Intrinsic strength of silicon crystals in pure- and combined-mode fracture without precrack

Measurement of the critical fracture strength of single-crystal silicon was carried out by contact-free laser-based excitation and detection of nonlinear surface acoustic wave (SAW) pulses. The three crystallographic geometries Si(112)111[over ], Si(112)1[over ]1[over ]1, and Si(110)11[over ]1 were examined. A comparison of the optically detected SAW transients and numerically calculated stress-strain fields allowed an estimate of the intrinsic mechanical strength without using an artificial precrack. Depending on the geometry, the critical strength varied between 5 and 7 GPa.     

Impulsive fracture of silicon by elastic surface pulses with shocks

During nonlinear evolution of surface acoustic waves (SAWs) stress increases with propagation, and may cause fracture of brittle materials. This effect was used to evaluate the strength of crystalline silicon with respect to impulsive load in the nanosecond time scale without using seed cracks. Short SAW pulses propagating in the [11(macro)2] direction on the Si(111) plane induce fracture at significantly lower SAW amplitudes than the mirror symmetric wave propagating in the [112(macro)] direction. This effect is explained by the differences in elastic nonlinearity of the two propagation directions.     

Measurement of the elastic properties of evaporated C60 films by surface acoustic waves

Surface Acoustic Wave (SAW) pulses were excited in C₆₀ films deposited on quartz and silicon substrates using pulses from excimer lasers with wavelengths of 248 nm and 308 nm for excitation. An optical beam-deflection technique and polymer electret transducers were utilized to detect the propagation of the SAW pulse with high spatial and temporal resolution, allowing an accuracy of better than 0.1% for SAW velocity measurements. With this technique the frequency dependence of the SAW velocity was determined for a number of fullerite films and density, as well as elastic bulk properties of the films were derived by a theoretical analysis of the dispersion effect.     

Nonlinear surface acoustic waves: realization of solitary pulses and fracture

A laser-based technique for the contact-free generation and detection of strongly nonlinear surface acoustic wave (SAW) pulses with amplitudes limited by the materials strength has been developed. The effects of nonlinear propagation of short elastic surface pulses with finite strength in isotropic solids, such as fused quartz, anisotropic solids, such as silicon, and dispersive media were investigated. Solitary surface wave propagation was observed in layered structures for normal and anomalous dispersion. In addition, a SAW-based method for evaluating the critical fracture stress of anisotropic brittle solids, such as single crystal silicon, is introduced.     

Influence of oxygen on the elastic properties of nanocrystalline diamond films studied by laser-induced surface acoustic waves

Laser-induced surface acoustic waves (SAWs) were used to study the influence of oxygen on the elastic properties of nanocrystalline diamond films. A series of samples was grown by chemical vapor deposition (CVD) on the (100) plane of p-type silicon for 3h under different O(2)/(CH(4)+H(2)) flow ratios from 0% to 8%. The elastic properties of these nanocrystalline diamond films were determined from the SAW dispersion curves. The maximum frequency realized was about 310 MHz in wideband SAW experiments and about 700 MHz using a narrowband mask method with several higher harmonics. It was found that the densities of all samples were surprisingly high, approaching the ideal value of diamond, whereas the Young's modulus increased from 700 to 950 GPa with the addition of oxygen. It is concluded that oxygen has a significant positive effect on the elastic properties of nanocrystalline diamond.     

State-of-the-art surface acoustic wave linear motor and its future applications

Two merits of the surface acoustic wave (SAW) device are its high energy density and small size. However, the driving frequency is around 10 MHz or higher. In spite of the difficulties involved with high frequency, the high energy density is attractive for actuator applications. The SAW linear motor's no load speed and maximum output force were 1.1 m/s and 3.5 N using a silicon slider. The silicon slider dimensions were 4 x 4 x 0.3 mm3. We made a lot of 30 microns diameter projections on the silicon surface. The acceleration was 1000 m/s2. The SAW motor is expected to be a high speed, quick response, high resolution microactuator, and much more. High driving voltage was a problem. Our newly designed electrode proved that the driving voltage was reduced to less than 10 V to excite the traveling wave. For actual applications, the SAW device will be placed in a slider. This design is effective in terms of performance and cost. The nanotribology of the SAW motor is also an important and interesting subject.     

Visualization of 50 MHz Surface Acoustic Wave Propagation Using Stroboscopic Phase-Shift Interferometry

A Stroboscopic phase-shift interferometry has been developed to visualize surface acoustic wave (SAW) propagation on SAW devices quantitatively. The developed interferometry is a Fizeau-type one with a multi-mode semiconductor laser diode and an optical isolator. With the laser light illuminating stroboscopically, observed interference intensity gives information about average displacements of the vibrations. Fifty MHz SAW propagation on the surface of the SAW device has been measured with the interferometry. Distribution of the SAW along the propagation path has been observed, whose amplitude is 3 nm (p-p). Repetition accuracy evaluated with the root mean square method is 1/2500 of the laser wavelength. The system is useful for estimating and improving performances of micro-devices.Presented at 1996 International Workshop on Interferometry (IWI ‘96), August 27-29, Saitama, Japan.     

