Competition between strain and interface energy during epitaxial grain growth in Ag films on Ni(001)
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Epitaxial Grain Growth (EGG) is an orientation-selective process that can occur in polycrystalline thin films on single crystal substrates. EGG is driven by minimization of crystallographically anisotropic free energies. One common driving force for EGG is the reduction of the film/substrate interfacial energy. We have carried out experiments on polycrystalline Ag films on Ni(OOl) substrates. The orientation dependence of the Ag/Ni interfacial energy has been previously calculated using the embedded atom method. Under some conditions, EGG experiments lead to the (111) orientations calculated to be interface- and surface-energy-minimizing. However, when Ag films are deposited on Ni(OOl) at low temperature, EGG experiments consistently find that (111) oriented grains are consumed by grains with (001) orientations predicted to have much higher interface and surface energy. The large elastic anisotropy of Ag can account for this discrepancy. Strain energy minimization favors growth of (001) grains and can supersede minimization of interfacial energy if sufficient strain is present and if the film is initially unable to relieve the strain by plastic deformation.
I. INTRODUCTION Polycrystalline thin films find application in device interconnects, FET gate metallurgies, high-r c superconductors, magnetic storage media, ferroelectrics, and tribological coatings. The properties of these films often depend sensitively on the average grain size and on the preferred orientations of the grains, both of which evolve when grain growth occurs, either during film deposition or as a result of post-deposition processing. Grain growth is a coarsening process driven by the reduction of the grain boundary energy per unit volume. Additional driving forces for grain growth often act concurrently with grain boundary capillarity. Of particular interest here are the driving forces due to the anisotropic surface and interface energy of a quasi-two-dimensional solid such as a thin film. Interface-energy-driven grain growth in thin films is an orientation-selective process resulting in the development of preferred orientations in an initially randomly oriented film.1"7 If the film resides on a single crystal substrate, the preferred orientation^) can have specific orientation relationships with the substrate lattice. We call this process Epitaxial Grain Growth (EGG). 810 Thin films often exhibit high stress levels after deposition. Under some circumstances the reduction of the strain energy density is another potentially impor-
^Currently at Sandia National Laboratories, Albuquerque, New Mexico 87185. J. Mater. Res., Vol. 9, No. 9, Sep 1994 http://journals.cambridge.org
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tant driving force for orientation-selective grain growth. However, the orientations that minimize elastic energy need not also minimize the interfacial energy. This investigation explores the effect of interfaceenergy-anisotropy and film strain on epitaxial grain growth in polycrystalline Ag films on (001) Ni thin film substrates. The dependence of t
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