A novel inorganic leveler of sodium azide for high aspect ratio through-holes uniform Cu thickening with high throwing power by electronic plating

The uniform thickening of high aspect ratio (AR) through-holes (HAR-TH, AR: 10–12:1) by Cu electronic plating (Cu-EP) is essential in printed circuit boards fabrication, and is often accomplished with organic levelers and at low current densities (6–15 mA/cm 2 ). Nevertheless, the organic levelers are often unstable and the Cu-EP at low current densities presents low production efficiency. In this study, we demonstrate that inorganic compound sodium azide (SAD) can act as a novel leveler in acidic Cu-EP for achieving uniform thickening of HAR-THs (AR: 12:1) even at a high current density of 20 mA/cm 2 . The throwing power using SAD for HAR-THs (AR: 12:1) thickening reaches as high as 94.7 %. Galvanostatic measurements and cyclic voltammetry tests on Pt rotating disk electrode (Pt-RDE) reveal that the competitive adsorption among the three additives of SAD, PEG and SPS occurs only at high rotating rate, referring to a strong inhibition of cupric ions reduction at violent convection area of the HAR-THs mouth. In situ FTIR spectroscopy analyses combined with DFT theoretical calculations illustrate that the synergistic effect of SAD and PEG is manifested by the formation of a PEG-SAD co-adsorption layer. The key to achieving uniform thickening lies in the difference in competitive adsorption abilities between the PEG-SAD co-adsorbed layer and SPS at both mouth and middle part of the HAR-THs. The PEG-SAD co-adsorption layer occupies all active sites of the HAR-THs mouth, preventing SPS from adsorption and thus reducing Cu deposition rate. However, the SPS firmly occupies active sites of middle part of the HAR-THs, facilitating to maintain a consistent Cu deposition rate inside the HAR-THs, consequently achieving uniform Cu thickening of the HAR-THs.