Cobalt electrochemical deposition, with its bottom–up growth properties, is a core technology for creating metal interconnects. Additives are crucial during electrodeposition to control the quality of deposits by adsorbing onto the Co surface. The functional groups of additive molecules are the key to tailoring the adsorption behavior. This study focuses on heteroaromatic thiol additives, including 2-mercaptobenzimidazole (MBI), 2-mercapto-5-benzimidazolesulfonic acid sodium salt dehydrate (MBIS), and 2-mercaptobenzoxazole (MBO). Cyclic voltammetry, chronopotentiometry, quantum chemical calculations, and characterization tests were employed to investigate the adsorption behavior of additive molecules with different functional groups on cobalt. The results indicate that the inhibitory strength of the three additives on electrodeposition follows the following order: MBI > MBIS > MBO. The strong inhibitory effect of MBI primarily stems from the adsorption of the thiol group, the pyridine-like nitrogen in the heterocycle, and the benzene ring. MBIS exhibits reduced inhibitory capability due to the combined effects of the sulfonic acid group and hydrolysis ionization. MBO, with the introduction of an oxygen atom in the heterocycle, shows the weakest adsorption and inhibitory capability among the three.

Copper interconnect technology faces limitations due to the electron’s mean free path and electromigration, driving the adoption of cobalt alternatives. This study proposes a novel mechanism to achieve superfilling by tuning the adsorption energy of additive molecules on cobalt surfaces. The adsorption energies of additives are tailored by changing molecular structures with different functional groups. Computational results reveal that carbon–carbon triple bonds critically strengthen adsorption, while ether bonds further enhance binding on distinct cobalt crystallographic planes. Specifically, 1,4-bis(2-hydroxyethoxy)-2-butyne (BEO) containing both triple bonds and ether groups exhibits the highest adsorption energy (−22.62 eV). Replacing ether with hydroxyl groups in 2-butyne-1,4-diol (BOZ) reduces the adsorption energy to −10.39 eV, while eliminating triple bonds in 1,4-butanediol diglycidyl ether (BDE) further decreases it to −8.43 eV. Experimental studies demonstrate that BOZ and BEO preferentially adsorb on the (101) and (110) planes of hexagonal close-packed cobalt (HCP-Co) due to their differential adsorption energies. This selective suppression promotes preferential growth along the densely packed (002) orientation. This leads to a trench-filling process dominated by the most densely packed plane, resulting in better electrical performance. Superfilling is achieved when molecular adsorption energies are in the range of 5–8 eV. The work establishes a functional group design strategy to regulate additive adsorption, enabling crystallographic control for advanced cobalt electrodeposition processes.