Surface Acoustic Wave Filters Research Articles

An Improved Method to Compute the Mutual Capacitance between Interdigital Transducers in Radio Frequency Surface Acoustic Wave Filters

This paper proposes an improved method to calculate the mutual capacitance between interdigital transducer (IDT) electrodes to enhance the accuracy of the traditional coupling-of-modes (COM) model, which is commonly used to simulate surface acoustic wave (SAW) filters and duplexers. In this method, the boundary element method (BEM) is adopted to obtain the capacitance per unit length in a layered medium, while the partial capacitance (PC) method is used to derive the effective relative permittivity of the multi-layered IDT. Numerical results from commercially available software are provided for comparison with the results calculated using the proposed method. The consistent results verify the validity and accuracy of this method, which also demonstrates significantly faster calculation speed compared to commercially available software. Precise electrical response prediction of a dual-mode SAW (DMS) filter can be achieved by applying this method to the COM model, and this ultra-fast calculation method can also be included in filter design optimization.

Study on the Fabrication and Acoustic Properties of Near-Stoichiometric Lithium Tantalate Crystal Surface Acoustic Wave Filters

Near-stoichiometric lithium tantalate (NSLT) wafers with different Li contents were prepared by vapour transfer equilibrium (VTE) method and fabricated into surface acoustic wave filters. The temperature coefficient of frequency, insertion loss, and bandwidth of the surface acoustic wave filters were tested using a special chip test bench and a network analyzer. The results show that the temperature coefficient of frequency shows a trend of first decreasing and then increasing with the increase in Li content, and the temperature stability of the surface acoustic wave filters is best when the Li content is 49.75%. It is also found that the surface acoustic wave filter fabricated from NSLT wafers has 21.18% lower temperature coefficient of frequency, 7.3% lower insertion loss, and 2.8% lower bandwidth than those fabricated from congruent lithium tantalate wafers. Therefore, NSLT crystals are more suitable for applications in acoustic devices, providing a new idea for performance enhancement of 5G communication devices.

Plasma-enhanced chemical vapor deposition a-SiOCN:H low-Z thin films for bulk acoustic wave resonators

The 5th generation (5G) wireless telecommunication standards with newly defined frequency bands up to 6 GHz are currently established around the world. While outperforming surface acoustic wave (SAW) filters above 1 GHz, bulk acoustic wave (BAW) resonators in multiplexers for radio-frequency front-end (RFFE) modules continuously face higher performance requirements. In contrast to free-standing bulk acoustic resonators (FBARs), solidly mounted resonator (SMR) technology uses an acoustic Bragg mirror, which has already been successfully applied for several GHz applications. In this work, we investigate the potential of amorphous hydrogenated silicon-oxycarbonitride (a-SiOCN:H) thin films synthesized with low-temperature plasma-enhanced chemical vapor deposition (PECVD) as a low acoustic impedance (low-Z) material. Compared to the state-of-the-art where in Bragg mirrors up to now SiO2 is used as standard, the acoustic impedance ratio against the high-Z material tungsten (W) is enhanced for a better device performance. To limit the expected increase in viscous loss when the acoustic impedance is reduced, to a minimum, predominantly the mass density was reduced while keeping the mechanical elasticity high. By doing so, acoustic impedance values as low as 7.1 MRayl were achieved, thereby increasing the impedance ratio of high-Z to low-Z materials from 8:1 up to 14:1.

Passive bandpass filters for modern microwave communication systems

Background. Bandpass filters are an integral part of any radio engineering systems and modern communication systems. Research and development of new passive components is due to growing need for such elements for modernization and creation of new modern communication systems.
 Aim. A brief overview of passive bandpass filters. Their classification according to the type of implementation is given.
 Methods. Results of experimental research and design of different passive bandpass filters are considered.
 Results. Lumped elements filters, microstrip filters, filters on high-temperature superconductor films, filter on dielectric resonators, surface acoustic wave filters, bulk acoustic wave filters are considered. At a qualitative level, the main advantages and disadvantages are considered from point of view of electrical parameters and size indicators. Examples of topological and constructive implementation are given.
 Conclusion. The passive bandpass filters considered in the paper make it possible to implement frequency selection devices at operating frequencies up to 6 GHz and higher, taking into account modern system trends and requirements for devices of this type.

