Etching mechanisms of Cu thin films under the exposure of an argon ion beam and halogen molecules (Cl2, Br2) were examined in a UHV system using a quartz crystal oscillator and x-ray photoelectron spectrometer. Chlorine exposure at room temperature produced a very stable chlorine overlayer on the Cu surface. The passivation of Cu by chemisorbed Cl reduced the etch rate of Cu film under argon ion bombardment for pressure up to 10−3 Torr of chlorine. Cuprous chloride was formed at 210 K, where multilayers of Cl2 are adsorbed on the Cu surface. Br2, however, diffused into the Cu substrate at 10−6 Torr at room temperature. A linear growth rate of the halide film was observed. The etch rates of copper were not enhanced by ion bombardment in the presence of either halogen. Large amounts of free halogens were incorporated in the halide films. These results suggest that the halogenation of a copper surface is dominated by the unactivated place exchange mechanism with constant sticking coefficient. The relative flux ratios of neutral and ion species as well as surface temperature were found to play important roles in halide formation and in the etching of copper.