Seedless Copper Electrodeposition onto Tungsten Diffusion Barrier

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Copper was electrodeposited directly onto a sputter-deposited tungsten diffusion barrier layer in an acidic copper–ammonium–citrate bath. The nucleation characteristic of copper deposits according to applied cathodic potential was investigated. The nucleation mode, nuclei density, and dominant copper species to be preferentially reduced were critically dependent on the applied cathodic potential. Pure instantaneous or progressive nucleation of copper was not observed, but instead, mixed nucleation mechanisms appeared. For low cathodic potentials, the copper deposits, which were reduced mainly from cupric ions, showed consequently a progressive nucleation characteristic despite initial instantaneous nucleation. For large cathodic potentials, the instantaneous nucleation of copper was achieved from the reduction of copper complexes following the early stage of progressive nucleation. The copper nuclei density of calculated for filling the sub-70 nm damascene structures was approached by applying large cathodic potentials.