Mechanical Properties Of Ultra-thin Films Research Articles

Nanomechanical and structural study of Au38 nanocluster Langmuir-Blodgett films using bimodal atomic force microscopy and X-ray reflectivity

HypothesisLangmuir-Blodgett (LB) technique allows the deposition of gold nanoparticles and nanoclusters (atomically precise nanoparticles below 2 nm in diameter) onto solid substrates with an unprecedented degree of control and high transfer ratios. Nanoclusters are expected to follow the crinkle folding mechanism, which promotes the formation of trilayers of nanoparticles but kinetically disfavors the formation of the fourth layer. ExperimentsLB films of Au38(SC2H4Ph)24 nanocluster were prepared at a range of surface pressures in the bilayer/trilayer regime and their internal structure was analyzed with X-ray Reflectivity (XRR) and Grazing-Incidence Wide-Angle X-ray Scattering (GIWAXS). Bimodal atomic force microscopy (AFM) imaging was used to quantify the elastic modulus, which can be correlated with the topography at the same point on the surface. FindingsNanocluster bilayers and trilayers exhibited the elastic moduli of ca. 1.2 GPa and 0.9 GPa respectively. Films transferred in the 20–25 mN/m surface pressure regime displayed a particular propensity to form highly vertically organized trilayers. Further compression resulted in disorganization of the layers. Crucially, the use of two cantilevers of contrasting stiffness for bimodal AFM measurements has demonstrated a new approach to quantify the mechanical properties of ultrathin films without the use of deconvolution algorithms to remove the substrate contribution.

Size-dependent frequency of simply supported elastic ultra-thin films with surface effect under periodic vibration

Although surface effects play an important role in the mechanical properties of ultra-thin films, the nonlinear vibrations of ultra-thin films influenced by surface effects have not been fully understood. This paper develops an analytical framework for studying the nonlinear vibrations of simply supported ultra-thin films with surface effects. The framework is based on the modified Kirchhoff plate theory. The surface stress effects are treated by the Gurtin–Murdoch surface elasticity model and the motion equations include the effects of curvature and classical inertia. The dimensionless frequency of forcibly vibrated ultra-thin films with a simple support and surface effects is explicitly deduced through a series of perturbation procedure. Finally, the surface effects are evaluated in two numerical examples. In these demonstrations, the surface effects significantly influenced the dimensionless frequency when the film thickness reduced to one micrometer or less.

Full Characterization of the Mechanical Properties of 11-50 nm Ultrathin Films: Influence of Network Connectivity on the Poisson's Ratio.

Precise characterization of the mechanical properties of ultrathin films is of paramount importance for both a fundamental understanding of nanoscale materials and for continued scaling and improvement of nanotechnology. In this work, we use coherent extreme ultraviolet beams to characterize the full elastic tensor of isotropic ultrathin films down to 11 nm in thickness. We simultaneously extract the Young's modulus and Poisson's ratio of low-k a-SiC:H films with varying degrees of hardness and average network connectivity in a single measurement. Contrary to past assumptions, we find that the Poisson's ratio of such films is not constant but rather can significantly increase from 0.25 to >0.4 for a network connectivity below a critical value of ∼2.5. Physically, the strong hydrogenation required to decrease the dielectric constant k results in bond breaking, lowering the network connectivity, and Young's modulus of the material but also decreases the compressibility of the film. This new understanding of ultrathin films demonstrates that coherent EUV beams present a new nanometrology capability that can probe a wide range of novel complex materials not accessible using traditional approaches.

Modeling the mechanical properties of ultra-thin polymer films

A modeling method to extract the mechanical properties of ultra-thin films (10–100 nm thick) from experimental data generated by indentation of freestanding circular films using a spherical indenter is presented. The relationship between the mechanical properties of the film and experimental parameters including load, and deflection are discussed in the context of a constitutive material model, test variables, and analytical approaches. Elastic and plastic regimes are identified by comparison of finite element simulation and experimental data.

