Kinetics and mechanisms of metal silicide formation

Kinetics and mechanisms of metastable (C49-TiSi$\sb2)$ and stable titanium disilicides (C54-TiSi$\sb2)$ have been investigated by thermally annealing Ti/polycrystalline Si bilayers using a combination of in situ sheet resistance measurement, in situ stress measurement, x-ray diffraction, transmission electron microscopy (TEM), scanning transmission electron microscopy, and Auger electron spectroscopy (AES). Studies of the initial stage of the C49-TiSi$\sb2$ formation during isothermal anneal at 530$\sp\circ$C and anneal at 10$\sp\circ$C/m show that the C49 phase first individually nucleates at the interface between amorphous silicide and crystalline silicon, then followed by rapid simultaneous lateral and vertical growth until the formation of a continuous layer of polycrystalline C49-TiSi$\sb2.$ Local chemical analysis indicates that the composition range of the amorphous silicide is narrowed due to the appearance of C49 phase.Microstructural study of the C49-TiSi$\sb2$ to C54-TiSi$\sb2$ polymorphic transformation has been performed to elucidate the microstructural evolution and possible mechanism of the phase transformation. It has been shown that the nucleation of the C54-TiSi$\sb2$ is heterogeneous, and preferentially takes place at grain boundaries. The interphase interfaces between C49 and C54 disilicides are often ragged with incoherent characteristics. The growth of the C54 phase is found to proceed by advancing the highly mobile incoherent interfaces in all directions toward the heavily faulted C49 phase. No rigorous orientation relationships are found between two phases. It has been suggested that the C49-to-C54 polymorphic transformation occurs via a massive mechanism.The effect of the C49-TiSi$\sb2$ film thickness on the kinetics and mechanism of the C49-to-C54 polymorphic transformation has been studied using in situ resistance measurement, in situ stress measurement, x-ray diffraction, and transmission electron microscopy. The transformation rate is found to be a strong function of the film thickness and temperature, and can be described by exp (E$\rm\sb a$/kT) with an activation energy of E$\rm\sb a$ = 3.73, 4.44, and 5.08 ($\pm$ 0.07) eV for samples starting with 1000, 550, and 250 A Ti, respectively. An anomalous nucleation-mode transition between nucleation sites, from two-grain junctions (normal grain boundaries) to three-grain junctions (grain edges) is observed as a result of the thickness scaling. The stress measurements show that thin film is under higher tensile stress than thick film after the complete formation of C49-TiSi$\sb2.$ It has been suggested that the nucleation of the C54-TiSi$\sb2$ is the rate-limiting step in the overall transformation. The observed kinetics difference and nucleation-mode change due to the reduction in film thickness is discussed based upon the energetics consideration of nucleation at different geometrical sites, nucleation site density, and the effects of surface and stress.Evolution of the Ti/a-Ge/Si trilayer reactions has been investigated using TEM and AES. The amorphous phase formation is eliminated by introducing an a-Ge interlayer. The crystalline Ti$\sb6\rm G\sb5$ is the first phase observed during the reaction. Preceding the equilibrium C54-Ti(Si,Ge)$\sb2,$ a substitutional solid solution type C49-Ti(Si,Ge)$\sb2$ forms upon annealing at $550\sim600\sp\circ$C, regardless of the replacement of amorphous phase by the crystalline phase. The same type of C49-to-C54 polymorphic transformation occurs at ${\sim6}50\sp\circ$C. The reaction path is also correlated with the change in film resistance obtained from a four-point sheet resistance measurement. (Abstract shortened by UMI.)U of I OnlyETDs are only available to UIUC Users without author permissio