A model based on physical phenomena has been developed to improve the filling by pulse plating of copper into high aspect ratio features by using a copper complex which dissolves in solution and acts as a buffering agent. Mass transfer pulse-plating models are formulated to account for this effect and solved numerically to show the competition between step coverage and deposition rate. An important parameter, the Damkohler number, applied to filling nanoscale features for the first time, has been identified from the model and its effect on step coverage and deposition rate has been systematically studied. An analytical model is also developed that assumes linear deposition kinetics and neglects boundary movement due to deposition. It describes the initial part of the fill and is used to obtain important trends to guide the numerical simulations. A new criterion, based on the pulse-plating parameters, to obtain the optimal plating conditions has also been developed. I-D and 2-D profile simulations are performed to demonstrate the significant improvement in gap filling in the presence of a buffering agent. Copper has been deposited into submicrometer features using an alkaline ammoniacal bath. Model results have been compared to experimental observations with good correlation between them.