Surface Acoustic Wave Propagation Direction Research Articles

Acoustic-driven magnetic skyrmion motion.

Magnetic skyrmions have great potential for developing novel spintronic devices. The electrical manipulation of skyrmions has mainly relied on current-induced spin-orbit torques. Recently, it was suggested that the skyrmions could be more efficiently manipulated by surface acoustic waves (SAWs), an elastic wave that can couple with magnetic moment via the magnetoelastic effect. Here, by designing on-chip piezoelectric transducers that produce propagating SAW pulses, we experimentally demonstrate the directional motion of Néel-type skyrmions in Ta/CoFeB/MgO/Ta multilayers. We find that the shear horizontal wave effectively drives the motion of skyrmions, whereas the elastic wave with longitudinal and shear vertical displacements (Rayleigh wave) cannot produce the motion of skyrmions. A longitudinal motion along the SAW propagation direction and a transverse motion due to topological charge are simultaneously observed and further confirmed by our micromagnetic simulations. This work demonstrates that acoustic waves could be another promising approach for manipulating skyrmions, which could offer new opportunities for ultra-low power skyrmionics.

Laser Ultrasound Investigations of AlScN(0001) and AlScN(11-20) Thin Films Prepared by Magnetron Sputter Epitaxy on Sapphire Substrates.

The laser ultrasound (LU) technique has been used to determine dispersion curves for surface acoustic waves (SAW) propagating in AlScN/Al2O3 systems. Polar and non-polar Al0.77Sc0.23N thin films were prepared by magnetron sputter epitaxy on Al2O3 substrates and coated with a metal layer. SAW dispersion curves have been measured for various propagation directions on the surface. This is easily achieved in LU measurements since no additional surface structures need to be fabricated, which would be required if elastic properties are determined with the help of SAW resonators. Variation of the propagation direction allows for efficient use of the system’s anisotropy when extracting information on elastic properties. This helps to overcome the complexity caused by a large number of elastic constants in the film material. An analysis of the sensitivity of the SAW phase velocities (with respect to the elastic moduli and their dependence on SAW propagation direction) reveals that the non-polar AlScN films are particularly well suited for the extraction of elastic film properties. Good agreement is found between experiment and theoretical predictions, validating LU as a non-destructive and fast technique for the determination of elastic constants of piezoelectric thin films.

Properties of higher-order surface acoustic wave modes in Al1−xScxN/sapphire structures

In this work, surface acoustic wave (SAW) modes and their dependence on propagation directions in epitaxial Al0.68Sc0.32N(0001) films on Al2O3(0001) substrates were studied using numerical and experimental methods. In order to find optimal propagation directions for higher-order SAW modes, phase velocity dispersion branches of Al0.68Sc0.32N on Al2O3 with Pt mass loading were computed for the propagation directions <112¯0> and <11¯00> with respect to the substrate. Experimental investigations of phase velocities and electromechanical coupling were performed for comparison with the numerical results. Simulations carried out with the finite element method and a Green function approach allowed identification of each wave type, including Rayleigh, Sezawa, and shear-horizontal wave modes. For the propagation direction <11¯00>, significantly increased wave guidance of the Sezawa mode compared to other directions was observed, resulting in enhanced electromechanical coupling (keff2=1.6%) and phase velocity (vphase=6km/s). We demonstrated that selecting wave propagation in <11¯00> with high mass density electrodes results in increased electromechanical coupling without significant reduction in phase velocities for the Sezawa wave mode. An improved combination of metallization, Sc concentration x, and SAW propagation direction is suggested that exhibits both high electromechanical coupling (keff2>6%) and high velocity (vphase=5.5km/s) for the Sezawa mode.

