Microstructure evolution of gold thin films under spherical indentation for micro switch contact applications

RF MEMS (Radio Frequency Micro Electro Mechanical System) switches are promising devices but their gold-on-gold contacts, assimilated for this study to a sphere/plane contact, represent a major reliability issue. A first step toward understanding failure mechanisms is to investigate the contact metal microstructure evolution under static and cyclic loading. After static and cyclic loading of sputtered gold thin films under spherical indentation, high-resolution Electron Back Scatter Diffraction (EBSD) is used to investigate the contact area. Grain rotation against {111} fiber texture of 1-μm-thick sputtered gold thin film is a signature of plastic deformation. Grain rotation is observed above 1.6 mN under static loading using a spherical diamond indenter with 50-μm tip radius. The heterogeneity in grain rotation observed corresponds to a greater plastic deformation in the middle of the indent than at the edge. A 30° grain rotation due to cyclic work hardening is observed for a half-million mechanical cycles under 300 μN load using a spherical gold tip (20 μm radius). The same test in hot switching mode induces a grain growth in the contact area. Therefore, thermal effects occurring during hot switching are underlined.     

Material characterization by instrumented spherical indentation

It was illustrated by the author in the previous work that combinations between material properties and indentation parameters can be used as mixed parameters in dimensionless functions to capture the sharp indentation response of materials. These issues are further extended for spherical indentation in the present study. Instrumented spherical indentation was performed by a parametric finite element analysis for a wide range of materials with maximum indentation depth-indenter radius ratios rising from 0.01 to 0.3 to investigate several fundamental features within the frame work of limit analysis. Frictional effects are taken into account. Regarding dimensional analyses and using a Taylor series expansion, a new set of dimensionless functions is constructed for spherical indentation parameters and hardness associated to a 70.3° conical indenter. Based on formulated functions, a reverse analysis procedure is suggested to extract material properties and hardness from spherical indentation force-depth curves with respect to two different indentation depth-indenter radius ratios. Effects of indenter compliance on indentation parameters and reverse results are considered. The accuracy of the proposed method is studied and discussed by carrying out reverse and sensitivity analyses for 22 representative materials with rigid and deformable indenters.     

Fracture mechanics analysis of thin coatings under spherical indentation

Spherical indentation of a thin, hard coating bonded to a thick substrate is investigated. The bending of the coating over the softer substrate induces concentrated tensile stresses on the lower and upper coating surfaces, from which transverse cracks may ensue. This work is primarily concerned with ring cracks originating from the top surface of the coating. In-situ indentation tests are carried out on a model glass/polycarbonate bi-layer, with the coating thickness and the indenter radius being the main test variables. As the coating thickness is decreased, the critical load to initiate ring cracks progressively departs from that associated with a critical surface stress, the effect that increases with increasing the indenter radius. A fracture mechanics approach in conjunction with the FEM technique is used to elucidate the onset of cylindrical ring cracks in thin-film bi-layer structures due to spherical indentation. The analysis, conducted as a function of the coating thickness and the indenter radius, reveals the existence of bending-induced compression stress regions ahead of the crack tip, which tend to shield the crack or increase the fracture resistance. The specific behavior is dictated by a complex interplay between the contact radius, a, the coating thickness, d, and the crack length, c. An interesting manifestation of this shielding mechanism is that when the coating surface contains flaws of various sizes, small flaws in this population may be more detrimental than large ones. Incorporation of this aspect into the analysis led to a good correlation with the experimental results. In the limit case of point-load, a closed-form, approximate solution for the stress intensity factors and the critical loads is obtained. This solution constitutes a lower bound for the critical loads, and is furthermore directly applicable to finite size indenters provided da. In the limit c/d/to0, a failure stress criterion may be used irrespective of the ball radius, r. The analysis in this case reveals that decreasing either d/r or the coating/substrate modulus ratio tend to favor ring cracking over radial type cracking. The transition between these two failure modes is identified explicitly as a function of the system parameters.     

Elastic-plastic indentation stress fieldsusing the finite-element method

The finite-element method is used to model the elastic-plastic indentation response of a flat extensive specimen for the case of a spherical indenter. The work highlights several interesting finite-element modelling techniques and provides insight into the physical processes involved in elastic-plastic indentation of certain structural ceramics. Full details of the stress distribution are given and compared with the results of elastic formulae. This work has particular application to the modelling of physical phenomena of deformation in ceramic materials in machining, wear, bearings and hardness testing. This revised version was published online in November 2006 with corrections to the Cover Date.     

