Surface Acoustic Wave Velocity Research Articles

Correlation between adhesion of diamond-like carbon film on LiTaO 3 substrate and SAW velocity

Coating diamond-like carbon (DLC) film to LiTaO 3 substrate is theoretically expected to stiffen the substrate surface so as to accelerate the surface acoustic wave (SAW) at the surface due to its high elastic constant and low mass density. However, it has been reported that DLC film on LiTaO 3 substrate led to a reduction of SAW velocity probably due to poor adhesion between the DLC film and LiTaO 3 substrate. In this study, the adhesion strength between DLC film and LiTaO 3 substrates has been enhanced by introducing a SiO 2 interlayer. Scratch test shows that relatively thick DLC films with an appropriated thickness SiO 2 interlayer exhibit a good adhesion. The increase in SAW velocity correlates with the adhesion strength of the DLC film to the LiTaO 3 substrate, indicating that the adhesion plays an important role of joining the two materials together. In addition, with an improved adhesion, SAW phase velocity in the DLC/SiO 2/LiTaO 3 structure increases with increasing DLC film thickness and with decreasing SiO 2 interlayer thickness.

AC-hopping conductance of self-organized Ge/Si quantum dot arrays

Dense ( n = 4 × 10 11 cm - 2 ) arrays of Ge quantum dots in a Si host were studied using attenuation of surface acoustic waves (SAWs) propagating along the surface of a piezoelectric crystal located near the sample. The SAW magneto-attenuation coefficient, Δ Γ = Γ ( ω , H ) - Γ ( ω , 0 ) , and change of velocity of SAW, Δ V / V = ( V ( H ) - V ( 0 ) ) / V ( 0 ) , were measured in the temperature interval T = 1.5 –4.2 K as a function of magnetic field H up to 6 T for the waves in the frequency range f = 30 –300 MHz. Based on the dependences of Δ Γ on H, T and ω , as well as on its sign, we believe that the AC conduction mechanism is a combination of diffusion at the mobility edge with hopping between localized states at the Fermi level. The measured magnetic field dependence of the SAW attenuation is discussed based on existing theoretical concepts.

Design considerations on surface acoustic wave resonators with significant internal reflection in interdigital transducers

This paper discusses, both qualitatively and quantitatively, the operation and the design principle of current surface acoustic wave (SAW) resonators in which the internal reflection within interdigital transducers (IDTs) is not negligible and lower capacitance ratio is necessary. For the purpose, the p-matrix expression is used with the help of the coupling-of-modes theory. The internal reflection causes: deformation of the IDT passband shape, frequency dependence of the effective SAW velocity within IDTs, and suppression of higher-order resonances. It is shown that these features can be effectively used to enhance performances of both one-port SAW resonators and two-port double-mode SAW (DMS) filters. In addition, under proper designs accounting for the internal reflection, most of all structural discontinuities can be removed, and is most preferable for the reduced scattering loss at the discontinuity. Design criteria also are presented for DMS filters of wide bandwidth, and it is demonstrated how device performances are improved by proper design accounting for the criteria.

Wireless measurement of temperature using surface acoustic waves sensors

Surface acoustic wave (SAW) devices can be used as wireless sensor elements, called SAW transponders, for measuring physical quantities such as temperature that do not need any power supply and may be accessed wirelessly. A complete wireless sensor system consists of one or more such SAW transponders and a local radar transceiver. The SAW transponder receives an RF burst in the VHF/UHF band transmitted by the radar transceiver. The reader unit performs a radar measurement of the impulse response of the SAW transponder via a high-frequency electromagnetic radio link. A temperature variation changes the SAW velocity and thereby the response pattern of the SAW device. By analyzing the time delay between backscattered pulses with different time delays we get a rough estimation of the temperature of the SAW transponder. By using this information the ambiguity of +/-2pi in the phase differences between the pulses can be eliminated, which provides an overall and unambiguous temperature resolution of +/-0.2 degrees C.