A generic "micro-Stoney" method for the measurement of internal stress and elastic modulus of ultrathin films.

Accurate measurement of the mechanical properties of ultra-thin films with thicknesses typically below 100 nm is a challenging issue with an interest in many fields involving coating technologies, microelectronics, and MEMS. A bilayer curvature based method is developed for the simultaneous determination of the elastic mismatch strain and Young's modulus of ultra-thin films. The idea is to deposit the film or coating on very thin cantilevers in order to amplify the curvature compared to a traditional "Stoney" wafer curvature test, hence the terminology "micro-Stoney." The data reduction is based on the comparison of the curvatures obtained for different supporting layer thicknesses. The elastic mismatch strain and Young's modulus are obtained from curvature measurements of cantilevers before and after the film deposition. The data reduction scheme relies on both analytical and finite element calculations, depending on the magnitude of the curvature. The experimental validation has been performed on ultra-thin low pressure chemical vapor deposited silicon nitride films with thickness ranging between 54 and 133 nm deposited on silicon cantilevers. The technique is sensitive to the cantilever geometry, in particular, to the thickness ratio and width/thickness ratio. Therefore, the precision in the determination of the latter quantities determines the accuracy on the extracted elastic mismatch strain and elastic modulus. The method can be potentially applied to films as thin as a few nanometers.

Mechanical properties of C60 thin films at the air–water interface

Understanding the mechanical properties of ultra-thin films formed by the self-assembly of molecules/nanoparticles/colloids at fluid–fluid interfaces is central to many technological applications. Here, we have carried out interfacial rheology measurements to systematically investigate the concentration dependent viscoelastic response of 2D films of Fullerene C60 at the air–water interface. With increasing C60 concentration, amplitude sweep measurements show that the films undergo a transition from viscoelastic liquid-like to viscoelastic solid-like behaviour. Interestingly, for high C60 concentrations's, the loss modulus G′′ reaches a maximum before the onset of power-law shear-thinning in G′, the storage modulus, and G′′. The power-law exponents have a ratio 2. This response is typical of systems that show soft glassy behaviour. We also observe a power-law increase in G′ and G′′ at low frequencies in the frequency response measurements and a transition from Newtonian to shear-thinning behaviour, with increasing shear rate, in steady shear measurements. Our results are in qualitative agreement with the phenomenological soft glassy rheology model.

A non-destructive method for measuring the mechanical properties of ultrathin films prepared by atomic layer deposition

The mechanical properties of ultrathin films synthesized by atomic layer deposition (ALD) are critical for the liability of their coated devices. However, it has been a challenge to reliably measure critical properties of ALD films due to the influence from the substrate. In this work, we use the laser acoustic wave (LAW) technique, a non-destructive method, to measure the elastic properties of ultrathin Al2O3 films by ALD. The measured properties are consistent with previous work using other approaches. The LAW method can be easily applied to measure the mechanical properties of various ALD thin films for multiple applications.

Determination of Young's modulus and Poisson's ratio of thin films by X-ray methods

Accurate determination of Young's modulus and Poisson's ratio is critical when characterizing the mechanical properties of ultra-thin films. In this work, a method to simultaneously measure the Young's modulus and Poisson's ratio of thin films by X-ray techniques combined with a four-point bending fixture is presented. The four-point bending fixture was carried out to purposely induce stresses on the thin film and cause a curvature of the film/wafer system. The induced stresses were measured by the modified conventional X-ray diffraction for film measurements. At the same time, the curvature of the film/wafer system was measured by the X-ray rocking curve. With the film stress derived from the curvature according to the Stoney formula, both the Poisson's ratio and the Young's modulus were evaluated from the plot of lattice d-spacing changes (strain) versus cos2α sin2ψ. The method was applied to determine thin copper films (~180nm). The estimated Poisson's ratios ranged from 0.18 to 0.24. Compared to the 0.34 for bulk Cu, the deviation between the bulk and the film properties was above ~30%. The Young's modulus of the films is 116.7±2.9GPa, which was comparable to those reported in the literature for ~128GPa.