Acoustic wave devices based on piezoelectric/ferroelectric thin films for high-frequency communication systems and sensing applications

Surface acoustic wave (SAW) and bulk acoustic wave (BAW) based radio-frequency (RF) filters dominate in handsets markets mainly because they have miniature size so multiple filters can be used but take up very small board area. Besides, due to their inherent advantages, such as low insertion loss, relatively high resonant frequency, and miniature size, their application in the physical and biochemical sensing fields has also gradually expanded. The successful implementation requires specific knowledge of acoustic wave properties, the working mechanisms, material properties, and device design. So, in this review, we survey the SAW and BAW filter technology by discussing the working principles and the comparison between the operation frequency band allocations. In addition, the approaches to enhance the performance including the selection of single crystal/ferroelectric materials for BAW and the structure design optimization for SAW are introduced with discussions for further improvement. Then, flexible SAW and BAW devices are also presented and their new application opportunities in the fields of skin-like electronics and wearable health monitoring devices can be foreseen. Finally, the potential challenges of high frequency, wide bandwidth, miniaturization, and compatibility with the integrated circuits (IC) manufacturing process are overviewed as well.

Concept for Parameter Design and Optimization of Surface Acoustic Wave Devices

Concept for Parameter Design and Optimization of Surface Acoustic Wave Devices

Modeling for High-Frequency Spurious Responses in Incredible High-Performance Surface Acoustic Wave Devices.

To ensure that surface acoustic wave (SAW) filters fulfill the requirements of Carrier Aggregation (CA) applications, the development of modeling tools that can forecast and simulate high-frequency spurious responses has been necessary. This paper presents an advanced methodology for extending the coupling-of-modes (COM) model to obtain precise modeling of the high-frequency spurious responses of incredible high-performance surface acoustic wave (I.H.P. SAW) devices. The extended COM (ECOM) model is derived by modifying the conventional COM model and extending it accordingly. The parameters used in this model are determined through numerical fitting. For validation, firstly, the ECOM model is applied to a one-port synchronous I.H.P. SAW resonator, and the simulation and measurement results match. Then, the structural parameters of the ECOM model are varied, and the accuracy of the model after the structural parameters are varied is verified. It is demonstrated that this model can be applied to the design work of SAW filters. Finally, the ECOM model is applied to the design of the I.H.P. SAW filter based on a 42°YX-LiTaO3 (LT)/SiO2/AlN/Si structure. By using this method, the I.H.P. SAW filter's high-frequency spurious response can be predicted more accurately.

Microstructure evolution and the deformation mechanism in nanocrystalline superior-deformed tantalum.

The temperature-controlled relationship between the mechanical properties and deformation mechanism of tantalum (Ta) enables the extension of its application potential in various areas of life, including energy and electronics industries. In this work, the microstructure and deformation behavior of nanocrystalline superior-deformed Ta have been investigated in a wide temperature range. The structural analysis revealed that the high-performance Ta consists of several different substructures, with an average size of about 20 nm. The tensile behavior of nanocrystalline Ta (NC-Ta) was analysed and simulated at various temperatures from 100 K to 1500 K by the molecular dynamics (MD) method. It is shown that with increasing average grain size, the elastic modulus of NC-Ta linearly increases, and the impact factor reaches a value close to 1.8. The critical grain size, as obtained from the Hall-Petch relationship, was found to be about 8.2 nm. For larger grains, the flow stress follows the Hall-Petch relationship, and the thermal behavior of twin bands determines the deformation process. On the other hand, when grains are smaller than the critical size, the relationship between the flow stress and structure transforms into the inverse Hall-Petch relationship, and the deformation mechanism is controlled by grain rotation, boundary sliding or atomic migration. The results of numerical simulations revealed that temperature significantly affects the critical grain size for the plastic deformation of NC-Ta. In addition, it is demonstrated that both the elastic modulus and dislocation density decrease with increasing temperature. These findings provide guidance for the design of polycrystalline tantalum materials with tailored mechanical properties for specific industrial applications such as heat exchangers and condensers in aerospace, bone substitutes in biomedicine, and surface acoustic wave filters or capacitors in electronics.

