Surface Acoustic Wave Spectroscopy Research Articles

Surface acoustic wave spectroscopy for non‐destructive coating and bulk characterization at temperatures up to 600°C enabled by piezoelectric aluminum nitride coated sensor

Surface acoustic wave spectroscopy has been established as non‐destructive and fast method for characterization of mechanical properties of surfaces and bulk materials in both research and industry. The present work shows that by application of a novel and robust aluminum nitride (AlN) coated piezoelectric contact sensor the advantages of the method can be extended from room temperature to at least 600°C. An overview of sensor concepts and applications of the method is discussed first, followed by theoretical and practical considerations for design and coating of a novel temperature stable contact sensor. After fabrication of such a sensor using magnetron sputtering, it was tested in a modified surface acoustic wave spectroscopy setup with an incorporated heating table concerning signal amplitude and frequency range. The AlN coated sensor was found to perform well up to 600°C, with temperature limited by the specification of the heating table. At room temperature, performance was acceptable when compared with a conventional contact sensor using a PVDF piezoelectric foil. Application of the high temperature capabilities of the setup was demonstrated by measuring temperature stability of hydrogen‐free amorphous carbon coatings (a‐C and ta‐C) depending on their sp3 carbon ratio. In another example, high precision temperature dependent measurement of Young's modulus for ultrasonic fatigue test specimen was taken, achieving an accuracy better than 1%. Use of the developed sensor opens up new possibilities in material science for in situ study of temperature depending mechanical properties for coatings and surfaces.

Benzene bridged hybrid organosilicate films with improved stiffness and small pore size

Critical properties of porous periodic mesoporous silica (PMO) low-k dielectric with a different ratio of benzene bridges and methyl terminal groups are studied by using various advanced instrumentations such as Ellipsometric Porosimetry (EP), Surface Acoustic Wave Spectroscopy (SAWS), Specular X-ray reflectivity and others. It is shown that the pore size and surface roughness of the films decrease with an increase in the concentration of the benzene bridge, although at a concentration of benzene bridges >25 mol %, the pore size sharply decreases and changes little with a further increase in the concentration of bridges. The concentration of the benzene bridge also increases dielectric constant and improves the mechanical properties. The increase in Young's modulus with the benzene bridge concentration has percolation type behavior and sharply increases at a concentration of the benzene bridge close to 50 mol %. It has also been found that introducing 30 wt % porosity into benzene-bridged films without methyl terminal groups increases the Young's modulus. It is assumed that this unusual behavior is associated with the formation of crystal-like structure on the film framework. The increase of dielectric constant is associated with the higher polarizability of the benzene bridges in comparison with methyl terminal groups, as well as with their higher hydrophilicity and presence of adsorbed water.

Evaluation methods of mechanical properties for low-k dielectrics

This review introduces the study of state-of-art methods for assessing the mechanical properties of insulating materials with low dielectric constant. The main features of measuring Young’s modulus of thin films insulating materials with low dielectric constant are determined by usage of Brillouin light scattering, surface acoustic wave spectroscopy, picosecond laser-acoustic method, ellipsometric porosimetry, nanoindentation and atomic force microscopy in various modes. The author estimated the optimum lateral and optimum depth resolution for each above method. The review analyzes the degree of sample preparation complexity for the measurements by these methods and describes what methods of measurement are destructive for the samples. Besides, the review makes a comparison for the results of evaluating Young’s modulus of insulating materials with low dielectric constant achieved by different methods. Comparative analysis of the methods for assessing mechanical properties lead us to the conclusion that the method of atomic force microscopy is superior to other methods described above, both in lateral (8 nm) and optimum depth (10 nm) resolution. It is shown that due to the small impact force of the atomic force microscope probe on the surface, the method does not have a destructive effect on the sample. In addition, there is no need to create special conditions for the experiment (e.g., the cleanliness level of the premises, the possibility of an experiment under environmental conditions, etc.). This makes the experiment relatively simple in terms of preparing the object of research. It has been also established that the method of atomic force microscopy in the mode of quantitative nanomechanical mapping allows forming a map of the distribution of the Young’s modulus of the insulating material as part of the metallization system of integrated circuits.

