Achieving high nucleus density is key to the success of direct electrodeposition of copper in small, high-aspect-ratio features. The impact of the Fe(III)/Fe(II) redox couple on the nucleus density during the galvanostatic electrodeposition of copper is investigated. Scanning electron microscopy (SEM) is utilized to observe the copper nuclei on the sample surface. Experimental data show that the addition of the iron redox couple can increase the nucleus density by. a factor of 3 to 5. This enhancement however, is not strongly correlated with an increase in overpotential. Additionally, CuSO4 concentration, substrate material, suppressor type and current density have also been shown to influence the nucleus density. (C) 2013 The Electrochemical Society. All rights reserved.