Surface Acoustic Wave Velocity Research Articles

Epitaxial PbZr.52Ti.48O3 films on SrTiO3/(001)Si substrates deposited by sol–gel method

We report on the sol–gel deposition and characterization of high-quality, epitaxial films of PbZr.52Ti.48O3 (PZT) on (001)Si substrates, with a thickness range of 400 Å to 1 μm. The epitaxial growth of PZT on (001)Si is achieved using a thin template layer of SrTiO3, grown by molecular-beam epitaxy. The sol–gel PZT films have a typical surface roughness of 5 Å and exhibit well defined reflective high-energy electron diffraction patterns characteristic of smooth, epitaxial films. Using high-resolution transmission electron microscopy and double-crystal x-ray diffraction, we find that the PZT films are oriented with the c axis normal to the (001)Si plane and with the a axis lying along 〈110〉Si direction. Finally, we measure the electromechanical coupling coefficients and the surface acoustic wave velocities for our films as a function of thickness and compare our experimental data to previously published theoretical values for this system.

Characterization of Stress at a Ceramic/Metal Joined Interface by the Vz Technique of Scanning Acoustic Microscopy

It is well known that the process of heating and then cooling dissimilar materials introduces considerable stress at and near the interface. In this article, first, the surface wave velocity distributions obtained with the Vz curve technique were found to compare well with residual stress distribution measured by the finely collimated X-ray diffraction technique. Second, a delamination was introduced at the interface. The Vz curve technique was then used again to measure the surface acoustic wave velocity along the interface. The defective specimens showed significantly different patterns of surface acoustic wave velocities. Thus, this study presents useful guidelines in discriminating between sound and defective ceramic/metal joints by scanning acoustic microscopy.

Characterization of AlN Films on Y-128° LiNbO3 by Surface Acoustic Wave Measurement

Y-128° LiNbO3 is used extensively for the development of surface acoustic wave (SAW) devices. AlN thin films are an attractive material that have some excellent characteristics, such as high surface acoustic wave (SAW) velocity, piezoelectricity, high-temperature stability, and stable chemical properties. In this research, AlN films were sputtered on Y-128° LiNbO3 to yield a new piezoelectric substrate for SAW devices. The X-ray diffraction (XRD) method and SAW measurement were used to evaluate the material and acoustic properties of this structure. The experimental results exhibited that highly c-axis-oriented AlN films were prepared on Y-128° LiNbO3. AlN films on Y-128° LiNbO3 can effectively enhance the SAW velocity and improve the poor temperature stability, but reduce the electromechanical coupling coefficient (k2) values. As the ratio (film-thickness/acoustic-wavelength) increased, the SAW velocities and the temperature coefficient of frequency (TCF) all increased, but the electromechanical coupling coefficient (k2) values decreased.

Acoustic study of adsorbed liquid layers

Interference measurements of small variations in the velocity and attenuation of surface acoustic waves (SAWs) are used to investigate water layers up to 15 nm thick adsorbed on the surface of a lithium niobate crystal. The frequency dependence of the relative variation of the SAW velocity with the adsorption of water vapor is measured in the range from 40 to 400 MHz. Acoustic techniques are used to experimentally estimate the frequency dependence of the dielectric constant of adsorbed water and its dipole relaxation frequency along with the dependence of the adsorption layer thickness on the water vapor pressure in the surrounding medium. A simple expression is proposed for calculating the dispersion of the SAW velocity in a solid loaded with a thin liquid layer.

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.

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 plane-wave 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 LiTaOa]. The accuracy is nearly the same as that for the constants determined only from the bulk wave velocities.

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 nonpiezoelectric 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 degrees 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.

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.