Design for SAW Antenna-Plexers with Improved Matching Inductance Circuits.

This study designs antenna-plexers, including a surface acoustic wave (SAW) extractor and an upper- and mid-high band (UHB + MHB) diplexer, for LTE 4G and 5G bands using carrier aggregation. The SAW extractor combines a bandpass filter (BPF) and a band-stop filter (BSF) in a single unit that consists of eight modified Butterworth-van Dyke (mBVD) resonators that resonate in parallel with an inductor and SAW resonators. This BSF behaves as a high-pass filter at frequencies lower than the designed WIFI band and as a capacitor at higher frequencies. The SAW extractor meets product specifications in the frequency range 0.7 to 2.7 GHz. The UHB + MHB diplexer, which is composed of a microwave filter, a SAW filter, and a simple matching inductor, uses frequency response methods to create an RF component for 2.4 GHz + WIFI 6E applications. The design uses a SAW's interdigital transducer (IDT) structure, and the experimental results are in agreement with the simulation results, so the design is feasible.

The Extraction of Coupling-of-Modes Parameters in a Layered Piezoelectric Substrate and Its Application to a Double-Mode SAW Filter.

This paper presents an advanced method that combines coupling-of-modes (COM) theory and the finite element method (FEM), which enables the quick extraction of COM parameters and the accurate prediction of the electroacoustic and temperature behavior of surface acoustic wave (SAW) devices. For validation, firstly, the proposed method is performed for a normal SAW resonator. Then, the validated method is applied to analysis of an I.H.P. SAW resonator based on a 29°YX-LT/SiO2/SiC structure. Via optimization, the electromechanical coupling coefficient (K2) is increased up to 13.92% and a high quality (Q) value of 1265 is obtained; meanwhile, the corresponding temperature coefficient of frequency (TCF) is -10.67 ppm/°C. Furthermore, a double-mode SAW (DMS) filter with low insertion loss and excellent temperature stability is also produced. It is demonstrated that the proposed method is effective even for SAW devices with complex structures, providing a useful tool for the design of SAW devices with improved performance.

Designed Structures of Interdigital Electrodes for Thin Film SAW Devices.

This paper studied the impact of the microstructure of interdigital electrodes on the performance of surface acoustic wave (SAW) resonators and proposed an innovative piston, dummy finger and tilt (PDT) structure, which was then applied to the GLONASS L3 band filters. Through the adoption of 3D finite element simulation (FEM), photolithography, and testing on an incredible high-performance surface acoustic wave (I.H.P. SAW) substrate, it is concluded that the total aperture length is 20T (T is period), resulting in a more optimal resonator performance; changing the width and length of the piston can suppress transverse modes spurious, but it does not enhance impedance ratio; to further improve the quality of the SAW resonator, the proposed PDT structure has been experimentally proven to not only effectively suppress transverse modes spurious but also possess a high impedance ratio. By utilizing a PDT structure within a "T + π" topology circuit, we successfully designed and manufactured a GLONASS L3 band filter with a bandwidth of 8 MHz and an insertion loss of 3.73 dB. The design of these resonators and filters can be applied to the construction of SAW filters in similar frequency bands such as BeiDou B2 band or GPS L2/L5 band.

SAW Filters on LiNbO3/SiC Heterostructure for 5G n77 and n78 Band Applications.

The 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 the corresponding spurious-free surface acoustic wave (SAW) filters exploiting large-coupling shear horizontal (SH) modes based on X-cut LiNbO3 (LN)/silicon carbide (SiC) heterostructure. Here, we initially investigated the suppression methods for spurious modes theoretically and experimentally and summarized an effective normalized LN thickness ( [Formula: see text] range of 0.15-0.30 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 [Formula: see text] 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 ( fc) of 3763 and 3560 MHz, 3-dB fractional bandwidths (FBW) of 24.8% and 15.6%, and out-of-band (OoB) rejections of 18.7 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.