Brillouin scattering and x-ray photoelectron studies of germanium nanoclusters synthesized in SiO2 by ion implantation

Brillouin scattering and x-ray photoelectron spectroscopy (XPS) have been utilized to characterize Ge+-implanted thermal SiO2 layers on a Si substrate with subsequent annealing at 500°C and 1100°C. Sputtering depth profiling in conjunction with XPS studies have been applied to identify the chemical state of elemental Ge and GeO2 precipitations in the SiO2 matrices. The presence of a subsurface GeOx zone as predicted in kinetic 3-dimensional lattice simulations has been confirmed. It is concluded that the intermediate step of Ge oxide formation seems necessary for the creation of Ge nanoclusters. The Ge atomic concentrations obtained from XPS were used to compute the bulk and shear moduli, and consequently the surface acoustic wave (SAW) velocities, for the Ge∕GeO2∕SiO2 systems. These calculations confirm the character of SAW velocity softening as determined from the Brillouin scattering investigations.

Dispersion and attenuation of surface shear acoustic waves with horizontal polarization on the free statistically rough surface of the hexagonal crystal

Expressions for dispersion of the phase velocity and inverse damping depth of surface acoustic waves with shear horizontal polarization are derived in an analytical form within perturbation theory using the modified mean-field method for the Z-cut hexagonal crystal with a free statically rough surface. Both two-and one-dimensionally rough surfaces are considered. The one-dimensionally rough surface is considered as a special case of the two-dimensionally rough surface. It is shown that shear surface waves with horizontal polarization cannot exist on the flat surface of the Z-cut hexagonal crystal. The derived expressions are studied analytically and numerically in the entire frequency range accessible in perturbation theory. The long-wavelength limit (most interesting from the experimental point of view) is considered, where the wavelength is much longer than the roughness correlation radius. The conditions for the existence of SH-polarized waves are determined for both roughness types. It is shown that dispersion and attenuation of SH polarized waves are qualitatively similar in character to those we considered previously for an isotropic medium.

Role of Si-doped Al0.3Ga0.7As layers in the high-frequency conductivity of GaAs/Al0.3Ga0.7As heterostructures under conditions of the quantum hall effect

The complex high-frequency conductivity of GaAs/Al0.3Ga0.7As heterostructures that are δ-doped and modulation-doped with silicon was investigated by acoustic methods under conditions of the integer quantum Hall effect. Both the real (σ1) and imaginary (σ2) parts of the complex conductivity σ(ω, H)=σi−iσ2 were determined from the dependences of the absorption and velocity of surface acoustic waves on magnetic field. It is shown that, in the heterostructures with electron density ns=(1.3–7)×1011 cm−2 and mobility μ=(1–2)×105 cm2/(V s), the high-frequency conductivity near the centers of the Hall plateau is due to electron hopping between localized states. It is established that, with filling numbers 2 and 4, the conductivity of the Al0.3Ga0.7As:Si layer efficiently shunts the high-frequency hopping conductivity of the two-dimensional interface layer. A method of separating the contributions of the interface and Al0.3Ga0.7As:Si layers to the hopping conductivity σ(ω, H) is developed. The localization length of electrons in the interface layer is determined on the basis of the nearest neighbor hopping model. It is shown that, near the centers of the Hall plateau, both σ(ω, H) and ns depend on the cooling rate of a GaAs/Al0.3Ga0.7As sample. As a result, the sample “remembers” the cooling conditions. Infrared light and static strain also change both σ(ω, H) and ns. We attribute this behavior to the presence of two-electron defects (so-called DX− centers) in the Al0.3Ga0.7As:Si layer.

Evaluation of mass-produced commercial LiTaO/sub 3/ single crystals using the LFB ultrasonic material characterization system

