MECHANISM FOR ION-BEAM PASSIVATION OF COPPER SURFACES

Irradiation by energetic ion beams has been observed to produce resist ance to oxidation of the surface of copper and other metals. The effec t was initially assumed to derive from a carbonaceous buildup produced when the surface was irradiated in a poor vacuum. However, we have fo und substantial protection of copper surfaces to be associated with a surprisingly thin surface layer containing carbon and having a thickne ss less than 30 Angstrom. The evidence suggests that the ion beam serv es to dissociate the contaminant layer of hydrocarbon adsorbates found on metal surfaces exposed to room atmosphere prior to implantation, l eading to blocking of sites on the coffer surface where oxygen would o therwise be adsorbed. In this experiment, the role of carbon in copper passivation has been tested by comparing the effect of ion bombardmen t of a copper surface during which carbon is totally excluded, The res ulting lack of passivation lends to the conclusion that sub-monolayer ion bombarded carbon plays a critical role in ion beam passivation of copper surfaces. A model for the process is proposed in terms of graph itic carbon lodged at preferred sites on the copper surface.