Surface Acoustic Wave Velocity Research Articles

Theoretical and experimental characterisation of a SAW delay line through its Y ‐matrix

Surface acoustic wave (SAW) delay lines are commonly used devices, and it is important to characterise them to find the causes of deviations between the experimental and theoretical behaviour in order to consider them during design. The authors present the theoretical characteristics and the characterisation of a SAW delay line through X-ray diffraction, energy dispersive X-ray fluorescence, scanning electron microscopy, atomic force microscopy and the measurement of S-parameters. Using the S-parameters, the Y-parameters were calculated and comparing them with those obtained theoretically, they found disagreement in their magnitudes and also that the experimental SAW velocity was 2.8% larger than the theoretical one. The magnitude of experimental Y11 is smaller than that obtained theoretically because of the ill-defined profile and the metallisation ratio of the electrodes is not ideal due to inherent limitations in the fabrication process, and besides Y21 is smaller than expected because the attenuation of the SAW when it propagates through the delay line. When the electrode defects, the experimental SAW velocity and the attenuation coefficient of SAW in this material are considered in the theoretical calculations, agreement is found between theoretical and experimental results. This procedure is based in the comparison between experimental and theoretical Y-parameters and could be used to estimate attenuation, electrode capacitance and SAW velocity.

Mechanical back-action of a spin-wave resonance in a magnetoelastic thin film on a surface acoustic wave

Surface acoustic waves (SAWs) traveling on the surface of a piezoelectric crystal can, through the magnetoelastic interaction, excite traveling spin-wave resonance in a magnetic film deposited on the substrate. This spin-wave resonance in the magnetic film creates a time dynamic surface stress of magnetoelastic origin that acts back on the surface of the piezoelectric and modifies the SAW propagation. Unlike previous analyses that treat the excitation as a magnon-phonon polariton, here the magnetoelastic film is treated as a perturbation modifying boundary conditions on the SAW. We use acoustical perturbation theory to find closed form expressions for the back-action surface stress and strain fields and the resultant SAW velocity shifts and attenuation. We demonstrate that the shear stress fields associated with this spin-wave back-action also generate effective surface currents on the piezoelectric both in-phase and out-of-phase with the driving SAW potential. Characterization of these surface currents and their applications in determination of the magnetoelastic coupling are discussed. The perturbative calculation is carried out explicitly to first order (a regime corresponding to many experimental situations of current interest) and we provide a sketch of the implications of the theory at higher order.

Performance comparison of Rayleigh and STW modes on quartz crystal for strain sensor application

In this study, we compare two kinds of strain sensors based on Rayleigh wave and surface transverse wave (STW) modes, respectively. First, we perform a strain-and-stress analysis using the finite element method, and we consider the contribution to a surface acoustic wave (SAW) velocity shift. Prior to fabrication, we use a coupling-of-modes model to simulate and optimize two-port SAW resonators for both modes. We use a network analyzer to measure and characterize the two devices. Further, we perform an experiment using a strain-testing system with a tapered cross-section cantilever beam. The experimental results show that the ratio of the frequency shift to the strain for the Rayleigh wave mode is −1.124 ppm/με in the parallel direction and 0.109 ppm/με in the perpendicular direction, while the corresponding values for the STW mode are 0.680 ppm/με and 0.189 ppm/με, respectively.

Analysis of contributions of nonlinear material constants to temperature-induced velocity shifts of quartz surface acoustic wave resonators

