The diffusivity of Cu in single-crystal (NaCl-structured) and polycrystalline TaN thin films, grown by pulsed-laser deposition, was investigated (figure 5). Polycrystalline TaN films were grown directly on Si(100), while single-crystal films were grown over TiN buffer layers. Both poly- and mono-crystalline films with Cu overlayers were annealed at 500, 600, 650 and 700C under vacuum in order to study Cu diffusion characteristics. The diffusion of Cu into TaN was studied by using scanning transmission electron microscopy Z-contrast, where the contrast was proportional to Z2 (atomic number), and transmission electron microscopy. The diffusion distances were found to be about 5nm at 650C for 0.5h annealing. The diffusivity of Cu into single-crystal TaN could be described by:

D (cm2/s) = 1.60 x 102 exp[-3.27(eV)/kT]

at 600 to 700C. It was observed that Cu diffusion in polycrystalline TaN thin films was non-uniform, with enhanced diffusivities along the grain boundary.

Copper Diffusion Characteristics in Single-Crystal and Polycrystalline TaN. H.Wang, A.Tiwari, X.Zhang, A.Kvit, J.Narayan: Applied Physics Letters, 2002, 81[8], 1453-5