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Mechanisms of Long-Range Edge Retraction of Au/Ni Bilayer Films


Dewetting is an aspect of materials science that directly affects the reliability of microelectronics and the formation of nanostructures with applications in storage, semiconductors, and plasmonics. Dewetting initiates at triple phase lines between film, substrate, and vapor and propagates away from the triple lines through the process of surface self-diffusion due to film curvature towards flat areas of the film (i.e. away from the edge of a film). As edge retraction starts to occur, other instabilities further the process through allowing agglomeration of the retracting film to form islands of high aspect ratio. The solid-state dewetting and long range edge retraction of Au/Ni thin films from a SiO2/Si substrate are driven by the minimization of the free surface and film/substrate interface. While dewetting processes in monolayer thin films have been studied extensively, there are only a few studies involving bilayer thin films and only one that reports long-range edge retraction.

This thesis explores the agglomeration and long-range edge retraction of Au/Ni bilayer films from SiO2/Si substrate. The films were deposited in sequences of Au deposited on top of Ni and Ni deposited on top of Au. As-deposited films were subsequently annealed at 545 ⁰C, 675 ⁰C, and 730 ⁰C. They were characterized by scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDX) to identify the mechanisms and driving forces for long-range edge retraction. Annealing of Au/Ni bilayer films supported by a SiO2/Si substrate displayed long-range edge retraction occurring, as well as the formation of seven morphologically distinct regions. The formation of each region was reliant on the dominant mechanisms for retraction at play. Interface and volume diffusion were determined to be the dominant mechanisms in the regions closest to the edge of the substrate (regions P6 and P7), while in the transition region P5, interface diffusion was the sole dominating mechanism. Surface diffusion dominated regions P3 and P4. The semi-circular shape in which retraction occurred was indicative of a corner instability, the primary mechanism of long-range edge retraction. This is the first reported instance of a corner instability initiating without a corner, especially for long-range edge retraction. Line-like features of high aspect ratio indicated a secondary retraction mechanism of a fingering instability. The comparison between both deposition sequences revealed the stabilizing effect a native NiOx has on the bilayer film through hindering edge retraction and the critical role that interaction between Au and Ni plays in edge retraction. A uniform area of dark contrast indicating the Si substrate at the P2-P3 interface was observed in all Au/Ni bilayer films. It was proposed that this interface is a result of void formation and nucleation propagated by a thickness variation in region P2 due to the diffusion of Ni to region P3. However, another explanation is the occurrence of a pinch-off event due to the agglomeration of Au and Ni in regions P3 to P5 reaching a critical height. However, additional data is required to ascertain whether that is occurring.

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