Highly sensitive thick diaphragm-based surface acoustic wave pressure sensor

Diaphragm-based Surface acoustic wave (SAW) pressure sensors are of great interest to fulfill high-pressure sensing requirements in industrial and commercial applications. These sensors are developed with a thick diaphragm, can endure considerable pressure but adversely result in low sensitivity and affect device accuracy. The trade-off between diaphragm thickness and sensitivity is the major problem in designing pressure sensors for high-pressure sensing ranges. This work proposed a highly sensitive pressure sensor design based on one port SAW resonator with AlN/Mo/SOI multilayer structure. The sensor utilizes a 50 μm thick rectangular diaphragm and has been fabricated with simple microfabrication techniques, making it cost-effective and easy to implement in pressure sensing equipment. Finite element analysis (FEA) has been performed for analyzing diaphragm to locate IDTs based on bending, uniformity of stress distribution and induced strain type. The developed sensor exhibits good performance characteristics, including high sensitivity (up to 129.17 ppm/MPa), wide pressure sensing range (up to 2 MPa), and good repeatability with linear behavior for applied pressure range. Additionally, this work has presented a novel approach to enhance pressure sensitivity based on increased velocity without compromising sensing range. The obtained sensitivity has been enhanced up to 228.46 ppm/MPa by modifying the primarily developed sensor design such that the SAW propagation direction becomes parallel to the induced strain direction. With the proposed novel concept, the sensitivity has been significantly improved by 76.87 % compared to the primarily developed sensor. Besides improved sensitivity, the modified sensor also shows the best repeatability and linearity. All these performance characteristics pave the way to make this design a good choice for high-pressure sensing applications.

Dynamic Behavior of Droplet Impact on Inclined Surfaces with Acoustic Waves.

Droplet impact on arbitrary inclined surfaces is of great interest for applications such as antifreezing, self-cleaning, and anti-infection. Research has been focused on texturing the surfaces to alter the contact time and rebouncing angle upon droplet impact. In this paper, using propagating surface acoustic waves (SAWs) along the inclined surfaces, we present a novel technique to modify and control key droplet impact parameters, such as impact regime, contact time, and rebouncing direction. A high-fidelity finite volume method was developed to explore the mechanisms of droplet impact on the inclined surfaces assisted by SAWs. Numerical results revealed that applying SAWs modifies the energy budget inside the liquid medium, leading to different impact behaviors. We then systematically investigated the effects of inclination angle, droplet impact velocity, SAW propagation direction, and applied SAW power on the impact dynamics and showed that by using SAWs, droplet impact on the nontextured hydrophobic and inclined surface is effectively changed from deposition to complete rebound. Moreover, the maximum contact time reduction up to ∼50% can be achieved, along with an alteration of droplet spreading and movement along the inclined surfaces. Finally, we showed that the rebouncing angle along the inclined surface could be adjusted within a wide range.

Large Nonreciprocal Propagation of Surface Acoustic Waves in Epitaxial Ferromagnetic/Semiconductor Hybrid Structures

Non-reciprocal propagation of sound, that is, the different transmission of acoustic waves traveling along opposite directions, is a challenging requirement for the realization of devices like acoustic isolators and circulators. Here, we demonstrate the efficient non-reciprocal transmission of surface acoustic waves (SAWs) propagating along opposite directions of a GaAs substrate coated with an epitaxial Fe$_3$Si film. The non-reciprocity arises from the acoustic attenuation induced by the magneto-elastic (ME) interaction between the SAW strain field and spin waves in the ferromagnetic film, which depends on the SAW propagation direction and can be controlled via the amplitude and orientation of an external magnetic field. The acoustic transmission non-reciprocity, defined as the difference between the transmitted acoustic power for forward and backward propagation under ME resonance, reaches values of up to 20%, which are, to our knowledge, the largest non-reciprocity reported for SAWs traveling along a semiconducting piezoelectric substrate covered by a ferromagnetic film. The experimental results are well accounted for by a model for ME interaction, which also shows that non-reciprocity can be further enhanced by optimization of the sample design. These results make Fe$_3$Si/GaAs a promising platform for the realization of efficient non-reciprocal SAW devices.

Unidirectional planar Hall voltages induced by surface acoustic waves in ferromagnetic thin films

The electromotive forces induced by surface acoustic waves (SAWs) are investigated in ferromagnetic thin films. CoFeB thin films deposited on LiNbO$_3$ substrates are patterned into Hall-bars to study the acoustoelectric transport properties of the device. The longitudinal and transverse dc voltages that develop in the Hall bars, which are parallel and orthogonal to the flow of the SAW, respectively, are measured under application of an in-plane magnetic field. The longitudinal voltage scales linearly with the SAW power and reverses its polarity upon changing the direction to which the SAW propagates, suggesting generation of a dc acoustic current via the SAW excitation. The magnetic field has little influence on the acoustic current. In contrast, the SAW induced transverse voltage shows significant dependence on the relative angle between the magnetic field and the SAW propagation direction. Such field angle dependent voltage resembles that of the planar Hall voltage induced by electric current. Interestingly, the angle dependent acoustic transverse voltage does not depend on the SAW propagation direction. Moreover, the magnitude of the equivalent angle dependent acoustic transverse resistance is more than one order of magnitude larger than that of the planar Hall resistance. These results show the unique acoustoelectric transport properties of ferromagnetic thin films.