Standardized evaluation of chemical compositions of LiTaO/sub 3/ single crystals for SAW devices using the LFB ultrasonic material characterization system

The line-focus-beam ultrasonic material characterization (LFB-UMC) system is applied to compare and evaluate tolerances provided independently for the Curie temperature T(C) and lattice constant a to evaluate commercial LiTaO3 single crystals by measuring the Rayleigh-type leaky surface acoustic wave (LSAW) velocities V(LSAW). The relationships between V(LSAW), and T(C) and a measured by individual manufacturers were obtained experimentally using 42 degrees Y X-LiTaO3 wafers as specimens from three crystal manufacturers. In addition, the relationship between V(LSAW) and SH-type SAW velocities V(SAW) that are actually used for the SAW device wafers was obtained through calculations, using the chemical composition dependences of the acoustical physical constants for LiTaO3 crystals reported previously. The result of a comparison between the T(C) tolerance of +/-3 degrees C and the a tolerance of +/-0.00002 nm through the common scale of V(LSAW) or V(SAW) demonstrated that the a tolerance is 1.6 times larger than the T(C) tolerance. Furthermore, we performed a standardized comparison of statistical data of T(C) and a for LiTaO3 crystals grown by two manufacturers during 1999 and 2000, using V(LSAW). The results clarified the differences of the average chemical compositions and of the chemical composition distributions among the crystal ingots between the two manufacturers. A guideline for the standardized evaluation procedure has been established for the SAW-device wafer specifications by the LFB-UMC system.

Visible–blind photoresponse of GaN-based surface acoustic wave oscillator

We describe the photoresponse of GaN-based surface acoustic wave (SAW) delay-line oscillator operating in the 200 MHz range. The decrease in oscillator frequency under ultraviolet illumination of GaN transducer is caused by the SAW velocity decrease due to the acoustoelectronic interaction with photoconductivity electrons. The oscillator frequency shift reaches its maximum value at 365 nm and drops to zero above 400 nm with visible/ultraviolet rejection ratio more than 100. The optical quenching of the photoconductivity in GaN was observed. These results demonstrate the potential of the GaN-based SAW oscillators for applications as visible–blind remote UV sensors.

Preparation of AlN and LiNbO3 thin films on diamond substrates by sputtering method

Diamond is attracting interest as a high velocity material because its elastic constant is the highest of all substances and the surface acoustic wave (SAW) velocity is more than 10000 m/s. Although diamond is not piezoelectric, its high acoustic propagation makes it a desirable substrate for SAW devices when coupled with piezoelectric thin films such as aluminum nitride (AlN) and lithium niobate (LiNbO3). Highly oriented AlN thin films and LiNbO3 thin films were prepared by a reactive sputtering method on polycrystalline diamond substrates. The average surface roughness (Ra) of the AlN thin films was below 2 nm by locating the diamond substrates at the position of 100 mm from the aluminum target. The full width at half maximum of the rocking curve for the AlN(002) peak of X-ray diffraction was approximately 0.2°. SAW characteristics with an interdigital transducer (IDT)/AlN/diamond structure were investigated. The average surface roughness of LiNbO3 thin films was 5 nm. If the highly oriented LiNbO3 films are deposited on a diamond substrate, the IDT/LiNbO3/diamond layered structure will be capable of wide-bandwidth application in SAW devices at high frequencies.

Acoustic microscopy measurement of elastic constants by using an optimization method on measured and calculated SAW velocities: effect of initial cij values on the calculation convergence and influence of the LFI transducer parameters on the determination of the SAW velocity

Abstract The determination of elastic constants in anisotropic solids by line-focus acoustic microscopy (LFAM) is discussed. The velocity of leaky surface acoustic waves (SAWs) has been obtained for different directions of propagation ( φ ) from the V ( z ) curves. The elastic constants are determined from the comparison of measured and calculated velocities. The principle is based on the minimization of the summation of the square difference ∑| V Φ c − V Φ m | 2 , by using a Simplex method. This paper deals with the case of cubic-crystallite solids defined by three independent elastic constants. The influence of the initial values of c ij on the final result is discussed in the case of MgO substrate and TiN deposited on MgO substrate. Experimental SAW velocity profile is measured on (100) silicon substrate as a function of φ between [100] and [010] directions. The influence of the transducer parameters and defocusing distance are also discussed.

