Study of Elastic Anharmonicity in Mo Thin Films

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Thin films have a larger residual stress than their bulk materials. The purpose of this project was to investigate a possible difference in the elastic moduli in thin films from the view poin of elastic anharmonicity. Experimentally, Mo thin films were sputter deposited on silicon substrates. The elastic properties of Mo thin films were studied for various deposition conditions. Deposition parameters included substrate temperature and a target power. Residual stress was measured by a laser curvature-measuring system. Residual strain was measured by a X-ray diffractometer. Elastic modulus and hardness were measured by a nano-indentation tester. The major obtained results were the following :(1)Mo thin films were under a compressive stress state for all deposition conditions. Residual stress were found to decrease with increasing substrate temperature and a target power. This tendency was attributed to the temperature dependence of the surface diffusivity and to the target power dependence of the kinetic energy of the deposited atoms.(2)Compressive stress and strain were found to be in a range of 0.4-1.2 GPa and 0.2-0.6%, respectively. The observed residual stress was twice as much as that of the bulk value of 0.4-0.6GPa. On the other hand, hardness and Young's modulus did not show any dependence on the deposition conditions and were approximately the same as the bulk values.These results indicate that residual stress and strain in thin films can be substantially affected by the deposition conditions and may take larger values than the bulk. In contrast, in the observed range of stress and strain, elastic anharmonicity can be neglected.