A 20 nm thick Cu_3Sn intermetallic compound overlayer on Cu interconnect surfaces was found to effectively block dominant surface diffusion paths, thus resulting in close to one order of magnitude improvement in electromigration lifetimes. This improvement may be explained on the basis of the terrace-ledge-kink model in which the supply of Cu adatoms by the dissociation of atoms from the kinks on the Cu surface steps is hindered by a stronger chemical binding of Sn atoms to the kink sites. The mode of electromigration failures seem to have changed from surface diffusion-induced void formation at the cathode via corner to interfacial and grain-boundary diffusion-induced void formation in the interconnect line.
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