Cobalt silicides formation through a diffusion barrier

Abstract

Silicon (Si) has various applications in different technological fields as a structural material or a semiconductor. Cobalt disilicide is an attractive silicide for contact with Si because it has favourable properties such as low resistivity. Recently, a great deal of interest has been shown in thin film silicides, produced by the reaction of alloys of cobalt with a silicon substrate. In these systems the actual concentration of Co is diluted and the reaction pathway is changed from that of pure Co and Si reaction. Another way of influencing the reaction pathway is by use of a diffusion barrier. This study investigates solid-state reactions between Co thin films (126 nm) and single-crystalline Si substrate. Specifically, it examines the formation of cobalt silicides through diffusion barrier interlayers composed of iron-zirconium (FeZr). Samples with the standard thickness of Co thin films and various thicknesses of the same composition diffusion barrier (Fe90Zr10) were prepared through utilisation of the molecular beam epitaxial (MBE) deposition technique on Si substrates. These samples were annealed at temperatures of 400 and 450 °C for durations of 3 and 24 hours under high vacuum conditions. The results of the solid-state reactions are analysed by Rutherford backscattering spectrometry (RBS) before and after annealing, X-ray diffraction (XRD) using CoK? radiation, and scanning electron microscope (SEM) and some structures were characterized by Auger electron spectroscopy (AES) depth profiling. The formation of the various cobalt silicides found at diffusion barrier interlayers is then interpreted in terms of the reduced flux of reactant atoms at the reaction interface, which is due to the different thicknesses of the diffusion barriers. The RBS results indicate that the unannealed spectra fit well with those annealed at 350 °C, thus showing that no reactions took place. The XRD results reveal that, at temperatures of 400 and 450 °C, Co reacted with the Si substrate and formed a mixed layer of cobalt silicides, namely, CoSi and CoSi2. The SEM images revealed that the unannealed Co thin films had a granular surface with an average granule diameter of 20 nm. After annealing, the average granule diameter was observed to be larger than the unannealed value. However, after some annealing periods the granule diameters were observed to generally become larger and this substantiates cobalt silicide formation compared to the unannealed images. In some images the crystals bore visible black spots due to the presence of carbon in the interface, which is confirmed by the AES results. A cross-sectional SEM image of the Si<100>|FeZr(53 nm)|Co(126 nm) sample, prepared by liquid nitrogen (LN2) fracture and annealed at 450 °C for 24 h, reveals three different observable layers. These results were in good agreement with the information gained from the AES analysis. Concentration-controlled phase selection in solid-state reaction has been proposed as a model to interpret first-phase formation occurring at solid interfaces. This is done in the context of the effective heat of formation (EHF) model. The EHF model predicts, as the initial phase, the formation of Co2Si at the Co/Si interface if there is no barrier layer. In the case of this study, the presence of FeZr as barrier layer caused CoSi to be formed as the initial phase, thus bypassing Co2Si formation. The EHF model shows that the diffusion barrier can reduce the effective concentration of the Co atoms to a value where the effective heat of formation of CoSi is more negative than that of Co2Si. Thus, first-phase formation of CoSi is thermodynamically favoured. The results obtained in this study fit well the model if the initial phase Co2Si has been bypassed.