Quantitative analysis of anisotropic edge retraction by solid-state dewetting of thin single crystal films

When single crystal thin films undergo solid state dewetting, film edges retract at a rate that is strongly affected by their crystallographic orientations. Lithographically patterned macroscopic edges with a limited number of specific in-plane crystallographic orientations remain straight as they retract. Macroscopic edges with other crystallographic orientations develop in-plane facets, whose in-plane normals are the same as those of kinetically stable edges. Therefore, a quantitative understanding of the retraction of kinetically stable edges can serve as the basis for understanding the retraction of edges with all other in-plane orientations. Measurements of the rates of retraction of kinetically stable edges for single crystal (100) and (110) Ni films on MgO are reported. Retracting edges develop out-of-plane facets that are generally consistent with the facets expected from the equilibrium Wulff shape. To capture the observed anisotropic character of the edge retraction rate, edge retraction through surface diffusion driven by the surface Laplacian of the weighted mean curvature of fully faceted edges has been modeled. The 2-dimensional model and experiments show a similar time scaling for the edge retraction distance (∼tn, with n ∼ 0.4) and the rim height and width (n ∼ 0.2). Also, they are consistent with the specific observed retraction rate anisotropy, within the range of known error of the surface energies and diffusivities used in the model. However, formation of valleys ahead of the rims is observed in the experiments on (110) films but not in the simulation.