In this paper, we examine the significance of the various higher-order effects regarding calculating temperature behavior from a set of material constants and their temperature coefficients. Temperature-induced velocity shifts have been calculated for quartz surface acoustic wave (SAW) resonators and the contributions of different groups of nonlinear material constants (third-order elastic constants (TOE), third-order piezoelectric constants (TOP), third-order dielectric constants (TOD) and electrostrictive constants (EL)) to the temperature-induced velocity shifts have been analyzed. The analytical methodology has been verified through the comparison of experimental and analytical results for quartz resonators. In general, the third-order elastic constants were found to contribute most significantly to the temperature-induced shifts in the SAW velocity. The contributions from the third-order dielectric constants and electrostrictive constants were found to be negligible. For some specific cases, the third-order piezoelectric constants were found to make a significant contribution to the temperature-induced shifts. The significance of each third-order elastic constant as a contributor to the temperature-velocity effect was analyzed by applying a 10% variation to each of the third-order elastic constants separately. Additionally, we have considered the issues arising from the commonly used thermoelastic expansions that provide a good but not exact description of the temperature effects on frequency in piezoelectric resonators as these commonly used expansions do not include the effects of higher-order material constants.

Enhancement of effective electromechanical coupling factor by mass loading in layered surface acoustic wave device structures

This paper describes a drastic enhancement of the effective coupling factor by mass loading in layered surface acoustic wave (SAW) device structures such as the ScAlN film/Si substrate structure. This phenomenon occurs when the piezoelectric layer exhibits a high acoustic wave velocity. The mass loading decreases the SAW velocity and causes SAW energy confinement close to the top surface where an interdigital transducer is placed. It is shown that this phenomenon is obvious even when an amorphous SiO2 film is deposited on the top surface for temperature compensation. This enhancement was also found in various combinations of electrode, piezoelectric layer, and/or substrate materials. The existence of this phenomenon was verified experimentally using the ScAlN film/Si substrate structure.

Impact of layer and substrate properties on the surface acoustic wave velocity in scandium doped aluminum nitride based SAW devices on sapphire

This paper investigates the performance of surface acoustic wave (SAW) devices consisting of reactively sputter deposited scandium doped aluminum nitride (ScxAl1-xN) thin films as piezoelectric layers on sapphire substrates for wireless sensor or for RF-MEMS applications. To investigate the influence of piezoelectric film thickness on the device properties, samples with thickness ranging from 500 nm up to 3000 nm are fabricated. S21 measurements and simulations demonstrate that the phase velocity is predominantly influenced by the mass density of the electrode material rather than by the thickness of the piezoelectric film. Additionally, the wave propagation direction is varied by rotating the interdigital transducer structures with respect to the crystal orientation of the substrate. The phase velocity is about 2.5% higher for a-direction compared to m-direction of the sapphire substrate, which is in excellent agreement with the difference in the anisotropic Young's modulus of the substrate corresponding to these directions.

Broadband Device for Measurement of Surface Acoustic Wave Velocities for Diagnostics of Residual Stress

A pulsed broadband device for measuring the velocity of surface acoustic waves intended for diagnostics of residual mechanical stresses in manufactured products is described. The output signal of the electromagneticacoustic receiver is produced by an inductor in the form of two closely spaced, straight, thin, parallel conductors. A method is proposed for measuring the time interval between two acoustic pulses.

Evaluation of Acoustic Properties for Ca3Nb(Ga0.75Al0.25)3Si2O14 Single Crystal Using the Ultrasonic Microspectroscopy System.

Acoustically related physical constants were experimentally determined for Ca3Nb(Ga0.75Al0.25)3Si2O14 (CNGAS) single crystal for the first time. Several plate specimens of the X-, Y -, Z -, 40.24 ° Y -, and 144.98 ° Y -cut were prepared from a CNGAS single crystal ingot grown by Czochralski technique. Elastic constants, piezoelectric constants, and their temperature coefficients for CNGAS were determined from longitudinal wave and shear wave velocities at around room temperature, measured by the ultrasonic microspectroscopy system. Dielectric constants, density, and coefficients of thermal expansion were also measured. It was demonstrated that the determined constants could provide calculation accuracy within ±0.12% in leaky surface acoustic wave velocity. The piezoelectric constants for CNGAS were a 7.2% increase in e11 and a 1.7% decrease in e14 due to Al-substitution effect, compared with those of Ca3NbGa3Si2O14. The appropriate cut angle for thickness-shear mode resonator with zero temperature coefficient of velocity was estimated to be around 150 ° Y -cut from calculations using the determined constants of CNGAS exhibiting electromechanical coupling factor k2 of 3.19% and power flow angle of -1.70°.

