High-accuracy standard specimens for the line-focus-beam ultrasonic material characterization system

We prepared standard specimens for the line-focus-beam ultrasonic material characterization system to obtain absolute values of the propagation characteristics (phase velocity and attenuation) of leaky surface acoustic waves (LSAWs). The characterization system is very useful for evaluating and analyzing specimen surfaces. The calibration accuracy of these acoustic parameters depends on the accuracy of acoustical physical constants (elastic constants, piezoelectric constants, dielectric constants, and density) determined for standard specimens. In this paper, we developed substrates of non piezoelectric single crystals (viz., gadolinium gallium garnet [GGG], Si, and Ge) and an isotropic solid (synthetic silica [SiO₂] glass) as standard specimens. These specimens can cover the phase velocity range of 2600 to 5100 m/s for Rayleigh-type LSAWs. To determine the elastic constants with high accuracy, we measured velocities by the complex-mode measurement method and corrected diffraction effects. Measurements of bulk acoustic properties (bulk wave velocity and density) were conducted around 23°C, and bulk wave velocities were obtained with an accuracy of within ±0.004%. We clearly detected differences in acoustic properties by comparing the obtained results with the previously published values; the differences were considered to be due to differences of the specimens used. We also detected differences in acoustic properties among four SiO₂ substrates produced by different manufacturers     

Characterization of all the elastic,dielectric, and piezoelectric constants of uniaxially oriented poled PVDF films

Polyvinylidene fluoride (PVDF), a piezoelectric material, has many useful applications, for example, as sensors, transducers, and surface acoustic wave (SAW) devices. Models of performance of these devices would be useful engineering tools. However, the benefit of the model is only as accurate as the material properties used in the model. The purpose of this investigation is to measure the elastic, dielectric and piezoelectric properties over a frequency range, including the imaginary part (loss) of these properties. Measurements are difficult because poled material is available as thin films, and not all quantities can be measured in that form. All components of the elastic stiffness, dielectric tensor, and electromechanical coupling tensor are needed in the models. The material studied here is uniaxially oriented poled PVDF that has orthorhombic mm2 symmetry. Presented are the frequency dependence of all nine complex elastic constants, three complex dielectric constants, and five complex piezoelectric constants. The PVDF was produced at Raytheon Research Division, Lexington, MA. Measurements were made on thin films and on stacked, cubical samples. The elastic constants c₄₄^D and c₅₅^D, the dielectric constants e₁₁^T and e₂₂^T , as well as the piezoelectric constants g₁₅ and g₂₄ reported here have not been published before. The values were determined by ultrasonic measurements using an impedance analyzer and a least square data-fitting technique     

A method of determining acoustical physical constants for piezoelectric materials by line-focus-beam acoustic microscopy

We developed a new method of determining acoustical physical constants (elastic constant, piezoelectric constant, dielectric constant, and density) of piezoelectric materials with high accuracy. This method acquires velocities of leaky surface acoustic waves (LSAWs) excited on the water-loaded specimen surface, measured by line-focus-beam (LFB) acoustic microscopy, and bulk velocities of longitudinal and shear waves, measured with planewave transducers replacing the LFB device in the same system, together with the dielectric constants and density measured independently, for a small number of specimens. For LiNbO₃ and LiTaO₃ crystals, we demonstrated that we could accurately determine the constants by choosing proper propagation directions of LSAWs and bulk waves for three principal X-, Y-, and Z-cut specimens and one rotated Y-cut specimen [(104) plate for LiNbO₃ and (012) plate for LiTaO₃]. The accuracy is nearly the same as that for the constants determined only from the bulk wave velocities     

