Elastic Properties Of Films Research Articles

Measurement of the elastic and viscoelastic properties of dielectric films used in microelectronics

The Brillouin light scattering technique has been exploited in order to study the elastic properties of dielectric films used in microelectronics, such as silicon oxide, silicon oxinitride and silicon nitride films deposited on (1 0 0)-Si by different techniques. In the case of films as thick as approximately 0.4 μm, a number of Rayleigh–Sezawa modes, polarised in the sagittal plane were detected, as well as Love modes which have a shear horizontal polarisation. In addition, the longitudinal resonance corresponding to a longitudinal wave travelling parallel to the free surface was revealed. In thicker films (0.8–0.9 μm) we could also measure the longitudinal bulk mode, travelling at an angle with respect to the film normal. The mass density of the films was accurately measured by X-ray reflectivity. A best fit procedure of the calculated acoustic phase velocities on the measured ones enabled us to determine the elastic properties of the films and to put in evidence their dependence on the particular deposition technique and on the annealing treatment. The obtained values of the elastic constants were used to reproduce, using finite element calculations, the measured stress vs. temperature, achieving an estimation of both the thermal expansion coefficient and the viscosity of the films.

In-situmeasurement of magnetostrictive coefficient and elastic properties for thin films during growth

A method to determine simultaneously the magnetostrictive coefficient and the elastic properties (Young’s modulus and Poisson ratio) of a thin film during the growth was developed based on minimization of the total elastic energy of a cantilever film-substrate system. Compared to other published measurement methods, an inaccuracy in the magnetostrictive coefficient, caused by assuming the elastic properties of the film as those of the bulk material, could be avoided. Only one elastic isotropic substrate is employed in the present method. The experimental data of an Fe-based amorphous thin film was analyzed using the model. The calculated dependence of the magnetostrictive coefficient on the external magnetic field was compared with the experiment, and the discrepancy between both results is explained. Furthermore, the elastic properties of the film were also obtained.

Surface Brillouin scattering of cubic boron nitride films

Surface Brillouin scattering has been used to determine the elastic properties of thin hard submicron cubic boron nitride (cBN) films grown on silicon by mass selected ion beam deposition. The elastic properties of the films have been determined by fitting experimental data to theoretical dispersion curves. A Green’s function method was used to predict Brillouin scattering spectra of the acoustic excitation at the free surface. Our results demonstrate that the effect of the thin hexagonal boron nitride interlayer located between cBN film and the Si substrate on the velocity of the surface acoustic wave does not exceed 2% for a thin (16 nm) film and is negligible for cBN films thicker than 100 nm. The elastic properties of the cBN films are not softer than those of bulk cBN.

Characterization of thin film elastic properties using X-ray diffraction and mechanical methods: application to polycrystalline stainless steel

The Young's modulus and Poisson's ratio of reduced thickness layers are generally unknown whereas simulation of mechanical behavior of thin film/substrate systems or stress determination by X-ray diffraction cannot be done in an accurate way without the knowledge of these values. In this paper, we present three types of experiments which are used in our laboratory for determining elastic constants in polycrystalline thin films elaborated by ion beam sputtering: A vibrating reed device, X-ray tensile testing and AFM buckling geometry analysis. Results obtained for metallic 304L stainless steel thin films are given.

Effect of nitrogen addition on the elastic and structural properties of amorphous carbon thin films

Nitrogen-doped amorphous hydrogenated hard carbon films were deposited by r.f. glow discharge from methane–nitrogen mixtures onto silicon substrates. The elastic properties of films obtained for different N 2 partial pressures were investigated by Brillouin light scattering, while Raman spectroscopy, X-ray photoelectron spectroscopy and X-ray reflectivity were exploited to study their structural and vibrational properties. Addition of a low percentage of nitrogen in the films induces marked modifications of the microscopic film structure, which induces a clear decrease of the elastic constants.

