Thermo-mechanical behavior of Fe thin films
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Thermo-mechanical behavior of Fe thin films Nail R. Chamsoutdinov, Amarante J. Böttger, Rob Delhez and Frans D. Tichelaar. Materials Science Department, Delft University of Technology, Rotterdamseweg 137, Delft, 2628AL, The Netherlands. ABSTRACT The thermo-mechanical behavior of magnetron sputtered Fe polycrystalline films of thickness between 50 nm and 400 nm has been investigated. The state of stress has been determined by means of wafer curvature and X-ray diffraction (sin2ψ-method). Both methods are in good agreement for layers of thickness above 200 nm. For specimens of smaller layer thickness, however, the average stresses as measured by X-ray diffraction are systematically higher than those observed by wafer curvature experiments. The results can be interpreted in terms of differences in micro-strain (estimated using X-ray diffraction peak width analysis) and grain size as obtained by transmission and scanning electron microscopy. Thermal cycling experiments were performed between RT and 873 K. The effect of microstructure on thermomechanical properties was shown to be crucial. INTRODUCTION The properties of magnetron sputtered iron thin films are of particular interest for many important applications such as recording media. The mechanical properties of iron define not only the stability of the film but also its other properties, magnetic performance for instance. The mechanical behavior of thin metallic films is quite different from bulk metals and still not fully understood. For example, the relationship between grain size and yield stress described by the Hall-Petch relation: σ=σ0+kd-0.5 does not hold for nano-crystalline materials. The physical origin behind the experimentally observed changes in mechanical behavior depending on grain size is still unclear. So far only very limited amount of studies is performed on sputtered iron films: a nanoindentation analysis of finely grained (10.3 – 21.0 nm) films of as-sputtered iron showed that no precise exponent could be assigned in the power law relation for the hardness data [1], and the analysis of the state of stress of 400 nm thick iron films as a function of the (deposition) substrate temperature revealed high tensile stresses up to 1 GPa [2]. In this work the state of stress in sputtered iron films of thicknesses from 50 to 400 nm is investigated by means of X-ray diffraction (XRD) and wafer curvature (WC) experiments. The evolution of the state of stress upon annealing is also studied. It was shown that the microstructure of the as-deposited films plays the most important role in the mechanical properties of (magnetron) sputtered iron. EXPERIMENTAL RESULTS Iron films are deposited on Si (100) 2-inch wafers (with 150 nm of thermal oxide) in a Leybold Z550 magnetron sputter-coater with a base pressure below 7·10-5 Pa and using a working Ar-pressure of 4.8 Pa. The films are sputtered at room temperature under RF-power of 500 W with growth rate of 3.1 nm/min. The thickness of the films ranges from 50 nm to 400 nm and is determined by weighing (Met
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