Characterization of 36°YX-LiTaO3 wafers byline-focus-beam acoustic microscopy

Application of line-focus-beam (LFB) acoustic microscopy is extended to characterization of substrates for SH-type SAW devices. Theoretical and experimental studies on a wave mode for characterization are carried out on 36°Y-cut LiTaO₃ wafers. A Rayleigh-type mode of leaky surface acoustic waves (LSAWs) must be employed instead of an SH-type mode of leaky pseudo-surface waves (LPSAWs). Experimental results show that the LSAW propagation should be directed along the X-axis because the LSAW velocities are more sensitive to chemical composition and elastic inhomogeneities. The relations among the LSAW velocities, densities, and Curie temperatures are determined. The LSAW velocity increases linearly at the rate of 0.52 m/s/°C with the Curie temperature. A chemical composition change of 0.03 Li₂ O-mol%, corresponding to temperature resolution of better than 0.3°C, is easily detected by the velocity measurements. Elastic inhomogeneities due to residual multi-domains, produced during the poling process during wafer fabrication, are interpreted quantitatively by this ultrasonic technology     

SAW velocity measurement of crystals and thin films by the phasevelocity scanning of interference fringes

We present principle and application of a novel noncontact velocity measurement of surface acoustic waves (SAW) on crystals and thin films using laser interference fringes scanned at the phase velocity of SAW. The scanning interference fringes (SIF) are produced by intersecting two laser beams with a frequency difference. The SAW velocity within the laser beam spot is measured as the ratio of observed SAW frequency and predetermined wave number of the SIF. The frequency measurement can be quite precise because of a large number of generated SAW carriers and amplitude enhancement effect. The SAW velocity measurement is free from the water loading effect accompanying the leaky SAW measurements. This principle was successfully applied to evaluate Si ₃N₄ and SiO₂ films deposited on Si (001) surface     

The experimental and theoretical characterization of the SAW propagation properties for zinc oxide films on silicon carbide

The surface acoustic wave (SAW) propagation properties of zinc oxide (ZnO) films on silicon carbide (SiC) have been theoretically and experimentally characterized in the film thickness-to-acoustic wavelength ratio range up to 0.12. The experimental characterization of the SAW propagation properties was performed with a linear array of interdigital transducer (IDT) structures. The measurements characterized the velocity and propagation loss of two surface modes, a generalized SAW (GSAW) mode with velocities between 6000 and 7000 m/s, and a high velocity Pseudo-SAW (HVPSAW) mode with velocities between 8500 and 12 500 m/s. The experimentally determined characteristics of the two waves have been compared with the results of calculations based on published data for SiC and ZnO. Simulation of wave characteristics was performed with various values of the elastic constant C(13), which is absent in the published set of material constants for SiC, within the interval permitted by the requirement of positive elastic energy in a hexagonal crystal. The best agreement between the measured and calculated propagation losses of the HVPSAW has been obtained for C(13) near zero. Although for the GSAW mode the calculated velocity dispersion has been found nearly insensitive to the value of C (13) and consistent with the experimental data, for the HVPSAW, some disagreement between measured and calculated velocities, which increased with ZnO film thickness, has been observed for any C(13 ) value. Theoretical analysis of HVPSAW has revealed the existence of a previously unknown high velocity SAW (HVSAW). The displacement components of this wave have been analyzed as functions of depth and confirmed its pure surface, one-partial character.     

Accurate measurements of the acoustical physical constants of synthetic alpha-quartz for SAW devices

Accurate measurements of the acoustical physical constants (elastic constants, piezoelectric constants, dielectric constants, and density) of commercially available and widely used surface acoustic wave (SAW)-grade synthetic α-quartz are reported. The propagation directions and modes of bulk waves optimal for accurately determining the constants were selected through numerical calculations, and three principal X-, Y-, and Z-cut specimens and several rotated Y-cut specimens were prepared from a single crystal ingot to determine the constants and to confirm their accuracy. All of the constants were determined through highly accurate measurements of the longitudinal velocities, shear velocities, dielectric constants, and density. The velocity values measured for the specimens that were not used to determine the constants agreed well with those calculated from the determined constants, within a difference of ±0.20 m/s (±0.004%)     

Q values of longitudinal leaky Saws propagating on rotated Y-cut LN substrates along the perpendicular to the X axis

Longitudinal leaky SAWs (LLSAWs), which can be excited in a thick aluminum (A1) grating on a rotated Y-cut lithium-niobate substrate, were analyzed in a simulation that combined both FEM and BEM. The Q-value dependence of an LLSAW on the rotated angle and the Al-grating thickness was clear. The simulation results were confirmed experimentally. These SAWs have potential applications in high-frequency SAW devices because their velocities range from 6100 to 6500 m/s and their optimum Q values are theoretically larger than 4000.     

