Laser-induced Surface Acoustic Waves Research Articles

Simulation on laser-induced surface acoustic wave on isotropic cylinders by finite element method

Based on the thermoelastic theory, a finite element model is developed to simulate the process of laser inducing ultrasonic field in isotropic cylinders, which can take the temperature dependence of thermal parameters into account. Using the finite element model, we have simulated the ultrasonic fields induced by a pulse laser line source impacting on the generatrix of aluminum cylinders with different diameters. And the intact waveforms of surface acoustic wave (SAW including cylindrical Rayleigh and Whispering gallery (WG) modes) are presented, which are in very good agreement with the calculated and experimental waveforms in other literatures. Furthermore, the dispersion properties of cylindrical Rayleigh waves are analyzed by the method of phase spectral analysis, and the results show that with the increasing frequency, the phase velocity of cylindrical Rayleigh wave rapidly increases to the maximum value, and then gradually decreases to that of plane Rayleigh wave. With the diameter of cylinder decreasing, the maximum value of phase velocity and the corresponding frequency increase.

Mechanical properties of nanocrystalline diamond films

Nanocrystalline diamond (NCD) films with thicknesses in the range of 0.12–1.5μm were deposited on silicon substrates in CH4∕H2∕O2 gas mixtures by microwave plasma-enhanced chemical vapor deposition. The morphology and structure of these NCD films were analyzed by field-emission scanning electron microscopy, x-ray diffraction (XRD), and ultraviolet-Raman spectroscopy. The lower limit of the grain size in the NCD films was estimated to be 10nm from the XRD measurements. These grains are embedded in a columnar-type structure. The elastic and mechanical properties of the NCD films were determined by measuring the dispersion of laser-induced surface acoustic waves. The densities were in the range of 3.41±0.11g∕cm3 and Young’s moduli varied between 674±34 and 953±48GPa, depending on the growth time and deposition conditions. It is concluded that oxygen may have a significant positive effect on the elastic properties of NCD films. The growth rate decreases sharply for an oxygen content in the source gas in excess of about 4%.

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 (1 0 0) plane of p-type silicon for 3 h 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.

Young’s modulus of silicon nitride used in scanning force microscope cantilevers

The Young’s modulus and Poisson’s ratio of high-quality silicon nitride films with 800 nm thickness, grown on silicon substrates by low-pressure chemical vapor deposition, were determined by measuring the dispersion of laser-induced surface acoustic waves. The Young’s modulus was also measured by mechanical tuning of commercially available silicon nitride cantilevers, manufactured from the same material, using the tapping mode of a scanning force microscope. For this experiment, an expression for the oscillation frequencies of two-media beam systems is derived. Both methods yield a Young’s modulus of 280–290 GPa for amorphous silicon nitride, which is substantially higher than previously reported (E=146 GPa). For Poisson’s ratio, a value of ν=0.20 was obtained. These values are relevant for the determination of the spring constant of the cantilever and the effective tip–sample stiffness.

Measurement of Electroplated Copper Overburden for Advanced Process Development and Control

AbstractWe used a non-contact metrology system based on laser-induced surface acoustic waves to evaluate the thickness uniformity of electroplated copper. Several wafers were prepared using different plating conditions that affect the copper overburden thickness and uniformity. Measurements of overburden were made on a variety of pattern features including trench arrays of several pattern densities, solid bond pads, and the surrounding field area. In general, the overburden was found to be thicker above the arrays of narrow linewidth, as compared to the surrounding field. However, the magnitude of this “mounding” effect is dependent on the plating process, pattern density, and position on the wafer. For example, as the accelerator in the plating solution increases, the mounding on a 50% density line array varies from zero to 5000 Angstroms. Under all plating conditions used, the mounding was found to be progressively higher as the pattern density of the array increases (in the range of 20% to 50% copper). The solid pads were found to have conformal filling, in all cases. It was also found that the overburden in the field area is reduced as the adjacent mounding increases, presumably because the reactant in the plating bath becomes depleted in the vicinity of the high overburden regions above the arrays.

