Solid-state dewetting of continuous and patterned single crystal Ni thin films

Abstract

Solid-state dewetting of thin films is a process through which continuous solid films agglomerate to form islands. This process is driven by capillary forces, often occurring via surface self-diffusion. Solid-state dewetting of single crystal films has the potential to produce a wide range of regular structures because it occurs in ways controlled by crystallographic symmetries in single crystal films. We demonstrate this potential by pursuing two major objectives: understanding the underlying physics of regular morphological evolution during dewetting of single crystal films, and guiding the phenomenon to reproducibly produce regular structures with various morphologies. We used single crystal Ni films grown on single crystal MgO substrates as a model system. Dewetting initiates with the nucleation of holes and proceeds through the retraction of film edges around holes. By analyzing the anisotropy of the edge retraction rate and facet morphologies, we show that the effect of the anisotropy of surface energy and surface diffusion on early-stage dewetting morphologies strongly depends on the initial film orientation and annealing ambient. A series of instabilities increases the complexity of morphological evolution in the latter stages of dewetting. These include inplane faceting of retracting edges, accelerated growth at concave corners, edge drag at convex corners, edge pinch-off, and Rayleigh-like instabilities. Clear identification of these instabilities leads to improved understandings of the kinetic mechanisms that govern the formation of complex dewetting morphologies. We also demonstrate that solid-state dewetting can be used to produce regular structures with specific shapes via dewetting of patches patterned from single crystal films. Initial patches were systematically designed on the basis of the dewetting mechanisms to form a variety of specific morphologies. Morphological evolution of these patches occurs in more deterministic ways because of geometric constraints, and leads to the formation of regular structures with smaller sizes and more complex shapes than the initial patches.