Surface Acoustic Wave Velocity Research Articles

Experiments for self-temperature-compensated characteristics of SAW gas sensor at fundamental and third-harmonic frequencies

A novel surface acoustic wave (SAW) gas sensor with self-temperature-compensated characteristics is proposed. Any type of piezoelectric crystal substrate irrelevant to their temperature coefficients of SAW velocities, can be adopted in this gas sensor. The sensing dynamic range can be extended utilising both the fundamental and third-harmonic frequency signals. Experimental results at the 75 MHz fundamental frequency and the 225 MHz third-harmonic frequency showed validity of the proposed sensor structure.

Laser ultrasonic surface wave dispersion technique for non-destructive evaluation of human dental enamel

This paper describes a novel optical system for clinical diagnosis of dental enamel based on its elasticity. Current examination techniques are typically destructive, and frequently impractical for in-vivo inspection. This paper describes the first application of a laser ultrasonic non-destructive evaluation (NDE) method for clinical dental diagnosis. It performs remote elasticity evaluation on small dimension samples. A focused laser line-source generates broadband surface acoustic wave (SAW) impulses which are detected with a simplified optical fibre interferometer. The measured SAW velocity dispersion spectrum was in turn used to characterise the elasticity of the specimen. Different metal structures were measured to verify the system performance. The results agree well with theoretical values and confirm the reliability and accuracy of the laser NDE system. This technique was then applied to evaluate the surface of sound natural human dental enamel. The measured dispersion spectra match theoretical expectations and the influences of both the enamel and the underlying dentin on the surface wave propagation were observed. This is the first time, to the best of our knowledge, that a laser based SAW velocity dispersion technique has been successfully applied on human dental enamel. As a remote, non-destructive technique it is applicable in-vivo and opens the way for early diagnosis of dental caries.

Measurement of Surface Acoustic Wave Velocity Using a Variable-Line-Focus Polyurea Thin-Film Ultrasonic Transducer

This paper presents the novel measurement method of the surface acoustic wave velocity by the variable-line- focus transducer using a polyurea piezoelectric ultrasonic transducer. First, a multiresonant polyurea thin-film ultrasonic transducer is fabricated by the vapor deposition polymerization process using 2 monomers. Second, the measurement system of surface acoustic wave velocity modified from the V(z) curve method is established. The system uses the fabricated polyurea thin film as a variable-line-focus transducer at the 30-MHz resonance frequency. The focal length is changed by varying the radius of curvature of the film transducer. To estimate the surface acoustic wave velocities from the measured data theoretically, the photographs of the transducer bent shapes are taken by using a digital microscope, and the bent transducer curvature is modeled by the 7th-order polynomial. To examine the performances of the variable-line-focus transducer, the surface acoustic wave velocities of an aluminum and a synthesized silica glass specimen have been measured. The measured surface acoustic velocities showed good agreement with the reference values.

High temperature shear modulus determination using a laser-ultrasonic surface acoustic-wave device

For radioactive thin specimens in high pressure or high temperature environments, the conventional approach for high precision elastic modulus measurements is not easy. Radiation damage to transducer’s bonds or the substantial changes in specimen dimensions induced by temperature, pressure, and phase transitions, reduce the intrinsic accuracy of the time-of-flight acoustic measurements. We describe here an alternative approach in which the surface acoustic-wave (SAW) velocity is measured directly with a fixed propagation distance, independent of specimen dimensions. Then, using the known relationship between SAW and shear velocities, it is possible to obtain the shear modulus with a high accuracy without measuring the bulk shear wave. Laser ultrasonics was used combining a pulsed laser source with a heterodyne interferometer. The proposed technique is validated for Al, Au, Ta, and Pb 0.8% Ca, TA6V4 without phase transitions, and for Sn and Co through the phase transitions.

