Effect of the functional group of polyethylene glycol on the characteristics of copper pillars obtained by electroplating

Abstract

Five polyethylene glycol (PEG)-based compounds with varying terminal functional groups were investigated as suppressors to investigate the effect of different terminal functional groups on the ability of PEG-based suppressors to inhibit Cu pillar electroplating. The inhibition ability of the suppressors increased with increasing steric hindrance of the aromatic functional group and length of the alkyl group. PEG-based compounds bearing terminal functional groups with large steric hindrances significantly impeded the diffusion of electroplating accelerator, thus enhancing the inhibitory effect. When the molecular weight of PEG was less than 1000, possessing a terminal functional group had a greater effect on the inhibitory ability than increasing the molecular weight. The wetting ability of the electrolyte increased as the inhibitory ability of the suppressor increased. The significant steric effect of PEG p-(1,1,3,3-tetramethyl butyl)-phenyl ether was confirmed, which contributed to the convex shape of the Cu pillars and the growth of small-sized Cu grains. Conversely, the interaction between PEG and Cu electrode surface was weak. Thus, flat-top Cu pillars were obtained when PEG was used as the suppressor.