The role of microstructure in nanocrystalline conformal Co _0.9 W _0.02 P _0.08 diffusion barriers for copper metallization

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 (∼10 nm thick) films of Co _0.9 W _0.02 P _0.08 . Electroless Co _0.9 W _0.02 P _0.08 is an effective barrier against Cu diffusion up to 450 °C 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 °C to hcp Co. The orthorhombic Co ₂ P phase nucleates at 420 °C, while the majority phase remains hcp Co. Since we have found that up to 450 °C there is no phase formation between Cu and the Co _0.9 W _0.02 P _0.08 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 Co _0.9 W _0.02 P _0.08 , Co _0.9 P _0.1 and PVD cobalt. Secondary ion mass spectrometry depth profile measurements were performed on the films after subjecting them to anneals at 400 °C resulting in type-C Cu grain boundary diffusion. The Cu diffusivity in the Co _0.9 W _0.02 P _0.08 film is lower than in Co _0.9 P _0.1 , 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 Co _0.9 W _0.02 P _0.08 films.