Nonlinear compression of giant surface acoustic wave pulses

The nonlinear propagation of very high-amplitude surface acoustic wave (SAW) pulses in polycrystalline aluminum and copper was studied. A nonlinear compression and an increase of the SAW pulse amplitude have been observed. SAW pulses were numerically simulated with a nonlinear evolution equation including local and nonlocal nonlinear terms.     

Effects of surface roughness on surface acoustic wave propagation in semiconductor materials

The effect of surface roughness on adhesion and tribological properties of films and interfaces is of key importance. Therefore, it is of utmost importance to be able to measure this quantity and to predict the effects that different roughness levels may cause. Roughness affects the propagation of surface acoustic waves on a material but there is little useful quantitative data on the topic. This work investigates the dispersive effect of roughness on surface acoustic wavepackets (30-200 MHz frequency range) for different degrees of nanometer roughness on silicon (0 0 1) and (1 1 1) surfaces, we show that the roughness-induced frequency dispersion effect is significant, and that although available theories agree qualitatively with the results, the theory is not adequate to predict the real SAW dispersion. These experimental results have considerable implications for design of SAW devices, for accuracy of Brillouin spectroscopy measurements, and for possible applications to non-destructive testing of materials. Previously unknown dispersive effects on anisotropic crystal surfaces are also demonstrated.     

Surface-acoustic-wave-induced space-charge waves in electron-hole systems

Space-charge waves in an electron-hole system are studied, which are excited by a moving grating provided by a surface acoustic wave (SAW). The SAW induces a constant current that may change its sign, when a constant electric field is applied opposite to the wave propagation direction. Current resonances are predicted to appear, when the SAW wavelength and frequency match the ones of the space-charge wave.     

Anisotropic effects in surface acoustic wave propagation from a point source in a crystal

Strong anisotropic effects in the propagation of surface acoustic waves (SAWs) from a point-like source are studied experimentally and theoretically. Nanosecond SAW pulses are generated by focused laser pulses and detected with a cw probe laser beam at a large distance from the source compared to the SAW wavelength, which allows us to resolve fine intricate features in SAW wavefronts. In our theoretical model, we represent the laser excitation by a localized impulsive force acting on the sample surface and calculate the far-field surface response of an elastically anisotropic solid to such a force. The model simulates the measured SAW waveforms very well and accounts for all experimentally observed features. Using the data obtained for the (111) and (001) surfaces of GaAs, we describe a variety of effects encountered in the SAW propagation from a point source in crystals. The most interesting phenomenon is the existence of cuspidal structures in SAW wavefronts resulting in multiple SAW arrivals for certain ranges of the observation angle. Cuspidal edges correspond to the phonon focusing directions yielding sharp peaks in the SAW amplitude. A finite SAW wavelength results in internal diffraction whereby the SAW wavefront spreads beyond the group velocity cusps. Degeneration of a SAW into a transverse bulk wave is another strong effect influencing the anisotropy of the SAW amplitude and making whole sections of the SAW wavefront including some phonon focusing directions unobservable in the experiment. The propagation of a leaky SAW mode (pseudo-SAW) is affected by a specific additional effect i.e. anisotropic attenuation. We also demonstrate that many of the discussed features are reproduced in powder patterns, a simple technique developed by us earlier for visualization of SAW amplitude anisotropy.Received: 17 June 2003, Published online: 15 October 2003PACS: 43.35.+d Ultrasonics, quantum acoustics, and physical effects of sound - 68.35.Gy Mechanical properties; surface strains - 62.65.+k Acoustical properties of solidsA.M. Lomonosov: On leave from the General Physics Institute, 117942 Moscow, Russia     

环形定子的激光致表面波机理及可视化探测研究

开展了用于新型激光驱动马达的环形定子的激光致表面波机理及实验研究. 提出一种带有凹槽阵列结构的环形定子新设计, 建立了激光在环形定子表面激发表面波的物理模型, 揭示了影响表面波幅值的关键因素; 采用一种新颖的激光致表面波可视化探测方法, 在波长1053 nm, 脉宽30 ns, 单脉冲能量1 mJ的激光激发下, 对表面波在铜质环形定子表面的传播特性进行了可视化探测实验. 理论与实验研究表明: 当激发光斑的位置紧邻凹槽阵列时, 沿着圆环向凹槽方向传播的表面波会被齿状凹槽阵列迅速衰减和吸收, 而沿着圆环向远离凹槽方向传播的表面波能够持续传递, 从而首次实现了激光致表面波在环形定子上的单向传播; 而对没有凹槽阵列结构的圆环进行的对比实验表明, 激光致表面波在圆环表面双向传播, 最终因相互混叠和串扰等而处于混乱状态. 由激光在该种环形定子表面激发出的单向表面波, 可望在光致表面波马达及驱动机构中获得应用. 关键词： 激光致表面波 环形定子 表面波可视化 激光驱动     