Correlation of surface, mechanical and microproperties of tetrahedral amorphous carbon films deposited under different magnetic confinement conditions

The effect of using a magnetic field to confine and focus the carbon plasma in a filtered cathodic vacuum arc (FCVA) deposition system was investigated in the preparation of tetrahedrally bonded amorphous carbon (ta-C) thin films. The design of the magnetic field was such that the plasma can be confined into a high-density focussed spot of ∼2 cm diameter or de-focussed into a wide beam of ∼10 cm diameter. The microstructural, optical, tribological and surface properties of the ta-C films grown in the high-density magnetic field were subsequently studied in detail. Under a high-density magnetic field, ta-C thin films were deposited on Si and quartz substrates. Laser-induced surface acoustic wave (L-SAW) measurements confirmed an increase in the Young’s Modulus and the bulk density of these films as compared to ta-C films deposited under no or low magnetic field [Phys. Rev. B 48 (1993) 4777; J. Appl. Phys. 79 (1996) 7239]. XPS results showed a high >85% sp 3 content on the surface while combined EELS and Raman measurements showed high and constant sp 3 content of >85% in bulk of all deposited films. An increase in optical bandgap was observed, from 3.6 to 3.9 eV as the density of the plasma (and hence, the films) was increased. In addition, the overall surface free energy was also observed to decrease from 44 to 40 dyn/cm. This confirms that under a high external magnetic field, the carbon plasma is confined and focussed, and is thus able to deposit highly densified ta-C thin films with high optical bandgap and low sp 2 defect density. Tribological measurements showed that by comparing the high-density ta-C films with lower density ta-C films, the former have a better adhesion to the substrate, as well as a better coefficient of friction and wear rate.

Theoretical study on line source laser-induced surface acoustic waves in two-layer structure in ablative regime

The generation of ultrasound in film–substrate system by a laser line source is studied in the case of ablation mechanism, which can be realized by adding a liquid layer at the excitation point. The time domain displacement can be yielded by the numerical jointed inversed Laplace–Fourier transformation technique. The typical surface acoustic waves (SAW) of two layer structures, slow film on fast substrate and fast film on slow substrate, are obtained and the effect of the propagation distance and the thickness of the film on the SAW are given.

Detection of Voids in Tungsten Interconnect Vias Using Laser-Induced Surface Acoustic Waves

AbstractTest wafers were prepared under different process conditions of barrier and seed film deposition, in order to study the effects on tungsten plug filling. Tungsten fill percentage was measured using laser-induced surface acoustic wave metrology on plug arrays of varying plug diameter and the results were verified by SEM cross-section. The data show that, for via diameters of 0.12 micron or less, the seed deposition process is a critical factor in effective (void free) plug filling. Seed deposition by ALD and low-temperature CVD were found to be far more effective than conventional CVD. Changing the thickness of the TiN barrier layer was found to have minimal effect on the occurrence of voids. For via diameters greater than 0.12 microns, all of the seed processes led to relatively little voiding.

レーザー励起広帯域表面波を用いた微小き裂サイジング手法

A new method to estimate crack depth based on laser-induced surface acoustic wave (SAW) is proposed. Laser ultrasonic technique that uses laser beams for generation and detection of ultrasonic waves allows remote and non-contacting inspection. Moreover, since laser-induced SAW has a wide frequency band, it also offers crack-depth measurement using a frequency analysis. To confirm the performance, some feasibility tests using artificial slits were performed. Simulated surface cracks were machined on the type 304 stainless steel plates; each of them had a width of about 100μm and a depth of from 0.2mm to 1.5mm. The location and the depth of the slits were estimated from the detected waves by both the pulse-echo measurement and a frequency analysis. Results of the frequency analysis show that a surface slit behaves as the low pass filter to the laser-induced SAW and the step response time of the transfer function has linear relationship with the slit-depth. Consequently, it is possible to detect small cracking having 0.2mm depth and good agreement can be achieved between measured and nominal depths.

Laser-induced surface acoustic waves for evaluation of elastic stiffness of plasma sprayed materials

The elastic properties of plasma sprayed deposits have been evaluated using a laser-excited surface acoustic wave (SAW) technique and an inversion processing analysis. The SAWs including Lamb and Rayleigh waves were generated in plasma sprayed NiCoCrAlY and ZrO2, respectively, and their group velocity dispersions were used to determine the elastic properties (i.e.Young's modulus, Poison's ratio and density) of the deposits. Estimated elastic moduli from the velocity dispersions of A0-mode Lamb waves are in the range of 40–140 GPa for the deposits, which are much lower than the values 220–240 GPa of the comparable dense materials. The dramatic reductions in modulus and density of ZrO2 deposit have been attributed to the presence of high porosity and particularly microcracks. Moreover, this study has emphasized on exploiting the applicability of each kind of the SAWs for the elastic property evaluation of different sprayed materials. Both Lamb and Rayleigh wave dispersions are useful for the estimation of APS and VPS-deposited NiCrAlY, but S0-Lamb and Rayleigh waves are exceptional for that of sprayed ZrO2, because of its characterization of high acoustic attenuation and inconsequent displacement across the weak bonded interface of ZrO2 and substrate.

