During high temperature treatments, thin solid films with submicron thicknesses have a tendency to break up into islands in a bid to lower the resultant free energy. Generally, this phenomenon is referred to as “agglomeration” or “dewetting”. Research has revealed that the said phenomenon is usually driven by the minimization of the total energy, including the film surface energy, the substrate-surface energy and the film-substrate interfacial energy. These thin films have a large surface area to volume ratio which induces a high driving force for agglomeration making such films more susceptible to agglomerate. With all factors considered, it can be agreed that dewetting/agglomeration is a vital process in material science since it controls the stability of thin films or can be used for film nano-structuration by formation of islands.
Models developed for dewetting usually assume diffusion at the interface and/or at the surface but no direct evidence of such diffusion has so far been demonstrated. Worse off, majority of such models mainly deal with elemental materials and not with compounds in which several elements can diffuse. To sum it up, till now the mechanism responsible for agglomeration of polycrystalline compound thin films is yet to be fully understood.
In a recent publication, IM2NP Research Institute scientists Dr. Ting Luo and Pr. Christophe Girardeaux together with Pr. Hartmut Bracht at University of Münster and Dr. Dominique Mangelinck from CNRS, Aix-Marseille University investigated the mechanism of agglomeration for the Ni monosilicide using silicon isotope multilayers and atom probe tomography (APT). They focused on revealing the agglomeration mechanism behind nickel monosilicide (NiSi), a binary compound largely used as contacts for advanced devices. Their work is currently published in the research journal, Acta Materialia.
To begin with, the research team started by employing APT to determine the redistribution of the silicon (Si) isotopes, both in NiSi and in the Si substrate in a bid to map out the distribution of atoms in three dimensions (3D) and distinguish between different isotopes with an atomic resolution. Next, they engaged in a direct experimental approach whose aim was to offer evidence for Si diffusion along the NiSi/Si interface, which, as a result, would give an access to the agglomeration mechanism of NiSi thin films. Models to establish diffusion paths during the agglomeration process were also developed.
The authors chiefly reported on the diffusion of Si, the less mobile species in NiSi. In particular, this diffusion at the NiSi/Si interface was demonstrated through comparison between the three-dimension redistribution of the Si isotopes determined by APT and models taking into account grooving and agglomeration. Additionally, they also reported that the multilayer structure was maintained in some regions of NiSi showing a limited diffusion of Si in NiSi.
In summary, the study determined the 3D redistribution of Si isotopes using APT for an agglomerated film of NiSi obtained by the reaction between a 15nm nickel film and the Si isotope multilayers. Generally, the study demonstrated that diffusion of the less mobile species is of crucial importance for agglomeration of compounds. Indeed, quantitative prediction of dewetted shapes necessitates improved models and simulations of dewetting through the knowledge of the mechanism. Altogether, results reported the associated model provide a new way for simulations of dewetting and thus for producing complex structures
T. Luo, C. Girardeaux, H. Bracht, D. Mangelinck. Role of the slow diffusion species in the dewetting of compounds: The case of NiSi on a Si isotope multilayer studied by atom probe tomography. Acta Materialia, volume 165 (2019), page 192-202.Go To Acta Materialia