Phononic crystals for Love waves based on thin-film lithium niobate

This paper presents a type of surface acoustic wave (SAW) phononic crystals based on thin-film lithium niobate (LN). They are created by forming micro-pillar or micro-well structures on the LN, resulting in significant Rayleigh and Love SAW bandgaps. Especially for Love waves, they offer an irreplaceable advantage because they overcome the inability of conventional electrodes to reflect Love waves effectively. This enables the creation of high-quality, compact, high electromechanical coupling coefficient, stable and power-resistant acoustic resonators based on Love waves, potentially leading to a new generation of high-performance SAW filters and sensors. In this paper, we demonstrate the feasibility of such phononic crystals using xy-cut LN-on-SiC. However, it is worth noting that other piezoelectric materials such as lithium tantalate can also be used instead of LN, and high acoustic velocity substrates such as sapphire and diamond can be substituted for SiC.

LiNbO3 Surface Acoustic Wave Resonators with Large Effective Electromechanical Coupling

This paper reports an LNO surface acoustic wave (SAW) resonator based on a shear horizontal mode with high operating frequency over 3 GHz, large electromechanical coupling of 33.54%, Q factor of 380, and a relatively good figure of merit (FOM) of 127. Combing crystal-ion-slicing (CIS) technology with a room temperature bonding method, a 4-inch single crystalline LNO thin film on silicon is prepared successfully. The influence of damaged LNO film on crystalline quality and SAW performance is comprehensively analyzed. After totally removing the damaged layer, the electromechanical coupling and Q factor is significantly improved. The high-performance SAW resonator possesses the potential to meet the requirements of SAW filters for the fifth-generation (5G) communication in terms of high frequency, large bandwidth, and a high-quality factor.

Preparation, Properties, and Applications of Near Stoichiometric Lithium Tantalate Crystals

Lithium tantalate crystal is widely used in optical devices, infrared detectors and surface acoustic wave devices because of its excellent piezoelectric, acousto-optic and nonlinear optical properties. The Li content of near stoichiometric lithium tantalate (NSLT) crystal is higher than that of congruent lithium tantalate (CLT) crystal. Therefore, the performance of NSLT crystal is better than that of CLT crystal in some aspects. This article reviews the physical properties, preparation methods and current research status in acoustics and optics of NSLT crystals. It also looks forward to the improvement of NSLT crystal preparation methods and their applications in surface acoustic wave (SAW) filters and optics. With the increase of Li content, the acoustic performance of NSLT crystals is expected to be comprehensively improved, achieving the application of SAW filters in 5G communication.

Synthesized Improvement of Die Fly and Die Shift Concerning the Wafer Molding Process for Ultrafine SAW Filter FOWLP

As the surface acoustic wave (SAW) filters incline to ultrafine, the failures resulting from the wafer molding process have become increasingly prominent. A methodology for coupling the mechanisms of die fly and die shift for the SAW filter miniatured with 737 μm × 517 μm × 200 μm is developed for the trade-off between reliability and yields. In terms of die fly and die shift, the former occurs before the epoxy molding compound (EMC) is cured in the temperature rise period, while the latter occurs in the cooling stage after being cured. The die fly is induced by the fluid flow force in the high-temperature stage of heat compression, which is fatal for the scrap. Followed by the cooling stage, the CTE (coefficient of thermal expansion) misalignment between the die and epoxy molding compound (EMC) seriously affects the die shift and the following lithography process yields. The debonding critical energy in the mixed mode is employed to avert the die fly. Then, the die fly can be shunned by fine-tuning the die thickness, die layout, and EMC layout. A methodology to measure die shift was conducted, by which a total of 47,568 dies were embedded using compression molding. The mechanical error of the mounter and the die shift law are comprehensively leveraged, indicating that the die shift can be controlled within 50 μm for 8-inch wafer-level packaging.