Evaluation of Mechanical Properties of Porous OSG Films by PFQNM AFM and Benchmarking with Traditional Instrumentation.

Characterization of mechanical properties of thin porous films with nanoscale resolution remains a challenge for instrumentation science. In this work, atomic force microscopy (AFM) in the PeakForce quantitative nanomechanical mapping (PFQNM) mode is used for Young's modulus measurements of porous organosilicate glass films. The test samples were prepared by sol-gel techniques using silicon alkoxide and methyl-modified silicon alkoxide to prepare films with different CH3/Si ratios. The film porosity was engineered by using a Brij 30 template and the evaporation-induced self-assembly technique. The chemical composition, pore structure, and modification during air storage and thermal annealing were studied using FTIR spectroscopy and ellipsometric porosimetry (EP). Since PFQNM AFM was first used for evaluation of Young's modulus of thin porous films, the obtained results are benchmarked using nanoindentation (NI), surface acoustic wave (SAW) spectroscopy, and EP. The results have good agreement with each other, but PFQNM and NI give slightly larger values than SAW and EP. The difference is in agreement with previously reported data and reflects the different physical meaning of the obtained values. It is shown that the presence of physically adsorbed water strongly influences the results generated by PFQNM AFM, and therefore, reliable water removal from the studied materials is necessary.

Lattice swelling and modulus change in a helium-implanted tungsten alloy: X-ray micro-diffraction, surface acoustic wave measurements, and multiscale modelling

Using X-ray micro-diffraction and surface acoustic wave spectroscopy, we measure lattice swelling and elastic modulus changes in a W-1% Re alloy after implantation with 3110appm of helium. An observed lattice expansion of a fraction of a per cent gives rise to an order of magnitude larger reduction in the surface acoustic wave velocity. A multiscale model, combining elasticity and density functional theory, is applied to the interpretation of observations. The measured lattice swelling is consistent with the relaxation volume of self-interstitial and helium-filled vacancy defects that dominate the helium-implanted material microstructure. Larger scale atomistic simulations using an empirical potential confirm the findings of the elasticity and density functional theory model for swelling. The reduction of surface acoustic wave velocity predicted by density functional theory calculations agrees remarkably well with experimental observations.

Effect of Pore Structure of Nanometer Scale Porous Films on the Measured Elastic Modulus

The impact of pore structure of nanoporous films on the measured elastic modulus is demonstrated for silica-based nanoporous low-k films that are fabricated using an alternative manufacturing sequence which allows a separate control of porosity and matrix properties. For this purpose, different experimental techniques for measuring the elastic properties were compared, including nanoindentation, laser-induced surface acoustic wave spectroscopy (LAwave), and ellipsometric porosimetry (EP). The link between the elastic response of these nanoporous materials and their internal pore structure was investigated using positronium annihilation lifetime spectroscopy (PALS), EP, and diffusion experiments. It is shown that the absolute value of the Berkovich indentation modulus is very sensitive to the local pore structure and stiffness of the substrate and can be influenced by densification and/or anisotropic elasticity upon indentation, while on the other hand spherical indentation results are less sensitive to the local pore structure. The comparison of Berkovich and spherical indentation results combined with finite element simulations can potentially reveal changes in the internal structure of the film. For nanoporous films with porosity above the percolation threshold, the elastic modulus results obtained with LAwave and EP agree very well with spherical indentation results. On the other hand, below the percolation threshold, the elastic modulus values determined by these techniques deviate from the spherical indentation results. This was explained in terms of specific technique related effects that appear to be sensitive to the specific arrangement and morphology of the pores.

Nanometer-Thick Textured ZnO Films: Preparation, Characterization and Interaction with Ethanol Vapor

A study was carried out on the effect of the reaction temperature in atomic layer deposition on the structural and piezoelectric properties of thin ZnO films (5–50 nm) by scanning electron microscopy, X-ray diffraction, piezoresponse force microscopy, and surface acoustic wave (SAW) spectroscopy. The piezoresponse was shown to be mainly a function of ZnO grain size and the monocrystalline structure perpendicular to the substrate plane. The results demonstrate promise for the use of textured ZnO films as sensitive coatings for acoustic gas sensors in instruments based on the SAW transducer amplification-frequency response.