A mass-production line of lithium tantalate (LiTaO3) crystals with a maximum charge number of 60 for surface acoustic wave (SAW) devices was evaluated with the line-focus-beam (LFB) ultrasonic material characterization system. Some serious problems associated with chemical compositions were observed and resolved by measuring the velocities of Rayleigh-type leaky surface acoustic waves (LSAWs), VLSAW, for two groups of LiTaO3 wafers: 21 36 degrees Y X-LiTaO3 wafers selected randomly from crystal ingots grown with different charge numbers in different furnaces, and 14 42 degrees Y X-LiTaO3 wafers obtained at the top, middle, and bottom parts from 5 crystals selected from 39 crystals grown successively in the same furnace and crucible. Using the measured VLSAW and the predetermined relationship between VLSAW and Li2O concentrations, M(Li2O), we estimated the average M(Li2O) controlled in the current mass-production line to be about 48.77 mol% with a maximum difference of 0.75 mol%. The composition for each crystal ingot increased linearly about 0.04 mol% from the top to the bottom, and no dependence on the charge number was observed, as the melt composition used for the mass production was controlled through Curie temperature (TC) measurements. A nearly true congruent composition of 48.49 Li2O-mol% was obtained through the precise VLSAW data for the 42 degrees Y X-LiTaO3 wafers, that was about 0.3 mol% less than the melt composition in the production line. It was also pointed out that the TC measurement conditions, including room temperatures surrounding the measurement systems, should be re-examined for reliable production control. A guideline for more efficient mass production of the crystals has been established concerning the true congruent composition as the starting material.

Laser-ultrasonic surface wave dispersion measurements on surface-treated metals

Surface acoustic wave (SAW) velocity spectroscopy has been long considered to be one of the leading candidates for nondestructive characterization of surface-treated metals because of its ability to probe the material properties at different penetration depths depending on the inspection frequency. We developed a high-precision laser-ultrasonic technique to study the feasibility of SAW dispersion spectroscopy for residual stress assessment on shot-peened metals. This technique is capable of measuring SAW dispersion with a relative error of 0.1% over a frequency range from 2 to 15 MHz. Our experimental results obtained from shot-peened aluminum 2024-T351 samples indicate that the dispersion of the surface wave is a superposition of different effects of surface treatment in the material, including surface roughness, compressive residual stress, and cold work. Although the surface roughness induced component is often the dominating part of the overall dispersion, the experimental results also indicate that it is feasible to observe a perceivable change in the dispersion of the SAW when the specimen is heat-treated at different temperatures, which has no perceivable effect on the surface roughness. The part of the dispersion, which changes during annealing via thermal relaxation, is due to near-surface residual stresses and the decay of texture, although at high frequencies nonuniform grain coarsening could also play a significant role.

SAW characteristics of AlN films sputtered on silicon substrates

This article is focused on the analysis of the electroacoustic response of surface acoustic wave (SAW) filters made of aluminium nitride (AlN) thin films on various types of Si wafers. AlN films with (0 0 · 2) orientation were deposited by RF reactive sputtering of an Al target in Ar and N 2 admixtures on Si(1 0 0) and (1 1 1) wafers with resistivities ranging between 10 and 2000 Ω cm. The electroacoustic response of SAW filters with an acoustic wavelength of 40 μm was analysed by measuring the S ij parameters with a network analyser. We have determined that the out-of-band loss is directly related to the Si substrate resistivity, varying from 26 dB for 10 Ω cm to 55 dB for 2000 Ω cm. The SAW velocity depends on the orientation of the Si wafer, being approximately 4700 m/s for Si(1 1 1) and 5100 m/s for Si(1 0 0). The electroacoustic responses of the SAW filters were fitted by computations based on a simple circuital model that takes into account parasitic effects such as airborne electromagnetic coupling and conduction through the substrate. This procedure provides accurate values of the electromechanical coupling factor k 2 even for devices with poor characteristics. Good quality SAW filters of AlN on high resistivity Si(1 0 0) wafers with k 2 larger than 0.12% are demonstrated.

An impulsive stimulated scattering study of surface acoustic waves under high pressure

Impulsive stimulated scattering (ISS) offers an accurate spectroscopic method of obtaining surface acoustic velocities of materials. High-pressure ISS results are obtained for polycrystalline aluminum in a diamond anvil cell (with fluid nitrogen as the pressure-transmitting medium) and these results are compared with analytical solutions for an isotropic (polycrystalline) solid in contact with an isotropic fluid. The choice of an isotropic solid–isotropic fluid interface permits a straightforward analysis of the effect of pressure on both the Scholte and generalized Rayleigh (GR) surface acoustic modes intrinsic to such a system. The present study is the first to measure and analyze both the Scholte and GR modes under high pressure. Pressure dependent surface acoustic wave (SAW) velocities are demonstrated as a spectroscopic probe of the equation of state of materials under extreme conditions. In addition, directionally resolved ISS measurements on single crystal surfaces of Ni are compared with numerical calculations to determine the origin of dramatic changes in SAW velocities on crystal surfaces of slightly lower symmetry. The calculations show that the SAW velocity becomes strongly mixed with the bulk transverse acoustic mode for deviations of just a few degrees away from the (100) surface.

