Leaky Surface Acoustic Wave Velocity Research Articles

Evaluation of glass materials by using the line-focus-beam ultrasonic-material-characterization system

We developed experimental procedures to evaluate glass materials using the line-focus-beam ultrasonic-material-characterization (LFB-UMC) system. We prepared 28 specimens of a commercial borosilicate glass from random lots, and measured the velocities of leaky-surface acoustic waves (LSAWs) and leaky-surface-skimming compressional waves (LSSCWs), VLSAW and VLSSCW, using V(z) curve measurements at 225 MHz and 23 degrees C. The velocities for VLSAW ranged from 3121.83 m/s to 3149.77 m/s, with a maximum deviation of 27.94 m/s. The velocities for VLSSCW ranged from 5547.7 m/s to 5585.0 m/s, with a maximum deviation of 37.3 m/s. To investigate these observed variations in VLSAW and VLSSCW, we measured the bulk acoustic wave (BAW) properties, viz., longitudinal and shear velocities, then the densities and the chemical compositions of 8 of the 28 specimens. The LFB-UMC measurements confirmed that decreases in VLSAW and VLSSCW occur mainly with the B2O3 dopant concentrations, corresponding to the decrease of shear-wave and longitudinal-wave velocities that are caused by the decrease of the stiffness constants c44 and c11, respectively, rather than with decreased densities. The sensitivities are -6.36 x 10(-2) wt%/(m/s) for VLSAW and -4.87 x 10(-2) wt%/(m/s) for VLSSCW. This demonstrates that the LFB-UMC system is effective for evaluating glass materials and controlling production processes, by analyzing variations in chemical composition through the super-accurate velocity measurements of LSAWs and LSSCWs.

A Super-Precise CTE Evaluation Method for Ultra-Low-Expansion Glasses Using the LFB Ultrasonic Material Characterization System

A super-precise method of evaluating the coefficient of thermal expansion (CTE) of ultra-low-expansion glasses for future extreme ultra-violet lithography (EUVL) systems was developed using the line-focus-beam ultrasonic material characterization (LFB-UMC) system. Evaluation was demonstrated for two commercial glasses, TiO2-SiO2 glass (C-7971) and Li2O-Al2O3-SiO2 glass ceramic (Zerodur). For the C-7971 specimens, the sensitivity and resolution in the velocity measurement of leaky surface acoustic waves (LSAWs) for the CTE were estimated to be 4.40 (ppb/K)/(m/s) and ±0.77 ppb/K for ±2σ (σ: standard deviation). LSAW velocity differences caused by different TiO2 concentrations and distributions or striae in each specimen were successfully detected and evaluated. For the Zerodur specimens, LSAW velocity differences associated with the chemical compositions and crystallization conditions were observed among different ingots and specimens. This ultrasonic method is expected to be an extremely useful and effective CTE evaluation technology and to contribute to improving and developing EUVL-grade glass materials.

I-3 Accurate measurement of leaky surface acoustic wave velocity for TiO_2-SiO_2 ultralow expansion glasses by the LFB ultrasonic material characterization system

I-3 Accurate measurement of leaky surface acoustic wave velocity for TiO_2-SiO_2 ultralow expansion glasses by the LFB ultrasonic material characterization system

Leaky surface acoustic waves in Z-LiNbO3 substrates with epitaxial AIN overlays

The properties of leaky surface acoustic waves (LSAW) in MBE grown AIN layer on Z-cut LiNbO3 structures have been studied by numerical simulation and experimental measurements and compared with those of Rayleigh waves in the same structure. In the range of AIN layer thicknesses studied (0<kh<0.145) the measured velocity of LSAW propagating along the X axis of LiNbO3 substrate was essentially constant at around 4400m∕s. The measured electromechanical coupling coefficients (K2) for the LSAW are roughly 1/4 of the predicted values, which might be due to the strong attenuation of the leaky wave unaccounted for during the parameter extraction. The thin AIN film slightly improved the measured temperature coefficient of frequency for the LSAW over that attained for the Z-cut, X-propagating LiNbO3 substrate alone.

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.

