Ductility Of Thin Films Research Articles

A method of determining the strength of ductile thin films

A method is presented for measuring the strength of thin films deposited on brittle substrates. The method has been successfully applied for Al and Cu films on quartz or Si substrates. A free-standing film region is produced through carefully breaking the substrate below the film starting from a diamond saw-cut. Due to delaminations along the interface the influence of the substrate is minimized. Because substrate breakage can be realized without damage of the ductile film an almost intact free-standing film covers the cracked substrate.The strength behavior of the thin metallic film is measured through applying a bending moment to the specimen, which loads the film by opening the substrate crack. The strength of the film is derived from the maximum of the load–deflection curve. The calculated strength values amount to about 2.3 GPa for e-beam evaporated 200 nm Al film, and 1.7 GPa for thermally evaporated 200 nm Al film. For 800 nm Al film a strength level of 0.74 GPa and for 800 nm Cu film 0.93 GPa is observed. These high strength values are suggested to be due to the size-effect of high geometrical constraint resulting from both the small thickness and active specimen length owing to difficult dislocation generation at sub-micron length scales.The proposed method proved to be suitable for the investigation of the strength properties of thin films of ductile materials on brittle substrates.

Microbridge tests: II. On buckled thin gold films

In the present work, we report a microbridge testing method for microbridge beams initially buckled by a residual compressive stress. A theoretical formula is derived in closed form with the consideration of substrate deformation. Measuring the profile of a buckled microbridge beam, one can evaluate Young's modulus and the residual compressive stress of the beam. Alternatively, Young's modulus and the residual stress can also be evaluated from the microbridge test under low loads, i.e., from the elastic load–deflection curve. Moreover, we introduce a microbridge testing approach to determine the yield strength of a ductile thin film. Experimentally, microbridge samples were fabricated with a 0.48 µm thick gold film deposited on a silicon wafer by electroplating. Young's modulus, the residual stress and the yield strength of the gold film were determined from the microbridge test to be 66.2 ± 2.9 GPa, −8.5 ± 1.0 MPa and 162.4 ± 5.9 MPa, respectively.

Microbridge testing of thin films

In the present work, we describe a novel microbridge testing method for thin films. The single-layer, bilayer and trilayer thin film microbridge samples were prepared with the microelectromechanical fabrication technique such that they were easy to handle. The microbridge test was conducted with a load- and displacement-sensing nanoindenter system equipped with a microwedge probe. In the mechanics analysis, we considered residual stress in each layer, modeled the substrate deformation with three coupled springs and derived the load–deflection formula in a closed form. For brittle thin films, the microbridge testing method allows us to evaluate simultaneously the Young's modulus, the residual stress and the bending fracture strength. In addition, the microbridge test can characterize the yield strength for ductile thin films.

Residual stress effects on plastic deformation and interfacial fracture in thin-film structures

Residual stress in ductile thin films is shown to significantly affect the extent of film plasticity and interfacial fracture energy during debonding in thin-film structures. Specifically, the interfacial fracture resistance of a TaN/SiO 2 interface in a thin-film structure containing an adjacent ductile Cu film with varying residual stress is compared to predictions from computational models. The fracture energy of the TaN/SiO 2 interface increased by 50% for structures containing a Cu film in compression compared to similar structures with the film in tension. The effect of residual stress on interfacial adhesion is rationalized in terms of the onset of yielding within the Cu film, which affects the contribution from plastic energy dissipation to the total interface fracture energy.

Modeling nonlinear peeling of ductile thin films––critical assessment of analytical bending models using FE simulations

Three analytical double-parameter criteria based on a bending model and a two-dimensional finite element analysis model are presented for the modeling of ductile thin film undergoing a nonlinear peeling process. The bending model is based on different governing parameters: (1) the interfacial fracture toughness and the separation strength, (2) the interfacial fracture toughness and the crack tip slope angle, and (3) the interfacial fracture toughness and the critical Mises effective strain of the delaminated thin film at the crack tip. Thin film nonlinear peeling under steady-state condition is solved with the different governing parameters. In addition, the peeling test problem is simulated by using the elastic–plastic finite element analysis model. A critical assessment of the three analytical bending models is made by comparison of the bending model solutions with the finite element analysis model solutions. Furthermore, through analyses and comparisons for solutions based on both the bending model and the finite element analysis model, some connections between the bending model and the finite element analysis model are developed. Moreover, in the present research, the effect of different selections for cohesive zone shape on the ductile film peeling solutions is discussed.