球面扁壳在径向脉冲荷载作用下的弹塑性动力响应研究

球面扁壳在径向朝外脉冲荷载作用下,可能发生反直观行为;在径向朝内荷载作用下,则可能发生动力屈曲.通过大量的数值分析发现,两种动力响应除各自拥有自己的响应规律外,还拥有许多共同的特点.研究结果表明:在冲击荷载作用下,当荷载为径向朝内并达到一定值时该结构发生动力屈曲,当荷载为径向朝外并达到一定值域时,该结构发生反直观行为;反直观行为和动力屈曲的位移时程曲线均具有突变现象;反直观行为的最终变形模态和荷载方向相反,动力屈曲的最终变形模态与荷载方向相同;扁壳在冲击荷载作用下,发生反直观行为和动力屈曲时,对荷载的微小扰动都极其敏感;发生反直观行为和动力屈曲时的主要内力是薄膜力;反直观行为发生机理与动力屈曲机理基本类同.     

The effects of thin compressive films on indentation fracture toughness measurements

Thin compressive films have been shown to decrease the lengths of radial cracks produced by a Vickers indentation in a variety of non-metallic materials. The intrinsic stress of submicrometre thick films deposited by reactive ion beam sputtering was measured by a cantilever technique. The change in the apparent indentation fracture toughness of the coated material was correlated with film thickness and stress, indentation load, and the nature of the substrate.     

Mechanical properties of thin glassy polymer films filled with spherical polymer-grafted nanoparticles

It is commonly accepted that the addition of spherical nanoparticles (NPs) cannot simultaneously improve the elastic modulus, the yield stress, and the ductility of an amorphous glassy polymer matrix. In contrast to this conventional wisdom, we show that ductility can be substantially increased, while maintaining gains in the elastic modulus and yield stress, in glassy nanocomposite films composed of spherical silica NPs grafted with polystyrene (PS) chains in a PS matrix. The key to these improvements are (i) uniform NP spatial dispersion and (ii) strong interfacial binding between NPs and the matrix, by making the grafted chains sufficiently long relative to the matrix. Strikingly, the optimal conditions for the mechanical reinforcement of the same nanocomposite material in the melt state is completely different, requiring the presence of spatially extended NP clusters. Evidently, NP spatial dispersions that optimize material properties are crucially sensitive to the state (melt versus glass) of the polymeric material.     

Tem characterization of nanodiamond thin films

The microstructure of thin films grown by microwave plasma-enhanced chemical vapor deposition (MPCVD) from fullerene C₆₀ precursors has been characterized by scanning electron microscopy (SEM), selected-area electron diffraction (SAED), bright-field electron microscopy, high-resolution electron microscopy (HREM), and parallel electron energyloss spectroscopy (PEELS). The films are composed of nanosize crystallites of diamond, and no graphitic or amorphous phases were observed. The diamond crystallite size measured from lattice images shows that most grains range between 3–5 nm, reflecting a gamma distribution. SAED gave no evidence of either sp²-bonded glassy carbon or sp³-bonded diamondlike amorphous carbon. The sp²-bonded configuration found in PEELS was attributed to grain boundary carbon atoms, which constitute 5–10% of the total. Occasionally observed larger diamond grains tend to be highly faulted.     

Elastic-plastic analysis of indentation damage: cyclic loading of copper

This is the second in a two-paper series examining the elastic-plastic deformation of a metallic target material under repeated impact. This paper examines the effect of statically cyclic loading on copper and numerical calculations have been performed up to ten cycles. The extent of the loading phase in each cycle is specified by a constant external work done by the indenter. The deformed configuration of the target material, the locus of material flow, and elastic-plastic boundaries due to cyclic loading are presented. There is a saturation of the stress field near the bottom of the indented crater after the first few cycles of loading. A residual tensile stress field in the direction normal to the target surface exists underneath the indenter in every cycle, which is responsible for the formation of subsurface layer cracks. Results of the coefficient of restitution obtained from the analysis and experiments are also presented.     

Analysis of data from various indentation techniques for thin films intrinsic hardness modelling

Due to the influence of the substrate, direct measurement of the hardness of thin films by standard micro-indentation tests is not always possible. In such situation, determination of the intrinsic film hardness requires the analysis of a set of experimental apparent hardness values obtained for different indentation loads. A number of mathematical equations based on various assumptions were proposed in literature for that purpose.Most of the models were established on the basis of standard Vickers indentation. Using these models to process the data obtained by Knoop indentation does not provide the same intrinsic hardness value, even after Knoop/Vickers standard conversion, than the one obtained from Vickers indentation. The same problem arises when processing the data coming from depth-sensing indentation. A method to obtain comparable hardness values is proposed in the present work by considering an “equivalent” Vickers hardness in the case of Knoop indentations and the corresponding Martens hardness for depth-sensing indentation. This method has been used to determine the intrinsic hardness of titanium nitride film.     

