A controlled atmosphere polishing (CAP) system was used to determine the effects of various chamber gases on copper chemical mechanical polishing (CMP) in the presence and absence of NH4OH and H2O2. Using 500 kPa oxygen or nitrogen has only slight effects on copper removal rates in the presence of 1 wt % H2O2. Polishing without H2O2, performed with controlled oxygen partial pressure, demonstrates removal rates that are 4 times higher than using nitrogen. Polishing using inert gases alone demonstrates an oxidant-starved system that reflects little dependence on wafer pressure or velocity. Addition of NH4OH (pH 10) to experiments using oxidizing gases, such as oxygen and air, increases removal rates up to 3 X. Removal rates vary linearly with oxygen partial pressure using oxidizing gases for experiments using NH4OH at pH 10. A trend indicating a transition from chemical to mechanical control is observed when NH4OH concentration is increased at constant oxygen pressure. A copper removal mechanism in the presence of dissolved oxygen has been developed that highlights a buildup of oxidized copper at the wafer surface. The ability to perform CMP in a pressurized gaseous environment has shown that copper removal is a process of mechanical removal, dissolution of abraded material, and copper-oxygen reactions at the wafer surface. (c) 2005 The Electrochemical Society.