Mechanical properties and density of BCxNy films grown by low-pressure chemical vapor deposition from triethylamine borane

Boron carbonitride (BCxNy) films of different compositions have been grown by low-pressure chemical vapor deposition using triethylamine borane as a single-source precursor and ammonia as an additional nitrogen source. Experiments were performed at various initial vapor compositions. The resultant films have been characterized by ellipsometry, IR spectroscopy, energy dispersive X-ray spectroscopy, scanning electron microscopy, nanoindentation, and surface acoustic wave spectroscopy. The mechanical properties of the films are shown to correlate with their density and chemical composition. With increasing initial ammonia partial pressure in the vapor phase, the elemental composition of the films moves away from boron carbide, approaching boron nitride, which is accompanied by a reduction in the Young’s modulus, hardness, and density of the films.

Influence of Grain Orientation on the Microstructural Characterization in Cu During (self)-Anneal using a Surface Acoustic wave Technique

AbstractThis paper describes the influence of microstructure, or more specifically grain orientation and grain size, on the in-line monitoring of copper interconnect properties during (self)-anneal using surface acoustic wave spectroscopy (SAWS). In electroplated Cu after (self)-anneal the SAWS frequency is lower for samples annealed at higher temperature because of the lower porosity induced elasticity. In sputtered Cu, the SAWS frequency shows a clear correlation with grain size, which is induced by a strong re-orientation of the copper film from the as-deposited (111) texture (E=190 GPa) to a strongly (100) textured super grain structure (E=78GPa).

Assessment of thermal expansion coefficient of methylsilsesquioxane thin film

For the methylsilsesquioxane film whose optical birefringence is almost zero, it was recently reported that its vertical thermal expansion coefficient (CTE) was approximately one order of magnitude larger than the lateral CTE. Though the birefringence is not an absolute predictor of anisotropic behavior, the discrepancy in both the CTEs was so remarkable that it was essential to investigate whether the anisotropy was intrinsic property or not. If the effect of Poisson's ratio is considered in the calculation of the vertical CTE and when elastic modulus measured by surface acoustic wave spectroscopy is used in the assessment of the lateral CTE, both the CTEs are coincident with each other. Therefore, it can be concluded that the discrepancy in the CTEs can be attributed to a higher in-plane polymer chain orientation but it can also arise from the misleadingly assumed modulus and Poisson's ratio. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3109–3120, 2006

Anisotropic elastic properties of nanocrystalline nickel thin films

The elastic properties of a nickel film approximately 800 nm thick were measured with nanoindentation, microtensile testing, atomic force acoustic microscopy (AFAM), and surface acoustic wave (SAW) spectroscopy. Values for the indentation modulus (220–223 GPa) and Young’s modulus (177–204 GPa) were lower than predicted for randomly oriented polycrystalline nickel. The observed behavior was attributed to grain-boundary effects in the nanocrystalline film. In addition, the different measurement results were not self-consistent when interpreted assuming elastic isotropy. Agreement was improved by adopting a transversely isotropic model corresponding to the film’s 〈111〉 preferred orientation and reducing the elastic moduli by 10–15%. The SAW spectroscopy results indicated that the film density was 1–2% lower than expected for bulk nickel, consistent with models for nanocrystalline materials. Similar reductions in modulus and density were observed for two additional films approximately 200 and 50 nm thick using AFAM and SAW spectroscopy. These results illustrate how complementary methods can provide a more complete picture of film properties.

Effects of residual stress on the thin-film elastic moduli calculated from surface acoustic wave spectroscopy experiments

We describe a method to examine how residual stress affects the values of thin-film elastic moduli determined from surface acoustic wave spectroscopy experiments. To illustrate our approach, we apply it to the case of an elastically isotropic thin film under equibiaxial stress. The five test samples consisted of TiN films deposited on single-crystal Si substrate (film thickness 0.287–3.330 μm and assumed compressive stress 0.5–5.4 GPa). With increasing thickness and decreasing stress, the effective second-order moduli C 11 increased and C 13 decreased. The natural second-order elastic moduli c 11 and c 13 and the natural third-order elastic moduli c 111, c 112, and c 123 in the unstressed state were computed by fitting the model equations to the measured data. Our results show how surface acoustic wave measurements may be analyzed to obtain additional information about the mechanical properties of thin films with residual stress.