Investigation of titanium nitride coating by broadband laser ultrasonic spectroscopy

We present a laser ultrasonic method to investigate a titanium nitride (TiN) coating specimen. The technique is based on the principle of surface acoustic wave (SAW) dispersion during acoustic propagation on a half-space with the presence of a thin layer. Due to the high efficiency of laser line-source excitation, we have been able to generate and detect a SAW with an excellent signal-to-noise ratio in a wide frequency band. An inverse fitting algorithm was employed to extract simultaneously the thickness and the elastic parameters of the TiN coating from the experimental SAW velocity dispersion curve.

Fabrication of c-axis oriented ZnO/AlN thin films prepared by radio frequency reactive sputtering and development of ZnO/AlN layered structure surface acoustic wave devices

ZnO/AlN films have been continuously deposited on silicon wafers by rf reactive magnetron sputtering method. The c-axis preferred orientation of ZnO film with respect to the properties such as crystallinity and interface smoothness of AlN film at bottom has been examined by x-ray diffraction (XRD). The surface acoustic wave (SAW) filters with a wavelength of 52.8 μm using these ZnO/AlN bilayered structure have been fabricated for the first time and their SAW velocities and electromechanical coupling coefficients have been measured by a network analyzer. It is observed that as the thickness of AlN film increases, the average roughness of AlN film increases and the degree of the c-axis preferred orientation of ZnO film reduces. From these results, it is suggested that the factor determining the preferred orientation of top ZnO layer is the surface smoothness of bottom AlN layer. By measuring SAW characteristics of the ZnO/AlN bilayered structure, the phase velocity is calculated to be 4300 m/s and the electromechanical coupling coefficient is 3.23%, which are high enough for commercialization. It is also found that the velocity and the coefficient follow the better one of two films. The SAW filter fabricated in this work reveals good characteristics as the thickness of ZnO film increases and that of AlN film decreases.

Properties of Surface Acoustic Waves in AlN And GaN

ABSTRACTThe surface acoustic wave velocities, electromechanical coupling coefficients, and the spatial distributions of both elastic displacement and electric potential have been calculated for various configurations of gallium nitride and aluminum nitride. The electromechanical coupling coefficient values of 0.13 % in GaN and 0.29 % in AlN have been predicted. The maximum electromechanical coupling coefficient values of 0.24 % at Euler angles (0, 54°, 90°) in GaN and 1.08 % at (0, 53°, 90°) in AlN have been found. For GaN layer-on- sapphire substrate structures, the SAW velocity and electromechanical coupling coefficient have been calculated as functions of layer thickness and acoustic wavelength. The experimentally measured values of the surface acoustic wave velocity and electromechanical coupling coefficient are in satisfactory agreement with the calculation results.

Surface acoustic wave resonator from thick MOVPE-grown layers of GaN(0001) on sapphire

ABSTRACTWe report on fabrication and measurement of a surface acoustic wave resonator prepared on ∼10m thick GaN(0001) films. The films were grown by metal-organic vapor phase epitaxy on a c-plane sapphire substrate. The surface morphology of the films were examined with scanning electron and atomic force microscopy. A metallic bilayer of Al/Ti was subsequently evaporated on the nitride film surface. Definition of the resonator interdigital transducers, designed for a wavelength of λ=7.76m, was accomplished with standard UV lithography and lift-off. S-parameter measurements showed a resonator center frequencyf0=495MHz at room temperature, corresponding to a surface acoustic wave velocity of 3844m/s. The insertion loss at center frequency was measured at 8.2dB, and the loaded Q-factor was estimated at 2200. Finally, measurements of the resonator center frequency for temperatures in the range 25–155°C showed a temperature coefficient of -18ppm/°C. The intrinsic GaN SAW velocity and electromechanical coupling coefficient were estimated at νSAW=383 1m/s andK2=1.8±0.4·10−3.

