Improved diffusion barriers for copper metallization obtained by passivation of grain boundaries in electroless deposited cobalt-based films

The mechanism for improved barrier properties against copper diffusion in electroless deposited Co0.9W0.02P0.08 and Co0.9P0.1 thin films compared to physical vapor deposition (PVD) of cobalt is quantitatively explained. Secondary ion mass spectrometry depth profile measurements were performed on the films deposited on copper substrates after subjecting them to thermal anneals at approximately half the melting temperature of cobalt. A steady-state mode was observed in the form of concentration plateaus which originate from a combined contribution of grain-boundaries' saturation and copper solubility in the grains. The difference in plateau heights between the samples is assigned to the varying degrees of grain-boundaries' passivation. For Co0.9W0.02P0.08, the copper concentration in the grain boundaries is negligible and the solubility in the temperature region between 550 and 700 degreesC may be described as C(S)similar or equal to6x10(2)xexp(-0.52 eV/kT) at. %. The higher-copper concentration plateaus in the Co0.9P0.1 films are a result of a low, non-negligible copper concentration at the grain boundaries, estimated at less than 10 at. %. The copper concentration plateaus in PVD cobalt were significantly larger due to saturation of the grain boundaries. (C) 2002 American Institute of Physics.