Surface Acoustic Wave Filters Research Articles

CuAlO2/AlN double-layer thin film prepared by the spin coating approach

Delafossite CuAlO2/AlN thin films have been prepared by a facile and affordable spin coating method. The main purposes of this experiment were to explore the preparation of CuO on AlN film by a spin coating method and to form CuAlO2 by high-temperature annealing and also to explore the properties resulting from different temperature treatments. The CuAlO2/AlN film was identified by X-ray diffraction studies, and field emission scanning electron microscopy studies. The cross-section of the CuAlO2/AlN film and thickness of the CuAlO2 and AlN film were 346 nm and 625 nm, respectively. Because, AlN has various excellent characteristics, such as a good dielectric constant, low energy consumption, and acoustic wave components, it has considerable potential in various applications, such as bulk acoustic wave filters, surface acoustic wave filters, and semiconductors. This experiment demonstrates that CuAlO2/AlN film can be prepared by a simple and inexpensive spin coating method.

Development and Application of SAW Filter.

With the in-depth advancement of the fifth generation (5G) mobile communication technology, the technical requirements for filters are also constantly improving. Surface acoustic wave (SAW) filters are widely used in home TV, mobile communications, radio frequency filters and radar due to their simple structure, few mask layers, easy miniaturization, and low cost. Through the continuous improvement of communication technology, SAW has developed into various high-performance acoustic filters from bulk SAW with the support of some new architectures, new materials and advanced modeling techniques. This paper analyzes and reviews the research situation of SAW filter technology.

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.

SAW-Less Direct RF Transmitter With Multimode Noise Shaping and Tri-Level Time-Approximation Filter

This article presents a multimode direct radio frequency (RF) transmitter capable of a high in-band dynamic range and a low out-of-band (OOB) noise floor. It features a hybrid digital-to-analog converter (DAC) with a reconfigurable dual-band delta–sigma modulator to lower the quantization noise floor at one or two frequency bands. Additionally, a tri-level time-approximation filter is proposed to further suppress OOB noise with high tunability and stopband attenuation. The objective is to demonstrate a low-noise floor without using a surface acoustic wave (SAW) filter. For a 256-quadratic-amplitude modulation (QAM) signal with 17.4-dBm average output power at a 2.2-GHz carrier, the silicon prototype achieves −40-dB error vector magnitude (EVM) and −169-dBc/Hz noise spectral density (NSD) at 68-MHz offset by consuming a total power of 1.2 W.

Development of 3-D Wafer Level Packaging for SAW Filters Using Thin Glass Capping Technology

The introduction of 5G mobile communication created the need to support a large number of radio frequency (RF) bands. This required high performance and reduced size for microacoustic filters. Based on these requirements, a new three-dimensional wafer-level packaging (3-D WLP) solution based on the predecessors’ developed technology was presented to resolve reliability issues for surface acoustic wave (SAW) filter packages with large cavities. In the 3-D WLP, vertical interconnects were realized using through glass vias (TGVs). To enable such a process scheme, thin glass caps with TGVs were formed through laser-induced chemical etching. Then, glass capping dies were bonded on the corresponding LiTaO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (LT) device wafer to generate a cavity structure. Partial filling of the vias on the pad was used, and a metal trace was formed on the glass. Ball grid arrays (BGAs) were formed on top of the glass. Finally, an SAW filter WLP was fabricated. This glass capping packaging avoids the outgassing problem of conventional thin-film acoustic packaging at high temperatures, which would cause the contamination of the interdigital transducers (IDTs). In addition, a numerical study based on the 3-D finite element (FE) model has been conducted to analyze the reliability of the package. The built model was applied to perform parametric studies on the effects of package geometry, material properties, and the processing conditions on reliability. Based on the above studies, the optimized SAW filter WLP successfully passed the reliability test of pre-con level 3 and temperature cycling test (TCT, −40 °C ~ 125 °C).

Wave attenuation in 1-3 phononic structures with lead-free piezoelectric ceramic inclusions

The unit cell wave attenuation of elastic Bloch waves in 1-3 piezoelectric phononic structures with BaTiO3 inclusions in a polymeric matrix is reported. This attenuation can be observed by the evanescent Bloch waves in the complex dispersion diagram of the periodic piezoelectric ceramic structure. Full and complete Bragg-type band gaps are observed. The piezoelectricity of the periodic lead-free ceramic inclusions improves the band gap width and wave attenuation in a specific frequency spectrum. These characteristics can be used for the wave attenuation design using smart periodic structures, for surface acoustic wave (SAW) filter and 1-3 piezocomposite transducer design using piezoelectric phononic structures with BaTiO3 inclusions.

