Electroless deposition of diffusion barriers for Cu metallization is an attractive process as it is selective, deposits conformal films at a low temperature and enables seedless Cu deposition. We demonstrate electroless deposition of conformal, ultra-thin (similar to10 nm thick) films Of Co0.9W0.02P0.08. Electroless Co0.9W0.02P0.08 is an effective barrier against Cu diffusion up to 450 degreesC as opposed to physical vapor deposited (PVD) Co, which is a poor barrier. In this study, the role of microstructure in determining the barrier properties is discussed. The microstructure of the as-deposited layers consists of nanocrystallites of hexagonal close-packed (hcp) Co and an amorphous CoWP component. The amorphous component crystallizes at approximately 290 degreesC to hcp Co. The orthorhombic Co2P phase nucleates at 420 degreesC, while the majority phase remains hcp Co. Since we have found that up to 450 degreesC there is no phase formation between Cu and the Co0.9W0.02P0.08 film, we conclude that the mechanism of barrier failure is grain boundaries diffusion. The dependance of Cu grain boundary diffusivity on the microstructure is qualitatively demonstrated by comparing between electroless deposited Co0.9W0.02P0.08, Co0.9P0.1 and PVD cobalt. Secondary ion mass spectrometry depth profile measurements were performed on the films after subjecting them to anneals at 400 degreesC resulting in type-C Cu grain boundary diffusion. The Cu diffusivity in the Co0.9W0.02P0.08 film is lower than in Co0.9P0.1, and substantially lower than in PVD Co. The difference in Cu diffusivity is explained by varying degrees of grain boundaries' passivation due to the P and W alloying elements enriching the grain boundaries. This passivation effect is more pronounced in the Co0.9W0.02P0.08 films. (C) 2003 Elsevier Science B.V. All rights reserved.