DEFECT-MEDIATED TRANSFORMATIONS IN THIN METAL FILMS

Abstract

Thin metal films are widely used in industry, and often show strikingly different behavior than the same material bulk form. Two thin-film phenomena were explored in this work: the β-to-α phase transformation in tantalum thin films, and the (111)-to-(100) texture transformation in face-centered cubic metal thin films. Both are revealed to rely on specific defect structures. To understand the β-to-α phase transformation in Ta films, we first investigated the nature of the metastable β phase of tantalum. A series of Ta films was deposited in an ultra-high vacuum system under varying Ar sputter pressures to investigate the role of energetic deposition on phase formation, structure, and properties. By measuring the stress in each film, we unambiguously established that only the β phase of Ta appears, despite large variations in deposition energy. All films exhibit (002) fiber texture, and the while the breadth of this component increased with increasing sputter pressure, no other texture component appeared. In comparing these results with a detailed review of the literature, a theory of phase selection in Ta films was formulated that is consistent with virtually all reported results. As these films consist of 100% β-Ta with no evidence of contamination with either oxygen or the stable α phase, we were also able to provide representative measurements of the resistivity and Hall-Petch constants of pure β-Ta. When annealed, these films underwent the β-to-α phase transformation, and electron backscatter diffraction revealed that they formed an unusual microstructure with continuous orientation gradients and discontinuous grain boundaries. The variation in initial β-Ta microstructure resulted in substantial variation in the phase-transformed microstructure. The magnitude of the orientation gradient decreased with increasing sputter pressure, and the distance between high-angle orientation increased with increasing sputter pressure. We propose a simple rotation mechanism by which the orientation gradient might form, which is consistent with all observed variations in the gradient microstructure. Using a genetic algorithm, we identified the geometrically necessary dislocations needed to produce these orientation gradient patterns according to this mechanism. The (111)-to-(100) texture transformation is a well-known phenomenon occurring in FCC films deposited with (111) fiber texture. Upon annealing, films below some critical thickness retain (111) texture, while those above the critical thickness transform to a (100) texture. The transformation is often explained by a competition between strain energy and interface energy: thin films retain (111) texture because the (111) interface has low surface energy, while thick films transform to (100) because the (100) orientation is more compliant and therefore has a lower strain energy for a given strain. However, recent work has suggested that neither strain energy nor interface energy play a dominant role in the transformation. We investigated the driving forces involved in this transformation by using a bulge test to induce different strain energies in thin Ag films under identical annealing conditions, and observing the progression of the texture transformation using synchrotron x-ray diffraction. Despite a large change in strain energy, the applied stresses had no effect on the transformation. We therefore evaluated various defects as potential driving forces for the transformation, and show that nanotwins provide both an adequate driving force and a potential orientation selection mechanism for the (111)-to-(100) transformation.