Kinetic mechanisms of theC49-to-C54 polymorphic transformation in titanium disilicide thin films: A microstructure-scaled nucleation-mode transition

The kinetics and mechanism of the polymorphic transformation in titanium disilicide thin films from a base-centered orthorhombic structure (C49-${\mathrm{TiSi}}_2$) to a face-centered orthorhombic structure (C54-${\mathrm{TiSi}}_2$) were investigated with the emphasis on the effect of film thickness scaling on the phase transition using in situ resistance measurements, x-ray diffraction, and transmission electron microscopy (TEM). The C49 disilicide films of different thicknesses were prepared by annealing Ti/polycrystalline Si thin film couples of varying Ti thicknesses (250, 550, and 1000 Å). The transformation rate was found to be a strong function of the film thickness and temperature, and can be described by τ∝exp(${\mathit{E}}_{\mathit{a}}$/kT) with an activation energy of ${\mathit{E}}_{\mathit{a}}$=3.73, 4.44, and 5.08 (±0.07) eV for 1000, 550, and 250 Å Ti, respectively. An unusual transition between nucleation sites was observed as a result of the scaling: for samples with 250 Å Ti, the transformation proceeded by nucleating the C54 polymorph at grain edges (three-grain junctions) of the C49-${\mathrm{TiSi}}_2$ films while the C54 nuclei were predominantly formed at the grain boundaries (two-grain junctions) in thicker films, which are in good agreement with the predictions of the nucleation mode from kinetics analyses. It has been suggested that the nucleation of the C54-${\mathrm{TiSi}}_2$ is likely to be the rate-limiting step in the overall transformation. Based upon the microstructural evidences provided by TEM analyses, the significance of these observations due to the reduction in film thickness was discussed by considering energetics of nucleation at different geometrical sites, nucleation site density, and effects of surface and stress. It was demonstrated that the surface contribution and nucleation site density become important for nucleation in thinner films with largely increased surface-to-volume ratio. The nucleation-mode transition is driven by the morphological and microstructural changes associated with thickness scaling.