Accelerating Effect of Aluminum Oxide Films on Surface Acoustic Wave on Cystalline Quartz

Aluminum oxide (Al2O3) films have been successfully deposited on stable temperature (ST)-cut quartz substrates by electron beam evaporation without any interlayer to ensure a good adhesion of the Al2O3 films to the quartz substrates. The Al2O3 thin films increase the surface acoustic wave (SAW) velocity sufficiently, and can be used to improve the performance of SAW devices. Both the theoretical and experimental results show that the SAW phase velocity in the Al2O3/quartz structure was increased with the insertion of an Al2O3 film. The increase in SAW velocity with increasing thickness of the Al2O3 film results from the stiffening effect of the Al2O3 layer. The velocity change ratio predicted by the theoretical analysis of the Al2O3 (1.45 µm)/quartz sample was about 4.38% which is slightly larger than the experimental result of 3.28%. These results are better than those published in the literature for a diamond-like carbon/SiC buffer layer/quartz structure with a velocity change ratio of 2.3%.

Quantitative Evaluation of One-Dimensional Temperature Distribution on Material Surface Using Surface Acoustic Wave

A quantitative method for measuring a one-dimensional temperature distribution on a material surface using a surface acoustic wave (SAW) is presented. The principle of the method is based on the temperature dependence of the velocity of the SAW propagating on a material surface. An effective method consisting of a SAW velocity measurement and an inverse analysis coupled with a one-dimensional finite difference calculation is developed to determine the surface temperature distribution quantitatively. To demonstrate the practical feasibility of the method, surface temperature distributions for aluminum and steel plates are examined. The SAW of each plate during heating is measured using a laser interferometer based on photorefractive two-wave mixing. The surface temperature distributions determined by the ultrasonic method almost agree with those measured using an infrared camera.

Procedures for determining acoustical physical constants of class 6mm single crystals by ultrasonic microspectroscopy technology

We theoretically and numerically investigated procedures for accurately determining the acoustical physical constants of piezoelectric hexagonal (class 6mm) crystals, taking ZnO as an example, using bulk-wave velocities and leaky surface acoustic wave (LSAW) velocities acquired by ultrasonic microspectroscopy technology. Selection of appropriate propagation modes and directions for eight velocity measurements to facilitate accurately determining five elastic constants (c11E, c12E, c13E, c33E, and c44E) and three piezoelectric constants (e15, e31, and e33) was discussed in addition to measurements of two dielectric constants (ε11S and ε33S) and the density (ρ). Several determination procedures were obtained. Using only bulk-wave velocities measured for Y-cut, Z-cut, and several rotated Y-cut crystalline plane specimens provided the best determination accuracies for constants c13E, c33E, e31, and e33. Favorable determination accuracies were also achieved using several sets of two rotated Y-cut crystalline plane specimens such as (101) and (102). Conductive specimens of (001) and (102) with a resistive (103) specimen made the bulk-wave measurements simpler, resulting in greatly improved determination accuracies. The additional use of LSAW velocity with one conductive (001) specimen enabled the determination procedures to be simplified, resulting in acceptable accuracy for c13E. Measurements of bulk-wave velocities and LSAW velocities for just two resistive Y-cut and Z-cut specimens could determine all the constants, albeit with some deterioration in accuracy, especially for e31 and e33. This will be useful for preliminary determination of the constants for precious crystals.

Mass sensitivity analysis and designing of surface acoustic wave resonators for chemical sensors

The sensitivity of surface acoustic wave (SAW) chemical sensors depends on several factors such as the frequency and phase point of SAW device operation, sensitivity of the SAW velocity to surface mass loading, sensitivity of the SAW oscillator resonance to the loop phase shift, film thickness and oscillator electronics. This paper analyzes the influence of the phase point of operation in SAW oscillator sensors based on two-port resonator devices. It is found that the mass sensitivity will be enhanced if the SAW device has a nonlinear dependence on the frequency (delay ∼ frequency−1). This requires the device to generate and operate in a ωτg(ω) = const region in the device passband, where ω denotes the angular frequency of oscillation and τg(ω) denotes the phase slope of the SAW resonator device. A SAW coupled resonator filter (CRF) that take advantage of mode coupling is considered in realizing such a device to help in shaping the phase transfer characteristics of a high mass sensitivity sensor. The device design and simulation results are presented within the coupling-of-modes formalism.