Periodic Green's functions analysis of SAW and leaky SAWpropagation in a periodic system of electrodes on a piezoelectriccrystal

A method of periodic Green's functions with a propagation factor exp(iβx), unknown in advance, is used to calculate dispersion curves and attenuation coefficients for Rayleigh- and leaky- waves propagating in a periodic system of thin electrodes on a piezoelectric surface. To describe the charge distribution on the electrodes both a step approximation and Chebyshev polynomials are used, the last being more adequate in most cases. Numerically determined values of the Green's function are used and interpolated either linearly or using a modified variant of Ingebrigtsen's formula. Such basic parameters as stopband width, stopband center frequency, wave velocity and attenuation in the stopband are found. These parameters can be used in the coupling-of-modes (COM) analysis and design of SAW devices. The analysis includes bulk wave radiation and scattering. The dependence of the corresponding attenuation coefficient on frequency is determined. Results obtained allow the determination directly and properly of the COM parameters and the design of SAW devices having large number of electrodes most precisely and rapidly. Numerical results for Rayleigh waves on YZ-LiNbO₃ and leaky waves on 36°YX-LiTaO₃ substrates are presented     

Determination of mass density, dielectric, elastic, and piezoelectric constants of bulk GaN crystal

Mass density, dielectric, elastic, and piezoelectric constants of bulk GaN crystal were determined. Mass density was obtained from the measured ratio of mass to volume of a cuboid. The dielectric constants were determined from the measured capacitances of an interdigital transducer (IDT) deposited on a Z-cut plate and from a parallel plate capacitor fabricated from this plate. The elastic and piezoelectric constants were determined by comparing the measured and calculated SAW velocities and electromechanical coupling coefficients on the Z- and X-cut plates. The following new constants were obtained: mass density p = 5986 kg/m(3); relative dielectric constants (at constant strain S) ε(S)(11)/ε(0) = 8.6 and ε(S)(11)/ε(0) = 10.5, where ε(0) is a dielectric constant of free space; elastic constants (at constant electric field E) C(E)(11) = 349.7, C(E)(12) = 128.1, C(E)(13) = 129.4, C(E)(33) = 430.3, and C(E)(44) = 96.5 GPa; and piezoelectric constants e(33) = 0.84, e(31) = -0.47, and e(15) = -0.41 C/m(2).     

Influence of leaky surface acoustic wave velocity of glass substrates on frequency variation of ZnO/glass SAW filters

During the manufacture of ZnO/glass surface acoustic wave (SAW) filters, two kinds of problems sometimes arise. One is that the average frequency of the SAW filters changes greatly depending on the production lot of glass sheets. The other is that SAW filters made from the same production lot of glass sheets have largely separated double peaks in the frequency distribution. Previously, it had been considered that the frequency variation of ZnO/glass SAW filters depended on such factors as the ZnO film thickness and its elastic quality. The authors focused on the glass substrates as the cause of this variation and measured the leaky SAW (LSAW) velocity on the glass substrates using an ultrasonic microscope to clarify the mechanism. As a result, it was clarified that the LSAW velocities on the glass substrates showed a large variation within and between production lots of glass sheets, and the frequency of ZnO/glass SAW filters largely depended on the LSAW velocity on glass substrates. Moreover, the authors clarified the cause of the difference in the LSAW velocity between glass substrates and were able to reduce the variation of the LSAW velocity.     

