Leaky Surface Acoustic Wave Propagation Research Articles

Theoretical study of different attenuation measurement by acoustic microscopy

Many works are devoted to study the attenuation of surface waves in media, particularly, leaky surface acoustic waves (LSAW). In this work, a big part of the study is based on the intensity of the output signal, i.e., acoustic signature, V(z). The latter is obtained by the use of quantitative mode of acoustic microscopy in order to measure the velocity and the attenuation of those excited waves at the limit between the specimen and the coupling liquid. Our aim is to compare the attenuation values of the LSAW propagation in porous silicon obtained with three different methods. The first is obtained by resolving Viktorov equation. The second method is the spectral analysis acoustical signature V(z) curves. The third method uses the dark field. The obtained results are in a good agreement with those experiments.

Propagation properties of leaky surface acoustic wave on reverse‐proton‐exchanged 10° Y‐X LiNbO3

AbstractIn this study, to reduce leaky surface acoustic wave (LSAW) attenuation, a layered structure of air/bulk LiNbO3 (LN) layer/elastically softened LN substrate which can be fabricated by a reverse proton exchange (RPE) process was applied to 10° Y‐X LN with a larger coupling factor. First, the LSAW attenuation on a structure of air/bulk LN/modified LN was calculated with all material constants of the buried proton exchange (PE) layer taken into consideration. It was found that a condition under which the attenuation decreases as compared with the bulk value exists for the free and metallized surfaces. Next, the LSAW propagation properties were measured using IDT pairs with a 3.6‐μm wavelength on 10° Y‐X LN with the RPE layer and the buried PE layer. The propagation loss PL for the metallized surface was decreased by carrying out the RPE process from 0.44 dB/λ of a virgin wafer to 0.01 dB/λ. The decrease in PL for the shorted grating was also observed. © 2012 Wiley Periodicals, Inc. Electron Comm Jpn, 95(3): 19–27, 2012; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ecj.10412

Leaky-Surface-Acoustic-Wave Properties on Reverse-Proton-Exchanged LiNbO3

A proton-exchanged (PE) layer with an elastically soft property can be buried into a substrate by a reverse proton exchange (RPE) process. It is expected that an RPE layer with a property similar to that of bulk LiNbO3 will prevent the degradation of coupling factor, and that the buried PE layer will change the anisotropy of the substrate effectively and reduce the leaky surface acoustic wave (LSAW) attenuation as compared with bulk LN. In this study, first, a layered structure of air/bulk LN/softened LN was assumed for rotated Y–X LN and LSAW attenuation was calculated. By controlling the elastic constant of the softened LN, the rotation angle from the Y-axis giving zero attenuation shifted from 64 to 5° for a metallized surface, and the rotation angle at which the attenuation decreases as compared with bulk LN was drastically changed for a free surface. Next, the LSAW propagation loss PL was measured on 41° Y–X LN. The PL for the metallized surface was decreased by carrying out the RPE process from 0.036 dB/λ of a virgin sample to 0.015 dB/λ. The decrease in PL for the free surface was also observed.

Accurate Velocity Measurement of Periodic Striae of TiO2–SiO2 Glasses by the Line-Focus-Beam Ultrasonic Material-Characterization System

The evaluation of periodic striae layers in TiO2–SiO2 ultra-low-expansion glasses, formed during production, is discussed through the measurement of leaky surface acoustic-wave (LSAW) velocities by the line-focus-beam (LFB) ultrasonic material-characterization system. The averaging effect associated with an LSAW propagation region interacting with periodic striae on a specimen surface was examined at 225 MHz, taking several specimens prepared with inclination angles of θ=0, 2, 5, 10, 20, and 90° to the plane of striae layers with a periodicity of 0.16 mm. LSAW velocities corresponding to TiO2 concentrations in the glass were properly measured for the specimens with θ less than 10°, and the perpendicular specimen (θ=90°) was suitable for accurate determination of the striae periodicities for LSAW propagation perpendicular to the striae plane.

