Super-flat and uni-oriented cobalt film electrodeposited by modulating the crystal nucleation and growth behavior

A precise control of the microstructure of electrodeposited cobalt film is crucial for attaining the desired and reproducible electrical performance in edge nanoelectronic devices. Here we realize the super-flat and unioriented cobalt film through a one-step electrodeposition process using benzenesulfinic acid (BNS) as additive. We find that the unsaturated sulfite group in BNS plays a key role in the super-flat cobalt electroplating. Molecular activity analysis and electrochemical test results suggested that the electron donating ability of the sulfite group was higher than that of the sulfonate group, resulting in stronger adsorption of BNS on the cobalt surface and enhanced cathodic polarization. Nucleation model fitting and surface morphology evolution studies revealed that the cobalt nucleation with BNS shifted to progressive nucleation from instantaneous nucleation in the additive-free electrolyte, leading to a more uniform and compact film morphology. Furthermore, equilibrium adsorption calculations indicated that the selective and strong adsorption of BNS on cobalt planes was responsible for the preferential orientation of the (002) crystal plane growth. After annealing, cobalt films prepared with BNS exhibited substantial grain growth and impurity diffusion, which further improved the electrical performance.