Laser-ultrasonic measurements of thin-film properties using surface acoustic waves

High-frequency, laser-ultrasonic methods to evaluate the mechanical properties of thin films have been developed. The approach is based on the optical generation and detection of surface acoustic waves (SAWs) to determine the frequency dependence of the phase velocity (dispersion relation). Broadband or variable-frequency, quasi-plane wave SAWs are generated by a line-focused, 200-ps pulsed laser. The out-of-plane SAW displacement amplitudes are measured using a path-stablized Michelson interferometer with line-focus detection. The apparatus incorporates differential photodiode detection with a −3-dB bandwidth of 40 kHz–800 MHz. With this system, several displacement waveforms over a range of propagation distances are acquired and used to determine the SAW dispersion. Dispersion relations up to 200 MHz are presented for a variety of specimens, including a series of Si wafers with TiN films 0.2–1.3 micrometers thick and containing residual stress. The results illustrate the applicability of this technique to various conditions such as stiffening and loading films and isotropic and anisotropic substrates. The experimental dispersion curves obtained with this method are compared to theoretical predictions to evaluate the film elastic properties.

X-ray diffraction study of thin film elastic properties in relation with their microstructure

X-ray diffraction study of thin film elastic properties in relation with their microstructure

Determination of elastic properties of thin films by indentation measurements with a spherical indenter

Indentation is an important method for the determination of mechanical properties of surfaces and thin films. It is well known that the measurement results from thin layers are strongly influenced by the substrate properties. For hardness measurements it is frequently quoted that the indentation depth should be less than one-tenth of the film thickness (1/10th rule). This rule is often not practicable for thickness values below 1 μm. Therefore a correction method is required that allows the separation of substrate and film properties from the load-depth data. Moreover, the calculation is complicated if plastic deformation occurs. The use of a spherical indenter allows one to remain completely within the elastic range if the indenter radius is large enough and the load is low enough. In this case a novel analytical solution for the elastic deformation of a film on a flat substrate can be used to simulate the load-depth data. With this solution the determination of Young’s modulus of thin layers is possible independent of indentation depth and film thickness. Measurement data from a UMIS-2000 indentation system for different film substrate combinations are compared with theoretical results. It is shown, that a separation of the elastic film properties is possible. For metal films on Si the load-depth data did not differ from that of uncoated substrates. This can be explained mainly by delamination.

Cross-sectional nanoindentation: a new technique for thin film interfacial adhesion characterization

Interfacial adhesion is becoming a critical material property for improving the reliability of multilayer thin film structures used in microelectronics. Cross-sectional nanoindentation (CSN) is a new mechanical test especially designed for measuring the fracture toughness of thin film interfaces. Interfacial fracture is achieved by nanoindentation in the structure cross-section. A model based on the elastic plate theory has been developed to calculate numerically the interfacial critical energy release rate ( G ci) for ceramic–ceramic systems from CSN test results. The model inputs are the thin film elastic properties, thin film thickness, interfacial crack area and maximum thin film deflection during the test. Closed form analytical solutions, obtained for two limiting cases, are consistent with the numerical approach. This technique has been successfully applied to silicon nitride–silicon oxide thin films, commonly used as electrical isolators in microelectronic devices.

A comparative study of composition, structure and elastic properties of boron nitride films deposited by magnetron and ion beam sputtering

Abstract Sputtering from hexagonal BN targets, using a conventional magnetron (MS) or ion beam (IBS), produces films consisting of both sp 2 - and sp 3 -bonded BN and BN x . We present here the dependence of BN film composition, structure, morphology and elastic properties on the bias voltage ( V b ) applied on the substrate in the case of MS, compared with those of IBS-deposited BN films. The optical properties and the composition of the BN films were examined by spectroscopic ellipsometry (SE), whereas the chemical bonding was identified by Fourier Transform IR SE. The films' structure, morphology and density were also examined by X-ray diffraction and reflectivity and transmission electron microscopy. The elastic properties of the films were studied by nanoindentation techniques. Significant differences were found in composition, chemical bonding and structure of films grown by MS at various bias voltages.