Evaluation method of TiO/sub 2/-SiO/sub 2/ ultra-low-expansion glasses with periodic striae using the LFB ultrasonic material characterization system

Experimental procedures and standard specimens for characterizing and evaluating TiO2-SiO2 ultra-low expansion glasses with periodic striae using the line-focus-beam (LFB) ultrasonic material characterization system are discussed. Two types of specimens were prepared, with specimen surfaces parallel and perpendicular to the striae plane using two different grades of glass ingots. The inhomogeneities of each of the specimens were evaluated at 225 MHz. It was clarified that parallel specimens are useful for accurately measuring velocity variations of leaky surface acoustic waves (LSAWs) excited on a water-loaded specimen surface associated with the striae. Perpendicular specimens are useful for obtaining periodicities in the striae for LSAW propagation perpendicular to the striae plane on a surface and for precisely measuring averaged velocities for LSAW propagation parallel to the striae plane. The standard velocity of Rayleigh-type LSAWs traveling parallel to the striae plane for the perpendicular specimens was numerically calculated using the measured velocities of longitudinal and shear waves and density. Consequently, a reliable standard specimen with an LSAW velocity of 3308.18 +/- 0.35 m/s at 23 degrees C and its temperature coefficient of 0.39 (m/s)/degrees C was obtained for a TiO2-SiO2 glass with a TiO2 concentration of 7.09 wt%. A basis for the striae analysis using this ultrasonic method was established.

Effect of Substrate Thickness on Quasi-Longitudinal Leaky Surface Acoustic Wave Propagation on Quartz

Propagation properties of a quasi-longitudinal leaky surface acoustic wave (QLL-SAW) on rotated-Y cut quartz are described. It is found that by reducing the relative thickness of the quartz substrate, i.e., the ratio of the substrate thickness to the SAW wavelength, the Q-value of a QLL-SAW resonator can be improved. Its phase velocity is about 5704 m/s, which is close to the velocity of a longitudinal BAW (5750 m/s) and about 1.8 times that of a Rayleigh SAW. Propagation characteristics are simulated by a finite element method. The results obtained indicate that many guided wave modes exist near the velocity of the longitudinal BAW and strongly couple to QLL-SAW modes. Experimental results show that these guided wave modes affect QLL-SAW propagation and deteriorate resonator Q-values. By reducing the relative thickness of the quartz substrate, the frequency difference between the QLL-SAW and the guided wave modes is increased, which weakens the coupling. As a result, 700 MHz SAW resonators having a Q-value of more than 8000 are realized.

Accurate Calibration Line for Super-Precise Coefficient of Thermal Expansion Evaluation Technology of TiO2-Doped SiO2 Ultra-Low-Expansion Glass Using the Line-Focus-Beam Ultrasonic Material Characterization System

We obtained an accurate relationship between the leaky surface acoustic wave (LSAW) velocities and TiO2 concentrations. Such a relationship is needed for the line-focus-beam ultrasonic material characterization (LFB-UMC) system as a new technology for evaluating the coefficient of thermal expansion (CTE) of ultra-low-expansion glasses with extremely high precision. Using commercially available TiO2–SiO2 glass with periodic striae, averaged velocities for LSAW propagation parallel to the striae plane at 225 MHz and averaged TiO2 concentrations by X-ray fluorescence analysis were measured on the surface of seven specimen substrates prepared perpendicular to the striae plane. The obtained sensitivity of LSAW velocity to TiO2 concentration was -0.0601 wt %/(m/s), and the measurement resolution was estimated to be ±0.0084 wt % for ±2 σ (σ: standard deviation) from the LSAW velocity measurement resolution of ±0.14 m/s. The TiO2 concentration for the standard TiO2–SiO2 glass specimen with a LSAW velocity of 3308.18 m/s for system calibration was determined to be 7.09 wt %. This ultrasonic technology will contribute greatly to the development of the ultra-low-expansion glasses needed for extreme ultraviolet lithography (EUVL) systems.

Numerical analysis of surface and leaky surface acoustic waves in new piezoelectric KNBO3, PKN, and LGN crystals

We perform a numerical analysis of the properties of surface acoustic waves (SAW) as well as leaky surface acoustic waves (LSAW) in piezoelectric KNbO3, Pb2KNb5O15 (PKN) and La3Ga5.5Nb0.5O14 (LGN) crystals. We determine optimal thermostable cuts and directions in which the SAWs have high phase velocity, a very high coefficient of electromechanical coupling. and a small angle between the phase and group velocities. We also found the cuts and directions in which the LSAWs can exist. The characteristics of the LSAWs (velocity, the coefficient of electromechanical coupling, angle between the phase and group velocities, temperature coefficient of velocity, and coefficient of surface absorption along the LSAW propagation direction) are calculated.

