Influence of Gas Atmosphere on the Plasticity of Metal Thin Films
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Influence of Gas Atmosphere on the Plasticity of Metal Thin Films T. Wübben1, G. Dehm3, E. Arzt1,2 1
Max-Planck Institute for Metals Research, Stuttgart, Germany Institut für Metallkunde, University of Stuttgart, Stuttgart, Germany 3 now at Erich Schmid Institut für Materialwissenschaft, Österreichische Akademie der Wissenschaften, and Department für Materialphysik, Montanuniversität Leoben, Leoben, Austria 2
ABSTRACT Stresses in thin films are routinely measured by the so-called substrate curvature technique. These experiments are usually carried out in air or under a protective gas atmosphere. In this contribution we describe a new set-up capable of performing substrate curvature measurements under ultra-high vacuum conditions. The advantages are the absence of possible artifacts due to gas/film interactions, better control of gas composition, and the possibility to measure chemical effects on mechanical properties in a controlled way. We present first results that indicate an unexpected sensitivity even of polycrystalline Cu films to the gas environment. INTRODUCTION The substrate curvature technique is a widely used method for investigating the elastic and plastic properties of metal thin films. It makes use of the stress that is induced in the film due to thermal mismatch during temperature changes. The film stress exerts a bending moment on the substrate, whose curvature can be measured by different techniques. Substrate curvature experiments can in principle be carried out on every kind of material. However, most of the equipment in use today performs the required thermal cycles in high temperature furnaces without control of the atmosphere. To prevent the samples from oxidation, a flow of inert gas, e.g. nitrogen or argon, is usually sustained and a possible interaction of gas and sample is commonly neglected. In view of the high specific surface area of thin films, the surface state may strongly influence the experimental results. It is for instance known that a passivation such as an oxide layer on the sample surface can lead to an increased flow stress (1-4). The presence of an oxide layer during a temperature cycle, however, cannot be controlled under gas flow conditions. A controlled atmosphere during thermal cycling is also desirable for experiments investigating the stress development in a metal under catalytic conditions. This is of special interest for chemical applications, e.g. when metals are used as catalysts in the steam reformation or oxidation of methanol (CH3OH) in production lines for formaldehyde (CH2O). The influence of stress on the catalytic performance is currently under investigation in this community (5). In view of these limitations, we have developed a substrate curvature apparatus capable of operating under ultra high vacuum (UHV). Several design problems had to be solved to reach this goal. The new equipment now allows experiments under controlled ambient conditions.
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EXPERIMENTAL A schematic 3D representation of the system is shown in figure 1. The specimen is
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