Surface Acoustic Wave Velocity Research Articles

Impact of Spin-Wave Dispersion on Surface-Acoustic-Wave Velocity

The dependence of the velocity of surface acoustic wave (SAW) as a function of an external applied magnetic field is investigated in a Fe thin film epitaxially grown on a piezoelectric GaAs substrate. The SAW velocity is observed to strongly depend on both the amplitude and direction of the magnetic field. To interpret the experimental data a phenomenological approach to the relative change in SAW velocity is implemented. We find that the experimental velocity variation can be well reproduced provided that the spin wave dispersion is taken into account. The validity of this phenomenological model is attested by the comparison with a quasi-exact magnetoelastic one.

Dispersion of surface acoustic waves in thin films at extreme wavelength-to-thickness ratios.

Surface acoustic waves (SAWs) are sensitive to the presence of a layer on the surface of a material, even if this layer is extremely thin compared to their wavelengths. Given the very slow propagation velocities of SAWs compared to electromagnetic waves, their wavelengths are on the order of 40 μm for acoustic frequencies on the order of 100 MHz. However, it has been shown that these waves are dispersive for coatings whose thicknesses are more than 1000 times smaller than their wavelength. This sensitivity is verified by studying the dispersion of SAWs for a frequency range between 90 and 260 MHz.

Development of a Broadband (100-240 MHz) Surface Acoustic Wave Emitter Devoted to the Non-Destructive Characterization of Sub-Micrometric Thin Films.

In the ultrasonic non-destructive evaluation of thin films, it is essential to have ultrasonic transducers that are able to generate surface acoustic waves (SAW) of suitably high frequencies in a wide frequency range of between ten and several hundred megahertz. If the characterization is carried out with the transducer in contact with the sample, it is also necessary that the transducers provide a high level of mechanical displacement (>100 s pm). This level allows the wave to cross the transducer–sample interface and propagate over the distance of a few millimeters on the sample and be properly detected. In this paper, an emitter transducer formed of interdigitated chirp electrodes deposited on 128° Y-cut LiNbO3 is proposed. It is shown that this solution efficiently enables the generation of SAW (displacement level up to 1 nm) in a frequency range of between 100 and 240 MHz. The electrical characterization and a displacement field analysis of SAW by laser Doppler vibrometry are presented. The transducer’s significant unidirectionality is demonstrated. Finally, the characterization of two titanium thin films deposited on silicon is presented as an example. A meaningful SAW velocity dispersion (~10 m/s) is obtained, which allows for the precise estimation (5% of relative error) of the submicrometer thickness of the layers (20 and 50 nm).

Theoretical investigation of the piezoelectric and surface acoustic wave properties of GeSn alloys

The piezoelectric and dielectric constants of Ge1-xSnx with varying concentrations (x∈(0–1.0) were calculated using the first-principles method based on the density functional theory, and the surface acoustic wave (SAW) properties such as phase velocity, electromechanical coupling coefficient k2, and power flow angle (PFA) were obtained for Ge0.5Sn0.5 from using the Christopher equations in different orientations (0–90°, 0–90°, and 0–180°). The results demonstrate that the piezoelectric and dielectric constants of Ge1-xSnx deviate from Vegard's law when the x is varied, and the piezoelectric constant value reaches a maximum when x = 0.5. Additionally, the SAW velocity of Ge1-xSnx can be tuned by changing the Sn concentration. It can be seen from the contour maps of Ge0.5Sn0.5 at orientations (45°, 0–90°, and 0–180°), many orientations exhibit outstanding SAW performance. Orientations with large k2 and zero PFA are selected as promising candidates for fabricating SAW devices, which has important significance for experimental research.

Evaluation of Adhesion Properties of Thin Film Structure through Surface Acoustic Wave Dispersion Simulation.

