The presence of weak interfaces in Cu metallization, due to the fact that Cu does
not form a strongly adhering native oxide, means that optimization of interfacial
adhesion strength between Cu and the capping layer is critically important. Studies
have shown that improvements in interfacial adhesion strength will improve EM
performance. Also, when the interfacial adhesion is extremely good, as the case with Co-cladding of the Cu, the EM performance improves dramatically and the Cu EM
performance can then be primarily limited by bulk diffusion, with a corresponding
increase in activation energy Q.