Measurement of Young’s modulus and Poisson’s ratio of thin coatings using impact excitation and depth-sensing indentation

Poisson’s ratio is important for the characterization of coatings, as it is needed to determine the mechanical properties of coatings using techniques such as depth-sensing indentation and surface acoustic wave spectroscopy. However, until now it has been extremely difficult to measure the Poisson ratio of a coating and it is commonly assumed, often incorrectly, that it is the same as that of the bulk material. A new technique has therefore been developed following recent developments in impact excitation and depth-sensing indentation to measure the Poisson ratio of coatings. Impact excitation was first used to determine the Young modulus of the coatings. These results were then used in combination with the plane strain modulus obtained from depth-sensing indentation to determine the Poisson ratio of the coating. This technique has been successfully applied to determine the Poisson ratio of sputter deposited TiN coatings up to 2.66 μm thick. Consistent values of Young’s modulus (427±9 GPa) and Poisson’s ratio (0.23±0.03) were obtained for all the coatings, which agree with literature values for similar but thicker coatings obtained using acoustic microscopy.

Non-Destructive Measurement of the Density of Thin Coatings by a Combination of Instrumented (Nano) Indentation and Acoustical Techniques.

ABSTRACTNanoindentation is used to provide a measurement of coating plain strain modulus. The coating properties are then measured by surface acoustic wave spectroscopy (SAWS). It is shown that, by exploiting the relative insensitivity of the SAWS acoustical model to Poisson's ratio, it is possible to measure either the thickness or the density of a thin coating given the input of the other. Furthermore, the indentation and acoustical tests are non-destructive and the resulting value of thickness or density is of high precision. A case study is presented for the measurement of density for a 250 nm thick nano-crystalline Niobium coating on a Si (001) substrate. The density is found to be ρ = 8.4 ± 0.3 g cm−3, about 2 % below bulk values but the modulus is found to be only 85 GPa (for a Poisson's ratio of 0.4), which is 18 % below bulk.

Critical properties of nanoporous low dielectric constant films revealed by Brillouin light scattering and surface acoustic wave spectroscopy

Thin porous films with nanometer pore sizes are the subject of intense interest, primarily because of their reduced dielectric constant k. The lack of useful characterization tools and the reduction in film mechanical properties with increasing porosity have severely hindered their development and application. We show that both Brillouin light scattering and surface acoustic wave spectroscopy allow one to measure density, porosity and stiffness properties of nanoporous methylsilsesquioxane films of low-k value. Excellent correlations are observed among independent measurements of density, porosity and the Young’s modulus which show that the results obtained are reliable and reveal properties of the films which are difficult or impossible to obtain using other techniques.

Determination of Young’s Modulus of Porous Low-k Films by Ellipsometric Porosimetry

A method for determination of Young's modulus (E) of thin porous films using data obtained by ellipsometric porosimetry (EP) is reported. The calculated E values are in good agreement with data obtained by nondestructive methods like surface acoustic wave spectroscopy (SAWS) and Brillouin light scattering (BLS). These three methods (SAWS, BLS, and EP) use different physical ideas for determination of Young's modulus. Good agreement among these data suggests that the obtained data are more realistic than data obtained by nanoindentation that gives remarkably larger E values. © 2002 The Electrochemical Society. All rights reserved.

