We describe a process to selectively remove a preadsorbed 3-mercapto-1-propane-sulfonic acid accelerator from nonrecessed areas of a damascene Cu wafer by oxidation with aqueous O-3, prior to electroplating. Topographically, selective oxidation was achieved by exploiting competition between diffusion-controlled heterogeneous reaction and homogeneous decay of O-3 in a thin, viscous, hydrated coating on the surface of the wafer. For wide recessed features (10-100 mu m), the resulting differential enhancement of Cu plating approaches a limit governed only by the activity of the accelerator-suppressor combination. (C) 2008 The Electrochemical Society.