Modeling nonlinear peeling of ductile thin films??critical assessment of analytical bending models using FE simulations

Modeling nonlinear peeling of ductile thin films??critical assessment of analytical bending models using FE simulations

Continuum and atomistic modeling of electromechanically-induced failure of ductile metallic thin films

A multiscale modeling approach is presented for the analysis of electromechanically-induced void morphological evolution and failure in ductile metallic thin films, which are used for device interconnections in integrated circuits. Self-consistent mesoscopic simulations of surface morphological evolution are combined with atomistic calculations of surface properties and molecular-dynamics (MD) simulations of plastic deformation mechanisms in the vicinity of void surfaces. Results are presented that demonstrate a coupled mode of surface instability that leads to formation of electromigration-induced slit-like features and stress-induced crack-like features on void surfaces. In addition, MD results are presented for the dislocation-mediated mechanism and the kinetics of void growth in ductile metallic systems subject to hydrostatic and biaxial tensile strains. The incorporation of MD-derived constitutive information for plastic deformation into the mesoscopic analysis and simulation framework also is discussed.

Failure Mechanism Researches of Material Surface and Interface in Micro-Scratch Test

ABSTRACTIn the present research, the adhesion properties and failure mechanisms for a ductile thin film on a silicon substrate (Ni/Si) are studied experimentally, and are simulated theoretically. In the experimental research, the relations of the horizontal driving force, vertical displacement and the frictional coefficients with horizontal displacement are measured. Furthermore, the variation of the total energy release rate and the frictional coefficient between contact surfaces are measured through obtaining a frictional effect law. The law displays that the frictional influences on the energy release rate of the total system weakly depend on the thin film thickness. This conclusion leads to that the frictional effect can be eliminated in the toughness ratio relation approximately. So that one can directly obtain the interfacial adhesion toughness from measurements in the micro-scratching test. In addition, the micro-scratching process for the ductile thin film/brittle substrate systems is simulated using the double cohesive zone model. Prediction results of the energy release rate are obtained, and are compared with the experimental results obtained in the present research.

Measuring interface parameters and toughness—a computational study

Interface toughness of a ductile thin film sandwiched between substrates, Γ ss, can be evaluated from test data which exhibit steady crack growth characteristics. Γ ss is the sum of the adhesion energy of the interface Γ 0 and the plastic dissipation in the film and substrates Γ p. A proper characterization of the quality of interface adhesion requires knowledge of two quantities: the adhesion energy Γ 0 and the adhesion strength σ ̂ . In the present investigation based on a four-point bend geometry, the interface between the ductile film and the substrate is modelled by an adhesion law described in terms of Γ 0 and σ ̂ . The effects on interface toughness of σ ̂ , of Γ 0, and of the geometry and material properties of the layers are studied using a computational model of the test geometry. Through fitting model predictions against test data, a method of evaluating Γ 0 and σ ̂ is suggested.

On the Interaction Between Indentation Cracks on Metallized Silicon

AbstractThe metallization of Si represents a important industrial process and produces a bi-layered composite of a ductile metal film on a brittle substrate. The mechanical properties of such a composite are determined by the properties of the two layers and the interface and influenced by the fact that the metallized layer, being a very thin film, possesses properties different from those of a bulk material. The fracture toughness is also influenced by the nature and distribution of defects which may be generated during use of these materials, even if the manufacturing process produces a reasonably defect free material. Indentation cracking has been extensively used for the measurement of fracture toughness due to its small sample size requirements as well as a relatively good correlation with values obtained from traditional fracture mechanics tests. The indentation process, with its associated cracks, produces permanent plastic deformation and also introduces a residual stress field. This field influences the crack pattern generated in an adjacent indent and can be used as a methodology to model the influence of multiple defect sources.The present study was aimed at understanding the effect of a thin Ti alloy metallization layer sputtered on a Si wafer on the sizes of the cracks associated with the indents. It was also aimed at studying the interaction between cracks emanating from sequentially placed indentations. The distance between the indents which generated these cracks was varied from a level comparable to the crack size to a level where interaction could be ignored. This paper discusses the changes in the nature as well as the sizes of cracks due to the presence of the metallization layer as well as the interaction between the stress fields of the indents in this ductile thin film – brittle substrate composite and possible methodologies for delineating these effects.

