Spectroscopic ellipsometry study of thin diffusion barriers of TaN and Ta for Cu interconnects in integrated circuits

The objective of this work is to study the optical and electrical properties of tantalum nitride and tantalum barrier thin films used against copper diffusion in Si in integrated circuits using spectroscopic ellipsometry in the VUV and UV-visible range. Single layers of tantalum nitride and bilayer films of Ta/TaN were produced by reactive magnetron sputtering on Si(100) substrates covered with a native oxide layer. Ellipsometric measurements were performed in the energy range from 0.73 - 8.7 eV and the dielectric functions were simulated using Drude-Lorentz model and effective medium approximation (EMA) in order to obtain information regarding film thickness, film composition, free carrier plasma energy, mean relaxation time and electrical resistivity. The film thickness clearly affects the electrical resistivity and the electron mean free path. It was observed that for films of Ta on TaN even after maintaining the deposition condition suitable for theBeta-phase of Ta, it turned out to be a mixture of Alpha- and Beta-phases with higher contribution of the Alpha-phase. It is shown that even a very small intermixture of two different phases of Ta can be determined accurately using ellipsometry.