Solid Solution Alloy Effects on Microstructure and Indentation Hardness in Pt-Ru Thin Films
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SOLID SOLUTION ALLOY EFFECTS ON MICROSTRUCTURE AND INDENTATION HARDNESS IN PT-RU THIN FILMS Seungmin Hyun*, Oliver Kraft**, and Richard P. Vinci* * Department of Materials Science and Engineering, Lehigh University, Bethlehem, Pennsylvania 18015 ** Max Planck Institut fur Metallforschung, Stuttgart, Germany ABSTRACT The elastic moduli and flow stresses of as-deposited Pt and Pt-Ru solid solution thin films were investigated by the nanoindentation method. The influence of solid solution alloying was explored by depositing Pt-Ru solid solution thin films with various compositions onto Si substrates. The 200 nm films were prepared by DC magnetron cosputtering with a Ru composition range from 0 to 20wt%. As expected, the modulus and the flow stress both increased significantly with an increase in Ru. The experimental results compare favorably to predictions based on a simple dislocation motion model consisting of three strengthening terms: substrate constraint, grain size strengthening and solid solution strengthening. INTRODUCTION The general effects of solid solution alloying on microstructure development and strength are well known for bulk metals [1]. The increase in strength that is typically observed is the result of interaction between the stress field of a dislocation and the stress field associated with a solute atom. Strengthening mechanisms active in metal thin films have also been examined extensively [3,4]. However, the mechanical properties of solid solution alloy thin films have not been heavily studied because interest has traditionally focused on pure metals such as Al and Cu. As a result, the effectiveness of solid solution alloying relative to other strengthening mechanisms unique to thin films is not well understood. In this work, thin films of Pt alloyed with as much as 20 wt% Ru were studied with the goal of improving our fundamental understanding of thin film strengthening behavior. The equilibrium binary phase diagram of the Ru and Pt system [2] shows a homogeneous solid solution up to 46 wt% Ru at 1000oC, so we remained well within the solubility limit. In bulk, small amounts of Ru in Pt dramatically increase the hardness of the alloy [1]. It is therefore reasonable to expect that large strength gains should be found in our films if solid solution alloying can play a significant role compared to the other strengthening mechanisms that are active. EXPERIMENT Pt-20wt%Ru and pure Pt targets were used for DC-magnetron cosputtering at ambient temperature on to uncooled substrates at a base pressure of 4.5×10-9 Torr. All substrates were 3 inch diameter Si (100) wafers with a thermally grown oxide layer approximately 100 nm thick. The substrates were pre-cleaned with an ion gun to improve the adhesion between film and substrate. The deposition rates for the two targets were very similar: approximately 25nm/min at P3.2.1
160 W sputter power and about 8.3nm/min at 50 W sputter power. Approximate film composition was controlled by independently controlling the sputter power for the two targets. After fi
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