Rayleigh Mode Research Articles

Enhancement in performance of SAW resonator on LN/SiC substrate by take slowness asymmetry into account

Abstract Layered structures comprising high-coupling lithium niobate (LiNbO3, LN) piezoelectric thin plates bonded to high-velocity SiC substrates have attracted much attention of high-performance surface acoustic wave (SAW) devices. However, such these layered structures exhibit severe Rayleigh mode spurious responses that degrade the performances of SAW devices. This paper explores the generation mechanism of spurious modes on LN/SiC layered structure, and proposes an optimized layered structure with suppressed Rayleigh spurious modes. The investigation involves deriving the Euler angle range of LN by analysing the slowness surface of both LN and SiC. In addition, it is worth noting that slowness surface overlaps at some Euler angle ranges, which will cause coupling between different modes, thus generating spurious mode. Simulation result shows the frequency characteristics of SAW resonator at resonant and anti-resonant frequencies of 4.3 GHz and 4.88 GHz of SH mode, achieving a K 2 of up to 26.5% and a quality factor Q of 828. It is proved that the optimization structure offering larger K 2, higher velocity and improved Q values and Rayleigh spurious mode suppression, which is favourable for wideband and high frequency SAW devices applications.

High Order Rayleigh-Type SAW Devices for Advanced 5G Front-ends with Improved Performance

Abstract This paper presents a piezoelectric layered structure composed of a rotated LiNbO3 thin film combined with a diamond film for improved comprehensive performance SAW devices. Theoretical analysis of SAW resonator on the proposed structure is performed to calculate its SAW characteristics including electromechanical coupling coefficient (K2 ), frequency (f), and quality factor (Q). It is found that a high-order Rayleigh mode is effectively excited and exhibits not only high frequency but also large K2 and high Q. The influences of the Euler angles, layer stack configurations of LiNbO3, and the electrode on the SAW properties are investigated systematically to optimize the device performance of the proposed structure. The results show the resonator can operate at frequency of 6.6 GHz with a large K2 of 13.58% and high Q of 2134, which makes the proposed layered structure a good potential solution for high-performance wideband SAW filters operating over 5 GHz.

Acoustofluidic Diversity Achieved by Multiple Modes of Acoustic Waves Generated on Piezoelectric-Film-Coated Aluminum Sheets.

Excitation of multiple acoustic wave modes on a single chip is beneficial to implement diversified acoustofluidic functions. Conventional acoustic wave devices made of bulk LiNbO3 substrates generally generate few acoustic wave modes once the crystal-cut and electrode pattern are defined, limiting the realization of acoustofluidic diversity. In this paper, we demonstrated diversity of acoustofluidic behaviors using multiple modes of acoustic waves generated on piezoelectric-thin-film-coated aluminum sheets. Multiple acoustic wave modes were excited by varying the ratios between IDT pitch/wavelength and substrate thickness. Through systematic investigation of fluidic actuation behaviors and performances using these acoustic wave modes, we demonstrated fluidic actuation diversities using various acoustic wave modes and showed that the Rayleigh mode, pseudo-Rayleigh mode, and A0 mode of Lamb wave generally have better fluidic actuation performance than those of Sezawa mode and higher-order modes of Lamb wave, providing guidance for high-performance acoustofluidic actuation platform design. Additionally, we demonstrated diversified particle patterning functions, either on two sides of acoustic wave device or on a glass sheet by coupling acoustic waves into the glass using the gel. The pattern formation mechanisms were investigated through finite element simulations of acoustic pressure fields under different experimental configurations.

Spatial analysis of multi-frequency SAW beams excited by slanted IDTs on ZnO-SiC heterostructures

Slanted (or fan-shaped) interdigital transducers (IDTs) with broadband response allow the selective excitation of surface acoustic waves (SAWs) with narrow beam widths and pathways controlled by the excitation frequency. Such SAW-based spatial and frequency control is important for applications in microfluidics, as well as in emerging applications in semiconductor nanostructures. In this contribution, we generate both Rayleigh and Sezawa modes with slanted IDTs on a 4H-SiC substrate coated with a piezoelectric ZnO film. We directly measure the phase wavefronts of narrow SAW beams in the 1200–1260 MHz frequency bandwidth using high-resolution ( <1μm ) optical interferometry, and discuss the mechanisms that directly affect the propagation direction and phase profiles of the SAW beams. The combination of multimodal and multi-frequency SAW delay lines provides rich opportunities for SAW-based control of low-dimensional systems, such as electrons in epitaxial graphene or spin centers near the surface of the SiC substrates.

