In order to investigate the reactive ion etching mechanism of the copper films in CCl4/N-2 electron cyclotron resonance (ECR) plasma, the dependences of the copper etch rate on various etching parameters, the etch products as well as their depth distributions and the concentration of chlorine radicals in the plasma were examined. It was found that the etching species in CCl4 plasma is not CClx but atomic chlorine (Cl) and the etch product formed at the surface of the copper film is not CuCl but CuCl2. In order to carry out reactive ion etching of copper film in chlorine-based plasma the substrate temperature should be above 210 degrees C, below which the etch product has too low vapor pressure to be volatile. At the substrate temperature above 210 degrees C, the copper etch rate is not limited by the removal rate of the etch product but limited by its formation rate which depends on the concentration of chlorine radicals and the reaction rate between the etching species and the copper film. The etch rate is also increased by applying a negative bias to the substrate, the role of which is the enhancement of the formation rate of the etch product by activating chemical reactions due to energetic ion bombardment. Adding small amounts of CF4 to CCl4 plasma increases the etch rate dependence on ion bombardment energy by forming involatile copper fluoride on the etched surface.