Decoding the interfacial structure and functionality of an organic additive for superconformal cobalt electrodeposition

The interconnect metallization is crucial to attain signal and electrical transmission and thus a central process in chip manufacturing. Cu is the most commonly used interconnect metal by means of Damascene Cu plating, however, when a 14-nm (or below) process is adopted, electron scattering at external surfaces and grain boundaries leads to a sharp rise in the resistance of narrow Cu lines in the bottom layers. At the nanoscale, the resistivity of a metal is directly related to ρ_b×λ, where ρ_b represents the bulk resistivity of the metal and λ represents the electron mean free path. Co, Ru, Rh and Mo have lower ρ_b×λ values, thus are expected to exhibit lower resistances in narrow lines. In addition, their higher melting points compared to that of Cu, are also beneficial in reducing electromigration. In particular, Co interconnect by electrodeposition is highly promising for commercial applications because of its low cost and relative ease for plating among the four alternative metals. It is highly demanded to unravel the interfacial structure and functionality of the organic additives essential for electrochemical Co superfilling so as to develop new efficient ones.

Herein, interfacial electrochemistry for a model additive molecule MBIS (sodium 2−mercapto−5−benzimidazolesulfonate, validated for Co superfilling in the literature) at a Co electrode is initially investigated by applying in situ surface enhanced infrared absorption spectroscopy and electrochemical quartz crystal microbalance combined with density functional theory calculations and molecular dynamics simulations. The nearly vertical adsorption of MBIS on the Co surface via thiolate reduces the overall reduction current with a higher / lower faradaic efficiency (FE) towards HER / Co deposition due to the significant blockage of surface Co sites as well as the formation of Co(II) complex with MBIS and the increased interfacial free H₂O. Additionally, MBIS coverage exhibits dynamic responses to pH or potential variations, which may account for the location dependent functionality of MBIS in practical Co deposition in vias, qualitatively explaining cobalt's superfilling process. This research not only provides the first direct infrared spectroscopic characterization of interfacial adsorption of an additive for cobalt electroplating but also lays the theoretical foundation for constructing electro-superfilling simulation models based on adsorption kinetics through revealing dynamic coverage regulation mechanisms.