Velocimetry of GHz elastic surface waves in quartz and fused silica based on full-field imaging of pump–probe reflectometry

This study reports an imaging method for gigahertz surface acoustic waves in transparent layers using infrared subpicosecond laser pulses in the ablation regime and an optical pump–probe technique. The reflectivity modulations due to the photoelastic effect of generated multimodal surface acoustic waves were imaged by an sCMOS camera illuminated by the time-delayed, frequency-doubled probe pulses. Moving the delay time between 6.0nsto11.5ns, image stacks of wave field propagation were created.Two representative samples were investigated: wafers of isotropic fused silica and anisotropic x-cut quartz. Rayleigh (SAW) and longitudinal dominant high-velocity pseudo-surface acoustic wave (HVPSAW) modes could be observed and tracked along a circular grid around the excitation center, allowing the extraction of angular profiles of the propagation velocity. In quartz, the folding of a PSAW was observed. A finite element simulation was developed to predict the measurement results. The simulation and measurement were in good agreement with a relative error of 2% to 5%.These results show the potential for fast and full-field imaging of laser-generated ultrasonic surface wave modes, which can be utilized for the characterization of thin transparent samples such as semiconductor wafers or optical crystals in the gigahertz frequency range.

Thermal Annealing Impact on the Sensitivity of Piezomagnetic Surface Acoustic Waveguide to Applied Magnetic Field

AbstractMagnetostrictive thin films, exhibit significant utility as functionalization materials for Surface acoustic wave sensors designed for magnetic field measurements. This research investigates the influence of annealing under vacuum and magnetic field at various temperatures on the magnetic properties of the FeCo/TbCo2 thin film and therefore on the sensitivity of shear and Rayleigh acoustic waveguides functionalized with these magnetic layers. The observed effects include a reduction in magnetic anisotropy field and an increase in magnetostriction. XPS and TEM coupled with EDS microanalysis provide insights into the variations in the properties of the nanostructured magnetic film. To safeguard the magnetic film, a thin layer of SiO2 is deposited on top, serving as a protective shield. The inclusion of this layer amplifies sensitivity to applied magnetic fields for modes with shear polarization while reducing sensitivity for Rayleigh mode. In ST‐cut Quartz, the shear waves become waveguided upon the incorporation of magnetic thin films, with further enhancement achieved by introducing a SiO2 layer. Rayleigh waves are evanescent modes, the penetration depth is in the order of wavelength, and the addition of SiO2 decrease the wave confinement and therefore the sensitivity. The findings are supported by a theoretical model and experimentally validated results.

Optically Controlled Nano-Transducers Based on Cleaved Superlattices for Monitoring Gigahertz Surface Acoustic Vibrations.

Surface acoustic waves (SAWs) convey energy at subwavelength depths along surfaces. Using interdigital transducers (IDTs) and opto-acousto-optic transducers (OAOTs), researchers have harnessed coherent SAWs with nanosecond periods and micrometer localization depth for various applications. These applications include the sensing of small amount of materials deposited on surfaces, assessing surface roughness and defects, signal processing, light manipulation, charge carrier and exciton transportation, and the study of fundamental interactions with thermal phonons, photons, magnons, and more. However, the utilization of cutting-edge OAOTs produced through surface nanopatterning techniques has set the upper limit for coherent SAW frequencies below 100 GHz, constrained by factors such as the quality and pitch of the surface nanopattern, not to mention the electronic bandwidth limitations of the IDTs. In this context, unconventional optically controlled nanotransducers based on cleaved superlattices (SLs) are here presented as an alternative solution. To demonstrate their viability, we conducted proof-of-concept experiments using ultrafast lasers in a pump-probe configuration on SLs made of alternating AlxGa1-xAs and AlyGa1-yAs layers with approximately 70 nm periodicity and cleaved along their growth direction to produce a periodic nanostructured surface. The acoustic vibrations, generated and detected by laser beams incident on the cleaved surface, span a range from 40 to 70 GHz, corresponding to the generalized surface Rayleigh mode and bulk modes within the dispersion relation. This exploration shows that, in addition to SAWs, cleaved SLs offer the potential to observe surface-skimming longitudinal and transverse acoustic waves at GHz frequencies. This proof-of-concept demonstration below 100 GHz in nanoacoustics using such an unconventional platform might be useful for realizing sub-THz to THz coherent surface acoustic vibrations in the future, as SLs can be epitaxially grown with atomic-scale layer width and quality.

