TANTALUM AS A DIFFUSION BARRIER BETWEEN COPPER AND SILICON - FAILURE MECHANISM AND EFFECT OF NITROGEN ADDITIONS

The interaction of Cu with Si separated by thin (50 nm) layers of tantalum, Ta2N, and a nitrogen alloy of Ta has been investigated to determine the factors that affect the success of these materials as diffusion barriers to copper. Intermixing in these films was followed as a function of annealing temperature by in situ resistance measurements, Rutherford backscattering spectra, scanning electron microscopy, and cross-section transmission electron microscopy. Ta prevents Cu-silicon interaction up to 550-degrees-C for 30 min in flowing purified He. At higher temperatures, copper penetration results in the formation of eta"-Cu3Si precipitates at the Ta-Si interface. Local defect sites appear on the surface of the sample in the early stages of this reaction. The Ta subsequently reacts with the substrate at 650-degrees-C to form a planar hexagonal-TaSi2 layer. Ta silicide formation, which does not occur until 700-degrees-C in a Ta-Si binary reaction couple, is accelerated by the presence of Cu. Nitrogen-alloyed Ta is a very similar diffusion barrier to Ta. It was found that Ta2N is a more effective barrier to copper penetration, preventing Cu reaction with the substrate for temperatures up to at least 650-degrees-C for 30 min. In this case, local Cu-Si reaction occurs along with the formation of a uniform Ta5Si3 layer at the Ta2N-Si interface.