Surface Acoustic Waves in Ferroelectrics

We have considered the phase velocity dispersion of the Rayleigh waves on the surface of ferroelectric crystal and in a thin ferroelectric film deposited on a metal plate in the close vicinity of a structure phase transition.The drastic softening of the ferroelectric crystal elastic constants when its symmetry changes at the phase transition results in the critical slowdown of the Rayleigh waves. We have calculated temperature dependencies of the Rayleigh velocity and dispersion curves at some temperatures close to the critical value.As the temperature is cooled down to the critical value, the dispersion dependency of the surface acoustic wave velocity changed qualitatively its behavior. The effect can be used for investigations of the critical properties of various layered ferroelectric systems. In particular, it can be used for measurements of the critical temperatures of the structure phase transitions in ultrathin ferroelectric films with thicknesses less than 100 nm.

Rapid measurement of surface acoustic wave velocity on single crystals using an all-optical adaptive scanning acoustic microscope

An experimental method has been developed to measure the phase velocity of laser-generated and detected surface acoustic waves. An optical grating produced by a spatial light modulator was imaged onto the sample surface to generate the ultrasound whose frequency and wave front were controlled electrically by tailoring the grating. When the grating period matched the surface acoustic wavelength, strong excition of the surface wave was observed. Thus, the wavelength and, thereby, the phase velocity were determined. We present results with this method that allow the phase velocity and the angular dispersion of the generalized surface wave as well as the pseudosurface wave on the (100) nickel and (111) silicon single crystals to be measured, with the precision of approximately 0.2%. Those factors affecting the measurement precision are discussed.

High-frequency, high-sensitivity acoustic sensor implemented on ALN/Si substrate

AlN films, 1.6–6.3 μm thick, were sputtered at 200 °C on Si(100) and Si(111) substrates. The films were crack-free, uniform, and c-axis oriented. The experimental phase velocities of surface acoustic waves (SAW) propagating in the AlN/Si structures were estimated and showed only a small discrepancy (20–40 m/s) compared to the calculated theoretical values. A SAW resonator (SAWR)-based chemical sensor, operating at about 700 MHz, was implemented on AlN/Si. The SAWR surface was covered with a polymer film sensitive to relative humidity (RH) changes, already tested for RH sensing in previous works on SAW delay lines implemented on AlN/Si and ZnO/Si and operating at about 130 MHz. The RH mass sensitivity and the detection limit of the SAWR sensor improved by 38% and by one order of magnitude, respectively, compared to the delay line-based sensors previously tested.

Magnetoelastic surface waves in a semi-infinite ferromagnet in an external magnetic field

The magnetoelastic wave spectra are found for a semi-infinite Heisenberg ferromagnet in an external magnetic field. Near the surface of the ferromagnet the interaction of Rayleigh waves with surface spin waves gives rise to an additional activation energy in the spin-wave spectrum and leads to a decrease in the sound velocity. The magnetoelastic coupling increases the difference between the velocity of surface acoustic waves and that of bulk sound waves.

Comparison of acoustic properties between natural and synthetic α-quartz crystals

Accurate measurements of bulk and leaky surface acoustic wave (LSAW) velocities of natural α-quartz crystal were carried out using the line-focus-beam and plane-wave ultrasonic material characterization systems and then these measured velocities were compared with the velocities of synthetic α-quartz crystals. Longitudinal velocities of principal X-, Y-, and Z-cut specimens; shear velocities with X-axis polarized particle displacements of the Y- and Z-cut specimens; and LSAW velocities for these three specimens were precisely measured. We found all the velocities for the bulk waves and the LSAWs of natural quartz to be slightly smaller (within 1.00 m/s) than the velocities of synthetic quartz. Therefore, the acoustical physical constants of the natural quartz are very similar to those of synthetic quartz we reported recently.