Investigation of thin film shear modulus from spatially resolved measurement of Love wave velocity

The investigation of transversely polarized surface acoustic waves (SAWs) is strongly hindered by their missing out-of-plane oscillation component because commonly applied measurement techniques are based on access to that component. Nevertheless, these waves are very interesting for accurate extraction of thin film elastic properties as well as for SAW devices and sensor applications. In this contribution we report a new development of spatially resolved SAW analysis. The scanning acoustic force microscope (SAFM) has been extended in its sensitivity to in-plane polarized waves. A nonlinear mechanism could be found that couples the in-plane surface oscillations to torsional cantilever vibrations. This mechanism is used to transfer the rf SAW signals to kHz frequencies and to measure the SAW phase velocity. The new technique has been applied to study the Love wave propagation in the layered system fused silica on ST-cut quartz. The phase velocity could be derived from measurements of the phase shift along a propagation distance in the micron range. The obtained velocities show a clear spatial variation at elastic changes within the investigated surface. This is used to investigate the shear modulus of the layer material by numerically solving the inverse problem of SAW propagation.

Influence of stress in thin film modulus measurements by the vibrating reed technique

One method to measure thin film elastic properties is use of the vibrating reed technique to compare stiffness of uncoated and coated cantilever beams. Since the change of frequency is used to calculate the modulus, the change of frequency caused by curvature due to film stress has to be investigated. We present simultaneous measurement of resonant frequencies and curvature to show that curvature can cause large effects, depending on the geometry. An approximate calculation indicates in which cases stress may be neglected. A method to determine stress from combination of torsional and flexural vibrating frequency measurements is proposed.

Elastic properties of silicon dioxide films deposited by chemical vapour deposition from tetraethylorthosilicate

Dielectric silicate glass films have been obtained from tetraethylorthosilicate using both low-pressure chemical vapour deposition and plasma-enhanced chemical vapour deposition. The elastic properties of these films, which are deposited on (100)-Si wafers, have been studied by means of the Brillouin light scattering technique. The phase velocity of both the surface Rayleigh wave and the longitudinal wave in the film material have been measured and the two independent elastic constants c 11 and c 44 evaluated. This permitted us to derive the values of Young's modulus and Poisson's ratio which are useful quantities for the modelling of the elastic properties of multilevel structures used in electronics. Moreover, the substrate curvature method has been exploited in the case of the PECVD film, to study the evolution of the stress during subsequent thermal cycles. The values of the thermal expansion coefficient and of the water diffusion coefficient have thus been determined.

Characterization of the elastic properties of amorphous silicon carbide thin films by acoustic microscopy

A 590 MHz high frequency acoustic microscope has been used as a non-destructive local probe to measure the elastic properties of amorphous silicon carbide thin films supported as coatings on silicon substrates and as free-standing membranes. The films were amorphous hydrogenated and hydrogen free silicon carbide a-Si 0.5C 0.5(H) thin layers deposited by using two deposition techniques, namely plasma enhanced chemical vapor deposition (PECVD) and laser ablation deposition (LAD). Surface acoustic images showed a good homogeneity of the elastic properties of the films. Experimental Lamb mode velocities were deduced from the fast Fourier transform (FFT) treatment of the local acoustic response, V( z), and compared with values obtained from calculated models taking into account the different propagation modes of acoustic waves. The values of the longitudinal, V L, and shear, V T, velocities were determined to be, respectively, 9462 and 5853 m/s for a-Si 0.5C 0.5:H PECVD films and 10340 and 6397 m/s for a-Si 0.5C 0.5 LAD films. Elastic parameters such as Young modulus, E, and Poisson ratio ν were deduced from these velocity values. In good agreement with the results obtained with other techniques (bulge and indentation measurements), the Young modulus was shown to increase with the Si–C bond density of the a-SiC films with values of 196 GPa and 273 GPa for PECVD and LAD films, respectively. The Poisson ratio was found to be independent of the microstructure of the SiC films with a constant value of 0.19.

Emulsion copolymerization latices for interior and exterior coatings

Details the preparation of emulsion copolymerization of styrene (St)‐vinylacetate (VAc) with different molar ratio 33:67(A1), 50:50(A2) and 67:33(A3) using redox initiator system (k2S208/NaHS03) and investigates their film forming. Finds that the drying film characterization took on a brittle property which is attributed to the nature of styrene polymers. Explains that molar ratio 67:33 from butyl acrylate (BuA‐VAc) and vinyl acetate (B1, B2 and B3) in the presence of pluronic F87 was chosen as the coemulsifier as it gives good transparent and elastic film properties but has a tacky character owing to the nature of butyl acrylate polymer. Shows that styrene‐butylacrylate (St‐BuA) 67:33 molar ratio(C) has high conversion and a solid content of 51 per cent, and its film forming gives a transparent sheet that dries within 45 minutes. Notes that incorporation of these latices into interior and exterior coatings gives good characterization (hardness, ductility, adhesion and washability of 4,042 cycles) compared with the standard specifications for evaluation of latex paint.