A theoretical simulation study of the dispersion characteristic of the surface acoustic wave (Rayleigh wave) was conducted by modeling the adhesion interlayer with stiffness coefficients to evaluate the bonding properties of nano-scale thin film structures. For experimental validation, a set of thin film specimens were fabricated—637 nm, 628 nm, 637 nm, 600 nm, and 600 nm thick titanium (Ti) films were deposited on silicon (Si) (100) substrate using a DC Magnetron sputtering process with DC power from 28.8 W, 57.6 W, 86.4 W, 115.2 W, and 144 W. The thicknesses of the Ti films were measured using a scanning electron microscope (SEM). Surface acoustic wave velocity for each of the manufactured thin film specimens was measured by using a V(z) curve technique of a Scanning Acoustic Microscope. The measured velocity, transducer frequency, and thickness of the film were applied to dispersion characteristic simulation for a given stiffness coefficient to calculate adhesion strength of each specimen. To verify the simulation result, the adhesion force of each specimen was measured using a nano-scratch test and then compared with the calculated values from the dispersion characteristic simulation. The value of adhesion strength from the dispersion characteristic simulation and the value of adhesion force of the nano-scratch test were found to have a similar tendency according to the process variable of the thin film. The results demonstrated that the adhesion strength of a thin film could be evaluated quantitatively by calculating the dispersion characteristics with the adhesion interlayer stiffness model.

Firmness evaluation of watermelon and melon using velocity dispersion of surface-acoustic-wave

The surface-acoustic-wave (SAW) velocity was measured in the frequency range of 400–7000 Hz in watermelon and melon to evaluate the firmness nondestructively. We showed that the positive velocity dispersion (velocity increases with increasing frequency) observed was caused by the fruit structure, which consists of a hard pericarp and underlying soft flesh. In watermelon, the low-frequency limit of the velocity dispersion curve observed for the pericarp predicted the SAW velocity in watermelon flesh, which was measured to be independent of frequency. In melon, the positive velocity dispersions observed for the pericarp as well as the flesh suggested a distribution of elasticity in the depth direction. Ripening for fourteen days caused a decrease in the SAW velocity by 34%–57% depending on the frequency. The present results demonstrate that the SAW velocity dispersion is a good measure of the firmness and ripening of fruits.

Study of electronic band structure in a 75-nm-wide n-GaAs/AlGaAs quantum well by surface acoustic waves

From measured magnetic field dependences of attenuation and velocity of surface acoustic waves (SAW), we calculated the real part of electron the conductivity in a 75-nm-wide n-GaAs/AlGaAs quantum well in magnetic fields of up to 18 T at frequencies of 30–300 MHz in the temperature range 20–500 mK. We observed slow magneto-oscillations of the conductivity depended on SAW intensity and occurred due to elastic intersubband scattering between the symmetric (S) and antisymmetric (AS) subbands formed by Coulomb repulsion between the electrons. Our theoretical description of the magneto-oscillations allowed estimating the quantum and the intersubband scattering times. The experimentally obtained subband splitting value 0.42 meV was close to the theoretically found 0.57 meV. Additionally, in tilted magnetic fields of 0.5–1.5, T we observed crossings of the Landau levels between the S and AS subbands. According to the developed theory, the crossings at the in-plane field below 1.5 T occur due to different dependences of cyclotron energies in the subbands on the in-plane field. Namely, the cyclotron energy in the S (AS) subband decreases (increases) with the rising of the in-plane field. Work supported by “BASIS,” NSF DMR-1644779 and DMR-1420541, GBMF4420, and State of Florida.]

Measuring Velocity, Attenuation, and Reflection in Surface Acoustic Wave Cavities Through Acoustic Fabry-Pérot Spectra.

Surface acoustic wave (SAW) cavities have been widely applied as electronic bandpass filters, sensors, microfluidic tweezers, and, in recent years, as devices for coupling with quantum systems. Here we propose a novel method of analyzing acoustic Fabry-Pérot spectra, by analogy with optical cavities, to determine the free surface velocity and attenuation of SAW waves, as well as the reflection of interdigital transducers (IDTs), all of which are crucial design parameters. In our experiment, two-port SAW resonators, consisting of two IDTs laterally separated by a free surface cavity length, are used to generate SAWs on 128° Y-X lithium niobate that are trapped between the two IDTs which also act as Bragg reflectors. Resonant cavity peaks can be observed through the electrical S11 (reflection) spectrum measured on one IDT. The free spectral range and linewidths of cavity peaks are then measured to obtain the free surface SAW velocity, SAW propagation attenuation coefficient, and IDT reflection phase and amplitude. Our method of analyzing Fabry-Pérot spectra provides an intuitive approach for determining key characteristics of SAW waves and cavities.

Analysis of thin layers using surface acoustic wave-photonic devices in silicon-on-insulator.