Surface acoustic wave velocity and elastic constants of cubic GaN

We present high-resolution surface Brillouin scattering measurements on cubic GaN layers grown on GaAs substrate. By using a suitable scattering geometry, scattering by surface acoustic waves is recorded for different azimuthal angles, and the surface acoustic wave velocities are determined. A comparison of experimental results with numerical simulations of the azimuthal dependence of the surface wave velocity shows good agreement and allows a consistent set of elastic constants for c-GaN to be determined.

Spatially resolved acoustic spectroscopy for selective laser melting

Additive manufacturing (AM) is a manufacturing technique that typically builds parts layer by layer, for example, in the case of selective laser melted (SLM) material by fusing layers of metal powder. This allows the construction of complex geometry parts, which, in some cases cannot be made by traditional manufacturing routes. Complex parts can be difficult to inspect for material conformity and defects which are limiting widespread adoption especially in high performance arenas. Spatially resolved acoustic spectroscopy (SRAS) is a technique for material characterisation based on robustly measuring the surface acoustic wave velocity. Here the SRAS technique is applied to prepare additively manufactured material to measure the material properties and identify defects. Results are presented tracking the increase in the measured velocity with the build power of the selective laser melting machine. Surface and subsurface defect measurements (to a depth of ∼24μm) are compared to electron microscopy and X-ray computed tomography. It has been found that pore size remains the same for 140W to 190W melting power (mean: 115–119μm optical and 134–137μm velocity) but the number of pores increase significantly (70–126 optical, 95–182 velocity) with lower melting power, reducing overall material density.

Enlarged phase velocities of ultra-wideband surface acoustic wave devices with relaxor based ferroelectric single crystal/diamond layered structure

The surface acoustic wave (SAW) characteristics of Y-cut X propagating Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 (YX-PIMNT) single crystals on a diamond substrate have been theoretically calculated. The simulated results show that the phase velocity of the shear horizontal (SH) SAW may be greatly enhanced from the 1350m/s to 3350m/s by reducing the thickness of the PIMNT from 0.5λ to 0.05λ, with a corresponding decrease in the electromechanical coupling factor (K2) from 73.6% to 19.6%. The dispersion curves of phase velocity and K2 as a function of PIMNT thickness are given for the proposed layered structure. Besides the SH SAW, there are also higher order modes that would cause unwanted responses in the pass-band of wideband SAW filters. These were suppressed by properly controlling structural parameters including top electrode thickness, thickness and Euler angle (θ) of PIMNT substrate. The calculated results demonstrate the effectiveness of this approach to enlarge the phase velocity of the SH SAW without dramatically sacrificing its K2, which makes relaxor based ferroelectric single crystals promising for realizing ultra-wideband SAW devices working in ~GHz range.

The elastic tensor of monoclinic alkali feldspars

The full elastic tensors of two K-rich monoclinic alkali feldspars, Or 83 Ab 15 sanidine and Or 93 Ab 7 orthoclase, have been determined by using the Impulse Stimulated Light Scattering technique to measure surface acoustic wave velocities. The new data confirm that alkali feldspars exhibit extreme elastic anisotropy, so the bounds of their isotropic average properties span a wide range. The measured adiabatic moduli are, for Or 83 Ab 15 and Or 93 Ab 7 , respectively, K Reuss = 54.7(7), 54.5(5) GPa; K Voigt = 62.9(1.1), 64.4(0.6) GPa; G Reuss = 24.1(1), 24.5(1) GPa; and G Voigt = 36.1(5), 36.1(7) GPa. The small differences in moduli between the samples suggests that variations in composition and in state of Al, Si order only have minor effects on the average elastic properties of K-rich feldspars. The new measurements confirm that the earliest determinations of elastic wave velocities of alkali feldspars, widely used to calculate wave velocities in rocks, resulted in velocities systematically and significantly too slow by 10% or more.