Switched Dual-Band SAW Filters Using Hybrid and Monolithically Integrated Vanadium Oxide Switches

This article presents design and implementation of switchable dual-band filters using surface acoustic wave (SAW) resonators and vanadium oxide (VO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) switches imbedded within the filter using both hybrid and monolithic integration. The switched dual-band filter has two channels and four switching states: transmission only of the lower channel, transmission only of the upper channel, transmission of both the channels, and no transmission. The experimental results are presented for the first switchable dual-band filter of order 5 with two discrete VO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> switches strategically integrated within the proposed filter’s structure using hybrid integration. The five-pole dual-band filter demonstrates a measured insertion loss of 3.7 and 3.9 dB in the transmission states at 765 and 935 MHz, respectively. To improve the overall performance of the proposed switched dual-band filter, a second seven-pole filter with six VO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> switches, three for each channel, is designed and fabricated. VO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> switches are monolithically integrated with SAW resonators on the same substrate using a four-mask in-house fabrication process. The article also presents experimental results to study the impact of the monolithically integrated VO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> switches on the linear and power-handling performance of SAW resonators. VO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> monolithically integrated switched dual-band filter has two channels at 600 and 725 MHz. It exhibits a measured insertion loss of 3.23 and 3.22 dB for the two channels. Depending on the number of detuned resonators in the OFF state for each channel, an isolation of 20–40 dB is demonstrated. The monolithic integration of VO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> switches results in improved performance of the filter along with significant size reduction.

A spectrum analyzer system with wide bandwidth and high frequency resolution based on chirp transform

AbstractWith the advantages of low power consumption and high stability, chirp transform spectrometer (CTS) has become a powerful tool widely used in spectrum analysis today. In recent years, compared with other spectrum analyzers, CTS is limited by the lack of bandwidth, which severely restricts its application and development. In this article, a CTS system with a wide bandwidth is proposed. The chirp signal with 2 GHz bandwidth is generated by DAC and multiplier, and its dispersion characteristics are matched with a 1 GHz bandwidth surface acoustic wave (SAW) filter to build a spectrum analyzer with an analysis bandwidth of 1 GHz. Experimental results show that the system frequency resolution can reach up to 102 kHz, which is very close to the theoretical value (100 kHz).

Research in Nonlinearity of Surface Acoustic Wave Devices.

Surface acoustic wave (SAW) devices are one of the indispensable components in the radio frequency (RF) front-end of mobile phones. With the development of mobile communication technology, the requirements for linear specification of devices are more and more strict. Nonlinear distortions of SAW devices have a serious influence on the application of mobile RF modules. To satisfy the strict requirement of linearity of communication system, it is necessary to understand the generation mechanism of nonlinearity and study the accurate modeling, appropriate measurement methods, and nonlinear response elimination technology. In this paper, we summarize the research progress on the nonlinearity of SAW devices in recent years from four aspects: the generation mechanism, simulation methods, measurement system, and suppression technology. The nonlinear harmonics with the nonlinear Mason equivalent circuit model are simulated. Furthermore, harmonics and intermodulation signals of SAW filters are tested by the authors. Thanks to these research studies, it is of great significance to the development of future RF front-end modules with high linear SAW devices.

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.

Lateral resolution limit of laser Doppler vibrometer microscopes for the measurement of surface acoustic waves

The lateral or transverse resolution of single-point interferometers for vibration measurement is especially critical for microelectromechanical systems (MEMS) vibrating up to the gigahertz range. In this regime, the acoustic wavelengths are typically in the range of the size of the laser focus. Thus, a successful vibration measurement requires distinct knowledge about the lateral resolution limit and its dependencies with instrumentation parameters. In this paper, we derive an analytic approximation formula, which allows for estimation of the systematic measurement deviation of the vibration amplitude and, thus, a definition of the lateral resolution limit of single-point interferometers for vibration measurement. Further, a compensation and an optimum numerical aperture are proposed the reduce the measurement deviation. For this, the model includes a laser-interferometer microscope of Mach-Zehnder type with Gaussian laser beams considering the Gouy effect and wavefront curvature. As a measurement scenario, an unidirectional surface acoustic wave (SAW) is regarded. The theoretic findings have been validated in the experiment with a representative vibration measurement on a SAW filter at 433,{mathrm {MHz}} with our heterodyne laser-Doppler interferometer with offset-locked semiconductor lasers. The provided formulas help instrument designers and users to choose suitable instrument parameters, especially the numerical aperture of the utilized microscope objective.