Strength-porosity relationship of nanoporous MSSQ films characterised by Brillouin Light Scattering and Surface Acoustic Wave Spectroscopy

AbstractNanoporous Methylsilsesquioxane films are a leading candidate for low dielectric constant (low-κ) materials for microelectronic interconnect. Mechanical strength reduces rapidly with lower κ (increasing porosity) however, and there is a lack of techniques to characterize these properties in the κ∼2 range. This work reports application of surface acoustic wave spectroscopy and Brillouin light scattering to characterization of density/porosity and Young's modulus values of a range of Methylsilsesquioxane films from different manufacturers. We show that the results are validated by independent measurements and that nanoindentation measurements consistently overestimate stiffness. The behaviour of 3 sets of films show different stiffness-porosity relationships, the initially stiffer materials declining more rapidly with increasing porosity than the softer materials. This has important consequences for stiffness properties in the κ<2 region.

Structure and elastic properties of amorphous silicon carbon nitride films

Amorphous ${\mathrm{SiC}}_{x}{\mathrm{N}}_{y}$ films with various compositions were deposited by ion-beam sputtering. The bonding characteristics, Young's modulus, and density of the films were investigated using x-ray photoelectron spectroscopy (XPS), Fourier-transform infrared (FTIR) spectroscopy, surface acoustic wave spectroscopy, x-ray reflection, and molecular-dynamics (MD) simulations. It was observed that the Young's modulus decreases from $260\ifmmode\pm\else\textpm\fi{}20$ to $85\ifmmode\pm\else\textpm\fi{}12\mathrm{GPa}$ while the density decreases from $3.45\ifmmode\pm\else\textpm\fi{}0.2$ to $2.3\ifmmode\pm\else\textpm\fi{}0.3{\mathrm{g}/\mathrm{c}\mathrm{m}}^{3}$ as the carbon content of the films increases from 0% to 68%. FTIR and XPS spectra indicate an increasing proportion of double and triple bonds with higher carbon content of the films. These experimental results were compared with the Young's moduli and the infrared spectra obtained from density-functional-based MD simulations. Also for these model calculations the Young's modulus drops from $237\ifmmode\pm\else\textpm\fi{}54$ to $109\ifmmode\pm\else\textpm\fi{}25\mathrm{GPa}$ as the carbon content increases from 0% to 69%. It can be seen that the formation of $\mathrm{C}\mathrm{=}\mathrm{C},$ $\mathrm{C}\mathrm{=}\mathrm{N},$ $\mathrm{C}\mathrm{\ensuremath{\equiv}}\mathrm{C},$ and $\mathrm{C}\mathrm{\ensuremath{\equiv}}\mathrm{N}$ bonds, together with the occurrence of terminating nitrogen atoms for the films with higher carbon content, are responsible for the degradation of the ${\mathrm{sp}}^{3}$ network, and therefore for a lower Young's modulus and density.

Thermal diffusivity in amorphous silicon carbon nitride thin films by the traveling wave technique

Thermal diffusivity (α) of amorphous silicon carbon nitride (a-SiCxNy) thin films on crystalline silicon, prepared by ion beam sputtering, has been studied using the traveling wave technique. The variation of thermal diffusivity with carbon content in amorphous silicon carbon nitride samples are reported. Thermal diffusivity decreased from ∼0.35 cm2/s for samples with carbon contents of less than 30 at. %, to about 0.15 cm2/s for a-SiCxNy films with a carbon content of ∼70 at. %. A similar variation was found for the film density as measured by surface acoustic wave spectroscopy as a function of the carbon content. The results on a-SiCxNy, elucidate the relation between thermal diffusivity and the bonding configuration, density and microstructure of the network.

324 無電解NiPメッキの力学特性におよぼす熱処理および粒子分散の影響(OS 薄膜特性)

Elastic property determinations of several kinds of electroless Ni-P coatings deposited on a steel substrate have been performed. First, Young's moduli of NiP composite coatings are determined using a surface acoustic wave spectroscopy and the ultrasonically determined ones are compared with both mechanically determined ones using a nanoindentation and theoretically estimated ones. The results have clearly shown that SiC particles dispersion causes an increase in Young's modulus of composite coatings and conversely, Teflon particles dispersion decreases Young's modulus. Next, Young's moduli for several NiP coatings heat-treated at various temperature are examined. The result has shown that Young's modulus of NiP coating increases up to about 33% as tempering temperature increases and it has a maximum value by heat-treating at 773K.