Acoustic properties of silica glass doped with fluorine

The acoustic properties, viz., longitudinal and shear wave velocities, and the density, were measured with high accuracy for silica glass specimens doped with different concentrations of fluorine, and further, their leaky surface acoustic wave (LSAW) velocity and elastic constants were calculated from the above measured values. The temperature dependences of the acoustic properties were also measured around room temperatures, viz., 20.0–26.0 °C. Remarkable changes in the acoustic properties caused by doping fluorine into vitreous silica have been obtained. As compared with vitreous silica glass, the longitudinal velocity, shear velocity, LSAW velocity, and the density of silica glasses doped with fluorine decrease with −66 (m/s)/mol%, −50 (m/s)/mol%, −43 (m/s)/mol%, and −2.4 (kg/m 3)/mol%, respectively. Since these acoustic properties are varying linearly with fluorine concentration, and since they exhibit higher sensitivities to the variation of fluorine concentration than do the optical properties, the fluorine concentration dependences of the acoustic properties obtained in the present study can be widely used in the ultrasonic characterization of glass materials and optical waveguides doped with fluorine.

Quantitative characterization of the fabrication processes of domain-inverted layers in LiTaO3 by the line-focus-beam ultrasonic material characterization system

Application of the line-focus-beam ultrasonic material characterization system was extended to evaluate Z-cut LiTaO3 substrates with domain-inverted layers fabricated by proton exchange and quick heat treatment (QHT), and their fabrication processes and systems. Three Z-cut LiTaO3 substrates were proton-exchanged at 260 °C for 20 min; heat-treated for 30 s at 520, 540, and 560 °C, with a temperature increase rate of 80 °C/s; and annealed at 420 °C for 6 h. Slab-type domain-inverted layers with 0.50, 1.94, and 3.26-μm depths on the −Z surfaces and proton-exchanged layers on both the −Z and +Z surfaces were formed. Leaky surface acoustic wave (LSAW) velocities for both surfaces were measured in a frequency range of 100–300 MHz and found to exhibit different dispersion characteristics. Intrinsic LSAW velocity changes caused by the formation of domain-inverted layers were obtained from differences in the LSAW velocities for both surfaces. The velocity changes decreased with increases in the product fH of the ultrasonic frequency f and the domain-inverted layer thickness H. Relationships among the LSAW velocity changes, thicknesses of the domain-inverted layers, and process temperatures in QHT were experimentally obtained. An LSAW velocity variation of 2.6 m/s for the Y-axis propagation at 225 MHz was detected by a homogeneity evaluation on a specimen QHT-processed at 540 °C and annealed. This corresponded to a layer thickness variation of 0.20 μm and a QHT temperature variation of 2.9 °C.

Evaluation of the annealing effect of proton-exchanged LiTaO3 optical waveguides by the line-focus-beam ultrasonic material characterization system

We established an experimental procedure and collected basic data to evaluate the annealing process and effects for proton-exchanged LiTaO3 optical waveguides using the line-focus-beam ultrasonic material characterization (LFB-UMC) system in a frequency range of 100 to 300 MHz. Twelve Z-cut LiTaO3 substrates were proton-exchanged at 260 °C for 14 min in a pyrophosphoric acid solution and annealed at 420 °C for various periods from 10 sec to 24 h. The leaky surface acoustic wave (LSAW) velocities were decreased by the proton exchange, and were then increased and recovered by annealing in all propagation directions as the annealing time increased. The Y-axis propagation is most useful for an evaluation. The LSAW velocities decrease with an increase of the product fH, obtained from the frequency dependences and proton-diffused layer depths analyzed by secondary-ion mass spectrometry. Gradients of the fH dependences of the LSAW velocities become gentler with increases in the annealing time, corresponding to the concentrations and distributions of hydrogen and lithium ions in the proton-diffused layers. The relationships among the LSAW velocities, proton-diffused layer depths, relative concentrations of hydrogen ions at the specimen surfaces, and the annealing times were experimentally obtained. The measurement resolutions of the LFB-UMC system at 225 MHz to the proton-diffused layer depth, the relative concentration of hydrogen ions, and the typical annealing time for 1 min were estimated to be 4 nm, 0.2%, and 0.6 sec.

Evaluation and improvement of optical-grade LiTaO3 single crystals by the LFB ultrasonic material characterization system.

