With the implementation of copper instead of aluminum as metallization layer in high performance integrated circuits, the use of gas assisted etching for focused ion beam (FIB) based failure analysis and circuit rewiring becomes increasingly important. In the present study the effects of exposing a copper substrate to a mixture of chlorine (Cl@sub 2@) and anhydrous ammonia (NH@sub 3@) during ion bombardment have been investigated. The exposure of the copper surface to chlorine or to NH@sub 3@-Cl@sub 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, gas pressure and the NH@sub 3@ to Cl@sub 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 an 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@sub 3@ and Cl@sub 2@ flux an up to 10-fold increase over physical sputtering could be achieved. With increasing Cl@sub 2@ flux the etch rate decreased and the maximum in the etch yield shifted to longer dwell times, indicating changes in the adsorption kinetics of the two gases.