Size Control of Copper Grains by Optimization of Additives to Achieve Flat-Top Copper Pillars through Electroplating

An acidic cupric sulfate bath containing chloride ions, bis(3-sulfopropyl) disulfide (SPS), polyethylene glycol (PEG), and Janus Green B was employed to control the brightness of Cu films and the size of Cu grains for flat-top Cu pillars during a fast plating process. The plating potential, brightness of Cu films, and size of Cu grains were dominated by SPS and slightly affected by PEG at a current density of 5 A⋅dm−2. A shiny mirror-like Cu film with small Cu grains was formed upon increasing the concentration of SPS and PEG to 9 and 600 ppm, respectively; however, larger Cu grains and nodules were formed on the electrode surface when a current density of 7.5 A⋅dm−2 was applied. Using pulse experiments, we demonstrated that the size of the larger Cu grains and nodules can be reduced if the PEG molecules have sufficient time to adsorb on the surface of the electrode. In addition, the grain size of Cu decreased with decreasing the agitation speed if DC plating was used. A flat-top Cu pillar was achieved at a current density of 7.5 A⋅dm−2 when the additives were used at an optimized concentration.