Deciphering the levelling mechanism of sorbitol for copper electrodeposition via electrochemical and computational chemistry study

A combined method using electrochemical analysis, molecular dynamic simulation and quantum chemical calculation was employed to reveal the effects of sorbitol on copper electrodeposition from the pyrophosphate bath. It was demonstrated that sorbitol is a desirable leveler. A possible mechanism responsible for the levelling improvement was that sorbitol displayed lower adsorption energy on the electrode surface than pyrophosphate, due to its smaller energy gap. Therefore, adding sorbitol into the pyrophosphate bath could significantly elevate the adsorption capacity of copper pyrophosphate complex ions {[Cu(P₂O₇)₂]⁶⁻} on cathode, thereby increasing the diffusion coefficient (D) of [Cu(P₂O₇)₂]⁶⁻ and the number density (N) of copper nuclei. And the increased D and N are beneficial to improve the bath's levelling power. It should also be pointed out that sorbitol did not influence the purity and electrical conductivity of copper deposit as well as its nucleation mechanism, i.e., an irreversible and instantaneous nucleation process with three-dimensional growth of nuclei controlled by the diffusion of [Cu(P₂O₇)₂]⁶⁻.