Amorphous semiconductors: deposition chemistry and bonding

The deposition chemistry and bonding of the hydrogenated amorphous materials a-C:H, a-Si:H and a-Ge:H are discussed. The general principles governing the gas phase reactions and surface chemistry during deposition are outlined. Data obtained on single-crystal surfaces can be used to extract information on the important molecular processes such as hydrogen desorption and the non-equilibrium nature of chemical vapour deposition. The dependence of the electronic and mechanical film properties on the molecular structure is considered. On the basis of the constraint-counting model, the average coordination number is estimated from elastic film properties by assuming a random network. The central role played by hydrogen during film growth and in modifying the properties of the amorphous film is discussed in detail. Hydrogen is considered as one of the key species in optimizing the deposition chemistry and material properties.

Elastic and nanostructural properties of Cu/Pd superlattices

A series of Cu/Pd superlattices with composition modulation wavelengths (Λ's) ranging from 1.6 to 3.5 nm and a strong [111] growth texture were prepared by electron beam evaporation. The elastic properties of the films were examined using the methods of uniaxial tension tests [a Young's modulus (1/s11), where sij is an elastic compliance] with the applied load parallel to the plane of the Cu/Pd interface and Brillouin scattering [a shear modulus (1/s44) with the shear waves parallel to the plane of the Cu/Pd interface]. Also, the films were characterized using both x-ray diffraction and high-resolution electron microscopy; this was done to assess the effect of the nanostructure on a possible “supermodulus effect.” The films are nanostructurally very similar to the superlattice films employed in previous studies at Northwestern in which a supermodulus effect was reported. But, contrary to previous studies, no anomalous behavior was observed for the measured elastic properties of the thin films. Therefore the present results negate the earlier results and cast a serious doubt on the existence of a supermodulus effect.

Effect of Initial Substrate Curvature on Nonlinear Bending Measurements of Thin-Film Stress.

ABSTRACTA recently reported nonlinear model of the bending of a thin-film/substrate bilayer provides a means for determining stress in thin films even for large deflection and ellipsoidal bending. This model replaces the usual Stoney's equation, which is valid only for small deflections. However, the model omits consideration of the commonly observed initial curvature of the substrate before deposition. In the small deflection regime the principle of superposition justifies simply subtracting the initial curvature from the final curvature after deposition, but for large deflections this is inappropriate, because the principle of superposition is no longer valid. The present paper presents a modified form of the nonlinear model incorporating initial substrate bending. The resulting equations show that initial substrate curvature causes magnified nonlinear effects and provide a means for determining film and substrate elastic properties in addition to thin-film stress.

The effect of substrate on the elastic properties of films determined by the indentation test — axisymmetric boussinesq problem

T he elastic solution of an axisymmetric mixed boundary value problem is considered. An elastic layer is assumed to be either in frictionless contact or perfectly bonded to a semi-infinite elastic half-space. The elastic field caused by the indentation of the elastic layer, by a rigid indenter is solved for spherical, conical and flat-ended cylindrical indenters. The results are obtained by solving a Fredholm integral equation of the second kind with a continuous symmetrical kernel which depends on the bonding conditions. Numerical results are given for several combinations of film and substrate elastic moduli and film thicknesses. These results provide a guideline for selecting the appropriate film thickness and substrate in order to determine the elastic constants of thin films.

Investigation of elastic properties of ZnO films by Brillouin scattering

The elastic properties of ZnO films have been investigated by means of the Brillouin scattering technique. Measurements were made of the phase velocity of surface phonons propagating on ZnO films ≊0.2 μm thick, deposited on both Si and Al2O3 substrates. A comparison of the experimental results with theoretical predictions shows that the elastic properties of the films, although appreciably different from those of the bulk material, are fairly close, however, to the properties observed in films almost two orders of magnitude thicker.