In-situ study of solid state dewetting in metallic thin films

Abstract

Thin films are fabricated in conditions far from the equilibrium, and they bear a high surface to volume ratio. Therefore, when thermally activated, they gain the required atomic mobility to reduce their free energy associated with surfaces, by transforming to a stable morphology of isolated islands. This phenomenon is called “solid-state dewetting”. Solid-state dewetting of thin films is a source of failure in microelectronic applications. In the first part of this thesis, we present an in-situ investigation on the integrity of a polycrystalline Pt thin film which was enhanced by applying ZnO as an adhesion layer between the film and the Si substrate. Besides the typical morphological evolution during dewetting of a continuous thin film e.g. hillocking, hole formation, hole growth and formation of isolated islands, we report on two distinct events that were captured in real-time. Sublimation of ZnO and secondary hole formation via break up of blisters of Pt thin film, in this case at ~ 1053 K, and formation of an amorphous platinum silicide phase and Pt3Si intermetallic phase in later stages of dewetting. We do however highlight that these observations are for films exposed to high temperatures under high vacuum conditions and caution must be exercised when applying these observations to other systems.

Moreover, solid-state dewetting can lead to the formation of complex submicron or nanostructures, depending on the original geometry of the film. In single crystal thin films, due to the structural symmetry and anisotropic surface properties, the ordered assembly of the islands occurs spontaneously during dewetting. There is an increasing interest in controlling the morphology of dewetting structures due to higher demand for the scaled-down devices. The morphological characteristics of these structures are manifested by instabilities that happen simultaneously during dewetting. Precise identification of these instabilities leads to an enhanced inference of the underpinning kinetic mechanisms that govern the formation of complex dewetting morphologies. In the second part of the thesis, we use pre-patterned single crystal Ni film on single crystal MgO substrate as a model system to study the effect of various parameters on final morphology of dewetted structures. More specifically, we investigate the effect of film's edge roughness, the in-plane crystallographic orientation of film patches, annealing temperature and annealing ambient on fingering instability. We also demonstrate that solid-state dewetting can be exploited as a self-assembly method to achieve the desired morphology via introducing a template to the edges of patches of single crystal Ni film. The template is systematically designed to facilitate the fingering instability and control the spacing of void fingers.