Behaviour of Leaky Surface Acoustic Wave Propagation at Discontinuous Region of Periodic Grating

This paper describes the leaky surface acoustic wave (LSAW) propagation characteristics at a discontinuous region in a grating structure. First, the dispersion relation of LSAWs propagating under a multielectrode-type grating is analysed. Then, the LSAW propagation on the discontinuous region is characterised by simply assuming that the LSAW behaviour under periodic part of the structure is identical with that under a totally periodic grating structure. It is concluded that the phase lag considerably decreases at the discontinuous region. This suggests that the energy storing effect may be weakened at the region where the periodic structure is broken. The simulation of one-port LSAW resonators shows that device performances are sufficiently evaluated only by taking account of the constant phase lag for a simulation tool based on the coupling-of-modes theory.

A method for calibrating the line-focus-beam acoustic microscopy system

Absolute accuracy of the line-focus-beam (LFB) acoustic microscopy system is investigated for measurements of the leaky surface acoustic wave (LSAW) velocity and attenuation, and a method of system calibration is proposed. In order to discuss the accuracy, it is necessary to introduce a standard specimen whose bulk acoustic properties, (e.g., the independent elastic constants and density) are measured with high accuracy. Single crystal substrates of gadolinium gallium garnet (GGG) are taken as standard specimens. The LSAW propagation characteristics are measured and compared with the calculated results using the measured bulk acoustic properties. Calibration is demonstrated for the system using two LFB acoustic lens devices with a cylindrical concave surface of 1-mm radius in the frequency range 100 to 300 MHz.

Theoretical and experimental investigation for LFB acoustic microscopy characterization of proton-exchanged LiTaO3 optical waveguides

Line-focus-beam (LFB) acoustic microscopy has been recognized as a new analytical technique of quantitative material characterization, by measuring the phase velocities of leaky surface acoustic waves (LSAWs) excited on the water-loaded specimen surface. Recently, application of this ultrasonic technique has been demonstrated successfully for characterization of proton-exchanged layers for LiNbO3 and LiTaO3 optical waveguides and for evaluation of the device fabrication processes. It is necessary and important to calculate LSAW propagation characteristics, viz., velocity and attenuation, for interpretation of experimental results and for evaluation of physical properties, such as optical and elastic properties, in proton-exchanged layers. In this paper, LSAW propagation characteristics for the structural model of water/piezoelectric layer/piezoelectric substrate are analyzed. The propagation characteristics are interpreted with calculations of spatial distributions of mechanical displacements and electric potential. Specimens of Z-cut LiTaO3 substrates, with a layer thickness of 0.5 μm, were prepared. Experimental data of LSAW propagation characteristics, measured in the frequency range 100 to 300 MHz, are discussed in detail comparing with the calculated results.

Evaluation of spatial resolution of spherical-planar-pair lenses for elasticity measurement with microscopic resolution

The spatial resolution of spherical-planar-pair (SPP) lenses used in an ultrasonic microspectrometer (UMSM) was evaluated by scanning the lenses over a sharp edge of a specimen. The variation of the output observed in experiments was in good agreement with the theory. In the direction perpendicular to the propagation of a leaky surface acoustic wave (SAW), the best spatial resolution was obtained. The distance a/sub 90-10/ over which the magnitude decreased from 90% to 10% was 5 /spl mu/m at the frequency of 260 MHz. In the direction parallel to the leaky SAW propagation, the spatial resolution was severely affected by the excitation of the leaky SAW.

Characterization of 36/spl deg/YX-LiTaO/sub 3/ wafers by line-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/spl deg/Y-cut LiTaO/sub 3/ 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//spl deg/C with the Curie temperature. A chemical composition change of 0.03 Li/sub 2/O-mol%, corresponding to temperature resolution of better than 0.3/spl deg/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. >

A model for evaluating the efficiency of an interdigital transducer for bulk wave excitation into liquid

An equivalent circuit model analysis of a liquid-loaded interdigital transducer is described. The electric loss related to the complex relative dielectric constant and the electroacoustic conversion efficiency are represented in the form of a function of the specific acoustic impedance (ρv) of the loaded liquid. The conversion efficiency for ultrasound into the liquid is estimated from only the acoustic impedance and the relaxation time of the substrate material, which are obtained relatively easily from the water measurement. The estimated theoretical efficiency is in good agreement with the experimental results for various liquids. These results also coincide with the results from the wave equation for the leaky surface acoustic wave propagation at the solid–liquid boundary.