With the implementation of copper instead of aluminum as the metallization layer in high performance integral-ed circuits, the use of gas assisted etching of copper for focused ion beam (FIB) based failure analysis and circuit rewiring becomes important. In the present study the effects of exposing a copper substrate with a mixture of chlorine (Cl-2) and anhydrous ammonia (NH3) during ion bombardment have been investigated. The exposure of the copper surface to Cl-2 or to NH3/Cl-2 mixtures leads to the formation of a reaction layer. The thickness of this layer and its texture depends on the FIB parameters such as ion beam dwell time, Cl-2 pressure, and the NH3 to Cl-2 flux ratio. In addition, the experiments indicate that the formation of the reaction layer is enhanced in areas that have been previously exposed (i.e., damaged) with the ion beam. The etch yield shows a strong dependence on the ion beam dwell time and the gas flux. For short dwell times and low NH, and Cl-2 flux a 12-fold increase over physical sputtering could be achieved. With increasing Cl-2 flux the etch race decreased and the maximum in the etch yield shifted to longer dwell times, indicating a change in the rate limiting step of the ion induced reaction sequence.