Surface acoustic wave characteristics of AlN thin films grown on a polycrystalline 3C-SiC buffer layer

In this study, AlN thin films were deposited on a polycrystalline (poly) 3C-SiC buffer layer for surface acoustic wave (SAW) applications using a pulsed reactive magnetron sputtering system. AFM, XRD and FT-IR were used to analyze structural properties and the morphology of the AlN/3C-SiC thin film. Suitability of the film in SAW applications was investigated by comparing the SAW characteristics of an interdigital transducer (IDT)/AlN/3C-SiC structure with the IDT/AlN/Si structure at 160 MHz in the temperature range 30–150 °C. These experimental results showed that AlN films on the poly (1 1 1) preferred 3C-SiC have dominant c-axis orientation. Furthermore, the film showed improved temperature stability for the SAW device, TCF = −18 ppm/°C. The change in resonance frequency according to temperature was nearly linear. The insertion loss decrease was about 0.033 dB/°C. However, some defects existed in the film, which caused a slight reduction in SAW velocity.

Miniature high efficiency transducers for use in ultrasonic flow meters

The development of a miniature high efficiency transducer for use in ultrasonic flow meters is described. The transducer is based on conversion of energy between surface acoustic waves (SAWs) and bulk acoustic wave (BAWs). It consists of a thin plate of a suitable piezoelectric material whose SAW velocity is greater than velocity of BAW in the fluid. Under these conditions a surface wave traveling on the substrate radiates a bulk acoustic wave in the fluid. We show that with proper design efficient conversion of energy from SAW to BAW and vice versa can be obtained. For example, using lead zirconate titanate as the piezoelectric material and water as the fluid, total conversion loss (SAW to BAW and back from BAW to SAW) of less than 3 dB has been obtained.

Diagnostics of residual stress in metals with wideband surface acoustic pulses

A method for diagnostics of biaxial residual stress in metals with the use of short pulses of surface acoustic waves (SAWs) is discussed. Laser thermoelastic excitation of SAW pulses with a length of about 30 ns and piezoelectric detection by a specially designed wedgelike receiver with a high time resolution are used. Relative variations in the phase and group SAW velocities due to the presence of a weld are measured experimentally in two mutually perpendicular directions. The frequency range of measurements was 5–25 MHz, which corresponds to a depth of SAW penetration of about 120–600 μm. Measurements were conducted on samples made of 12X18H10T stainless steel with planar dimensions of 105 × 76 mm and thickness h = 8 mm, which were welded along the axis by an electron beam. The weld width was 5–7 mm. After the measurements, the samples were investigated by the fracture method based on the technique of a complete relief. The results of measurements are compared with the data obtained by fracture tests. The potential of the studied method is analyzed, and the measurement errors are estimated.

Dielectric, elastic and piezoelectric properties of RCa 4O(BO 3) 3 ( R=rare-earth elements) crystals with monoclinic structure of point group m

Rare-earth calcium oxoborate RCa 4O(BO 3) 3 ( R=La, Gd, Y) piezoelectric single crystals were successfully grown by a conventional RF-heating Czochralski (Cz) technique. They belong to the monoclinic system with point group m, and have 27 independent material constants. We developed the determination scheme (cuts and vibration modes) of the material constants in point group m. Their material constants were evaluated on the basis of the determination scheme and systematically compared. Also, the Rayleigh surface acoustic wave (RSAW) velocities v and coupling factors K 2 for the fundamental X-, Y- and Z-cuts were theoretically simulated in their crystals by Campbell and Jones's method, which showed a good agreement with the measured values. Moreover, from the calculated result in their all propagation direction, it was confirmed that (180°, 120°, 167°)-cut of the RCa 4O(BO 3) 3 ( R=Y) and (180°, 146°, 167°)-cut of the RCa 4O(BO 3) 3 ( R=La), indicated excellent RSAW propagation properties when compared with existing piezoelectric crystals.

