Stress and texture in sputter deposited Cr films
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Stress and texture in sputter deposited Cr films S.Yu. Grachev1, J.-D. Kamminga1 and G.C.A.M. Janssen2 1
Netherlands Institute for Metal Research, Rotterdamseweg 137, 2628 AL Delft, the Netherlands e-mail: [email protected] 2 Department of Materials Science, Delft University of Technology, Rotterdamseweg 137, 2628 AL Delft, the Netherlands ABSTRACT Intrinsic stress in coatings is often responsible for its performance. We studied tensile stress in sputter deposited chromium films as a function of film thickness and Ar pressure during deposition. We correlate the stress evolution to the grain growth in the polycrystalline films. Both grain growth and stress evolution obey the same power law dependence on thickness. We conclude that the tensile stress is generated at the grain boundaries. The power law exponent did not depend on pressure of Ar and remained 0.36. However, texture and microstructure in the layers changed when pressure was increased from 2x10-2 to 6x10-2 mbar. Texture switched from 110 to 111 fiber type. Grooves and sharp star-like grains were observed at higher pressure. We explain changes in terms of suppressed shadowing and less surface diffusion.
INTRODUCTION Cr films are often used for coating steels. Understanding the origin of stress and texture in chromium films is of vital importance for the optimizing of these conditions. It is generally accepted that the main sources for the stress are ion-peening and grain-boundary shrinkage for compressive and for tensile stresses, respectively [1-2]. Typically, films of body centered cubic (bcc) metals exhibit 110 out-of-plane fiber texture and columnar microstructure. The widely used evolutionary model by van der Drift [3] describes the mechanism of formation of oriented grains in terms of growth rate of the different facets of a crystal. This growth rate depends on the probability for arriving atoms to be adsorbed as a function of the angle of incidence, surface diffusion and thermodynamic probability to crystallize. All these quantities are different for each crystal face. This results in different rates of growth of the crystal faces. The fast growing crystals expand on the expense of the slow ones resulting in lateral grain growth and texture formation. Unfortunately, the quantities mentioned are not readily accessible and in rare cases can be obtained from direct measurements. The purpose of this study is to understand how fundamental surface processes (like shadowing) relate the deposition conditions with microstructure and stress in thin films. The power law dependence between the grain size and the thickness was predicted and observed before [4]. Recently, it was shown that the density of grain boundaries and tensile stress can be described by the same power law [5]. However, the relationship of the power law exponent and the growth conditions is still unclear. This paper presents preliminary results on the influence of Ar pressure on stress and microstructure.
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EXPERIMENTAL DETAILS Sputter deposition of Cr films was per
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