonators employing the SiO ₂ /LiNbO ₃ structure. The proposed IDT configuration includes primary metal dots at the edges of the IDT aperture region, and additionally, secondary metal dots placed near the bus-bars with addition of short dummy electrodes. This double metal dots IDT configuration enables us to effectively suppress two kinds of spurious resonances: the transverse resonances that are common and the ones that are predominantly excited in the gap regions between IDT finger edges and busbars. The impact of geometric parameters on suppressing spurious responses is studied using periodic three-dimensional finite element simulations. Then, it is shown both theoretically and experimentally that the proposed mechanism can effectively suppress almost all spurious responses without affecting the critical performance metrics such as the quality factor and electromechanical coupling coefficient.

As an essential functional block in radio frequency (RF) front-ends, surface acoustic wave (SAW) filters with large fractional bandwidth (FBW), high center frequency ( $f_{c})$ , and low loss are highly sought-after in 5G new radio (NR). To address this end, 32 $^{\circ}$ Y-X LiNbO $_{3}$ /SiO $_{2}$ /Si heterostructure with high electromechanical coupling coefficient of 23% was constructed, while the propagation characteristics as well as spurious resonance were analyzed through finite element method (FEM) and one-port resonators measurement. It is revealed that by using thin electrodes in the series resonator and thick electrodes in the parallel resonator, spurious modes are successfully eliminated out of the passband. As a result, the SAW filters formed by the above resonators are capable of several superb RF performances: a large bandwidth of 730 MHz (FBW of 19.5%) at $f_{c}$ of 3.74 GHz, a small insertion loss (IL) of 3.03 dB, and a flat passband. Furthermore, since our devices were fabricated by standard photolithography process, this work advances commercial implementation of SAW filter in 5G NR.

This research aims to overcome the tradeoff between bandwidth, temperature stability of frequency, and loss in surface acoustic wave filters. A novel multilayered structure, SiO ₂ /Cu-grating/ $15^{\circ }~{Y}$ - ${X}$ LiNbO ₃ /SiO ₂ /sapphire, is proposed. Resonators based on the multilayered structure are studied theoretically and experimentally. The results show that the multilayered structure can sufficiently suppress leaky wave and spurious responses, which significantly degrade the performance of devices based on conventional SiO ₂ /Cu-grating/ $15^{\circ }~{Y}$ - ${X}$ LiNbO ₃ structure. Fabricated resonators exhibit high performance including low frequency drift, large coupling, and large quality factor. A multilayered structure filter is designed, fabricated, and measured, resulting in a flat and clean passband with a small minimum insertion loss of only 1.01 dB. A large 3-dB fractional bandwidth of 14.74% is achieved, as well as a low temperature coefficient of frequency of approximately −18 ppm/°C. This research proposes a useful solution for high-performance filters in fifth-generation networks.

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 development of 5G has put forward a higher demand for filters, and the purpose of this work is fabricating surface acoustic wave (SAW) filters with excellent temperature stability and high power handling using LiTaO ₃ /SiC bonded wafers. SAW resonators with different wavelengths are fabricated on LiTaO ₃ /SiC, and LiTaO ₃ /Si, bulk LiTaO ₃ as well. We evaluate these resonators, concluding that the bilayer substrates can enhance the performance. Subsequently, SAW filters are designed and fabricated. For the filters based on LiTaO ₃ /SiC, the center frequency is around 2.62 GHz and the minimum insertion loss is 1.90 dB. The whole passband is flat and larger than 200 MHz, and the return loss is larger than 10 dB. SiC is more effective than Si in the enhancements of temperature stability and power handling. SiC reduces the temperature coefficient of the frequency at the left side of the passband to about half of that on LiTaO ₃ /Si and one-sixth of that on bulk LiTaO ₃ , and enlarges the peak power handling to 35.7 dBm. The time-to-failure of the filters on LiTaO ₃ /SiC is 3.8 times as long as LiTaO ₃ /Si and 11.3 times the level of bulk LiTaO ₃ , when an input power of 30 dBm is applied ceaselessly. This work demonstrates the potential of SAW filters based on LiTaO ₃ /SiC for RF filters in 5G. [2021-0233]

Surface acoustic wave (SAW) devices based on multi-layer structures have been widely used in filters and sensors. The electromechanical coupling factor (K²), which reflects energy-conversion efficiency, directly determines the bandwidth of the filter and the sensitivity of sensor. In this work, a new configuration of dual-mode (quasi-Rayleigh and quasi-Sezawa) SAW devices on a ZnO/SiC layered structure exhibiting significantly enhanced K² was studied using the finite element method (FEM), which features in the partial etching of the piezoelectric film between the adjacent interdigitated electrodes (IDTs). The influences of piezoelectric film thickness, etching ratio, top electrodes, bottom electrodes, and the metallization ratio on the K² were systematically investigated. The optimum K² for the quasi-Rayleigh mode and quasi-Sezawa mode can exceed 12% and 8%, respectively, which increases by nearly 12 times and 2 times that of the conventional ZnO/SiC structure. Such significantly promoted K² is of great benefit for better comprehensive performance of SAW devices. More specifically, a quasi-Rayleigh mode with relatively low acoustic velocity (V_p) can be applied into the miniaturization of SAW devices, while a quasi-Sezawa mode exhibiting a V_p value higher than 5000 m/s is suitable for fabricating SAW devices requiring high frequency and large bandwidth. This novel structure has proposed a viable route for fabricating SAW devices with excellent overall performance.