Guiding characteristics of surface acoustic waves visualized using photoluminescence quenching

We restrict the surface acoustic wave (SAW) propagation on a GaAs surface within a waveguide by means of reduction of the SAW velocity realized by metal gratings. The spatial variation of the SAW amplitude is evaluated by mapping the efficiency of the quenching of photoluminescence by the SAWs. Confinement of the SAWs is accomplished at a frequency of 1.4 GHz when the angle of SAW incidence with respect to the waveguide boundary is kept less than ∼10° for SAW velocity reduction of 4%. Mapping also reveals interference stripes associated with the SAWs bounce at the waveguide boundary.

Surface acoustic wave propagation characteristics of aluminum nitride thin films grown on polycrystalline diamond

Diamond has the highest surface acoustic wave (SAW) velocity among all materials and thus provides substantial advantages for the fabrication of high-frequency SAW devices when combined with a piezoelectric thin film. The properties of layered film structures consisting of a piezoelectric material layer, aluminum nitride (AlN), and a polycrystalline diamond layer grown on a silicon substrate have been examined. Highly textured AlN thin films have been sputter-deposited onto polycrystalline diamond substrates at room temperature. X-ray diffraction analysis of the multilayer structure as well as atomic force microscope images of the AlN surface indicate that the deposited AlN films were c-axis oriented with a full width at half maximum of the rocking curve of the AlN-002-peak of 2.1°. The thickness of the AlN layer was of 4.3 μm, whereas the diamond layer was 50 μm thick and resting on a 3-in Si wafer. Standard one-port SAW resonators with aluminum (Al) metallization have been subsequently fabricated and evaluated. Experimental results indicate that the fundamental as well as higher Rayleigh SAW modes are excited. Thus, the phase velocities of the first, second, third, and fourth mode are found to be 6.850, 10.000, 11.800, and 14.450 m/s respectively. A very good out-of-band rejection of −50 dB was obtained.

Experimental and theoretical characterization of the surface acoustic wave propagation properties of GaN epitaxial layers on c-plane sapphire

Surface acoustic wave (SAW) propagation properties of gallium nitride (GaN) epitaxial layers on sapphire were theoretically and experimentally characterized. GaN thin films were grown on a c-plane sapphire substrate using a metalorganic chemical vapor deposition system. The experimental characterization of SAW propagation properties was performed with a linear array of interdigital transducer structures, while SAW velocities were calculated by matrix methods. Experimentally, we found pseudo-SAW and high-velocity pseudo-SAW modes in the GaN/sapphire structure, which had a good agreement with calculated velocities.

Calibration of Curie temperatures for LiTaO/sub 3/ single crystals by the LFB ultrasonic material characterization system

The line-focus-beam ultrasonic material characterization (LFB-UMC) system is applied to a standardized comparison and evaluation of the Curie temperatures, T/sub C/, exclusively used in evaluating the chemical compositions of commercial LiTaO/sub 3/ crystals by measuring the velocities of Rayleigh-type leaky surface acoustic waves (LSAWs), V/sub LSAW/. We measured V/sub LSAW/ and T/sub C/ (standardized) under the same T/sub C/ measurement conditions for 36/spl deg/Y X-LiTaO/sub 3/ single-crystal wafers produced by four manufacturers and related the results to the T/sub C/ (individual) measured by the individual manufacturers. The relationships between V/sub LSAW/ and T/sub C/ (individual) varied from one company to another, and a single straight line of the proportional relationship between V/sub LSAW/ and T/sub C/ (standardized) was obtained for all wafers regardless of the manufacturer. These experimental results clarify that the problem associated with T/sub C/ measurements lies in the measurement conditions and the absolute accuracy of the measurement instruments. Measurements of the center frequencies of SH-type surface acoustic wave (SAW) filter devices are compared with V/sub LSAW/ measurements. A method of calibrating T/sub C/ using this ultrasonic system is proposed to establish standardized specifications of SAW-device crystal wafers.