Investigation of azathioprine and its local-structure molecules as levelers for through-holes copper electroplating

Azathioprine (Aza) and its local-structure molecules of 1-methyl-4-nitroimidazole (IMI) and 6-mercaptopurine (Merc) were performed as potential levelers for through-holes metallization by copper electroplating. Galvanostatic measurement tests of above three levelers were conducted to find out that Aza exhibited the largest adsorption strength on the surface of copper. The adsorption of Aza was strongly related to convection intensity, particularly when concentration of Aza in the plating bath reached >7.2 μmol/L. Molecular dynamics simulations were carried out to gain insights into the adsorption behavior of Aza, IMI and Merc, and results revealed that when Aza adsorbed on the copper surface, the imidazole part of Aza played a significant role. This finding was consistent with quantum chemical calculations, which indicated that the lowest unoccupied molecular orbital (LUMO) of Aza was predominantly localized on the imidazole moiety, underscoring its importance in the electron transfer process. Electroplating tests were then performed and the throwing power (TP) of through-holes with aspect ratio of 10:1 reached 106 % with Aza-based formula, compared to the TP of 26.9 % obtained from leveler-free formula. Physical properties of copper deposits, including surface morphology, roughness, and structure orientation, indicated that Aza could decrease roughness of electroplating copper and promote growth along the Cu(111) orientation. In this way, Aza has been proven to be a more effective leveler in through-hole plating than its local-structure molecules of IMI and Merc.