Laser-induced surface acoustic waves for evaluation of elastic stiffness of plasma sprayed materials

The elastic properties of plasma sprayed deposits have been evaluated using a laser-excited surface acoustic wave (SAW) technique and an inversion processing analysis. The SAWs including Lamb and Rayleigh waves were generated in plasma sprayed NiCoCrAlY and ZrO₂, respectively, and their group velocity dispersions were used to determine the elastic properties (i.e.Young's modulus, Poison's ratio and density) of the deposits. Estimated elastic moduli from the velocity dispersions of A₀-mode Lamb waves are in the range of 40–140 GPa for the deposits, which are much lower than the values 220–240 GPa of the comparable dense materials. The dramatic reductions in modulus and density of ZrO₂ deposit have been attributed to the presence of high porosity and particularly microcracks. Moreover, this study has emphasized on exploiting the applicability of each kind of the SAWs for the elastic property evaluation of different sprayed materials. Both Lamb and Rayleigh wave dispersions are useful for the estimation of APS and VPS-deposited NiCrAlY, but S₀-Lamb and Rayleigh waves are exceptional for that of sprayed ZrO₂, because of its characterization of high acoustic attenuation and inconsequent displacement across the weak bonded interface of ZrO₂ and substrate.     

Accurate measurements of the acoustical physical constants of LiNbO (3) and LiTaO(3) single crystals

The acoustical physical constants (elastic constant, piezoelectric constant, dielectric constant, and density) of commercial surface acoustic wave (SAW)-grade LiNbO(3) and LiTaO(3) single crystals were determined by measuring the bulk acoustic wave velocities, dielectric constants, and densities of many plate specimens prepared from the ingots. The maximum probable error in each constant was examined by considering the dependence of each constant on the measured acoustic velocities. By comparing the measured values of longitudinal velocities that were not used to determine the constants with the calculated values using the previously mentioned constants, we found that the differences between the measured and calculated values were 1 m/s or less for both LiNbO(3) and LiTaO(3) crystals. These results suggest that the acoustical physical constants determined in this paper can give the values of bulk acoustic wave velocities with four significant digits.     

Some nonlinear and linear elastic properties of Ni40Ti50Cu10 near the martensitic transformation

Linear and nonlinear elastic effects occuring in a single crystal of the ternary alloy Ni₄₀Ti₅₀Cu₁₀ were determined with a modified ultrasonic pulse echo-overlap technique at different temperatures. All linear elastic coefficients could be measured above and below the martensitic transformation supplementing the results of [A. Alippi (Ed.), Proceedings of the International Conference on Acoustics, Rome, 2001]. A large jump of the elastic stiffness c₄₄ could be detected. First nonlinear elastic measurements were performed. No substantial increase of anharmonicity could be found. The maximum stress amplitude was held far below the yield stress of the material.     

Mass sensitivities of shear horizontal waves in three-layer platesensors

Explicit velocity and mass sensitivity formulas for shear-horizontal (SH) plate wave sensors loaded symmetrically on both sides of a plate are presented. The sensor geometry is a composite plate which consists of a central isotropic plate sandwiched symmetrically between two identical layers of isotropic solids. It is demonstrated that if the side layers are considered as the mass loading, for the lowest SH mode (SSo) the sensitivity decreases by a factor of (1-(μ ₂/ρ₂)/(μ₁/ρ₁)) due to the elasticity and by a factor of (1+ρ₂h/ρ₁d)^-1 due to the inertia of the mass loading layers, where μ₁,μ₂ ; ρ₁, ρ₂ and 2d, h are the shear moduli, densities and thickness of the central plate and of the side layers, respectively. For higher order modes, the behavior of sensors which are operated near cutoff frequency is analyzed. The mass loading decreases the cutoff frequencies of the higher order modes and near the cutoff frequencies the mass sensitivities are very high but decrease dramatically as the mass loading increases. Specific examples are given for the case of a fused silica plate sandwiched between two thin lucite layers     

Line-focus acoustic microscopy measurements ofNb2O5/MgO and BaTiO3/LaAlO3thin-film/substrate configurations

Line-focus acoustic microscopy has been used to measure the phase velocities of surface acoustic waves on bare MgO and bare LaAlO₃ , and on Nb₂O₅/MgO and BaTiO₃/LaAlO₃ thin-film/substrate configurations. The thin films are polycrystalline materials. The substrates are anisotropic single-crystals. The measured angular variation of the surface acoustic wave velocities has been used to determine the elastic constants of MgO substrate and Nb₂O₅ thin-film. It has been assumed that the Nb₂O₅ films may be considered as essentially isotropic. The measurements for LaAlO₃ and BaTiO₃/LaAlO₃ show anomalies which are attributed to twinning in the LaAlO₃ substrate     