Experimental Analysis of Rayleigh and Sezawa Modes Resonance Frequencies in SAW Devices Manufactured on Sc0.3Al0.7N/Si†

GHz operating single port SAW resonators have been manufactured on Sc0.3Al0.7N/Si substrate. The thickness of the ScAlN thin layer was 0.8 μm. Advanced nano‐lithographic techniques have been developed for devices manufacturing. Rayleigh and Sezawa modes have been evidenced on Sc0.3Al0.7N/Si structure. The targeted applications are temperature sensors for harsh environmental operations. The sensitivity and temperature coefficient of frequency (TCF) of the manufactured devices have been experimentally determined and analyzed. The mode shapes of the resonances that appear in the devices supported on Sc0.3Al0.7N/Si substrate were evidenced by finite element method (FEM) simulations. A technique that combines FEM with Coupling of Modes (COM) was used for the determination of the resonance frequencies. Simulations performed for the temperature dependence of the resonance frequency led to results close to the experimental ones for the Rayleigh mode as well as for the Sezawa mode for devices supported on Sc0.3Al0.7N/Si substrate. © 2024 Institute of Electrical Engineer of Japan and Wiley Periodicals LLC.

Analysis of the Effects of Parameters on the Performance of Resonators Based on a ZnO/SiO2/Diamond Structure

With the development of communications technology, surface acoustic wave (SAW) and bulk acoustic wave (BAW) devices have become hotspots of the competitive research in the frequency band above GHz. It imposes higher requirements on the operating frequency, temperature coefficient of frequency (TCF), and electromechanical coupling coefficient (k2) of SAW devices. In this work, we reported on a novel ZnO/SiO2/diamond-layered resonator structure and systematically investigated its propagation characteristics by using finite element methods. A comparative study and analysis of k2 and acoustic velocity (vp) for both the excited Rayleigh mode and the Sezawa mode were conducted. By selecting the appropriate ZnO piezoelectric film, SiO2, and electrode thickness, the Sezawa mode was chosen as the main mode, effectively improving both k2 and vp. It was observed that the k2 of the Sezawa mode is 7.5 times that of the excited Rayleigh mode and nearly 5 times that of piezoelectric single-crystal ZnO; vp is 1.7 times that of the excited Rayleigh mode and nearly 1.5 times that of piezoelectric single-crystal ZnO. Furthermore, the proposed multilayer structure achieves a TCF close to 0 while maintaining a substantial k2. In practical applications, increasing the thickness of SiO2 can compensate for the device’s TCF reduction caused by the interdigital transducer (IDT). Finally, this study explored the impact of increasing the aperture width and IDT pairs on the performance of the single-port resonator, revealing the changing patterns of quality factor (Q) values. The results reported here show that the structure has great promise for the fabrication of high-frequency and low-TCF SAW devices.

GHz surface waves in Al/LiTaO3/Si composite: Effect of the Drude electrode on dispersion, attenuation and mode shapes

In fifth generation (5G) mobile communication, radio frequency is in the super high frequency range (3–30 GHz). Thus, different from Ohm law, electrode loss at GHz frequency becomes frequency-dependent. As such, many properties, especially attenuation of piezoelectric acoustic devices, should be reevaluated. In this paper, the attenuation of GHz surface wave propagating in composite structure made of metal electrode, piezoelectric film and elastic half space is investigated based on the loss model of Drude electrode. Specifically, the Al/LiTaO3/Si composite is considered where Rayleigh mode and SH mode are fully coupled. Based on the Stroh-type formalism, the dispersion equations of the composite with the Drude electrode are derived, and compared with the traditional perfect conductor electrode which is under the short circuit boundary condition. The phase velocity, attenuation and wave mode shapes are obtained successfully by the multidimensional moduli ratio convergence method. Numerical results are presented to reveal some interesting intrinsic features, which include two analytical upper bounds of phase velocity, nonexistent validation of intersection points in dispersion curves, and relations among veering, attenuation jumps and wave-mode conversions. These features and relations provide an in-depth insight for GHz wave motion properties in piezoelectric composites.

Rayleigh Wave Dispersion and Inversion for Shallow Surface with a High-velocity Rigid Pavement

We investigate the rigid pavement influence on phase velocity dispersion of the Rayleigh wave by using numerical modeling. An overestimation of Rayleigh wave phase velocity is observed more than 10% deviation caused by high-velocity rigid pavement. Based on the generalized reflection and transmission (R/T) coefficient method, the linear eigenvalue problem is solved for vertically heterogeneous media with a rigid pavement structure. The secular function demonstrates the multi-valuedness which is different phase velocity at the same frequency for the single Rayleigh mode. To address mode misidentification problem, we adopt a new misfit function for multimodal dispersion inversion. A Markov chain Monte Carlo (MCMC) sampling algorithm is used to solve dispersion inversion. The synthetic test shows that Rayleigh wave dispersion inversion incorporating a pavement layer can improve estimation accuracy of shear wave velocity.