The analysis of thin layers deposited on various substrates is widely employed in thickness monitoring, materials research and development and quality control. Measurements are often performed based on changes to acoustic resonance frequencies of quartz micro-balance devices. The technique is extremely sensitive, but it is restricted to hundreds of MHz frequencies and requires electrical connectivity. In this work we propose and demonstrate the analysis of elastic properties of thin layers deposited on surface acoustic wave-photonic devices in standard silicon-on-insulator. The devices operate at 2.4 GHz frequency, and their interfaces are fiber-optic. The radio-frequency transfer functions of the devices are modified by sub-percent level changes to the group velocity of surface acoustic waves following deposition of layers. Layers of aluminum oxide and germanium sulfide of thickness between 10-80 nm are characterized. The analysis provides estimates for Young's modulus of the layers.

A nonlinear analysis of surface acoustic waves in isotropic elastic solids

With the fast evolution of wireless and networking communication technology, applications of surface acoustic wave (SAW), or Rayleigh wave, resonators are proliferating with fast shrinking sizes and increasing frequencies. It is inevitable that the smaller resonators will be under a strong electric field with induced large deformation, which has to be described in wave propagation equations with the consideration of nonlinearity. In this study, the formal nonlinear equations of motion are constructed by introducing the nonlinear constitutive relation and strain components in a standard procedure, and the equations are simplified by the extended Galerkin method through the elimination of harmonics. The wave velocity of the nonlinear SAW is obtained from approximated nonlinear equations and boundary conditions through a rigorous solution procedure. It is shown that if the amplitude is small enough, the nonlinear results are consistent with the linear results, demonstrating an alternative procedure for nonlinear analysis of SAW devices working in nonlinear state.

Effect of crystal orientation on the hardness and strength of piezoelectric LiNbO3 substrates for microelectronic applications

Piezoelectric single crystalline materials are paramount for high-speed data transfer in 5G technologies. The functionality of the end-devices demands temperature independent frequency filtering and high surface acoustic wave velocities, which are associated with the orientation dependent thermo-physical properties of the piezoelectric substrate material. Single crystalline Lithium Niobate (LiNbO3), cut in particular directions, has proven to have outstanding functional properties, yet its brittle character along with the highly anisotropic mechanical properties may limit its use in demanding applications.In this study, the effect of crystal orientation on hardness and on mechanical strength is demonstrated by comparing nanoindentation results and finite element analysis supported biaxial strength experiments for two LiNbO3 samples with different orientation. It is demonstrated that the crystal anisotropy leads to differences in hardness up to ∼ 20% between both orientations, with the characteristic strength being double in the harder direction. The observed correlation is rationalized based on the effect of surface finish and distinct sub-surface damage in the corresponding crystal orientations. Additional strength measurements on nano-scratched samples revealed a significantly higher remaining strength for the harder orientation due to less (sub-) surface damage. These findings can be exploited in future design of single crystalline substrate materials with higher reliability.

Surface acoustic wave temperature properties in alpha-GeO2 crystal

In this study, the surface acoustic wave (SAW) temperature properties in flux-grown α-GeO2 crystal are numerically investigated. It is shown that the SAW velocity temperature change substantially depends only on the temperature coefficient of three elastic constants: C66, C44 and C14 for crystal cuts and wave propagation directions, where SAW has high electromechanical coupling coefficient. The SAW temperature coefficient of delay (TCD) for these crystal cuts are in the range from -40 ppm /°C to -70 ppm /°C. In contrast to alpha-quartz, the surface wave TCD values are not equal to zero in Z-, Y- , and Z- rotated cuts of α-GeO2 single crystal. Its values are comparable in the magnitude with the surface wave TCD values in lithium tantalate. In the crystal grown from the melt, the interdigital transducer (IDT) conductance has two times larger amplitude than that in hydrothermally grown a-GeO2. The leaky acoustic wave excited by IDT on Z+120°-cut and wave propagation direction along the X-axis, has an electromechanical coupling coefficient 5 times less than that for surface wave.

Analysis of surface acoustic wave properties in CaYAl3O7 single crystal

The success on the growth of new piezoelectric materials allows sufficiently increase the operating temperature of the surface acoustic wave (SAW) devices from 300°C to 1000°C. A new calcium yttrium aluminate (CaYAl3O7) single crystal of the tetragonal symmetry has piezoelectric properties up to the temperature of 1000°C. The paper presents a numerical study of the surface acoustic wave properties in the crystal. The SAW velocity, electromechanical coupling coefficient and power flow angle are studied for different crystal cuts of CaYAl3O7. It is shown that the maximum value of SAW coupling coefficient (0.24%) is on the Z+60°-cut and wave propagation direction along the X-axis of the crystal. For the Z-cut and wave propagation direction along the X+45°-axis of crystal, the SAW coupling coefficient is equal to 0.2%. These two cuts of the crystal are potentially useful for SAW device applications.