Chemical composition characterization of Ca3Ta(Ga0.5Al0.5)3Si2O14 single crystal by the line-focus-beam ultrasonic material characterization system

A new method for evaluation of homogeneity of Ca3Ta(Ga0.5Al0.5)3Si2O14 (CTGAS) single crystals was established based on leaky surface acoustic wave (LSAW) velocity measurements performed by the line-focus-beam ultrasonic material characterization (LFB-UMC) system. Three plate specimens cut perpendicular to X-, Y-, and Z-axes were prepared from the CTGAS crystal ingot and LSAW velocity distributions were examined for these specimens. LSAW velocity changes due to Al-substitution effect were successfully extracted by using a relationship between two LSAW velocities propagating along different directions for Ca3TaGa3Si2O14 (CTGS) and Al-substituted CTGS. Comparison of measured LSAW velocities and the results of chemical composition analysis performed by electron probe microanalysis (EPMA) demonstrated that LSAW velocity is mainly affected by Al-content change in CTGAS. Maximum velocity variation was observed in radial direction of the crystal ingot through the Z-axis propagating LSAW velocity measurements for Y-cut CTGAS specimen corresponding to Al-content change of 0.226mol%. Accuracy of evaluation of Al content by velocity measurement for Y-cut Z-propagating LSAW is estimated to be ±0.0047mol% and is superior to that by EPMA.

Investigating the change in surface acoustic wave velocity due to the change in adhesion of a SU-8 thin film using a SU-8/AlN/Si SAW sensor

This study investigates the change in surface acoustic wave (SAW) phase velocity due to the change in adhesion of a SU-8 thin film using an Aluminum Nitride (AlN)/Silicon (Si) SAW sensor. The variation in the stress field at the SU-8/AlN interface at different levels of SU-8 adhesion is also investigated and correlated to the change in SAW phase velocity. A theoretical model is developed to determine the change in wave dispersion profile due to the change in adhesion of the SU-8 film on the surface of the AlN/Si sensor. The interface of the SU-8/AlN layers is represented by a layer of massless springs with interface stiffness K (N/m3). The results illustrate that as the adhesion of the SU-8 film weakens on the surface of the AlN/Si SAW sensor the velocity dispersion profiles fluctuate. The calculated stress field at the SU-8/AlN interface also fluctuates because the weakened interface cannot sustain the increased stress due to the confinement of the wave near the interface. Therefore, the mode of wave propagation varies between the perfect bond case where the SU-8 layer is perfectly bonded to the AlN/Si surface and when the SU-8 is completely de-bonded. Four SAW sensor designs operating in the frequency range of 84–208MHz are developed to measure the shift in the center frequency values and the corresponding change in SAW phase velocities due to the change in adhesion of the SU-8 layer. The adhesion of the SU-8 film on the surface of the AlN/Si SAW sensor is changed by using different adhesion layers. The adhesion layers used are a gold film and an omnicoat coated gold film. Omnicoat is an adhesion promoter used to improve the adhesion of SU-8. The results illustrate that as the adhesion of the SU-8 film improves for the sensors with omnicoat the wave velocity shifts to a lower value. The wave velocity values are fitted with the wave dispersion profiles for different values of the interface spring stiffness K. The interface spring stiffness values for the SAW sensors with omnicoat coated gold and gold films are 8.1×109N/m3 and 7.95×109N/m3, respectively. The use of omnicoat improves the adhesion of the SU-8 film and corresponds to a higher interface spring stiffness value. The stress and displacement fields generated in the SU-8 layer for different values of the interface spring stiffness K are plotted. The results illustrate that as the adhesion of the SU-8 layer improves the stress transmitted to the SU-8 layer increases and the wave is more confined in the SU-8 layer, which justifies the drop in wave velocity since the Rayleigh wave velocity in SU-8 is much lower in comparison to AlN and Silicon; 1166, 5600, 5000m/s, respectively.

