The effect of cupric ion concentration on the nucleation and growth of copper on RuTa seeded substrates

The electrochemical nucleation and growth of copper on a ruthenium-tantalum (RuTa) alloy from acid copper sulfate solutions has been studied. The mechanism of copper nucleation and growth during galvanostatic deposition was investigated as a function of cupric ion concentration, [Cu2+], and deposition current density. The island density (Np) increased significantly when lowering [Cu2+] while keeping the other parameters constant. An increase in the deposition current density resulted in higher island density up to a certain limit where no significant change in the Np was observed. Both effects are explained by a direct correlation between the island density and the copper deposition overpotential (η). It is concluded that the use of low copper concentration baths could be beneficial for direct plating of small features for future interconnect metallization schemes.