Mechanism of SPS acceleration in a PEG containing copper plating bath
Deposition of copper in Through Silicon Vias (TSVs) is achieved by addition of organic compounds, which act as suppressors and accelerants to copper electroplating solutions. The focus of this study is on the acceleration effect of bis-(3-sodiumsulfopropyl) disulfide (SPS) which is believed to go through an electrochemical reaction to form sodium 3-mercapto-1-propanesulfonate (MPS), the effective accelerant. The accelerant is believed to operate by forcing the suppressor off the copper surface through competitive adsorption. The kinetics of MPS adsorption onto copper was studied by addition of MPS to solutions containing the suppressor molecule, polyethylene glycol (PEG), under constant overpotential and measurement of the resulting current increase. The electrochemical cleavage of SPS to form MPS was also studied. SPS was placed in an acid solution in the absence of cupric ions and an overpotential was applied to split the molecule. A new method for measuring MPS concentration was developed and used to determine the amount of MPS generated. An improved model for the acceleration effect of SPS on copper deposition was developed. First SPS diffuses to the metal surface at a rate that can be quantified by Fick's laws of diffusion. When the molecule reaches the electrode, it splits at the sulfur-sulfur bond in a reaction which is electrochemically driven. Higher overpotentials drive faster reaction rates. The resulting molecule adsorbs rapidly enough that coverage of the surface can be considered to be completely mass transfer controlled. The area covered by one MPS molecule on the saturated metal surface was determined to be 47 +/- 9 A&sup2;. This value is higher than would be expected only considering steric hindrance which suggests that repulsion between negatively charged sulfonate ends contributes to the spacing.
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