A superhard diamond-like carbon film

A superhard hydrogen-free amorphous diamond-like carbon (DLC) film was deposited by pulsed arc discharge using a carbon source accelerator in a vacuum of 2×10−4 Pa. The growth rate was about 15 nm/min and the optimum ion-plasma energy was about 70 eV. The impact of doping elements (Cu, Zr, Ti, Al, F(Cl), N) on the characteristics of DLC films deposited on metal and silicon substrates was studied aiming at the choice of the optimum coating for low friction couples. The microhardness of thick (≥20 μm) DLC films was studied by Knoop and Vickers indentations, medium thick DLC films (1–3 μm) were investigated using a ‘Fischerscope’, and Young's module of thin films (20–70 nm) was studied by laser induced surface acoustic waves. The bonds in DLC films were investigated by electron energy loss spectroscopy (EELS), X-ray excited Auger electron spectroscopy (XAES), and X-ray photoelectron spectroscopy (XPS). The adhesion of DLC films was defined by the scratch test and Rockwell indentation. The coefficient of friction of the Patinor DLC film was measured by a rubbing cylinders test and by a pin-on-disk test in laboratory air at about 20% humidity and room temperature. The microhardness of the Patinor DLC film was up to 100 GPa and the density of the film was 3.43–3.65 g/cm3. The specific wear rate of the Patinor DLC film is comparable to that of other carbon films.

Elastic constants of ultrathin diamond-like carbon films

The determination of all the elastic constants of a supported film of thickness smaller than 1 mm is an unsolved problem. Various experimental procedures can be found in the literature, such as nano-indentation and laser spectroscopy methods based on the propagation of surface acoustic waves (SAWs), namely Surface Brillouin Scattering (SBS) and laser-induced SAWs. For the first time, the elastic constants of isotropic diamond-like films (ta-C and ta-C:H ) have been determined by SBS from a modified Rayleigh wave and longitudinal guided mode. In SBS, the nano-mechanical response of the film is spontaneously probed by surface acoustic phonons scattering the light beam. A new fitting procedure gives a precise evaluation of the elastic constants even in the extremely diYcult case of hard on soft structures. In contrast to nano-indentation, the Young’s modulus, E ,i s determined directly. This provides a useful validation of the diverse approaches adopted to analyse nano-indentation data. © 2000 Elsevier Science S.A. All rights reserved.

Non-destructive testing of damage layers in GaAs wafers by surface acoustic waves

Slicing of GaAs wafers from ingots causes a damage layer which must be completely removed by polishing. For this purpose a non-destructive control of the machining process is very desirable. Laser-induced surface acoustic waves are demonstrated to be a promising method to characterize the state of the surface. The dispersion of this wave mode has been used as a measure of the thickness of the damage layer. The investigations were performed on wafers which were stepwise polished to remove the damage layer. In this way the damage layer was found to have a thickness of about . This result is in good agreement with investigations performed with positron annihilation. The damage layer was found to have a considerably lower elastic modulus (by up to about 18%) than the bulk material of GaAs. With increasing polishing depth the modulus of the damage layer approaches that of the bulk material. The elastic modulus allows us to estimate the defect volume in the damage layer by applying a theory of heterogeneous materials. A defect volume fraction of 0.48% was estimated for the as-sawn state of the wafer. It decreases to 0.05% for a polishing depth of and goes to zero at a depth of .

Observation of wavelength-dependent generation efficiency of laser-induced ultrasonic surface acoustic waves on ceramic materials

In this work, the efficiency of laser generation of surface acoustic waves (SAWs) in ceramic materials is reported to be wavelength dependent. A pulsed Nd:YAG laser operating at wavelengths of 1064 or 532 nm was used to generate SAWs on ceramic (silicon nitride and silicon carbide) and metal substrates. It was observed that 1064 nm radiation is more efficient than 532 nm radiation for SAW generation on ceramics, whereas the opposite is the case for metals. While the wavelength dependence of SAW generation efficiency in metals is due to stronger optical absorption at the shorter wavelength, the dependence for the ceramics is attributed to a longer absorption length at the longer wavelength.

Real-time detection of laser-induced transient gratings and surface acoustic wave pulses with a Michelson interferometer

Laser-induced transient gratings (LITGs) at surfaces of absorbing materials were utilized to generate narrowband surface acoustic waves (SAWs). In these experiments, SAWs were excited thermoelastically by two crossed picosecond laser pulses and detected with an actively stabilized Michelson interferometer by measuring transient surface displacements in the sub-angstrom range in real time. In addition, coherent broadband SAW pulses with frequencies up to 350 MHz were excited by sharply focusing the laser beam with a cylindrical lens system onto the sample surface. The LITG experiments provide an extension of the frequency range achieved with the broadband SAW pulse technique. From the measurements of the dispersive SAW phase velocity for a 650 nm aluminum film on fused silica in the frequency range 10 MHz–1 GHz the density and elastic constants were determined by fitting the experimental data to the exact solution of the wave equations taking into account the boundary conditions.