Visualization of surface acoustic wave scattering by dislocations

Stroboscopic X-ray topography at the synchrotron beam line was used to visualize the propagation of a 580 MHz surface acoustic waves (SAW) in LiNbO3 crystals. For this purpose, the X-ray bursts coming from the synchrotron storage ring with periodicity of 5.68 MHz were synchronized with the SAW frequency in a phase-locked mode. This method allowed us to "stop" the SAW in time and to observe the X-ray diffraction contrast caused by the dynamic deformation field of SAW. The X-ray topographic images showed well-resolved individual acoustic wave fronts of 6 microm SAW as well as their distortions due to SAW scattering by linear dislocations. Some of the images revealed an exceptional contrast of the concentric rings about the dislocation line, which is caused by coherent interaction of the secondary elastic waves. This contrast is similar to the Fresnel zones in optics, and this conclusion is confirmed by direct summation of secondary waves emitted by local elements of a vibrating dislocation string.     

Hypersound waves at the surface of CdZnTe crystals: The role of surface orientation and scattering at twin boundaries

The patterns of propagation of gigahertz surface acoustic waves (SAWs) are obtained for cuts of CdZnTe single crystals with different crystallographic orientations. By comparing the experimental patterns with the calculated ones, the existence of at least two SAW modes, one of which is a Rayleigh mode, is demonstrated. It is shown that the anisotropy of propagation of different SAW modes makes it possible to determine the local crystallographic orientation of the CdZnTe surface and detect local imperfections of the crystal structure. Strong anisotropic scattering of the Rayleigh wave by coherent twin boundaries is found.     

Mechanical and elastic properties of amorphous hydrogenated silicon films studied by broadband surface acoustic wave spectroscopy

Mechanical and elastic properties of a-SiH films were measured by broadband Surface Acoustic Wave Spectroscopy (SAWS). In the frequency range achieved, the SAW dispersion curves extend to 300 MHz, which allowed the density, Young's modulus, and Poisson's ratio to be evaluated for films grown by laser CVD or plasma CVD with different hydrogen concentrations. The films deposited by either method have the best mechanical and elastic properties. at a hydrogen concentration of about 10 at. %. For this material, a density of (2300±20) kg/m³ and Young's modulus of (134±5) GPa was determined. The network structures of amorphous silicon are discussed by applying the constraint-counting model to estimate the mean coordination number.     

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.     

Development of human IgE biosensor using Sezawa-mode SAW devices

This paper reports Sezawa-mode surface acoustic wave (SAW) devices with via-isolated cavity to construct the allergy biosensor. To fabricate Sezawa-mode SAW devices, the RF magnetron sputtering method for the growth of piezoelectric ZnO thin films are adopted and influences of the sputtering parameters are investigated. The optimal substrate temperature of 300 °C, RF power of 120 W and sputtering pressure of 2 Pa were used to deposit piezoelectric ZnO films with a smooth surface, uniform grain size and strongly c-axis-orientated crystallization. A back-etched SAW resonator is used in this study. The wet etching of (100)-oriented silicon wafers is used to form a back-side cavity which is critical to the formation of a hopper cavity for holding bio-analytes. The remaining membrane structure silicon thickness was 25 μm. In this report, the chrome (Cr, 12 nm)/gold (Au, 66 nm) layer was initially deposited onto the sensing area of SAW devices as the binding layer for biochemical sensor. The resonance frequency of the Sezawa-mode SAW device is 1.497 GHz. The maximum sensitivity of the Sezawa-mode is calculated to be 4.44 × 10⁶ cm²/g for human immunoglobulin-E (IgE) detection. The stability for human IgE detection is calculated to be 80% and the variation of the stability ±3% was obtained after several tests.     

Anomalous acoustoelectric effect in La0.67Ca0.33MnO3 films

We have studied the acoustoelectric (AE) effect produced by surface acoustic waves (SAW) in a monolithic layered structure, composed of a piezodielectric LiNbO3 substrate and a La0.67Ca0.33MnO3 film. The experiments unexpectedly revealed in the longitudinal AE effect an anomalous contribution, invariant upon reversal of SAW propagation, which coexists with the ordinary (odd in wave vector) effect. The anomalous effect dominates near the metal-insulator transition, while the ordinary effect prevails at high and low temperatures. We show that the anomalous effect is caused by strong modulation of the film conductivity produced by the SAW elastic deformations.     

斐索光纤干涉仪在超声检测中的应用

针对激光超声中固体表面波的传播特性,研究了一套斐索光纤干涉仪系统并实现了对YAG激发的铝材料表面波的检测.该光纤干涉仪基于共光路干涉原理,消除了其他表面波检测手段的诸多弊端,具有环境要求低、结构简单、易于调节、频响高且为非接触式测量等优点,实验表明该干涉仪适合于激光超声检测.