Optimized determination of elastic constants of crystals and their uncertainties from surface Brillouin scattering

Surface Brillouin scattering of light allows the angular-dependent velocities of Rayleigh surface acoustic waves (SAW), pseudo-SAW and longitudinal lateral waves (L) on the surface of an opaque crystal to be measured, and the elastic constants thereby determined. Closed form expressions exist for the surface wave velocities in high symmetry directions on crystallographic symmetry planes, and these have been exploited in the past for obtaining the values of the elastic constants. This paper describes a procedure for obtaining an optimized set of elastic constants from SAW, pseudo-SAW and L velocities measured in arbitrary directions in the (001) and (110) surfaces of cubic crystals. It does so by affecting a linearization of the numerically determined angular-dependent SAW and pseudo-SAW velocities near the best fit, and using analytic expressions for the L velocity. The method also generates covariance ellipsoids, from which the uncertainties in the determined values of the elastic constants can be read off. The method is illustrated using surface Brillouin scattering data to obtain the room-temperature elastic constants C11, C12 and C44 of the cubic crystals VC0.75 and Rh3Nb.

Czochralski growth of 2 in. Ca3Ta(Ga,Al)3Si2O14 single crystals for piezoelectric applications

Growth of 2-in. diameter Al-substituted Ca3TaGa3Si2O14 crystals by Czochralski method is reported. The crystals were grown from the melt of Ca3TaGa1.5Al1.5Si2O14 composition and had langasite structure. No inclusions of secondary phases were detected in these crystals. The Ca3Ta(Ga,Al)3Si2O14 mixed crystals produced using non-substituted Ca3TaGa3Si2O14 seeds were defective. They had cracks and/or poly-crystalline structure. However, those grown on the seed of approximately Ca3TaGa1.5Al1.5Si2O14 composition were defect-free. Phase diagram of the Ca3TaGa3Si2O14–Ca3TaAl3Si2O14 pseudo-binary system and segregation phenomenon are discussed in some details. Homogeneity of the crystals was evaluated by measuring 2D-mapping of leaky surface acoustic wave (LSAW) velocities for Y-cut Ca3TaGa1.5Al1.5Si2O14 substrate. Although some inhomogeneities were observed due to slight variations in chemical composition, the crystal had acceptable homogeneity for applications in acoustic wave devices exhibiting the LSAW velocity variation within ±0.048%.

Elasticity of plagioclase feldspars

AbstractElastic properties are reported for eight plagioclase feldspars that span compositions from albite (NaSi3AlO8) to anorthite (CaSi2Al2O8). Surface acoustic wave velocities measured using Impulsive Stimulated Light Scattering and compliance sums from high‐pressure X‐ray compression studies accurately determine all 21 components of the elasticity tensor for these triclinic minerals. The overall pattern of elasticity and the changes in individual elastic components with composition can be rationalized on the basis of the evolution of crystal structures and chemistry across this solid‐solution join. All plagioclase feldspars have high elastic anisotropy; a* (the direction perpendicular to the b and c axes) is the softest direction by a factor of 3 in albite. From albite to anorthite the stiffness of this direction undergoes the greatest change, increasing twofold. Small discontinuities in the elastic components, inferred to occur between the three plagioclase phases with distinct symmetry ( , , and ), appear consistent with the nature of the underlying conformation of the framework‐linked tetrahedra and the associated structural changes. Measured body wave velocities of plagioclase‐rich rocks, reported over the last five decades, are consistent with calculated Hill‐averaged velocities using the current moduli. This confirms long‐standing speculation that previously reported elastic moduli for plagioclase feldspars are systematically in error. The current results provide greater assurance that the seismic structure of the middle and lower crusts can be accurately estimated on the basis of specified mineral modes, chemistry, and fabric.

