The impact of organic additives and pulse-plating parameters on the initial stages of copper electrodeposition on Ru is characterized. Microscopy is used to observe 7-15 nm thick Cu deposits, prior to complete coverage of the substrate. Because the nucleus density is very high for the conditions studied here, the counting of individual Cu islands is difficult and an alternative method to analyze the images is presented. Results are compared for different additives for continuous plating and pulse-piating conditions. Pulse plating has a significant impact on the nucleus density. Replacement of Cl- ions with Br- ions of the same concentration yields an increase in the nucleus density at the same current density. A PEG-PPG-PEG block copolymer, when used instead of PEG as a suppressor, appeared to result in high nonuniformity in particle size. (C) 2008 Elsevier Ltd. All rights reserved.