Surface acoustic waves on nanocrystalline diamond

Surface acoustic wave (SAW) devices based on polycrystalline diamond have recently achieved success as microwave filters. This is due in part to the large acoustic wavelength of diamond at microwave frequencies, a consequence of its high surface wave velocity, and the resulting ability to use photolithography for transducer fabrication. Since nanocrystalline diamond has a smooth surface and is elastically isotropic, it may offer considerable advantages over thick films of polycrystalline diamond. We have studied the propagation of surface waves on nanocrystalline diamond prepared by microwave plasma chemical vapor deposition (CVD) on silicon substrates. Films were synthesized on 75-mm Si wafers using input gas mixtures consisting of Ar with 1% CH4 and 0–4% H2. The deposition parameters studied included pressure, 2.45 GHz microwave power, and total gas flow rate. Film thicknesses up to 23 μm were produced. SAW transducers were fabricated by photolithography on as-grown nanocrystalline diamond surfaces covered with a 1–3 μm overlayer of oriented polycrystalline piezoelectric ZnO prepared by reactive dc sputtering. The device response was analyzed with frequency and time domain methods. The resonant frequencies of the devices agree with the results of numerical solutions for sound propagation in layered media. Several surface acoustic modes exist at frequencies between 0.5 and 1 GHz that exhibit appreciable dispersion. We have propagated surface waves in nanocrystalline diamond over distances varying from 0.1 to 3 mm with low attenuation. For a film with mean grain size of approximately 30 nm, the SAW velocity is similar to test devices on thick polycrystalline diamond. We conclude that nanocrystalline diamond is a highly attractive substrate material for SAW devices, possessing the high sound velocity of diamond but requiring less materials processing.

The acoustoelectric effect in double layer AlGaAs/GaAs 2D hole systems

The surface acoustic wave velocity shift and acoustoelectric effect have been studied in a double layer GaAs/AlGaAs 2D hole system. The transverse acoustoelectric field exhibited bipolar peaks centred at integer filling factor, in qualitative agreement with the single layer theoretical prediction. Anomalous bipolar peaks have been observed in the longitudinal acoustoelectric effect centred at ν=2 (at 0.6– 0.8 GHz ) and ν=6 and 8 (at 0.8 GHz ).

SAW modes on ST-X quartz with an AlN layer

AlN thin films are an attractive material that have some excellent characteristics, such as high surface acoustic wave (SAW) velocity, piezoelectricity, high-temperature stability, and stable chemical properties. The Y-rotated, X-propagating (ST-X) cut of quartz is used extensively for the development of SAW devices. Besides the commonly used generalized SAW (GSAW) mode, this crystal cut also supports a pseudo-SAW (PSAW) and a high-velocity pseudo-SAW (HVPSAW) mode. In this paper, thin AlN films were sputtered on ST-X quartz to study the SAW modes. The X-ray diffraction (XRD) pattern exhibited that the AlN films were c-axis-oriented structures. The experimental result shoed that the added AlN layer can effectively raise the GSAW and HPSAW velocities.

Passband widening of transversely coupled resonator filters using the fundamental symmetric and antisymmetric modes.

This paper describes a method for widening the passband of transversely coupled resonator filters (TCF) using only the fundamental symmetric and antisymmetric modes. The coupling of modes analysis in the transverse direction is applied to the TCF design to investigate the passband width. As a result, it is found that the passband width can be increased by making the surface acoustic wave (SAW) velocity difference between the interdigital transducer (IDT) region and the resonator gap region smaller. It is proposed that a grating structure be applied to the common ground bar, instead of the uniform metal, to reduce the SAW velocity difference. Using the grating-type common ground bar, filters are fabricated on ST-quartz substrate. The passband of a single filter with a center frequency of 248 MHz is widened up to 410 kHz without any increase of the insertion loss. The effect of the impedance mismatch at the junction of two cascaded devices is investigated. It is shown that the filter performance is improved by reduction of the small parasitic capacitance existing at the cascade point. Experimentally, the capacitance formed between the bus bar of the IDT and the bottom surface of the ceramic package is reduced. The insertion loss is reduced by 0.6 dB, and 3-dB passband is widened by 8 kHz for a filter with a center frequency of 248 MHz. On the basis of these two improvements, cascaded TCFs are fabricated. For a filter with a center frequency of 248 MHz, an insertion loss of 5.5 dB and a 3-dB passband width of 270 kHz are obtained.