Capillary Instabilities in Cobalt Silicide Thin Films
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INTRODUCTION The term capillary instability has been used to define severe shape changes in thin films which are driven by surface energetics. In polycrystalline thin films, the process of capillary instability takes place by the mechanism of grain boundary grooving. Capillary forces, driven by the reduction in the grain boundary energy, induce the grain boundary grooving upon heating[l]. As the sample is heated, the grooves at grain boundaries tend to deepen while maintaining the equilibrium groove angles, where the equilibrium groove angles or dihedral angles, are determined by the balance of surface tension forces[2]. These "shape changes" lead to capillary instability or thermal instability when the deepening of grain boundary grooves leads to islanding or pinchoff (also defined as film agglomeration) of the film and to the formation of discrete spherical globules. Locally, the movement of atoms away from grain boundary grooves is due to the existence of a gradient in chemical potential[1]. In the 1950's, Mullins[1,3] formulated the kinetic theory of grain boundary grooving (based on chemical potential gradients which arise due to curvature
differences), the model describes the time dependence of various groove size parameters on annealing duration and diffusivity. Mullins' model (which assumes infinitely large grains and isotropic surface energy) provided an important benchmark. By plotting groove depth, d, vs. ti/n (where, t is annealing duration and n can be 2, 3 or 4), one can obtain the mode of atomic transport involved during the grooving process. However, it was also experimentally observed[4] that capillary instability (or grain boundary groove depth) is strongly dependent on the ratio of grain size to film thickness. To explain this effect, numerous geometrical agglomeration models[4-7] were formulated and a critical ratio of grain size to film thickness ratio, Dc, was defined. The geometrical agglomeration models state that if a polycrystalline layer has a D value (ratio of grain diameter to film thickness) larger than or equal to Dc, such a layer would be thermally unstable upon heating and would suffer from severe capillary instability. However, it was soon observed[8-9] that the silicide layers formed on polycrystalline substrates tend to suffer from severe agglomeration, even if their respective D values (=I) were significantly less than the critical value Dc for the silicide layer. It was noticed that grooving of these silicide layers formed on polycrystalline silicon can be explained by Mullins' kinetic model. Two independent models, Mullins' kinetic model and Geometrical agglomeration model, have been used to explain capillary instabilities. Thus the need for a single grooving model is paramount and is a focus of this paper. Capillary instabilities in cobalt silicide films in the context of a new empirical model is presented here. 2.
RESULTS AND DISCUSSION This section has been divided into three parts. In first part, inadequacies of separate kinetic model and geometrical agglomeration models
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