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

Abstract

In the as-deposited state, thin films are generally far from equilibrium and will agglomerate or dewet to form arrays of islands when sufficient atomic motion is allowed. Dewetting can occur well below the films' melting temperature in the solid-state. The dewetting process begins by formation and motion of film-substrate-vapor three-phase boundaries. These film edges retract via capillarity-driven mass transport. In the absence of film or substrate patterning, the dewetting morphology of polycrystalline films is not well ordered. However, dewetting in single crystal films leads to a much more regular morphology, due to surface and interfacial energy anisotropy and surface self-diffusivity anisotropy. When dewetting of such films is templated by pre-patterning, dewetting patterns much smaller than the original template patterns can be generated. This makes templated dewetting a potential self-assembly method for generation of complex structures with sub-lithographic length scales. However, control of such patterns in single crystal films requires a significant degree of quantitative understanding of anisotropic dewetting in the solid-state. As a starting point for quantitative research on solid-state dewetting of single crystal films, dewetting of thin single crystal films that were pre-patterned to have edges with specific in-plane orientations were quantitatively characterized and their observed behavior was modeled. Edges aligned to specific crystallographic orientations remain straight as they retract, while edges with other crystallographic orientations develop in-plane facets composed 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 orientations. Measurements of the rates of retraction of kinetically stable edges for single crystal (100) and (110) Ni films on single crystal MgO are reported. In cross section, the 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, capillarity-driven edge retraction through atomic surface self-diffusion was modeled in 2 dimensions using the crystalline formulation method developed by Carter and coworkers. The model and experiments show a similar time scaling for the edge retraction distance. Also, the magnitudes of the predicted retraction rates are consistent with the specific observed retraction rate anisotropy given the large range of error in parameters used in the model. Other possible sources of error include the fact that actual edges are not fully facetted and are sometimes bound by non-equilibrium facets.