Numerical simulations are employed to estimate the importance of parameters such as leveling-additive mobility and concentration on a process's capability of filling trenches or vias of a given size. Geometries relevant to ultralarge scale integration-copper technologies are considered. Simulation results are presented within the context of a suggested experimental protocol. The importance of controlling the operating conditions is demonstrated. For example, simulations indicate that a poorly controlled additive concentration can lead to an increase in void size over that which would be obtained in an additive-free bath. Finally, the design of robust processes is discussed in terms of the need to account for apparently random variations in the process.