Frequency effect of pulse plating on the uniformity of copper deposition in plated through holes

High- and low-frequency pulse current (PC) and pulse-reverse current (PR) were employed to the plated-through-hole (PTH) process. To understand the role of additives in PC and PR, chloride ions, polyethylene glycol (PEG), and 3-mercapto-1-propanesulfonate (MPS) were used as additives and compared to baths without additives. Linear sweep voltammetry, cyclic voltammetry, and impedance analysis were employed to characterize this system on a rotating disk electrode. In the bath without additives, it was found that deposition uniformity in PTH with high-frequency PC and PR at 20 mA/cm2 average current density were better than that with low-frequency PC and PR, even better than that of dc at 10 mA/cm2. Based on impedance analysis, plating at low frequency is dominated by the diffusion of cuprous or cupric ion while plating at high frequency is dominated by Cu2+ + e- - Cu+, resulting in the deposition improvement by high-frequency PC and PR. In the presence of PEG + MPS + Cl, the uniformity improves with dc at 20 or 10 mA/cm2. The trend in the baths with additives using PC and PR plating at high and low frequency is similar to that in the bath without additives. However, the improvement by high-frequency PC and PR is insignificant, since the plating is under adsorption control.