The electrochemical behavior of levelers was studied and compared for two commercial Cu plating chemistries in an effort to correlate the electrochemical behaviors with their impacts on bottom-up filling, impurity incorporation, and grain structures. While a strong complexing between leveler and accelerator resulted in a leveler-sensitive bottom-up filling rate and low impurity level in the deposit, a traditional non-interacting leveler showed little impact on the filling performance and yielded a high impurity incorporation. An oscillatory behavior was reported for the strongly-interacting leveler chemistry during galvanostatic plating, this oscillation manifested itself in both the potential and impurity incorporation. High impurity incorporation is known to inhibit the Cu grain growth; a laminated structure with alternating layers of big and fine Cu grains was obtained by annealing the Cu films plated with the oscillatory behavior. © 2012 The Electrochemical Society. All right reserved.