Co-alloy films with various solution compositions [CoB, CoWB, and CoW(B)P] were deposited with an electroless technique on Cu films without Pd activation, and their oxidation barrier performance was analyzed. The degrees of oxidation of all films were intensively studied. CoB showed excellent capping performance as an oxidation barrier, whereas CoWB and CoW(B)P exhibited even poorer oxidation resistance than the case of bare Cu at 400 degrees C. The depth profile of the film compositions and chemical states of the CoB film before and after oxidation was investigated, the results of which suggested that the oxidation of the B component in the film had a clear role in the prevention of continuous Cu diffusion to the surface. The multilayer structure of CoB/CoW(B)P/Cu for obtaining both electromigration and oxidation resistance was optimized, showing excellent oxidation resistance comparable to a single-composition CoB film. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3133219] All rights reserved.