Nanomechanics of Thin Films: Emphasis: Tensile Properties
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NANOHECHANICS OF THIN FILMS: EMPHASIS: TENSILE PROPERTIES R. W. HOFFMAN Department of Physics, Case Western Reserve University, Cleveland, OH 44106 ABSTRACT Tensile properties of thin films may be interpreted as a structure sensitive plastic region superposed on an elastic background in a manner similar to bulk specimen tensile testing. However, the limitations of both the material and tensile instrument have not usually been tested in detail. We report our experience with aluminum and alumina films some 100 nm thick prepared by evaporation of Al followed by anodization for the alumina film. Self-supporting films are glued to glass "jaws" of the nanotensilometer and force-elongation data recorded. Mounting thickness, glue slippage, instrument calibration, and other possible artifacts will be examined in detail. A typical Al stress-strain curve has an initial small curved region interpreted as a mounting artifact, followed by a primarily elastic (near linear) region and increasing plastic deformation until failure occurs. Alumina films fail in a brittle manner. Characterization techniques include TEM, RBS, and other surface spectroscopies; selected examples will be reported. Strain rate and preliminary annealing data are presented with a microscopic interpretation. In general, thin metal films are less ductile than their bulk counterparts, grain sizes are much smaller, and they may possess large stresses and unexpected impurities, but have mechanical properties that can be modelled. INTRODUJCTION The previous papers in this volume assess the role of growth stresses in thin films. It is obvious that the material of the thin film responds to the total local stress and its spatial distribution. Hence, one approach to thin film mechanics is to apply the same treatments that have been so successful in the bulk case. We in no way wish to reduce the importance of stresses in thin films. Indeed, the sum of the growth stresses and the stresses whose origin lies in differential thermal expansion plus their control in magnitude and location is the key to their utilization in critical applications. This has been the objective since the early papers and carries through to the present volume. With the application of finite element analyses to thin film structures came the realization that film material parameters are not so readily available. In some cases major changes in the composition and microstructure may be responsible. In general thin films have a smaller grain size and are more brittle than their bulk counterparts. It is the purpose of this paper to both review the literature and describe techniques for thin film mechanical property measurements and critique the results to date for the case of free standing aluminum films. It is well known that both elastic and plastic properties of bulk materials are anisotropic. For films essentially the same thing is true, even for polycrystalline films a preferred, perhaps tilted, fiber texture is possible. The use of Si wafers for substrates and the mechanical anisotropies in E/l-v
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