Electromechanical sensitivity of ZnO thin films at high-pressure regime for SAW strain sensor applications

This paper employs Density Functional Theory (DFT) to investigate the influence of high hydrostatic pressure on the piezo-elastic properties of ZnO with a specific focus on its repercussions for Surface Acoustic Wave (SAW) devices. Our findings reveal a significant enhancement in the piezoelectric coefficient d33 under high-pressure conditions, particularly reaching 13.09 pC/N at 8 GPa compared to the unpressurized state. This noteworthy improvement directly contributes to an overall enhancement in the electromechanical coupling coefficients (K2), emphasizing for the first time the potential of high-pressure conditions to positively influence the performance of ZnO/Al2O3 SAW devices. To confirm that, we utilized the Finite Element Method (FEM) to analyze the performance of a Surface Acoustic Wave (SAW) device based on ZnO for both propagating modes, namely Rayleigh and Sezawa, under elevated pressure conditions. We observed an improvement in the electromechanical coupling factor (K2) for the Rayleigh and the Sezawa modes, with enhancements of approximately 23% and 52%, respectively, under an applied pressure of 8 GPa compared to 0 GPa. Furthermore, we found that the Sezawa mode exhibits eight times greater sensitivity to pressure variations compared to the Rayleigh mode. These findings provide designers with a better understanding of the electromechanical responsivity and sensitivity of the ZnO-SAW devices to high level of applied pressure.

High performance SAW resonator with spurious mode suppression using double-layer electrode transverse modulation

This work aims to solve the problem of tradeoff between various properties and spurious mode suppression in surface acoustic wave (SAW) resonators. A high-angle rotated Y-cut LiNbO3 (LN)/SiO2/Si multilayered structure was proposed to balance the electromechanical coupling coefficient (K 2) and temperature coefficient of frequency (TCF), and the propagation characteristics of Rayleigh mode were simulated by the finite element method. For the widely existing spurious modes, the shear-horizontal wave and longitudinal modes were eliminated by optimizing the cut angle of LN and electrode thickness, and a method of double-layer electrode transverse modulation was proposed to suppress the transverse modes. This method reduces the mass loading effect by replacing the electrode from Cu to Cu/Al. Moreover, the Al thicknesses in different regions are changed to perform the transverse modulation, and thus a widespread suppression of transverse modes is achieved by exciting the piston mode and enhancing the energy constraint, with a significant improvement on quality factor at the resonance frequency. Eventually, the spurious-free SAW resonator has the K 2 of 9.5% and the TCF close to zero. This work provides a feasible scheme for the design of high performance SAW resonators with spurious mode suppression.

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.

Mode Characterization and Sensitivity Evaluation of a Surface Acoustic Wave (SAW) Resonator Biosensor: Application to the Glial-Fibrillary-Acidic-Protein (GFAP) Biomarker Detection.

Biosensors based on surface acoustic waves (SAWs) offer unique advantages due to their high sensitivity, real-time response capability, and label-free detection. The typical SAW modes are the Rayleigh mode and the shear-horizontal mode. Both present pros and cons for biosensing applications and generally need different substrates and device geometries to be efficiently generated. This study investigates and characterizes SAW resonator biosensors on lithium niobate in terms of modes generated and biosensing performance. It reveals the simultaneous presence of two typical SAW modes, the first around 1.6 GHz and the second around 1.9 GHz, differently polarized and clearly separated in frequency, which we refer to as slow and fast modes. The two modes are studied by numerical simulations and biosensing experiments with the glial-fibrillary-acidic-protein (GFAP) biomarker. The slow mode is generally more sensitive to changes in surface properties, such as temperature and mass changes, by a factor of about 1.4 with respect to the fast mode.

Simulation and Optimization of Surface Acoustic Wave Devices With High Electromechanical Coupling Factor and Zero Temperature Coupling Coefficient by Pattern Piezoelectric Layer