Flexible dual-mode surface acoustic wave strain sensor based on crystalline LiNbO3 thin film

This work presents the development of flexible dual-mode surface acoustic wave (SAW) sensor based on single crystalline thin film lithium niobate (TF-LN). Numerical modeling is conducted to investigate the SAW propagation and the effects on strain sensitivity. The dependence of strain sensitivity on angles between the applied strain and SAW propagation direction is analyzed numerically and experimentally, showing that the maximum strain sensitivity is at 45° rather than longitudinal direction. 128° Y-cut TF-LN (~50 µm), obtained by micromachining technique, is utilized as the piezoelectric substrate to fabricate the SAW strain sensors with dual-mode, namely Rayleigh mode and thickness shear mode. The sensor has excellent flexibility and demonstrates remarkable capability for an ultra-wide range strain measurement up to ±3000 . Temperature effects on resonant frequency and strain sensitivity are investigated in the range of 25 °C–100 °C, and similar temperature characteristics are observed for the dual modes. A method of beat frequency between the dual modes is introduced which is able to eliminate the temperature effect on strain sensing, an on-chip temperature influence removing capability. All the results clearly show that this sensor exhibits great potential for applications in flexible electronics and microsystems.

Three-dimensional finite element simulation of Love mode surface acoustic wave in layered structures including ZnO piezoelectric film and diamond substrate

Three-dimensional (3D) finite element method (FEM) was used to simulate the surface acoustic wave (SAW) propagation properties in layered structures including ZnO piezoelectric film and diamond substrate. The excitation conditions of Love mode and Rayleigh mode SAWs are discussed considering the ZnO crystal orientation and the SAW propagation direction which are specified using Euler angles (α, β, γ). The phase velocity and electromechanical coupling coefficient K2 of Love mode SAW in the ZnO/diamond structure are provided as functions of the normalized ZnO layer thickness hZnO/λ and the Euler angles (α, β, γ). The results show that the maximum K2 of 4.26% is reached at hZnO/λ = 0.3 and (α, β, γ) = (0°, 90°, 0°) for Love mode SAW; the associated phase velocity is 3436 m/s and the temperature coefficient of frequency (TCF) value is −26.5 ppm/°C. The TCF properties are greatly improved by adding a SiO2 layer between the ZnO film and the diamond substrate. A zero TCF can be obtained when the normalized SiO2 thickness hSiO2/λ is 0.25 and the hZnO/λ is 0.2, where the corresponding K2 is 3.26% and the phase velocity is 3126 m/s.

Diaphragm shape effect on the sensitivity of surface acoustic wave based pressure sensor for harsh environment

Aluminum Nitride (AlN) based surface acoustic wave (SAW) pressure sensors for harsh environment applications are of great interest in recent years. Such sensor employs a thick diaphragm (∼50 μm) to endure the high pressure, but this seriously limits the sensitivity of these devices. Understanding of the working mechanism and the effect of geometrical parameters will yield the design principles to achieve improved sensitivity. In this letter, the effect of diaphragm on the performance of SAW pressure sensors is studied. AlN based SAW resonators on (100) wafer with different diaphragm shapes are fabricated, packaged, and characterized. Pressure coefficient of frequency (PCF) of pressure sensors with circular diaphragm, rectangular diaphragm (small aspect ratio) and rectangular diaphragm (large aspect ratio) is found to be 0.071 ppm/psi, 0.038 ppm/psi, and −0.171 ppm/psi, respectively. The longitudinal and lateral strains along the SAW propagation direction (〈100〉 direction) have the opposite effects on the frequency change, i.e., longitudinal strain increases the resonant frequency while lateral strain decreases the resonant frequency. Hence, the measured PCF is a combined effect of the two strains, whereby increase in lateral strain results in lower PCF. The ratio of longitudinal/lateral strain is determined by the diaphragm shape. The rectangular diaphragm (large aspect ratio) with only lateral strain thus shows negative PCF. Additionally, by changing the wafer plane, SAW propagation direction and ratio of longitudinal/lateral strains may help to enhance the primary strain effect and minimize the reverse strain effect. This approach could further improve the sensitivity of pressure sensor without sacrificing its high pressure sensing range for harsh environment applications.

