Mechanical And Microstructural Properties Of Metallic Thin Films On Compliant Substrates

Micro-electro-mechanical systems(MEMS)and flexible electronic devices play an increasing significant role in new applications which include microtransducer,paper-like electronic displays and electronic sensitive skins.They have drawn more and more attention from both academic and industrial points of view.As an elementary substructure among them,functional metallic thin films are often supported by a deformable polymer substrate.The lifetime and performance of such systems are strongly dependent on their mechanical and microstructural properties since they are subject to complex thermomechanical loadings in use.This work is mainly devoted to the elastic properties and cyclic behavior of thin metal films on compliant substrates.An original procedure is proposed to characterize the in-plane elastic modulus of sub-micrometer thin films.With two coating layers bonded symmetrically to half polyimide substrates,a dual digital image correlation(DIC)system has been developed to measure the macroscopic strains of the film-substrate composite and the uncoated substrate simultaneously during a tensile testing.The strain difference between them allows us to extract the elastic properties of thin films with high precision.In the case of W,Ni,Cu and W/Cu films,the experimental values obtained agree well with the bulk ones.Furthermore,numerical analysis,finite element method(FEM),has been performed to simulate the mechanical behavior of the specimens.High consistency with theoretical and experimental results is verified.In order to study the strain transfer from a substrate to a film,we also perform uniaxial tensile tests on polyimide-supported thin metal films with thicknesses varying from 50 nm to 1 ?m.By means of our updated dual DIC system,the macroscopic strain of the film and the substrate can be measured simultaneously.For strains below 2%(far beyond the films' elastic limit),a continuous deformation through the interface,regardless of the film thickness,residual stress,microstructure and cracks,is observed.The macroscopic strain transfer from a substrate to the film is considered to be controlled only by the interfacial adhesion,and thus can be used to qualitatively evaluate the adhesion.Moreover,the curvature evolution of film surface as a result of Poisson's ratio mismatch is quantitatively studied thanks to an optical three-dimensional(3D)profiler.On the other hand,a new experimental method using uniaxial cyclic testing is presented to study the Bauschinger effect in metallic thin films deposited on stretchable substrates.Thanks to our custom-designed pre-stretching setup,300 nm thick Ni films are deposited on pre-tensile substrates and thus can be deformed alternately in tension and compression relative to the as-deposited residual stress state.The elastic intra-granular strain and true strain of polycrystalline thin films are measured in situ by X-ray diffraction(XRD)and DIC respectively.From the lattice stress/strain-true strain curves,the mechanical response of the Ni film is analyzed in view of the complete loading history and the presence of residual stresses and crystallographic texture.The Bauschinger effect is observed in the film with a nanometric grain size of ?35 nm,while it shows little or no cyclic hardening during the plastic deformation.