Wideband Surface Acoustic Wave Filters Research Articles

Large-Range Spurious Mode Elimination for Wideband SAW Filters on LiNbO₃/SiO₂/Si Platform by LiNbO₃ Cut Angle Modulation.

A 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 lead 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 the 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 ( [Formula: see text]) ranging from 0.1 to 0.19 when θ is between 19° and 22° on the LN/SiO2/Si platform, in contrast to the quite narrow REW on bulk LN restricted to some specific [Formula: see text]. 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. In addition, the typical spurious-free wideband SAW filter with [Formula: see text] nm based on the 20°YX-LN/SiO2/Si platform demonstrates high performance with a center frequency ( [Formula: see text]) 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 fifth-generation (5G) applications.

Fractional Bandwidth up to 24% and Spurious Free SAW Filters on Bulk 15°YX-LiNbO3 Substrates Using Thickness-Modulated IDT Structures

To cope with ubiquitous wireless connectivity and the increased and faster data delivery in 5G communication, surface acoustic wave (SAW) filters are progressively requiring wider bandwidths. Conventional bulk 15°YX-LiNbO3 substrates with a large coupling coefficient (K2) are attractive for the low-cost mass production of wideband SAW filters, but these generally suffer from spurious responses, limiting their practical application. In this work, a novel and simple SAW configuration is proposed that uses thickness-modulated interdigital transducer (IDT) structures to overcome the limitations set by spurious responses. Different from the conventional design where the thicknesses of the IDT electrodes in the series and parallel resonators generally kept the same, the proposed configuration adopts IDT electrodes of different thicknesses in the series and shunt resonators to suppress or remove unwanted spurious Rayleigh modes from the filter passband. Two different ultra-wideband SAW filter designs employing thickness-modulated IDTs were designed and fabricated to validate the effective suppression of spurious modes. The SAW filters experimentally featured spurious-free responses in the passband as well as a large 3 dB fractional bandwidth (FBW) in the 18.0% and 24.1% ranges and low insertion losses below 1 dB. This work can significantly broaden the range of applications for SAW devices and can open a pathway to commercialize ultra-wideband SAW filters in 5G communication systems.

Investigation of Surface Acoustic Wave Propagation Characteristics in New Multilayer Structure: SiO2/IDT/LiNbO3/Diamond/Si.

Surface acoustic wave (SAW) devices are widely used in many fields such as mobile communication, phased array radar, and wireless passive sensor systems. With the upgrade of mobile networks, the requirements for the performance of SAW devices have also increased, and high-frequency wideband SAW devices have become an important research topic in communication systems and other application fields. In this paper, a theoretical study for the realization of a layered SAW filter based on a new SiO2/IDT/128°YX-LiNbO3/diamond/silicon layered structure using the modeling software COMSOL Multiphysics is presented. The effects of lithium niobate (LiNbO3), an interdigital transducer (IDT), and SiO2 thin films on the evolution of the phase velocity, electromechanical coupling coefficient (k2), and temperature coefficient of frequency were studied by employing a finite element method simulation. Furthermore, a longitudinal coupling resonator filter was designed. To investigate the SAW characteristics of the filter, a transient analysis was conducted to calculate the electrical potential and particle displacement under the resonance condition and to analyze the frequency response. The study concluded that this new multilayer structure can be applied to design and manufacture a variety of high-frequency and wideband SAW filters with a temperature compensation function, for operation above the GHz range.

Microwave Irradiation Process for Al\u2013Sc Alloy Production

Scandium is being explored as an alloying element for aluminium alloys, which are gaining importance as high-performance lightweight structural alloys in the transportation industry. Sc-rich ScAlN thin films show strong piezoelectricity and can be fabricated on a hard substrate for use as wideband surface acoustic wave filters in next-generation wireless mobile communication systems. However, the use of ScAlN thin films in microelectromechanical system devices is limited by the high cost of metallic Sc, which is due to the difficulty in smelting of this material. Here, we propose a novel microwave irradiation process for producing Al-Sc alloys, with Mg ions as a reducing agent. Although scandium oxide is thermodynamically stable, intermetallic Al3Sc is obtained in high yield (69.8%) via a low-temperature (660 °C) reduction reaction under microwave irradiation. Optical spectroscopy results and thermodynamic considerations suggest a non-thermal equilibrium reaction with the univalent magnesium ions excited by microwave irradiation.