Vibration‐Induced Deagglomeration and Shear‐Induced Alignment of Carbon Nanotubes in Air

Carbon nanotubes (CNTs) are widely known to agglomerate into difficult to separate, 10–100 μm bundles, even after suspension in solution. Here, a dry and rapid (≈10 s) method to deagglomerate bulk, unbound multi‐walled CNT bundles due to surface acoustic waves (SAW) in a piezoelectric substrate is reported for the first time. The process first forms 1‐μm CNT bundles from extremely large (≈10 Mm/s2) mechanical accelerations due to the SAW; these bundles are consequently susceptible to SAW‐induced evanescent, quasistatic electric fields that couple into the bundles and form a mat of long (1–10 μm) individual CNTs on the substrate surface. These CNTs may then be aligned along the direction of shear provided by sliding a glass cover slip 10 mm across the CNT mat. This alignment is notably independent of the SAW propagation direction. Further, the intrinsic structure of the nanotubes is unaffected as verified using Raman spectroscopy. Uniquely simple, the approach avoids the many shortcomings of other CNT deagglomeration techniques—particularly surface modification and suspension in solution—to rapidly separate and align large numbers of CNTs, thereby overcoming a key limitation in their use for a diverse range of applications.

Double flow reversal in thin liquid films driven by megahertz-order surface vibration

Arising from an interplay between capillary, acoustic and intermolecular forces, surface acoustic waves (SAWs) are observed to drive a unique and curious double flow reversal in the spreading of thin films. With a thickness at or less than the submicrometre viscous penetration depth, the film is seen to advance along the SAW propagation direction, and self-similarly over time t 1/4 in the inertial limit. At intermediate film thicknesses, beyond one-fourth the sound wavelength λ ℓ in the liquid, the spreading direction reverses, and the film propagates against the direction of the SAW propagation. The film reverses yet again, once its depth is further increased beyond one SAW wavelength. An unstable thickness region, between λ ℓ /8 and λ ℓ /4, exists from which regions of the film either rapidly grow in thickness to exceed λ ℓ /4 and move against the SAW propagation, consistent with the intermediate thickness films, whereas other regions decrease in thickness below λ ℓ /8 to conserve mass and move along the SAW propagation direction, consistent with the thin submicrometre films.

Theoretical investigation of high velocity, temperature compensated Rayleigh waves along AlN/SiC substrates for high sensitivity mass sensors

The operation of electroacoustic devices based on surface acoustic waves (SAW) propagation along β-SiC/AlN and amorphous-SiC/AlN substrates is theoretically studied with respect to the AlN film thickness, the SAW propagation direction, temperature and electric boundary conditions. GHz-range, enhanced electroacoustic coupling coefficient, temperature compensated around 20 °C electroacoustic devices are the advantages of SiC/AlN composite structures. These structures are also suitable for the implementation of sensors with improved performances with respect to SAW devices based on bulk single crystal piezoelectric substrates. The structures feasibility was confirmed by structural investigation and quantitative analysis of sputtered amorphous-SiC and AlN films on Si substrates.

X-ray diffraction and surface acoustic wave analysis of BST/Pt/TiO2/SiO2/Si thin films

High dielectric constant and electrostriction property of (Ba, Sr)Ti03 (BST) thin films result in an increasing interest for dielectric devices and microwave acoustic resonator. Barium strontium titanate (Ba0.645Sr0.355TiO3) films of about 300 nm thickness are grown on Pt(111)/TiO2/SiO2/Si(001) substrates by rf magnetron sputtering deposition techniques. X-ray diffraction is applied for the microstructural characterization. The BST films exhibit a cubic perovskite structure with a dense and smooth surface. A laser acoustic waves (LA-waves) technique is used to generate surface acoustic waves (SAW) propagating in the BST films. Young’s modulus E and the Poisson ratio ν of TiO2, Pt and BST films in different propagation directions are derived from the measured dispersion curves. Estimation of BST elastics constants are served in SAW studies. Impact of stratification process on SAW, propagating along [100] and [110] directions of silicon substrate, has been interpreted on the basis of ordinary differential equation (ODE) and stiffness matrix method (SMM). A good agreement is observed between experimental and calculated dispersion curves. The performed calculations are strongly related to the implemented crystallographic data of each layer. Dispersion curves are found to be sensitive to the SAW propagation direction and the stratification process for the explored frequency ranges 50−250 MHz, even though it corresponds to a wave length clearly higher than the whole films thickness.