Power Durability Enhancement and Failure Analysis of TC-SAW Filter With Ti/Cu/Ti/Cu/Ti Electrodes

In response to the rapid development of wireless communication, enhancement of temperature stability and power durability for surface acoustic wave (SAW) devices is both scientifically and technologically important in the 5G era. This paper discusses the utilization of 5-layer electrode Ti/Cu/Ti/Cu/Ti to the development of high power and low loss temperature-compensated surface acoustic wave (TC-SAW) filters. By modulating the microstructure of 5-layer electrode, the power durability can be increased to 1.7 times that of the conventional 3-layer electrode, while the life time to failure can be enhanced by 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> times. Through the cross section analysis of devices before and after power test, it is found that the deformation mainly concentrates on the bottom edge of electrode, whose result matches well with finite element method (FEM) simulation. The enhancement of power durability can be attributed to high resistance to cyclic stress in the bottom fine grain area. Furthermore, substrate failure areas due to Ti diffusion were also observed, which may be caused by the high temperature due to the current concentration around the electrode deformation. Our results show that the 5-layer electrode configuration is a promising solution for high power TC-SAW filter without deterioration of radio frequency performance and temperature coefficient of frequency.

Near 30% fractional bandwidth surface acoustic wave filters with novel electrode configuration

With the rapid progress of the wireless communication technique and the fast growing of the radio frequency (RF) markets, the development of wideband surface acoustic wave (SAW) filter is intensely pursued in RF front-end platforms. In this study, a novel configuration of SAW device was proposed in 15°YX-LiNbO3 (300 ​nm)/SiO2 (600 ​nm)/Si heterostructure, where the series and shunt resonator innovatively adopt Cu electrodes of different thickness to suppress the unwanted Rayleigh mode efficiently. Series and shunt resonators with clean spectrum and high electromechanical coupling coefficients over 20% were experimentally achieved under the guidance of finite element method calculation. Based on the resonator results, a ladder-type SAW filter with a very large 3-dB fractional bandwidth near 30% was successfully designed and fabricated. The minimum insertion loss of device is as small as 1.1 ​dB while the in-band fluctuation is less than 0.3 ​dB. The unprecedented wideband property of SAW device indicates that the proposed configuration is expected to become one of the solutions of wideband filter in fifth-generation mobile communication, and possess great commercial application prospects.

A Multilayered Structure for Packageless Acoustic- Wave Devices With Ultra-Small Sizes

An increasing demand for surface acoustic wave filters with high performances contributes to the necessity of reducing sizes and costs. The present work proposes a multilayered structure of high-velocity layer/SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /interdigital transducers (IDTs)/piezoelectric-single-crystal substrate to further validate the strengths of packageless devices. Resonators based on the SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> /SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /IDTs(Cu)/15°Y-X LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> structure are simulated and analyzed. One-port resonators are designed, fabricated and tested on the basis of the simulation, and results validates the feasibility of no encapsulation. Finally, by using this multilayered structure, a packageless filter with a center frequency of 2.14 GHz and an ultra-small size of 0.695×0.490×0.122 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> is successfully designed and realized, maintaining well-balanced performances. This work has demonstrated the strengths of packageless devices which can be obtained by utilizing the proposed multilayered structure, and may promote the development of device miniaturization. [2021-0035]

Parameter detection for surface acoustic wave filter based on image processing

As core devices for the smartphone RF front end, surface acoustic wave (SAW) filters are in short supply. The detection of SAW filter parameters refers to the connection between layout design and process production, which is an important part of capacity improvement. This paper devises a method based on image processing to detect SAW filter parameters. We improve the morphological operator to recover an image and identify the number of interdigital transducer (IDT) finger bars under noise conditions. The feature points in the color image are selected to extract the scale. The Canny edge detection effect is enhanced through using an adaptive filter, increasing the gradient operator, bilinear interpolation, and other algorithms. We use a minimum shape to enclose the edges to determine the IDT width and period and apply Fourier transform to verify the detection results. The experimental results show that the number of IDT finger bars is accurately identified. The width and period are exactly the same as the real values at the 10th percentile, and the detection requirements are met.

