Electro less Copper Deposition on Metals and Metal Suicides
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MRS BULLETIN/AUGUST 1994
can treat the deposition process starting from this layer onward. Initiation of Electroless Copper Deposition To form the basis for the discussion of process compatibility, interfacial integrity, and evolution of film microstructure, we begin with the initiation process—the foundation from which the copper growth stems. Electroless deposition is a metallization
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Potential, V vs. SCE Figure 1. Catalytic activities of metals (the potentials at 10~4 A/cm2) for anodic oxidation of different reductants. Er: oxidation-reduction potentials of reductants. H2: reversible hydrogen potentials. Conditions: (a) 0.2 M NaH2PC>2 + 0.2 M Na-citrate + 0.5 M H3BO3, pH 9.0, 343 K; (b) 0.1 M HCHO + 0.175 M Na2EDTA, pH 12.5, 298 K; (c) 0.03 M NaBH4 + 0.175 M Na2EDTA, pH 12.5, 298 K; (d) 2.0 mg/cm3 DMAB + 0.2 M Na-citrate + 0.5 M H3BO3, pH 7.0, 298 K; (e) 1.0 M N2H4 + 0.175 M Na2EDTA, pH 12.0, 298 K. (Reference 12, with permission to reprint from the Journal of The Electrochemical Society.)
method in which the electrons required for the metal reduction are supplied by the simultaneous oxidation of a reducing agent in the solution. The process must be thermodynamically favorable; i.e., the redox potential of the reducing agent must be more negative than that of the metal being plated. In addition, for the process to take place heterogeneously, the metallizing surface must be energetically favorable for the oxidation of the reductant, and it must also be electronically conducting for the transfer of electrons to take place. Electroless copper deposition on copper is autocatalytic. By definition, this means that the reductant can continually adsorb and oxidize on the depositing surface, thereby indefinitely sustaining the coupled oxidative-reductive reactions. Usually, however, the surfaces to be metallized in silicon integrated circuits are not copper covered. Therefore, we must consider the initiation of copper deposition on a dissimilar surface. Deposition can begin in a number of ways. The most well-known method is substrate catalyzation. This refers to the deposition from a seed layer which has the catalytic properties to invoke the initial anodic oxidation of the reductant. These are generally metals that are known to be hydrogenation-dehydrogenation catalysts. For the reductant formaldehyde, Bindra and Roldan11 related this catalytic activity to the enthalpy of formation of the metal formate, which they proposed to be a reaction intermediate in the oxidation pathway. Ohno et al.u also evaluated this catalytic activity of various metals and ordered them for different reductants in various electrolytes based on the electrode potentials measured at a refere
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