Pulsed Galvanostatic Electrodeposition of Copper on Cobalt Using a pH-Neutral Plating Bath and Electroless Seeds

Due to its notable retention of electrical conductivity in nanoscale features, cobalt is a leading material candidate for diffusion barriers in the sub-14 nm node copper interconnects. In this application, Co also facilitates high aspect ratio gap-filling by serving as a substrate for direct electrochemical deposition (ECD) of Cu. However, the Co-Cu ECD system is associated with several technical challenges, which include dissolution of Cu seeds (and Co) in conventional acidic baths, interference of hydrogen evolution, and solution-sensitive nucleation barriers. We investigate here this ECD system by using pulsed galvanostatic ECD in a neutral plating bath of CuSO4, with in situ electroless deposition of Cu seeds. Both seeding and ECD of Cu are enhanced with the use of a carbon based catalytic activator. These observations are explained in terms of a proposed mixed-potential mechanism. Nucleation of the ECD-Cu is detected with voltammetry, and further quantified as an instantaneous step by measuring the associated kinetic parameters with chronoamperometry. Spatially uniform layers of pulse-deposited Cu on Co are detected by electron microscopy, and the deposition rates are determined using energy dispersed X-Ray spectroscopy. Corrosion tests performed in a glycine-based solution for Cu planarization confirm the general CMP compatible electrochemistry of the ECD-Cu.