Crystallographic Orientation Determination of Hexagonal Structure Crystals by Laser Ultrasonic Technique

Spatially resolved acoustic spectroscopy (SRAS) is a laser ultrasonic technique that shows qualitative contrast between grains of different orientation, illustrating the sensitivity of acoustic waves to the material structure. The technique has been improved significantly on determining the full orientation of multigrain cubic metals, by comparing the measured surface acoustic wave (SAW) velocity to a pre-calculated model.In this paper we demonstrate the ability of this technique to determine the orientation of hexagonal structure crystals, such as magnesium and titanium based alloys. Because of the isotropy of the SAW velocity on the basal plane (0001) of hexagonal crystals, the slowness surface is shown as a circle. As the plane moves from (0001) towards (112̄0) or towards (101̄0), the slowness surface gradually turns into an oval. These acoustic properties increase the difficulty in orientation determination.The orientation results of a grade 1 commercially pure titanium by SRAS is presented, with comparison with electron backscattered diffraction (EBSD) results. Due to the nature of SAWs on hexagonal structure crystals, only the results of Euler angles 1 and 2 are discussed. The error between SRAS and EBSD is also investigated.

Theoretical study on thermal and acoustic surface wave properties of Ga3PO7 crystal at high temperature

The high-temperature piezoelectric crystal Ga3PO7is a versatile functional material widely used in many electromechanical devices. As the Curie temperature of this crystal is as high as 1346 ℃, it can break through the current temperature limitations(1200 ℃) and then be used in extremely high-temperature condition. However, it is very difficult to explore its properties in such a high-temperature environment. Moreover, the relevant theoretical research has not been reported to date. Aiming at this problem, the density function theory combined with quasi harmonic approximation theory is used to investigate the structural, thermal and surface acoustic wave (SAW) properties of Ga3PO7. Firstly, the Gibbs energies of Ga3PO7 crystal with different stains are calculated, and the equilibrium structures of Ga3PO7 crystal at different temperatures (from 0 ℃ to 1200 ℃) are found according to minimal energy principle. Secondly, based on the result above, we optimize Ga3PO7 crystal at different temperatures, and then, the thermal and elastic properties of Ga3PO7 crystal within 0-1200 ℃ are calculated using CASTEP package based on the density functional theory in the generalized gradient approximation. The results show that its lattice constants increase almost linearly as temperature increases while its density decreases. Owing to anisotropy, its lattice constant along the c axis increases much more greatly than along the a axis. The coefficients of thermal expansion along the a and c axis are evaluated to be 1.6710-6 K-1 and 3.5810-6 K-1, respectively, and the volumetric heat capacity is evaluated to be 2.067 J/gK. These values all agree well with the experimental values. Finally, the elastic constants, bulk modulus and SAW properties of Ga3PO7 crystal at different temperatures (from 0 ℃ to 1200 ℃) are calculated. The results show that the bulk modulus can reach 175 GPa, and it changes very little as temperature increases. The fluctuation of elastic constants has slight influences on SAW velocity and the electric-mechanical coupling factor. When the propagation angle is 151, it possesses the stablest SAW properties and the largest electric-mechanical coupling factor which can reach 0.7%. The comprehensive analyses of the thermal, mechanical and SAW properties show that Y-cut Ga3PO7 possesses a greater potential application in high temperature environment.

Evaluation of near-surface stress distributions in dissimilar welded joint by scanning acoustic microscopy

This paper presents the results from a set of experiments designed to ultrasonically measure the near surface stresses distributed within a dissimilar metal welded plate. A scanning acoustic microscope (SAM), with a tone-burst ultrasonic wave frequency of 200MHz, was used for the measurement of near surface stresses in the dissimilar welded plate between 304 stainless steel and low carbon steel. For quantitative data acquisition such as leaky surface acoustic wave (leaky SAW) velocity measurement, a point focus acoustic lens of frequency 200MHz was used and the leaky SAW velocities within the specimen were precisely measured. The distributions of the surface acoustic wave velocities change according to the near-surface stresses within the joint. A three dimensional (3D) finite element simulation was carried out to predict numerically the stress distributions and compare with the experimental results. The experiment and FE simulation results for the dissimilar welded plate showed good agreement. This research demonstrates that a combination of FE simulation and ultrasonic stress measurements using SAW velocity distributions appear promising for determining welding residual stresses in dissimilar material joints.