Recently, a more popular patterned surface acoustic wave (SAW) device with better performance was proposed. This structure has a large electromechanical coupling factor. To make surface acoustic wave devices easy to be applied to integrated circuit technology, a multilayer structure based on patterned Al/ZnO/Si is designed by using finite element method (FEM). Firstly, dependence of ZnO piezoelectric material on electromechanical coupling factor is studied systematically. The results show that Al/ZnO/Si structure excites Rayleigh mode and Sezawa mode with high electromechanical coupling factor. The patterning of piezoelectric materials could be achieved by etching. The electromechanical coupling factor of Rayleigh mode is as high as 7.98%, and the electromechanical coupling factor of Sezawa mode is up to 9.68%. As we all know, temperature will affect the characteristics of SAW devices. Therefore, the temperature characteristics based on this structure are systematically studied. To achieve zero temperature coupling coefficient, SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> is used as an intercalation layer for temperature compensation. The structure of Rayleigh mode and Sezawa mode with the thinner SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> layer is easier to achieve zero temperature coupling coefficient than that of conventional structure. Meanwhile, the electromechanical coupling factor of Sezawa mode based on patterned Al/ZnO/SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Si structure does not change significantly with the increase of SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> thickness, with the maximum value of 10.78%, and the minimum value of 10.62%, which is 1.1% larger than that of structure without SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> .

Simulation of Flexible Dual-mode Surface Acoustic Wave Resonator for Strain Sensor Application

Based on the requirement of flexible electronic devices (FED)such as surface acoustic wave (SAW) sensors, finite element models for material analysis and SAW resonator design were established. Dual-mode of Rayleigh and Horizontal Shear mode on 0ºY cut LiNbO3 thin film was discovered. The influence of BCB thickness and PI substrate on resonant mode was analyzed. The flexible SAW resonator owned high sensitivity both in Rayleigh mode and Horizontal shear mode under tensile stress and compressive stress by simulation.

Ionospheric Response to the 6 February 2023 Turkey–Syria Earthquake

Two strong earthquakes occurred in Turkey on 6 February 2023, at 01:17:34 (nighttime, Mw = 7.8) and at 10:24:50 UT (daytime, Mw = 7.5). The seismo-ionospheric impact is an important part of the near-Earth environment state. This paper provides the first results on the ionospheric effects associated with the aforementioned earthquakes. We used data from global navigation satellite system (GNSS) receivers and ionosondes. We found that both earthquakes generated circle disturbance in the ionosphere, detected by GNSS data. The amplitude of the ionospheric response caused by daytime M7.5 earthquake exceeded by five times that caused by nighttime M7.8 earthquake: 0.5 TECU/min and 0.1 TECU/min, respectively, according to the ROTI data. The velocities of the earthquake-related ionospheric waves were ~2000 m/s, as measured by ROTI, for the M7.5 earthquake. TEC variations with 2–10 min periods showed velocities from 1500 to 900 m/s as disturbances evolved. Ionospheric disturbances occurred around epicenters and propagated to the south by means of 2–10 min TEC variations. ROTI data showed a more symmetric distribution with irregularities observed both to the South and to the North from 10:24:50 UT epicenter. The ionospheric effects were recorded over 750 km from the epicenters. Ionosonde located 420/490 km from the epicenters did not catch ionospheric effects. The results show significant asymmetry in the propagation of coseismic ionospheric disturbances. We observed coseismic ionospheric disturbances associated with Rayleigh mode and acoustic modes, but we did not observe disturbances associated with acoustic gravity mode.

The Rayleigh mode at the plane boundary between a liquid and a double porosity medium.

The Rayleigh mode (also called generalized Rayleigh wave) at the flat interface between a liquid and a double porosity medium saturated by the same liquid, is investigated. The double porosity medium obeys the phenomenological model of Berryman and Wang (2000). It allows the simultaneous propagation of three longitudinal waves and a transverse wave. Assuming open pore conditions at the interface, formulas are found for the coefficients of reflection in the liquid and of transmission in the double porosity medium. Considering only the angular variable in the study, numerical simulations based on data for Berea sandstone show the influence of frequency and the two types of porosity on the reflection coefficient and on the velocity of the generalized Rayleigh wave.

Fiber-Optic Airplane Seismology on the Northeast Greenland Ice Stream

Abstract We present distributed fiber-optic sensing data from an airplane landing near the EastGRIP ice core drilling site on the Northeast Greenland Ice Stream. The recordings of exceptional clarity contain at least 15 easily visible wave propagation modes corresponding to various Rayleigh, pseudoacoustic, and leaky waves. In the frequency range from 8 to 55 Hz, seven of the modes can be identified unambiguously. Based on an a priori firn and ice model that matches P-wave dispersion and the fundamental Rayleigh mode, a Backus–Gilbert inversion yields an S-wavespeed model with resolution lengths as low as a few meters and uncertainties in the range of only 10 m/s. An empirical scaling from S wavespeed to density leads to a depth estimate of the firn–ice transition between 65 and 71 m, in agreement with direct firn core measurements. This work underlines the potential of distributed fiber-optic sensing combined with strong unconventional seismic sources in studies of firn and ice properties, which are critical ingredients of ice core climatology, as well as ice sheet dynamics and mass balance calculations.