Trends and perspectives in solid-state wetting / Mouillage solide-solide : tendances et perspectives A model for solid-state dewetting of a fully-faceted thin film Un modèle pour le démouillage solide-solide d'un film mince complètement facetté

No abstract

Dewetting is a well-known degradation mechanism for thin films at elevated temperatures. It is driven by surface energy minimization and occurs while the film is solid. The dewetting process is characterized by the formation of holes, retracting edges, and the formation of thickened rims on retracting edges. In anisotropic single-crystal thin films, holes are initially faceted. It is often observed that the corners of the holes retract faster than the edges of the hole, leading to dendritic or star-shaped holes. This so-called “corner instability” is one of the defining morphological characteristics of the dewetting process, and an understanding of this instability may lead to new film patterning techniques. In this work, we present a study of the growth of natural and patterned initially square holes in single-crystal Ni thin films on MgO substrates. A characteristic structure near the corners of the holes was observed, and a model for the growth of faceted holes was developed based on these observations. Despite its simplicity, the model reproduces the observed phenomenology and is in quantitative agreement with experiments. The model reveals that the corner instability arises from a redistribution of mass to create a new hole perimeter, which can only be created at the corner. The consequence is that the corner reaches a steady-state constant retraction rate while mass accumulation at the rims causes their retraction rate to continuously decrease.

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.

During solid-state dewetting of thin single crystal films, film edges retract at a rate that is strongly dependent on their crystallographic orientations. Edges with kinetically stable in-plane orientations remain straight as they retract, while those with other in-plane orientations develop in-plane facets as they retract. Kinetically stable edges have retraction rates that are lower than edges with other orientations and thus determine the shape of the natural holes that form during solid-state dewetting. In this paper, measurements of the retraction rates of kinetically stable edges for single crystal (110) and (100) Ni films on MgO are presented. Relative retraction rates of kinetically stable edges with different crystallographic orientations are observed to change under different annealing conditions, and this accordingly changes the initial shapes of growing holes. The surfaces of (110) and (100) films were also characterized using low energy electron diffraction, and different surface reconstructions were observed under different ambient conditions. The observed surface structures were found to correlate with the observed changes in the relative retraction rates of the kinetically stable edges.