Nitrogen profiles of ion-implanted TiN films by electron energy loss spectroscopy

Titanium nitride (TiN) films are prepared by implanting 40 keV N2+ ions in silicon wafers coated with 60 and 80 nm thick titanium layers. These films are of interest as diffusion barriers in metallization schemes for very large scale integrated devices. After the implant, the specimens are annealed in vacuum either at 700 or 800 0C, giving rise to the formation of a TiSi2 film at the TiN/Si interface in the 80 nm thick nitride layer. No such silicide film grows in the case of the 60 nm thick nitride layer. This phenomenon is related to the different concentrations of nitrogen at the TiN/Si interface in the two cases. Measurements of the nitrogen profile by electron energy loss spectroscopy (EELS), performed in a dedicated STEM on cross-sectioned specimens, shows that in the thicker films, where the TiSi2 formation occurs, the nitrogen concentration at the TiN/Si interface is lower than about 35 at%; this is, in turn, reported to be the threshold concentration for the formation of the stable f.c.c. phase of titanium nitride. The thinner nitride films are subsequently coated with an aluminium overlayer, about 1 μm thick, and then annealed in N2 at 600 0C for 30 min; this treatment is known from previous works to result in the degradation of the diffusion barrier. The corresponding morphology of this Al/TiN/Si structure and the nitrogen profile are presented. (author)