Microelastic imaging of bone

Several high-frequency ultrasound techniques have been developed during the last decade with the intention of assessing elastic properties of bone at the tissue level. The basic measurement principles can be divided into: 1) measurement of the compressional wave velocity in thin tissue sections; 2) measurement of surface acoustic wave velocities in thick sections; and 3) derivation of the acoustic impedance from the confocal reflection amplitude in thick sections. In this paper, the 3 principles are described with example measurements given in the frequency range from 50 MHz to 1.2 GHz. The measurements were made with 2 microscopes operating in the pulse-echo mode, either with frequencies up to 200 MHz and time-resolved detection or between 100 MHz and 2 GHz and amplitude detection. The methods are compared and their application potentials and limitations are discussed with respect to the hierarchical structure of cortical bone. Mapping of the confocal reflection amplitude has superior capabilities for deriving quantitative elastic and structural parameters in the heterogeneous bone material. Even at low frequencies (50 MHz), the mineralized tissue matrix can be separated from the larger pores (Haversian canals), and the elastic coefficient in the probing direction can be measured in 2 dimensions. Depending on the type of sample surface preparation (flat or cylindrically shaped), local distribution of a single elastic coefficient or the average transverse isotropic stiffness tensor can be derived. With frequencies in the GHz range, the lamellar bone structure can be analyzed. However, at one GHz, the acoustic wavelength is still one order of magnitude larger than the individual mineralized collagen fibrils. Although the thickness of a lamellar unit can easily be assessed from the acoustic image, the derivation of the anisotropic elastic properties of the mineralized collagen fibrils as well as the detailed structure of a lamella can only be accomplished with further model assumptions.

Theoretical studies on a TeO2/ZnO/diamond-layered structure for zero TCD SAW devices

High-frequency surface acoustic wave (SAW) devices based on diamond substrate are useful because of their very high SAW velocity. In the present work, SAW propagation characteristics, such as phase velocity, coupling coefficient and temperature coefficient of delay (TCD) of a TeO2/ZnO/diamond-layered structure, are examined using theoretical calculations. The ZnO/diamond bi-layer structure is found to exhibit a high positive TCD value. A zero TCD device structure is obtained after integration with a TeO2 over layer having a negative TCD value. Introduction of a non-piezoelectric TeO2 over layer on the bi-layer structure (ZnO/diamond) increases the coupling coefficient. A relatively low thickness of TeO2 thin film (∼(1.6–3.1) × 10−3λ) is required to achieve temperature-stable SAW devices based on diamond.

Thermomechanical behavior of surface acoustic waves in ordered arrays of nanodisks studied by near-infrared pump-probe diffraction experiments and finite element simulations

The ultrafast thermal and mechanical dynamics of a two-dimensional lattice of metallic nanodisks has been studied by near-infrared pump-probe diffraction measurements over a temporal range spanning from 100 fs to several nanoseconds. The experiments demonstrate that in these systems a surface acoustic wave (SAW), with a wave vector given by the reciprocal periodicity of the two-dimensional array, can be excited by ̃120 fs Ti:sapphire laser pulses. We unambiguously show that the observed SAW velocity shift originates from the mechanical interaction between the SAWs and the nanodisks, while the correlated SAW damping is due to the energy radiation into the substrate. In order to clarify the interaction between the nanodisks and the substrate, numerical calculations of both the elastic eigenmodes and the time-dynamics of the system, following the impulsive heating excitation by the laser, are performed. Simulations based on finite-elements analysis, together with a wavelet analysis of our experimental data, suggest the opening of a band-gap at the centre of the super-Brillouin zone. The modes at the centre of the super-Brillouin zone are excited following laser excitation, as opposed to thermal population, of the elastic modes.

Effect of photoacoustic generation mechanisms on transient grating measurements of thin films on silicon

Laser induced transient gratings technique is widely used for non-contact measurements of surface acoustic waves (SAWs). One major application of such measurements is characterization of thin films used in microelectronics. A peculiar phenomenon has been observed in transient gratings measurements of thin films on a silicon substrate: if the film is transparent, there is a systematic discrepancy between the measured and expected SAW velocity values that cannot be accounted for by instrumental errors. However, as soon as the sample is coated by an opaque metal film, the discrepancy disappears. In this presentation, we will show that the origin of the effect is in the competition of thermal and electronic mechanisms of the optical generation of sound in silicon. We will also describe another effect of the same origin observed on very thin semi-transparent metal films: at a certain film thickness, the SAW signal vanishes as the thermal expansion of the film and the contraction of silicon caused by photoexcited carriers cancel each other.