High-frequency V-doped ZnO/SiC surface acoustic wave devices with enhanced electromechanical coupling coefficient

The rapid development of large-volume and high-speed mobile communication systems has increased the demand for high-frequency and wide-band surface acoustic wave (SAW) devices. In this work, ZnO films and V-doped ZnO (V:ZnO) films with (0002) orientation were grown on SiC substrates using a magnetron sputtering method. High-frequency SAW resonators with the resonant frequency ranging from 4 GHz to 6 GHz were fabricated on the above structures. V:ZnO/SiC SAW resonators exhibited a significantly increased electromechanical coupling coefficient (K2) in the range of 2.80%–5.12%, in a wide normalized thickness range, which is more than a 75% increase compared to that of ZnO-based SAW resonators. Besides, the high quality factor Q ranging from 431 to 593 and an improvement in the figure of merit value were observed for the V:ZnO/SiC SAW resonators operating at 4–6 GHz. Finally, 4.58 GHz SAW filters using V:ZnO films with a larger bandwidth and a lower insertion loss were achieved. This work clearly shows that the ZnO/SiC SAW properties can be improved by V doping, and the V:ZnO/SiC structures have great potential for application in high-frequency and wide-band SAW filters.

Enlarged phase velocities of ultra-wideband surface acoustic wave devices with relaxor based ferroelectric single crystal/diamond layered structure

The surface acoustic wave (SAW) characteristics of Y-cut X propagating Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 (YX-PIMNT) single crystals on a diamond substrate have been theoretically calculated. The simulated results show that the phase velocity of the shear horizontal (SH) SAW may be greatly enhanced from the 1350m/s to 3350m/s by reducing the thickness of the PIMNT from 0.5λ to 0.05λ, with a corresponding decrease in the electromechanical coupling factor (K2) from 73.6% to 19.6%. The dispersion curves of phase velocity and K2 as a function of PIMNT thickness are given for the proposed layered structure. Besides the SH SAW, there are also higher order modes that would cause unwanted responses in the pass-band of wideband SAW filters. These were suppressed by properly controlling structural parameters including top electrode thickness, thickness and Euler angle (θ) of PIMNT substrate. The calculated results demonstrate the effectiveness of this approach to enlarge the phase velocity of the SH SAW without dramatically sacrificing its K2, which makes relaxor based ferroelectric single crystals promising for realizing ultra-wideband SAW devices working in ~GHz range.

Ultra-wideband surface acoustic wave resonator employing Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 crystals

Theoretical analysis showed that high performance surface acoustic wave (SAW) resonator with an extremely large electromechanical coupling factor K2 could be realized on Y cut X propagating Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 (YX-PIN-PMN-PT) substrate. A one-port SAW resonator was designed, fabricated, and characterized on Cu-grating/YX-PIN-PMN-PT substrate structure. The experimental results demonstrate that very large K2 of 57.3% is realizable. Such an extremely large K2 makes PIN-PMN-PT single crystal great potential for realizing wideband SAW filters and tunable SAW filters. The influence of inhomogeneity and instability of the domain structure on the fabricated resonators has been discussed thoroughly for further improvement of the devices.