Asymptotic solution to axisymmetric indentation of a compressible elastic thin film

Using the perturbation theory, the axisymmetric indentation of a compressible elastic thin film bonded to a rigid substrate is analyzed for the contact radius much larger than the thickness of the thin film. Explicit expression of the nominal contact stiffness is obtained, which is proportional to the contact area and inversely proportional to the thickness of the thin film, independent of indenter. The effect of surface interaction on the indentation of compressible elastic films is also discussed by following the Johnson–Kendall–Roberts approach. Closed-form solutions are obtained for the indentation load–indentation depth relation under the action of the surface interaction. It turns out that, for spherical indenters, the pull-off force to detach the indenter from the surface of the thin film is independent of elastic properties of the thin film. This analysis provides us a new approach to evaluate mechanical properties of ultra-thin films by using the nanoindentation technique.

Brillouin light scattering studies of the mechanical properties of ultrathin low-k dielectric films

In an effort to reduce RC time delays that accompany decreasing feature sizes, low-k dielectric films are rapidly emerging as potential replacements for silicon dioxide (SiO2) at the interconnect level in integrated circuits. The main challenge in low-k materials is their substantially weaker mechanical properties that accompany the increasing pore volume content needed to reduce k. We show that Brillouin light scattering is an excellent nondestructive technique to monitor and characterize the mechanical properties of these porous films at thicknesses well below 200nm that are pertinent to present applications. Observation of longitudinal and transverse standing wave acoustic resonances and the dispersion that accompany their transformation into traveling waves with finite in-plane wave vectors provides for a direct measure of the principal elastic constants that completely characterize the mechanical properties of these ultrathin films. The mode amplitudes of the standing waves, their variation within the film, and the calculated Brillouin intensities account for most aspects of the spectra. We further show that the values obtained by this method agree well with other experimental techniques such as nanoindentation and picosecond laser ultrasonics.

A Novel Method to Determine The Mechanical Properties of Ultra-Thin Films.

ABSTRACTRecent experimental results based on x-ray reflectivity[1, 2], and ellipsometry[3] have demonstrated that physical properties of polymer films thinner than one micron may deviate significantly from bulk values[4]. The mechanical properties of the ultra-thin films (sub-micron) are experimentally difficult to determine with precision. The quartz crystal microbalance is an established technique for measuring properties of polymer thin films of a few microns thick. [5–7] Recently this quartz crystal microbalance technique has been modified for measuring the mechanical properties of sub-micron polymer films with high precision. The details and preliminary results from this recently modified quartz crystal microbalance technique will be presented.

Thermal-mechanical properties of a monomolecular film detected by a vibrating reed

Thin films of organic materials are finding many applications in modern technology. The microelectronics industry, in particular, uses very thin organic films both as photoresists and as insulating layers. The ultimate in organic thin films is represented by molecular monolayers of the order of 2.0 nm thick as deposited by the Langmuir–Blodgett technique1. By adapting a vibrating reed apparatus2 we have investigated the thermal–mechanical properties of a supported 6-nm thick monolayer film.Although a monolayer melting transition could not be found in the resultant spectra, at least two other, low-temperature peaks (at −150 and −20 °C) were detected which seem to be due to relaxation processes both within the film as well as at interfaces of the monolayer structure. Our main aim was to determine whether the mechanical properties of ultrathin films could be measured by the vibrating reed technique, and to detect any thermal–mechanical relaxations which may occur in such films, inparticular whether or not a melting transition could be detected at elevated temperatures. Such thermal–mechanical measurements have long been used in theplastics industry to characterize the mechanical properties of resin materials in bulk. The vibrating reed technique may provide an analogous service for very thin organic coatings.