Nondestructive Evaluation of Electroplating-Induced Hydrogen Embrittlement in Cadmium-Coated High-Strength Steel Using Ultrasonic Surface Waves

Development of a nondestructive evaluation (NDE) method to detect nascent hydrogen embrittlement (HE) in electroplated high-strength steel parts is becoming important for the aerospace industry. This research investigates the feasibility of surface acoustic waves (SAWs) measurements to distinguish between cadmium (Cd) plated SAE 4340 steel samples with low and high HE susceptibilities. SAWs were generated with a 10 MHz piezoelectric transducer and detected by line scans via a laser Doppler vibrometer setup. Using signal processing algorithms in MATLAB, SAW velocities as well as attenuation coefficients were estimated. Depth profiles of steel hardness near coatings were also evaluated using Vickers microindentation tests. Average steel hardness in not-baked samples was slightly increased. Cd coatings were characterized by laser and optical microscopy methods. Small variations found in thickness and surface roughness of the Cd coatings among the samples did not significantly affect the NDE results. On average, samples in the not-baked condition (high HE risk) exhibited lower SAW attenuation coefficients compared to immediately baked and late-baked conditions (low HE risk). However, it was not possible to distinguish between the manufacturing conditions of individual samples due to overlaps in attenuation measurement results. SAW velocities as estimated by the cross-correlation method were found to be not sensitive to manufacturing conditions.

Pd/Ni nanowire film coated SAW hydrogen sensor with fast response

A Pd/Ni nanowires (NWs) film-coated surface acoustic wave (SAW) device is proposed for sensing hydrogen with fast response. The gas adsorption in Pd/Ni NW film modulates the conductivity, and perturbs the SAW velocity, and the corresponding changes in phase can be collected as the sensing signal. 200 MHz SAW device with delay line pattern is prepared by using the lithographic technique on Y35oX quartz substrate. The larger specific surface of Pd/Ni NW contributes well to improvement of the response time. By using anode-aluminum-oxide (AAO) template-confined electrodeposition, the Pd/Ni NWs have been prepared, then dissolved in ethanol and finally deposited on the surface of the SAW device by drop coating method to build the SAW hydrogen sensor. The developed SAW sensor is integrated in a phase discriminating circuit, and experimentally characterized. Fast response less than 2 s, high sensitivity of 1.65 mV/% (0.3–3.5 v/v % in nitrogen gas), and low detection limit of 7 ppm have been achieved at room temperature (25 ℃). Further, the developed sensors show excellent selectivity and stability. Also, the film thickness - and humidity - dependent sensing performance was studied.

Scanning Electron Microscopy Investigation of Surface Acoustic Wave Propagation in a 41° YX-Cut of a LiNbO3 Crystal/Si Layered Structure

The propagation process of the surface acoustic waves (SAW) and the pseudo-surface acoustic waves (PSAW) in a bonded layered structure of a 41° YX-cut of a LiNbO3 crystal/Si(100) crystal was investigated. The scanning electron microscopy (SEM) method,in the low-energy secondary electrons registration mode, made it possible to visualize the SAW and PSAW in the LiNbO3/Si layered structure. The process of the SAW and PSAW propagation in a LiNbO3/Si layered structure and in a bulk 41° YX-cut of a LiNbO3 crystal were compared. It was demonstrated that the SAW velocities in the layered LiNbO3/Si structure exceed the typical SAW velocities for LiNbO3 and Si single crystals. In the layered structure, the SAW and PSAW velocities were 4062 m/s, 4731 m/s, and 5871 m/s. It was also demonstrated that the PSAW velocities are the same in the LiNbO3/Si layered structure and in the bulk 41° YX-cut of a LiNbO3 crystal.