Controlling the quality of thin films by surface acoustic waves

Abstract Techniques based on laser-induced surface acoustic waves have been developed with the potential for characterizing thin films. They enable variations of elastic parameters, density and film thickness to be detected. Previous work has demonstrated that from the measurement of the frequency dependence of the surface wave velocity, film parameters may be solved by using an inverse dispersion relation. In the present paper, a method is described that can be extended to the use of surface acoustic waves to the control of film quality in an on-line process. The method exploits the phenomenon that surface waves in coated materials show dispersion, giving rise to a characteristic deformation of wide-band surface wave impulses. Having traversed a fixed measuring distance on the test piece sample, an impulse signal is detected and compared with a reference signal from a standard sample. Signal processing yields two test functions: a cross-correlation of the test signal with a reference signal and the envelope of this cross-correlation function. If the film quality of the test sample agrees with that from the reference sample, the cross-correlation function is symmetric and its maximum coincides with the maximum of the envelope. Deviations from the required quality causes both test functions to shift against each other and gives rise to non-symmetric shape of the cross-correlation. The shifts and the deformation of the test functions are shown to contain information about the variations of film quality. As an example, results are presented for the case of TiN-coated steel samples having different film thickness. The sensitivity of the method is discussed and found to be dependent on signal-to-noise ratio (SNR) and bandwidth of the measurement. Tests were performed with two different types of laser-ultrasound instrumentation. They both used a pulse laser to generate the surface wave impulses. In the first type of instrumentation, ultrasonic impulses were detected with a piezoelectric transducer. It showed good signal-to-noise ratios (SNR) and high-frequency (60 MHz) bandwidth, but required mechanical coupling to the surface. The second type of instrumentation used a Fabry-Perot interferometer, which enabled a complete non-contact measurement to be performed up to a 15 MHz frequency bandwidth. The principle of the measurement method was again confirmed using the interferometer.

Non-destructive evaluation of diamond and diamond-like carbon films by laser induced surface acoustic waves

Young's modulus of diamond-like carbon films varies in a wide range which suggests the use of this material parameter to characterize the film quality. It can be obtained non-destructively by means of surface acoustic waves. A reliable and quick photoacoustic technique has been developed which enables the film modulus to be determined for rather small samples such as cutting tools. The technique is based on the measurement of the surface wave velocity vs. frequency. This spectrum is derived from laser induced surface wave impulses by Fourier transformation. Comparison with the theoretical results may provide Young's modulus, density, and film thickness simultaneously. Less information is obtained for very thin films so that less than three parameters can be derived. By taking into account an empirical relation between Young's modulus and density which has been found for diamond-like carbon films, two parameters can be obtained even for films about 100 nm thick. The informational content of the measurement depends on the bandwidth of the equipment and on the substrate material. Diamond and diamond-like carbon films were studied. They were deposited using different technologies onto silicon single crystals, steel, and WC-Co cemented carbide. The film thickness was in the range 60 nm < d < 6 m.

Application of Laser-Induced GHz Surface Acoustic Waves to Evaluate Ion-Implanted Semiconductors

The transient reflecting grating method has been used to characterize ion-implanted silicon layers and demonstrate its usefulness for nondestructive and remote evaluation of modified solid surfaces. The surface acoustic velocity, relaxation constant and signal intensity were measured as functions of ion dose. The results suggested that damage induced by implantation significantly affected the surface properties even under light dose conditions. The subnanosecond temporal resolution of the present method provided successful characterization of the implanted layers.

Laser-induced surface acoustic waves and photothermal surface gratings generated by crossing two pulsed laser beams

Gigahertz surface acoustic waves have been optically generated and detected on aluminum metallic films and silicon wafers using the laser-induced grating technique. The acoustic velocities were in good agreement with those of Rayleigh waves. Photothermal surface gratings could also be detected as a nonpropagating component of the thermoelastic waves. The relaxation time of the grating was related to the thermal diffusivity of the material. A simplified theory for the generation of surface transient gratings containing acoustic waves is presented in addition to the experimental results.

Surface acoustic wave interferometer — A high sensitivity detector for phase-modulated photoacoustics

A surface acoustic wave interferometer has proved to be a high sensitivity detector of laser induced surface acoustic wave phase shifts. It has been shown that the effect of the surface thermoelastic deformations must not be ignored at low thermal-modulation frequencies.