It was recalled that an improvement in the diffusion barrier performance for Cu metallization, by inserting a thin Al layer between two TiN layers, had been clearly demonstrated. The key idea behind the scheme was the so-called stuffing of grain boundaries of columnar TiN films by Al2O3. It was also found that the barrier property was at its best when the Al thickness was 1nm but, at above this value, the barrier performance degraded markedly when the upper TiN film was not pre-annealed. The question of why the barrier broke down at above 1nm of Al interlayer thickness was investigated here. High-resolution transmission electron microscopy, scanning transmission electron microscopy, and energy dispersive spectroscopy analyses revealed that the rapid diffusion of Cu in the presence of the free Al was the main reason for the failure of the present diffusion barrier scheme. These results were explained in terms of the differences between the movements of Al and Cu through the TiN film, and the differences between the solid solubilities of Al in Cu and in Si. The results showed that both Al interlayer thickness and the O content in TiN film should be properly controlled in order to take full advantage of the present multilayer diffusion barrier scheme.

Failure Mechanism of a Multilayer (TiN/Al/TiN) Diffusion Barrier between Copper and Silicon. S.H.Kim, K.T.Nam, A.Datta, K.B.Kim: Journal of Applied Physics, 2002, 92[9], 5512-9