Development and Reliability Study of 3-D Wafer Level Packaging for SAW Filter Using Thin Film Capping

The rapid growth of the advanced consumer 5G wireless mobile phones has propelled the demands of fabricating surface acoustic wave (SAW) filters with smaller size and high performance. Three-dimension wafer-level package (3-D WLP) for SAW filter is a promising solution to replace conventional surface mounted technology and chip-scale packaging. In this article, a new WLP for SAW filter with a size of 1.3 mm ×1.1 mm ×0.35 mm is developed using thin dry film capping. The new 3-D WLP significantly reduces the overall thickness which is particularly meaningful for low-profile smartphones and tablets. A number of key processes were developed. To improve the quality, finite element modeling (FEM) is used to simulate the mechanical stress to optimize cavity structure. Reliability tests were performed and the results indicated that there were no significant frequency response changes occurred and low insertion loss (IL) in pass band was less than 0.2 dB after pro-conditional level 3 test, unbiased highly accelerated stress (uHAST) test, and temperature cycling (TC) test on SAW filter packages. It can be concluded that 3-D WLP for SAW filter achieves miniaturization, good electrical performance, and reliability.

Accelerated numerical modeling of RF circuits using network characteristic mode analysis

AbstractA fast numerical modeling approach based on network characteristic mode analysis (CMA) is presented and investigated for analyzing electrical layouts in miniature RF filters, such as surface acoustic wave filters. In this approach, a generalized eigenvalue decomposition is performed on the Z‐parameters of an electrical layout at one or two sampling frequencies that can be computed and extracted with any numerical full‐wave method. The obtained eigenvalues are used to extract modal resistance, inductance, and capacitance matrices for each eigenmode. The frequency dependence of the modal resistance matrix can be assumed a priori or determined automatically, and the modal inductance and capacitance matrices are assumed frequency independent. These modal matrices are then used to approximate the Z‐parameters at any other frequencies to provide the response of the electrical layout, which can then be combined with the frequency responses of other components, such as resonators, to yield the electrical response of an entire RF filter. Compared with the previously developed analytic extension of eigenvalues, this fast CMA‐based method is less affected by the frequency variation of eigenmodes since the frequency dependences of the eigenmodes are implicitly canceled out in its formulation. The accuracy of this approach is evaluated by comparing with results from full‐wave analyses. For RF circuits whose electrical sizes are small and whose frequency range of interest is relatively small, the proposed CMA‐based fast frequency sweep approach is found to be sufficiently accurate and highly practical for engineering applications.

Dispersion Diagram of Trigonal Piezoelectric Phononic Structures with Langasite Inclusions

The dispersion relation of elastic Bloch waves in 1-3 piezoelectric phononic structures (PPnSs) with Langasite (La3Ga5SiO14) inclusions in a polymeric matrix is reported. Langasite presents promising material properties, for instance good temperature behaviour, high piezoelectric coupling, low acoustic loss and high quality factor. Furthermore, Langasite belongs to the point group 32 and has a trigonal structure. Thus, the 2-D bulk wave propagation in periodic systems with Langasite inclusions cannot be decoupled into XY and Z modes. The improved plane wave expansion (IPWE) is used to obtain the dispersion diagram of the bulk Bloch waves in 1-3 PPnSs considering the classical elasticity theory and D3 symmetry. Full band gaps are obtained for a broad range of frequency. The piezoelectricity enhances significantly the band gap widths and opens up a narrow band gap in lower frequencies for a filling fraction of 0.5. This study should be useful for surface acoustic wave (SAW) filter and 1-3 piezocomposite transducer design using PPnSs with Langasite.

Wideband and Low-Loss Surface Acoustic Wave Filter Based on 15° YX-LiNbO₃/SiO₂/Si Structure

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.

Advanced Chirp Transform Spectrometer with Novel Digital Pulse Compression Method for Spectrum Detection

Based on chirp transform and pulse compression technology, chirp transform spectrometers (CTSs) can be used to perform high-resolution and real-time spectrum measurements. Nowadays, they are widely applied for weather and astronomical observations. The surface acoustic wave (SAW) filter is a key device for pulse compression. The system performance is significantly affected by the dispersion characteristics match and the large insertion loss of the SAW filters. In this paper, a linear phase sampling and accumulating (LPSA) algorithm was developed to replace the matched filter for fast pulse compression. By selecting and accumulating the sampling points satisfying a specific periodic phase distribution, the intermediate frequency (IF) chirp signal carrying the information of the input signal could be detected and compressed. Spectrum measurements across the entire operational bandwidth could be performed by shifting the fixed sampling points in the time domain. A two-stage frequency resolution subdivision method was also developed for the fast pulse compression of the sparse spectrum, which was shown to significantly improve the calculation speed. The simulation and experiment results demonstrate that the LPSA method can realize fast pulse compression with adequate high amplitude accuracy and frequency resolution. Compared to existing digital pulse compression technology, this method can significantly reduce the number of required calculations, especially for measurements of sparse signals.