A novel weighted method for layered SAW filters using slanted finger interdigital transducers

In this paper, we propose a novel weighted method to improve the inclined pass-band problem that is intrinsic to surface acoustic wave (SAW) filters with slanted finger interdigital transducers (SFITs). By the dispersive characteristics of SAW in layered piezoelectric substrates, the present method could make the inclined pass-band flatter or the bandwidth wider. We derived a coupling-of-modes model for the calculation of the frequency responses of layered SAW devices with SFITs and calculated the dispersive characteristics of an AlN/silicon layered piezoelectric substrate, such as the phase velocity and the electromechanical coupling coefficient. Then we utilized the dispersive electromechanical coupling coefficient to flatten the inclined pass-band and the dispersive phase velocity to widen the bandwidth. Two designed examples of wide-band layered SAW filters using SFITs on AlN/silicon substrates are presented to prove the validity of the proposed method: one is designed to flatten the inclination of the pass-band; the other, to widen the bandwidth. Results show that the dispersions of layered piezoelectric media are favourable for improving the performance of SFIT-based SAW filters.

Research of Super-High Electromechanical Coupling Surface Acoustic Wave Substrates

The properties of surface acoustic wave (SAW) devices mainly depend on the choice of piezoelectric substrate materials. The important properties required for SAW substrates are a large electromechanical coupling coefficient (k2), small temperature coefficient of delay (TCD), and low propagation loss, etc. Especially, super-high coupling substrates are very important for obtaining wide-band SAW filters, wide-band voltage controll oscilater (VCO) and delay lines with small insertion loss. KNbO3 single crystal has an extremely large electromechanical coupling coefficient (k2=53%) and zero TCD at approximately room temperature. Large-scale single-crystal growth technologies are now under investigation. In this study, the propagation characteristics of SAW in PZNT and KTP substrates are investigated experimentally in order to obtain higher coupling substrates than those of KNbO3. The experimental results show that the substrates have large k2. Also, theoretical SAW propagation characteristics with a k2 of about 100% are analyzed and their physical phenomena are discussed.

Single crystal growth of KNbO3 and application to surface acoustic wave devices

Potassium niobate (KNbO 3 ) has large piezoelectric constants. The surface acoustic wave (SAW) substrates with high couplings are very important for wide-band SAW filters and SAW devices. Therefore, the large size KNbO 3 single crystals are required for SAW device applications. In this paper, the techniques of a large size of KNbO 3 using the top seeded solution growth techniques (TSSG) with large size of crucible of 60 mm φ platinum are described. The results show that 50×50×15 mm 3 single crystals are obtained. Also, the poling techniques of KNbO 3 crystal are investigated by using the nonlinear scanning dielectric microscope. The propagation characteristics of SAW and piezoelectric leaky surface waves in KNbO 3 single crystal were investigated theoretically. The results show that the electro-mechanical coupling coefficient of the surface wave propagating along the X -axis of the rotated Y -cut plane is extremely large: K 2 =0.53, compared to K 2 of 0.055 for LiNbO 3 . The experimental results of the above KNbO 3 substrates agree with the theoretical ones. Also the KNbO 3 substrates showed experimentally the zero temperature coefficients of frequency (TCF) around 20 °C.

Technique for low-loss and wideband surface acoustic wave filters

A ring surface acoustic wave filter on a high electromechanical coupling coefficient substrate is demonstrated. The insertion loss is 1.9 dB and the 3 dB fractional bandwidth is 30%. The filter also shows linear phase characteristics and low-passband ripples. The insertion loss is the lowest reported with such a wide bandwidth.

Superhigh Electromechanical Coupling and Zero-Temperature Characteristics of KNbO 3 and Wide Band Filter Applications

In this paper, a superhigh electromechanical coupling surface acoustic wave (SAW) propagating on a potassium niobate (KNbO3) substrate is described and its applications for low-loss, wide band and temperature stable SAW filters are investigated. KNbO3 has large piezoelectric constants. We obtained the electromechanical coupling coefficient (K2) of 0.53 for SAW in a Y-cut X-propagating substrate, theoretically and experimentally. It is observed to be about 10 times as large as that in LiNbO3. Moreover, we obtained the zero temperature characteristics around room temperature, in a rotated Y cut substrate. The experimental results of the wide band SAW filters show low loss and stable temperature characteristics. The bandwidth is about 20% and the insertion losses are less than 2–6 dB. The simulation results of a ladder-type filter using the Y-cut KNbO3 show a bandwidth of 40%.