A numerical fracture analysis of indentation into thin hard films on soft substrates

In this paper, finite element simulations of spherical indentation of a thin hard film deposited on a soft substrate are carried out. The primary objective of this work is to understand the mechanics of fracture of the film due to formation of cylindrical or circumferential cracks extending inwards from the film surface. Also, the role of plastic yielding in the substrate on the above mechanics is studied. To this end, the plastic zone development in the substrate and its influence on the load versus indentation depth characteristics and the stress distribution in the film are first examined. Next, the energy release rate J associated with cylindrical cracks is computed. The variation of J with indentation depth and crack length is investigated. The results show that for cracks located near the indenter axis and at small indentation depth, J decreases over a range of crack lengths, which implies stability of crack growth. This regime vanishes as the location of the crack from the axis increases, particularly for a substrate with low yield strength. Finally, a method for combining experimental load versus indentation depth data with simulation results in order to obtain the fracture energy of the film is proposed.     

Modification of composite hardness models to incorporate indentation size effects in thin films

The indentation hardness of copper films of thickness 25 to 1400 nm on oxidized silicon substrates is determined by nanoindentation. The load-displacement curves are analysed using the Oliver and Pharr method. An indentation size effect is observed at low relative indentation depths for the copper films. Composite hardness models are applied to the data without an indentation size effect and the Korsunksy and Puchi-Cabrera models are found to give very good fits to this data. The Nix and Gao mechanism based strain gradient plasticity (MSG) model is used to account for the indentation size effect. An attempt is made to modify the existing composite hardness models using the MSG model to incorporate indentation size effect. By modification of both the Korsunsky and Puchi-Cabrera models a good fit to the entire range of data is obtained.     

Structural characterization of thin amorphous Si films

We present a study of structural changes occurring in thin amorphous silicon (a-Si). The a-Si films were deposited on single-crystalline Si substrates held at room temperature or 200 °C by magnetron sputtering of a Si target in pure Ar atmosphere, and therefore the films were hydrogen-free. All samples were annealed in vacuum and subsequently studied by EPR and GISAXS. A strong decrease in the dangling bonds content at lower annealing temperatures, and then an increase of it at around 550 °C, suggested significant structural changes. In parallel the samples were studied by GISAXS which confirmed changes at the nanometric scale attributed to voids in the material. A nice correlation of the results of the two techniques shows advantages of this approach in the analysis of structural changes in a-Si material.     

Optical spectroscopic characterization of plasma-polymerized thin films

We report optical spectroscopic measurements of plasma-polymerized thin films coated on aluminum (Al) substrates over a spectral range from 0.01 to 6 eV. While the reflectance spectra, R(ω), remain almost unchanged in the infrared range as compared with the bare Al, R(ω) starts to decrease significantly in the higher energy above 2.5 eV. This decrease in R(ω) arises from electronic absorptions characteristic of the polymer films. Moreover, the details of R(ω) in the energy above 3 eV depend on the fabrication conditions of the polymer films. Our results, therefore, demonstrate that this optical measurement can be an excellent method to characterize the quality of polymer films deposited by the plasma polymerization process.     

Photothermal characterization of thin films and coatings

The principles of non-destructive, non-contact characterization of coatings by means of photothermal measuring techniques are briefly explained. A method of quantitative interpretation is presented, which relies on the relative extrema of the calibrated thermal wave phase lags measured as a function of the heating modulation frequency for coatings deposited by reactive magnetron sputtering. The application potential of this interpretation method with respect to the on-line control of coating deposition processes is discussed.     

Morphological characterization of TiO2 thin films

Films prepared by reactive magnetron sputtering always present some structural and morphological heterogeneities.In this work, optical parameters, n(λ), k(λ) and E₀, of TiO₂ thin films were obtained, using only optical transmittance measurements. Films were described according to Abèles's model. Using a mono-oscillator type dispersion curve for the refractive index and a Lorentzian type curve for the absorption coefficient, we were able to demonstrate that the films were optically equivalent to a porous layer, with some dispersion in film thickness.The detailed analysis of the experimental transmittance data, fitted between 330 nm to 2200 nm, also enabled us to correlate the effective refractive index of each film with its deposition conditions.