This paper describes the first demonstration for feeding back the results obtained by the line-focus-beam ultrasonic material characterization (LFB-UMC) system to the crystal growth conditions for optical-grade LiTaO3 crystals and for achieving much improved homogeneity of chemical composition. We evaluated a commercially available optical-grade LiTaO3 single crystal with a nominally congruent composition in detail, by measuring distributions of the velocities of leaky surface acoustic waves (LSAW) along the Y-axis direction for a Z-cut specimen plate prepared from the crystal grown in the Y-axis direction. We detected an increment of 0.66 m/s in LSAW velocity along the pulling axis direction corresponding to 0.024 mol% in Li2O content, and the compositional gradient was +0.346 x 10(-3) (Li2O-mol%)/mm. By experimentally obtaining the starting material composition dependence of the gradients, we developed a method of estimating the proper composition ratio that would lead to a more homogeneous crystal. We grew a new crystal with a Li2O content of 48.47 mol%, resulting in a very small compositional gradient of +0.046 x 10(-3) (Li2O-mol%)/mm and a compositional homogeneity of less than 0.012 Li2O-mol% in a Z-cut area of 50 mm x 50 mm used for device substrates.

Chemical composition dependences of the acoustical physical constants of LiNbO3 and LiTaO3 single crystals

We determined all the independent components of the acoustical physical constants (elastic constant, piezoelectric constant, dielectric constant, and density) of LiNbO3 and LiTaO3 crystals grown from the melts of three different starting materials with the Li2O contents set to 48.0, 48.5, and 49.0 mol %, and obtained the chemical composition dependences of the constants of each single crystal around the congruent composition. All the constants as well as the measured longitudinal, shear, and leaky surface acoustic wave (LSAW) velocities varied linearly with the composition ratios in the experimental range. The composition dependences of the LSAW velocities for the 128°YX-LiNbO3, X-112°Y-LiTaO3, and 36°YX-LiTaO3 substrates, previously obtained by line-focus-beam acoustic microscopy, were well matched with the calculated ones using the constants determined. Therefore the data of the composition dependences of the determined constants enable us to easily prepare the calibration lines for evaluating the crystals for any arbitrarily cut specimen surfaces, wave propagation directions, and modes by numerical calculations.

Development of the line-focus-beam ultrasonic material characterization system.

A line-focus-beam ultrasonic material characterization (LFB-UMC) system has been developed to evaluate large diameter crystals and wafers currently used in electronic devices. The system enables highly accurate detection of slight changes in the physical and chemical properties in and among specimens. Material characterization proceeds by measuring the propagation characteristics, viz., phase velocity and attenuation, of Rayleigh-type leaky surface acoustic waves (LSAWs) excited on the water-loaded specimen surface. The measurement accuracy depends mainly upon the translation accuracy of the mechanical stages used in the system and the stability of the temperature environment. New precision mechanical translation stages have been developed, and the mechanical system, including the ultrasonic device and the specimen, has been installed in a temperature-controlled chamber to reduce thermal convection and conduction at the specimen. A method for precisely measuring temperature and longitudinal velocity in the water couplant has been developed, and a measurement procedure for precisely measuring the LSAW velocities has been completed, achieving greater relative accuracy to better than +/- 0.002% at any single chosen point and +/- 0.004% for two-dimensional measurements over a scanning area of a 200-mm diameter silicon single-crystal substrate. The system was developed to address various problems arising in science and industry associated with the development of materials and device fabrication processes.

Quantitative evaluation of fabrication processes of proton-exchanged layers in LiTaO3 optoelectronic devices by the line-focus-beam ultrasonic material characterization system

Experimental investigations are conducted in order to collect basic data for evaluating proton-exchanged LiTaO3 optical waveguides and their fabrication processes and systems using the line-focus-beam ultrasonic material characterization system, in the frequency range 100–300 MHz. Seven Z-cut LiTaO3 substrates are proton exchanged at several process temperatures (220–280 °C) and times (5–30 min) in a pyrophosphoric acid solution. Leaky surface acoustic wave (LSAW) velocities, measured for all specimens, decrease for all propagation directions. The decrease rate is at maximum in the Y-axis propagation direction, in which the measurement sensitivity to the process conditions is highest. The fH dependences of LSAW velocities, obtained from frequency dependences of LSAW velocities and proton-exchanged layer depths analyzed by secondary-ion mass spectrometry, have almost constant gradients of −0.78 (m/s)/(Hz m). Normalized depth distributions of the elastic properties of proton-exchanged layers are nearly equal; only the depths differ. Also, the relationships among LSAW velocities, layer depths, process times, process temperatures, and diffusion coefficients are experimentally obtained. Homogeneity evaluation of a proton-exchanged, 2-in., Z-cut LiTaO3 wafer processed at 260 °C for 14 min is demonstrated, resulting in a maximum LSAW velocity variation of 1.3 m/s. This corresponds to a depth variation of 7.4 nm and a temperature variation of 0.8 °C for the whole surface.