Shear horizontal surface acoustic wave induced microfluidic flow

In this letter, we demonstrate that surface acoustic waves (SAWs) excited on 36°Y-X LiTaO3 have both Rayleigh and shear horizontal components, which can result in different acoustic streaming patterns in micro-droplets. The shear horizontal surface acoustic wave streaming pattern exhibits a quasi-stable counterflow in the SAW propagation direction with velocities of up to 16 mm/s, with minimal damping observed within the droplets. A pumping function was realized at a threshold power of 30 dBm with a large rotation vector on hydrophobically treated devices.

Magnetic Sensor Based on Surface Acoustic Wave Resonators

In this paper, we describe a new type of magnetic surface acoustic wave (SAW) sensor that consists of interdigital transducers made of a magnetostrictive material, Ni, on a quartz substrate. This sensor can be used as an element of a resonator-type wireless SAW sensor system because it has a high Q. Moreover, it has the merits of being fabricated easily using the existing process for SAW devices because its structure is the same as that of general SAW resonators. When an external magnetic field of about 100 mT was applied perpendicularly to the direction of SAW propagation, the frequency change obtained was 200 ppm. In addition, the developed sensor was applied in a passive and wireless open/close sensing system and the reading distance obtained was 3 m.

On the piezoelectric coupling constant of epitaxial Mg-doped GaN

The electromechanical coupling coefficient ( k 2) of an Mg-doped GaN epilayer has been evaluated by Lee et al. [IEEE Trans Electron Dev 2001;48:524–9] as (4.3 ± 0.3)%, which is much higher than those of other comparable III–V materials. As this is a surprising result, the present paper reports an independent determination of k 2 from a similar Mg-doped GaN epilayer for comparison. A 2 μm thick Mg-doped GaN epilayer ((0 0 0 1) orientation) was grown on a c-plane sapphire substrate. The atomic concentration of Mg was 1.2 × 10 18/cm 3. Surface-acoustic wave (SAW) delay-line filters were fabricated using evaporated aluminium and “lift-off” techniques, and consisted of pairs of interdigital transducers (IDTs). Each IDT had 150 pairs of double-fingers. The SAW propagation direction in the sapphire substrate was [ 1 ¯ 1 0 0 ]. Acoustic wavelengths of 32 μm, 40 μm, 48 μm and 56 μm were used in the present work. The insertion loss of these filters was found to be more than 80 dB. This allows an upper bound of the electromechanical coupling coefficient ( k 2) of the Mg-doped GaN epilayer to be calculated as 1 × 10 −4%. Our value, significantly lower than previously reported, shows that an Mg-doped GaN epilayer may not be such a promising material for SAW devices as appeared at first sight.

Surface acoustic wave mediated dielectrophoretic alignment of rolled-up microtubes in microfluidic systems

The alignment behavior of solution dispersed rolled-up microtubes by surface acoustic waves (SAW) is demonstrated. In contrast to the random alignment of rolled-up insulated silicon oxide tubes, metallic chromium tubes can be effectively aligned and assembled into “tube-chains” parallel to the SAW propagation direction. The experiments suggest that the tube orientation is mainly determined by the dielectrophoresis (DEP) forces acting on the tubes. The DEP forces arise from the induced dipole moment of the tubes in the SAW generated piezoelectric field on the LiNbO3 substrate.

Acoustic-counterflow microfluidics by surface acoustic waves

In this letter, we demonstrate an unexpected surface-acoustic-wave (SAW)-driven pumping effect in hydrophobic polydimethilsiloxane (PDMS)-lithium niobate (LiNbO3) microchannels. Atomization within the fluidic channel followed by SAW-assisted coalescence leads to liquid counterflow with respect to the SAW propagation direction. This physical mechanism is contrasted with the acoustic-streaming process driving isolated drop displacement on piezoelectric substrates. This principle is shown not to be readily applicable to the present microchannel case. The proposed device geometry can be exploited to integrate micropumps into complex microfluidic chips, improving the portability of micro-total-analysis systems.

Theoretical and experimental investigation of gigahertz-band, temperature-compensated electromechanical coupling configurations based on AlN films

Highly c-axis oriented piezoelectric AlN films were sputtered on (0001) Al2O3 substrates and four gigahertz-range surface acoustic wave (SAW) electroacoustic coupling configurations were realized by conventional photolithographic process. The thermal sensitivity of each device was experimentally estimated and found in good accordance with the theoretical predictions. Eight gigahertz-range, thermally compensated SAW devices were obtained for specific AlN thickness, SAW propagation direction, and electrical boundary conditions. A theoretical analysis of the acoustic and electrical behavior of the four coupling structures is proposed to match the achievable performances of the SAW devices with the design requirements.