GaN surface acoustic wave filter with low insertion loss

Surface acoustic wave (SAW) filter with a low insertion loss (IL) of 4.415 dB has been demonstrated on Carbon-doped semi-insulating c-plane bulk GaN without external lumped element matching. The center frequency, 3 dB bandwidth, out-of-band attenuation, return loss of the filter are 477.05 MHz, 0.308 MHz, 32.5 dB, and −9.72 dB, respectively. The electromechanical coupling coefficient (Kt2), and temperature coefficient of frequency (TCF) of the filter are 0.21 % and −26.0 ppm/°C, respectively. The impact of the number of interdigital transducers (NIDT) and acoustic propagation direction on filter performance has been studied. The IL of filters reduces from 16.07 dB to 4.415 dB with the increase of NIDT from 50 to 150 due to the enhanced acoustic superposition. The numerical distribution of elastic stiffness ([cij]), and piezoelectric constants ([eik]) of GaN has been calculated in Euler angle space, indicating that they are isotropic on c-plane. The small performance difference of filters along the m- and a-direction on c-plane bulk GaN can be attributed to the small offset angle of 0.5° of the bulk GaN wafer or IDT quality variation.

Dual-Passband SAW Filter Based on a 32°YX-LN/SiO2/SiC Multilayered Substrate.

To meet the demands of highly integrated and miniaturized radio frequency front-end (RFFE) modules, multi-passband filters which support multi-channel compounding come to the foreground. In this work, we proposed a new design of a dual-passband surface acoustic wave (SAW) filter based on a 32°YX-LiNbO3 (LN)/SiO2/SiC multilayered structure. The filter is of a standalone ladder topology and comprises dual-mode resonators, in which the shear horizontal (SH) mode and high-order SH mode are simultaneously excited through electrode thickness modulation. The impact of electrode thickness on the performance of the dual-mode resonator was systematically investigated by the finite element method (FEM), and resonators were prepared and verified the simulation results. The electromechanical coupling coefficients (K2) of the SH modes are 15.1% and 17.0%, while the maximum Bode-Q (Qmax) values are 150 and 247, respectively, for the fabricated resonators with wavelengths of 1 μm and 1.1 μm. In terms of the high-order SH modes in these resonators, the K2 values are 9.8% and 8.4%, and Qmax values are 190 and 262, respectively. The fabricated dual-band filter shows the center frequencies (fc) of 3065 MHz and 4808 MHz as two bands, with 3-dB fractional bandwidths (FBW) of 5.1% and 5.9%, respectively. Such a dual-band SAW filter based on a conventional ladder topology is meaningful in terms of its compact layout and diminished area occupancy. This work provides a promising avenue to constitute a high-performance dual-passband SAW filter for sub-6 GHz RF application.

Wideband Surface Acoustic Wave Filter at 3.7 GHz With Spurious Mode Mitigation

As an essential functional block in radio frequency (RF) front-ends, surface acoustic wave (SAW) filters with large fractional bandwidth (FBW), high center frequency ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$f_{c}$ </tex-math></inline-formula> ), and low loss are highly sought-after in 5G new radio (NR). To address this end, 32°Y-X LiNbO3/SiO2/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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$f_{c}$ </tex-math></inline-formula> 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.

Design of 3D SAW Filters Based on the Spectral Element Method

Surface acoustic wave (SAW) devices are key components in 5G communication systems, and the design of SAW filters with high frequency, high performance and high reliability is a necessary and challenging task. In this work, SAW resonators with <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LiNbO</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> / <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SiO</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> / <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Si</i> multi-layer structure are designed based on the spectral element method (SEM). Normally, however, massive computing resources are needed for solving a whole resonator directly by using full-wave numerical methods. To avoid simulating the large-aperture full structure of any SAW resonator, an equivalent small-aperture 3D structure can be modelled in the direction of the aperture by changing the internal resistance of the driving source to obtain the results of the full-scale aperture model. The one-port SAW resonators are then connected in the form of ladder networks to form band-pass SAWfilters. The numerical results show that the SEM is more computationally efficient than the finite element method under the same number of degrees of freedom, and the results of the 3D small-aperture model are consistent with the complete resonator.