Ultrasonic imaging, particle detection, and V(z) measurements in molten zinc using focused clad buffer rods

Ultrasonic imaging, particle detection, and V(z) measurements have been performed using focused clad buffer rods in molten zinc at temperature more than 600 °C. The focused ultrasonic waves are generated by a spherical or cylindrical acoustic lens which is fabricated at the end of the clad buffer rod. In order to evaluate its focusing ability, several experiments are carried out at 10 MHz in a pulse-echo mode. The lateral resolution at the focus of the spherical acoustic lens in molten zinc is quantitatively examined and compared with that in water using a thin stainless wire with a diameter of 380 μm. High resolution ultrasonic imaging is carried out by the common C-scan technique. The signal-to-noise ratio of the reflected signals from the flat sample surface at the focus is better than 35 dB. Ultrasonic images are obtained from the amplitude and time delay variations of the reflected signals. An attempt has also been made to detect particles suspended in molten zinc. Backscattered signals from particles are clearly visible at the focal region of the lens. For quantitative materials evaluation, V(z) curve measurements are performed using both spherical and cylindrical surface lenses and the leaky surface acoustic wave velocity of a ceramic (SiC) plate immersed in molten zinc is successfully determined.

Experimental study of construction mechanism of V(z) curves obtained by line-focus-beam acoustic microscopy

The propagation characteristics, viz., phase velocity and attenuation, of leaky surface acoustic waves (LSAWs), excited on the water/sample boundary are obtained through analyzing the V(z) curves measured by line-focus-beam acoustic microscopy. However, different values of these characteristics are obtained, depending upon different ultrasonic devices and operating frequencies employed. The construction mechanism of V(z) curves was investigated experimentally by measuring the amplitude and phase for Teflon to provide an understanding of the device performance for velocity measurements. A V(z) curve measured for Teflon, on which no leaky waves are excited when water is the coupling medium, can be used for the characteristic device response, depending only upon the device parameters and the operating frequencies. From the investigation of the ultrasonic device and the frequency dependences of the characteristic device responses, the phase gradient was found to be directly related to values of measured LSAW velocities. From this result, apparent frequency dependences in LSAW velocity measurements are explained quantitatively for a specimen of gadolinium gallium garnet.

Line-focus-beam acoustic microscopy characterization of optical-grade LiTaO3 single crystals

Basic data for evaluating homogeneities of LiTaO3 single crystals for optical use are investigated by line-focus-beam acoustic microscopy. First, the relationships among leaky surface acoustic-wave (LSAW) velocities, chemical compositions, Curie temperatures, densities, and lattice constants are experimentally investigated as the calibration lines for crystal evaluation using X-, Y-, and Z-cut substrates prepared from three LiTaO3 single crystals grown along the crystallographic Y axis with different Li2O contents ranging from 48 to 49 mol %. It is shown that as the Li2O content increases around the congruent composition, the LSAW velocities linearly increase for all specimen surfaces and all propagation directions, and the increase rate is maximum for Z-cut, Y-axis propagating (ZY) LiTaO3. Next, homogeneities of the above three crystals and a commercially available optical-grade crystal are evaluated using ZY-LiTaO3 specimens and the obtained calibration lines. The chemical composition variations along both the pulling direction and the diameter direction are successfully detected as LSAW velocity variations, due to the changes of crystal-growth conditions. In the commercial crystal, the LSAW velocity variations for the whole examined region (80 mm×60 mm) exhibit a maximum difference of 0.82 m/s, corresponding to the composition change of 0.026 Li2O mol %. It is demonstrated that this ultrasonic method has the unique and useful capabilities of detecting changes of the growth conditions and of evaluating local densities and the crystal-melt interface shape.

