Templated Solid-State Dewetting of Single Crystal Ni Thin Films

Abstract

Solid thin films are generally metastable in the as-deposited state and will undergo dewetting if they are heated. Dewetting can occur at temperatures well below the film’s melting temperature so that the film remains in the solid-state. Solid-state dewetting of thin films is driven by surface energy minimization and proceeds through retraction of film edges that develop rims that break down via various dewetting phenomena. While dewetting of polycrystalline thin films leads to irregular morphologies, dewetting of single crystal thin films leads to regular morphologies due to the uniformity in crystalline anisotropy. Moreover, for single crystal thin films, dewetting can be guided to form complex nanostructures in a highly reproducible manner by pre-patterning the film.

Lithographic pre-patterning of single crystal thin films also provides a framework for investigation of mechanisms of specific dewetting phenomena. These phenomena include the effects of in-plane crystallographic orientations of film edges on their morphological evolution during dewetting-induced retraction. Also, for a given prepatterned film, effects of materials and process parameters, including out-of-plane orientation of the film, initial film thickness and annealing ambient, on dewetting phenomena can be systematically investigated.

In this work, using pre-patterned single-crystal Ni films on MgO substrates as a model system, mechanisms and means of controlling dewetting phenomena were investigated, paying attention to effects of the above-mentioned parameters. The first part of this thesis focuses on a specific dewetting phenomenon called a fingering instability. The conditions that lead to fingering instead of an alternative phenomenon called rim pinch-off were investigated. It was found that an initial edge roughness leads to the development of fingering and that fingering can be induced by intentionally pre-patterning film edges with periodic roughness. Furthermore, it was found that within a range of periods, templating controls the steady-state finger characteristics (spacing, direction and propagation rate) and leads to the formation of periodic arrays of parallel nanowires. Templated fingering was investigated over a wide range of experimental variables (template wavelengths, in-plane edge orientations, and film thicknesses) and the finger characteristics were determined for each of the variables. The second part of this thesis focuses on how the steady-state finger characteristics are controlled by the wavelength of patterned roughness. A kinetic model that provides mechanistic insights into the effects of all of the investigated experimental parameters is developed. It is also shown that there is a lower limit and an upper limit for direct templating and dewetting behaviors of templated fingers outside the direct templating regime are reported and analyzed. Lastly, in the third part of this thesis, edge retraction and the pinch-off phenomenon was investigated in different ambient conditions, using two different types of reducing gas (H₂-based and CO-based). A profound effect of the ambient gas on the formation of valleys ahead of the rims on retracting edges, the origin of pinch-off behavior, was discovered and it is demonstrated that the observed differences can be attributed to changes in surface energy anisotropy.