Design methodology and experimental results for new ladder-type SAWresonator coupled filters

Design methodology and experimental results for a new type of SAW filter called an “extended SAW resonator coupled filter” used in 800 MHz and 0.6 to 1.0 watt cellular radios are presented. The new filters have very small nonlinearities as well as high-performance and high-power characteristics. We have proposed a new equivalent circuit model for IDTs with leaky SAWs, which includes the propagation losses due to leakage and conductances caused by bulkwave radiation. Synthesis procedures for the new SAW filters based on the new treatments of leaky SAWs are discussed, and simulation and experimental results are also given. The new filters satisfy not only the required frequency-response specifications but also the spurious response sensitivity for an antenna duplexer used in USA cellular radio     

Change in piezoelectric boundary acoustic wave characteristics with overlay and metal grating materials

This paper describes how the characteristics of shear-horizontal type piezoelectric boundary acoustic waves (PBAWs) change with combination of different overlay and metal grating materials. It is shown that PBAWs are supported in various structures provided that highly piezoelectric material(s) are employed as structural member(s). For verification, numerical simulation of different material combinations is done. The results are in good agreement with the qualitative prediction. That is, large electromechanical coupling factor K² is obtainable when materials having small mass densities shear modulus c₄₄ and shear velocity VBS; and materials having extremely large shear modulus c₄₄ are chosen, respectively, for overlay and metallic grating. When YX-LiNbO₃ is assumed as a substrate, for example, the best choice seems to be SiO₂ and Au for overlay and metallic grating, respectively. Although metals with extremely large ρ and c₄₄ such as W and Ta offer large K², they may not be acceptable for practical PBAW applications because of their large electric resistivity.     

Theoretical study on SAW characteristics of layered structuresincluding a diamond layer

Diamond has the highest surface acoustic wave (SAW) velocity among all materials and thus can provide much advantage for fabrication of high frequency SAW devices when it is combined with a piezoelectric thin film. Basic SAW properties of layered structures consisting of a piezoelectric material layer, a diamond layer and a substrate were examined by theoretical calculation. Rayleigh mode SAW's with large SAW velocities up to 12,000 m/s and large electro-mechanical coupling coefficients from 1 to 11% were found to propagate in ZnO/diamond/Si, LiNbO₃/diamond/Si and LiTaO₃/diamond/Si structures. It was also found that a SiO₂/ZnO/diamond/Si structure can realize a zero temperature coefficient of frequency with a high phase velocity of 8,000-9,000 m/s and a large electro-mechanical coupling coefficient of up to 4%     

ZnO films on {001}-cut <110>-propagating GaAs substrates forsurface acoustic wave device applications

A potential application for piezoelectric films on GaAs substrates is the monolithic integration of surface acoustic wave (SAW) devices with GaAs electronics. Knowledge of the SAW properties of the layered structure is critical for the optimum and accurate design of such devices. The acoustic properties of ZnO films sputtered on {001}-cut 〈110〉-propagating GaAs substrates are investigated in this article, including SAW velocity, effective piezoelectric coupling constant, propagation loss, diffraction, velocity surface, and reflectivity of shorted and open metallic gratings. The measurements of these essential SAW properties for the frequency range between 180 and 360 MHz have been performed using a knife-edge laser probe for film thicknesses over the range of 1.6-4 μm and with films of different grain sizes. The high quality of dc triode sputtered films was observed as evidenced by high K² and low attenuation. The measurements of the velocity surface, which directly affects the SAW diffraction, on the bare and metalized ZnO on SiO₂ or Si₃N₄ on {001}-cut GaAs samples are reported using two different techniques: 1) knife-edge laser probe, 2) line-focus-beam scanning acoustic microscope. It was found that near the 〈110〉 propagation direction, the focusing SAW property of the bare GaAs changes into a nonfocusing one for the layered structure, but a reversed phenomenon exists near the 〈100〉 direction. Furthermore, to some extent the diffraction of the substrate can be controlled with the film thickness. The reflectivity of shorted and open gratings are also analyzed and measured. Zero reflectivity is observed for a shorted grating. There is good agreement between the measured data and theoretical values