Sub- and supersonic elastic waves in an annular hole phononic metamaterial

Surface acoustic wave (SAW) devices are used in a wide range of applications including sensing and microfluidics, and are now being developed for applications such as quantum computing. As with photonics, and other electromagnetic radiation, metamaterials offer an exciting route to control and manipulate SAW propagation, which could lead to new device concepts and paradigms. In this work we demonstrate that a phononic metamaterial comprising an array of annular hole resonators can be used to realise frequency control of SAW velocity. We show, using simulations and experiment, that metamaterial patterning on a lithium niobate substrate allows control of SAW phase velocities to values slower and faster than the velocity in an unpatterned substrate; namely, to ~85% and ~130% of the unpatterned SAW velocity, respectively. This approach could lead to novel designs for SAW devices, such as delay lines and chirp filters, but could also be applied to other elastic waves.

Scattering Matrix Approach to Design of One-Port Surface Acoustic Wave Resonator Sensors Utilizing Reflectors as Sensing Element.

It has been recently demonstrated that one-port surface acoustic wave (SAW) resonators known for their high Q value and relatively small device footprint could be utilized for in-liquid mass loading sensing applications where only the reflectors of the device are coated with the sensing film, while the interdigital transducer (IDT) is isolated from the sensing environment. The sensor relies on changes induced in reflectivity and phase velocity of SAW in the region of the reflectors upon detection of the measurand and is particularly advantageous for SAW resonator-type sensors as any contact of the sensing film with the IDT could change its static capacitance during sensing and thereby introduce serious instability in the sensor response. Accordingly, in the present work, the existing scattering matrix approach to the design of one-port SAW resonator filters, which does not cater to the integration of sensing film on the resonator surface, is adapted to develop a method to design one-port SAW resonator sensors utilizing reflectors as sensing element. The reflector block of the one-port SAW resonator is readily split into sensing-active and sensing-inactive parts using the SAW grating transmission matrix in order to study the changes introduced in input admittance of the device for varying level of coverage of the sensing film. The theoretical design approach presented in this work could be used to fabricate high-performance one-port SAW resonator sensors operating at its point of highest sensitivity while utilizing one of the device reflectors as sensing element, without the use of additional impedance matching circuit elements.

Laser-Ultrasonic Characterization of Strongly Anisotropic Materials by Transient Grating Spectroscopy

Transient grating spectroscopy (TGS) is a laser-ultrasonic method allowing measurement of the surface acoustic wave (SAW) velocity in an examined material for a given direction of the wave vector. We explore the capability of TGS for determination of shear elastic coefficients ( $$c^\prime$$ and $$c_{44}$$ ) of strongly anisotropic cubic materials. TGS is tested on a set of single crystals with an anisotropy factor up to $$A=25$$ . Using a numerical simulation based on a Ritz-Rayleigh approach, we show that strong anisotropy may lead to significant coupling of SAWs with bulk shear waves, which complicates TGS measurements in specific directions. Based on the obtained TGS data, we discuss the possibility of also using the TGS technique for assessing the longitudinal elastic coefficient ( $$c_L$$ ). Despite the energy focusing and other effects originating from the strong anisotropy, the TGS method can be used to reliably determine the directional dependence of the SAW velocity in these materials, and the resulting experimental datasets are sufficient for inverse determination of both the soft shear elastic constant ( $$c^\prime$$ ) and the hard shear elastic constant ( $$c_{44}$$ ). The longitudinal coefficient can be determined with lower accuracy. TGS is a suitable experimental tool for contactless characterization of strongly anisotropic materials.

Single pixel camera methodologies for spatially resolved acoustic spectroscopy

Spatially resolved acoustic spectroscopy (SRAS) is a laser ultrasound technique used to determine the crystallographic orientation (i.e., microstructure) of materials through the generation and measurement of surface acoustic wave velocity on a sample. Previous implementations have used a grating pattern imaged onto the surface to control the frequency of the generated wave in a single direction—grain orientation can be computed by acquiring wave velocities in different directions on the surface (gathered by physically rotating the grating pattern). This paper reports an advance to this methodology, inspired by single pixel cameras, using a coded grating pattern, created using a spatial light modulator, to excite surface acoustic waves in multiple directions simultaneously. This change to the optical arrangement can simplify the overall system alignment, remove mechanical complexities, and is well suited for point-by-point full orientation imaging, potentially allowing for faster orientation imaging using SRAS microscopy. Improvements to the robustness of measurement may be expected to extend the applicability of SRAS in the materials science field. To demonstrate this methodology, experiments were conducted on isotropic and anisotropic samples.