A New Synchrotron-based Technique for Measuring Stresses in Ultrathin Metallic Films
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A New Synchrotron-based Technique for Measuring Stresses in Ultrathin Metallic Films Jochen Böhm1, Patric Gruber1, Ralph Spolenak2, Alexander Wanner3, and Eduard Arzt1,2 1 Institut für Metallkunde, Universität Stuttgart, Heisenbergstr. 3, D-70569 Stuttgart, Germany 2 Max Planck Institute for Metals Research, Heisenbergstr. 3, D-70569 Stuttgart, Germany 3 Institut für Werkstoffkunde I, Universität Karlsruhe (TH), D-76128 Karlsruhe, Germany ABSTRACT A novel synchrotron-based X-ray diffraction technique is presented by which it is possible to characterize the evolution of stresses in polycrystalline metallic films thinner than 100 nm. The film under investigation is deposited on a flexible polyimide substrate which is subjected to a uniaxial tensile test. The evolutions of longitudinal and transverse elastic strains in the film are monitored simultaneously by means of a high-resolution area detector. The strain resolution is better than 10-4. The samples are typically subjected to 6 % total strain and subsequently unloaded. First experiments carried out on Au films in the thickness range between 20 nm and 1000 nm show the usefulness and the power of this new technique. INTRODUCTION Most investigations of size effects on the mechanical properties of thin metallic films are based on wafer-curvature experiments involving thermal cycling. In order to better understand the fundamental mechanisms, it is favorable to decouple the changes in temperature and stress. A well-established isothermal technique to characterize the flow behavior of a crystalline thin film is to perform in situ tensile tests in an X-ray diffractometer [1]. The film of interest is deposited on a dog bone-shaped substrate. Film and substrate are strained simultaneously while the stress in the thin film is monitored by X-ray measurements. Polyimide has been shown to serve well as substrate material [2,3], because this amorphous polymer is compliant, can be deformed homogeneously up to very large strains (>5%), and is available with excellent surface quality. The standard X-ray technique applied so far to measure the stress evolution in the metal film is the “sin2ψ method”, an adaptation of which to thin films has been described extensively elsewhere [4-6]. This X-ray in situ tensile testing technique has been shown to work well for metal films with thickness in the micrometer range. Serious experimental problems arise, however, for films considerably thinner than 1 µm: The X-ray counting times required per data point increase to an impractical level as the gauge volume is decreased and the signal-to-noise ratio of the diffraction peaks becomes unfavorable, given that the amorphous substrate yields a considerable scattering background. Materials characterization by X-ray techniques is currently being revolutionized by the combined availability of three recently developed components: (1) 3rd generation synchrotron sources providing high-flux, tunable X-rays, (2) large-area X-ray detectors exhibiting low noise, high sensitivity and spatial resolution
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