Nucleation properties of magnetron-sputtered tantalum

The role of the crystalline and chemical nature of the substrate surface in the formation of either β-Ta or b.c.c. tantalum deposited by magnetron sputtering was studied. The β-Ta structure was observed on all the substrates studied (including platinum and gold) except titanium which has a crystal structure very close to that of b.c.c. tantalum. The results contradicted the nucleation theory that the presence of oxygen or OH on the substrate surface is necessary for the formation of β-Ta. However, the substrates with a high resistance to oxidation tended to nucleate the b.c.c. phase and the substrates which readily form oxides tended to yield β-Ta. The exposure of the substrate surfaces to the air resulted in films which contained a higher percentage of β-Ta. Conventional d.c. sputtering of tantalum onto substrates caused the diffusion of underlying materials into the newly deposited tantalum, due to the excessive substrate heating of the resultant alloy, and b.c.c. tantalum formation. However, a thin surface oxide on the substrate prevented the subsequently deposited tantalum from diffusing and the β-Ta phase appeared in the films. It is concluded that magnetron sputtering produces β-Ta film on non- oxidizing substrates as the low temperature deposition of tantalum in this case limits the diffusion between the films, resulting in low impurity levels in the tantalum films.