Elastic moduli at high temperatures with two different ultrasonic methods

“The elastic moduli and specially the shear modulus G is one of the parameters needed to implement the constitutive relations. These mechanical properties can be measured for a solid or a liquid with different ultrasonic methods. This paper deals with the determination of G(T) with two different methods: a contact delay‐line ultrasonic device which has been developed to measure the shear and compression waves velocities up to 1000 K on cylindrical specimens and a contactless (laser) ultrasonic device which can measure surface acoustic waves velocities in metals up to the melting point. Results on metals such as Al without phase transition and for Co through the phase transition will be presented.”

Characteristics of ZnO thin film surface acoustic wave devices fabricated using nanocrystalline diamond film on silicon substrates

The propagation characteristics of surface acoustic wave for the ZnO piezoelectric films and nanocrystalline diamond (NCD) films multilayer SAW devices on Si substrates were investigated. High surface acoustic wave velocity is achieved for a ZnO/NCD/Si multilayer structure excited by a bottom-electroded device configuration. The NCD films deposited on Si substrates (NCD/Si) not only possess smooth surface, but also show good compatibility with ZnO materials, such that [002] textured ZnO films can be directly grown on NCD/Si substrates without the necessity of using buffer layer. The surface acoustic wave velocity increased with the thickness of the NCD films. For the thickness of the ZnO and NCD films investigated, the 0th mode surface acoustic velocity in IDT/ZnO/NCD/Si structure achieves 5100 m/s, whereas the 1st mode SAW in the ZnO/IDT/NCD/Si structure reaches 8500 m/s.

Evaluation of material constants in NdCa4O(BO3)3 piezoelectric single crystal

A neodymium calcium oxoborate NdCa4O(BO3)3 piezoelectric single crystal that belongs to the monoclinic system with point group m was grown by the Czochralski technique. A practical evaluation method was developed to determine the 27 independent material constants for acoustic wave device applications. A longitudinal effect face–shear vibration was analyzed and used in the resonance–antiresonance measurement. This method avoided measuring d 11 and d 33 directly by use of X-bar and Z-bar, in which leak of electric field would cause large errors because of the very small dielectric constants. At room temperature, dielectric constants were $${{\varepsilon _{11}^{\text{T}} } \mathord{\left/{\vphantom {{\varepsilon _{11}^{\text{T}} } {\varepsilon _0 }}} \right.\kern-\nulldelimiterspace} {\varepsilon _0 }} = 9.9$$ , $${{\varepsilon _{22}^{\text{T}} } \mathord{\left/{\vphantom {{\varepsilon _{22}^{\text{T}} } {\varepsilon _0 }}} \right.\kern-\nulldelimiterspace} {\varepsilon _0 }} = 15$$ , $${{\varepsilon _{33}^{\text{T}} } \mathord{\left/{\vphantom {{\varepsilon _{33}^{\text{T}} } {\varepsilon _0 }}} \right.\kern-\nulldelimiterspace} {\varepsilon _0 }} = 10$$ and $${{\varepsilon _{13}^{\text{T}} } \mathord{\left/{\vphantom {{\varepsilon _{13}^{\text{T}} } {\varepsilon _0 }}} \right.\kern-\nulldelimiterspace} {\varepsilon _0 }} = - 0.8$$ , respectively. All the independent dielectric and elastic constants were determined in this work. The simulation of surface acoustic wave velocity showed a good agreement with the measured value.

A two-dimensional analysis of surface acoustic waves in finite elastic plates with eigensolutions

It is generally known that surface acoustic waves, or Rayleigh waves, have different mode shapes in infinite plates. To be precise, there are both exponentially decaying and growing components in plates appearing in pairs, representing symmetric and antisymmentric modes in a plate. As the plate thickness increases, the combined modes will approach the Rayleigh mode in a semi-infinite solid, exhibiting surface acoustic wave deformation and velocity. In this study, the two-dimensional theory for surface acoustic waves in finite plates is extended to include the exponentially growing modes in the expansion function. With these extra equations, we study the surface acoustic waves in a plate with different thickness to examine the coupling of the exponentially decaying and growing modes. It is found that for small thickness, the two groups of waves are strongly coupled, showing the significance of including the effect of thickness in analysis. As the thickness increases to certain values, such as more than five wavelengths, the exponentially decaying modes alone will be able to predict vibrations of surface acoustic wave modes accurately, thus simplifying the equations and solutions significantly.