The effects of acetic acid, tartaric acid, and Pb-mediated underpotential deposition (UPD) on the filling characteristics of copper electrodeposition in sub-micron trenches were studied. In the absence of other electrolyte additives, potentiostatic and galvanostatic depositions from electrolytes containing organic acids were capable of producing dense filling in sub-micron trenches due to higher deposition rates at the trench bottoms compared to the side-walls and adjacent surfaces surrounding the recessed features. The mechanism for dense filling of trenches from depositions in the organic acid containing electrolytes did not appear to match previously identified mechanisms for superfilling associated with surfactant effects. Instead, the differential deposition rates across the trench profile were attributed to textural and/or structural differences in the copper seed layer as the copper deposition rates were enhanced at both the bottom and adjacent surfaces surrounding the trenches, or sidewalls of the trench depending on the organic acid species present in the electrolyte. The organic acids induced conformal and smooth depositions improving with increasing overpotential. Pb mediated UPD yielded smooth, conformal films relative to non-mediated deposition under otherwise identical conditions. The density of the UPD mediated films was best at higher Pb coverage during Cu deposition. (C) The Author(s) 2017. Published by ECS. All rights reserved.