Development of Photo-Sensing Bulge Ductility Tester

The ductility of thin films is normally evaluated by conducting the bulge testing. However, the accuracy of the conventional type of mechanical bulge ductility testing unit is poor in determining the actual elongation. In this development, a photo-sensitive device is provided in the detecting portion to accurately measure the elongation. The sensitivity depends on the type of the materials tested. A Pyrex glass ball is utilized as a punching ball to provide equally distributed light. Pushing of the Pyrex glass is activated by a hydraulic media to control the punching velocity. Accuracy and reproducibility were primarily checked with copper thin foil deposited on polyimide substrates. Two types of dental articulating papers (paper-based and plastic-based) were tested. It was suggested that an appropriate selection of articulating paper to mark the occlusal interferences is essential for precise determination of occlusal harmony.

A new method for measuring the strength and ductility of thin films

A new method of measuring the mechanical strength of thin films is described. We prepare miniature arrays of four tensile specimens, each 0.25 mm wide, 1 mm long, and 2.2 μm thick, using deposition, patterning, and etching processes common to the semiconductor industry. Each array of four specimens is carried on and protected by a rectangular silicon frame. Thirty-six such specimens are produced on a single wafer. After a specimen frame is mounted, its vertical sides are severed without damaging the specimens. The load is applied by micrometers through a special tension spring. Tensile properties of a 2.2 μm thick Ti–Al–Ti film were determined.

The alignment of polar mesogens in electroactive polymers

We have studied the polar alignment produced in a range of comb polymers with a methacrylate backbone and oxynitrostilbene mesogenic groups attached to the backbone at variable densities by a 3 or 6 carbon spacer. These polymers, when poled, display second order non-linear optical activity, piezoelectricity and pyroelectricity. The magnitude of these effects depends upon the degree of polar ordering of the side groups and we have therefore investigated the efficiency of the poling process using the pyroelectric coefficient and thermally stimulated currents as primary diagnostic techniques. For constant poling conditions it is found that the polarization and pyroelectric response are not linearly related to the volume fraction of mesogen, i.e. samples of high mesogen concentration display lower polarization and pyroelectric activity than might be expected from extrapolation of low concentration data. Indeed, for the 6 carbon spacer material there would appear to be an optimum mesogen concentration around 30 mol%. Also, though the 3 and 6 carbon spacer materials are similar at low mesogen concentration, the latter material gives a lower response when it forms a liquid crystalline phase at high concentrations. We therefore suggest that the polar mesogens tend to pack anti-parallel at higher concentrations and that this interaction is not readily overcome by the applied field. Though these polymers are brittle at high mesogen concentration, ductile thin films can be cast if the density of side chains is reduced to 10 mol%. Despite the reduction in mesogen concentration, the pyroelectric activity is not proportionally reduced, implying more effective poling due to reduced interaction between the mesogens.

Fracture Toughness of Silicon and Thin Film Micro-structures by Wedge Indentation

ABSTRACTSilicon rooftop structures 30 µm high and of 1 µm linewidth were fabricated in a microelectronics facility using anistropic KOH etching. Aluminum-1% silicon was sputter-deposited to investigate the effects of a ductile thin film on the fracture toughness of a brittle material. The rooftop structures were indented by a Knoop indenter purposely placed off-center so that the indenter was wedge-shaped. Deformation and cracks were photographed by SEM. Crack threshold load per unit width are reported for normal cracks, running perpendicular to the roofline. Fracture toughness is calculated by first order approximation and finite element modeling using SEM measurements of the length of normal cracks. Without a thin film, values for silicon are in reasonable agreement with the literature. With a thin film of only 600 Å, fracture toughness is increased by 45%. Cracks nearly parallel to the roofline also appeared at higher load values. Characteristics of both types of cracks are discussed.