Ultrasonic microspectroscopy characterization of silica glass

Ultrasonic microspectroscopy technology, using the line-focus-beam and plane-wave ultrasonic material characterization system, is applied to characterization of silica glass. Eight types of commercial silica glasses fabricated by different production methods and conditions are used as specimens to measure their acoustic properties accurately, viz., acoustic velocity, density, elastic constant. The variations in those acoustic properties resulting from small amounts of impurities, such as hydroxyl (OH) and chlorine, incorporated during the production processes were quantitatively obtained. The longitudinal velocity, density, and elastic constant c11 of silica glass with 860 ppm OH ions were less by 0.28%, 0.06%, and 0.62%, respectively, than those for OH-free silica glass, while the temperature coefficient of the longitudinal velocity was greater by 7.4%. In contrast, the leaky surface acoustic wave velocity, shear velocity and elastic constant c44 of silica glass containing 1500 ppm residual chlorine were less by 0.23%, 0.30%, and 0.59%, respectively, than those of OH-free and chlorine-free silica glass, while the longitudinal velocity, elastic constant c11, and density increased slightly by 0.04%, 0.10%, and 0.02%, respectively. Further, it was found that the decrease in acoustic velocity due to OH or chlorine is mainly related to the decrease in the elastic constant, which also corresponds to the decrease in the viscosity of the SiO2 material. These results demonstrate that the ultrasonic method is very useful and effective for analyzing and evaluating both the glass properties and the production processes.

Influence of reflected waves from the back surface of thin solid-plate specimen on velocity measurements by line-focus-beam acoustic microscopy

We investigated the velocity measurements of leaky surface acoustic waves (LSAW) by line-focus-beam (LFB) acoustic microscopy of thin specimens for which the waves reflected from the back surface of the specimen (back reflection) must be included in the measurement model. The influence of back reflection resulted in a serious problem in measurement accuracy of the apparent changes of measured velocities. Using several samples of thin synthetic silica glasses, the determination of LSAW velocity affected by the reflected waves and the relationship between the specimen thickness and the apparent velocity change with a periodic frequency interval in the frequency dependence of measured LSAW velocities are discussed in detail. Three useful methods for eliminating that influence are proposed and demonstrated: first, separating the radio frequency (RF) pulsed wave signal from the specimen surface and the pulses reflected from the back surface by reducing the RF pulse width; second, scattering acoustic waves from the roughened back surface; and third, taking the moving average of measured frequency characteristics of LSAW velocities. It is shown that, among these methods, the moving average method is the most useful and effective as a general means to eliminate the influence and to determine intrinsic velocity values because this method needs no specimen process and no system change, and the same conventional V(z) curve measurement and analysis can be employed.

Elastic constants of multidomain LiTaO3 crystal

All the independent components of the elastic constant tensor of multidomain LiTaO3 crystal were determined by measuring bulk acoustic wave velocities and density for five multidomain LiTaO3 substrates perfectly depoled by heat treatment. Comparing these determined constants with the elastic constants of single-domain LiTaO3 crystal revealed that the elastic constants of multidomain crystal cij differ from both the elastic constants at constant electric field cijE and those at constant electric displacement cijD for single-domain crystal. The angular dependences of leaky surface acoustic wave velocity were measured on single and multidomain 36°Y-cut LiTaO3 substrates by line-focus-beam acoustic microscopy and compared with the calculated results using the determined constants. The measured results agreed well with the calculated results. The constants will be used to enhance theoretical understanding of the elastic properties of residual multidomains.

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.

Surface-acoustic-wave properties of MgO-doped LiNbO3 single crystals measured by line-focus-beam acoustic microscopy

Basic acoustic properties of five MgO-doped LiNbO3 rectangular parallelpiped specimens, having X-, Y-, and Z-cut planes, with different MgO dopant concentrations ranging from 0 to 13 mol % are investigated by line-focus-beam acoustic microscopy. The investigation is conducted to characterize the optical-use MgO:LiNbO3 crystals and wafers and to establish large-diameter crystal growth conditions. Leaky-surface-acoustic-wave (LSAW) velocities are measured on each crystalline plane of each specimen as a function of the wave propagation direction and are compared with the measured chemical composition ratios of Mg/Li/Nb, densities, and lattice constants. It is shown that, as the dopant concentrations increase in the experimental composition region, the LSAW velocities increase almost linearly for all the surfaces and all the propagation directions. The LSAW velocities are linearly proportional to the lattice constants, but inversely proportional to the densities.