Why is hydrochloric acid added to copper electroplating solution, and what is its function?
In a certain TSV copper wafer electroplating solution composed of: copper sulfate, sulfuric acid, hydrochloric acid, leveling agent, brightener, and suppressor, the role of hydrochloric acid is mainly to provide Cl⁻ ions, with two functions:
- An appropriate amount of chloride ions interacts with organic additives to achieve a bright and smooth coating.
- Cl⁻ promotes the dissolution of the copper anode and prevents the formation of passivating films such as Cu₂O, ensuring a continuous source of Cu²⁺ and avoiding anode polarization. Electrochemical reactions:
Cu⁰ + Cl⁻ → Cu⁺ + Cl⁻ → CuCl or CuCl₂⁻
Chloride ion concentration?
Cl⁻ concentration ≈ 30–80 ppm
What problems occur if chloride ion content is too low or too high?
Too few chloride ions or an excess of brightener can both result in a non-bright coating in low current density areas.
If there are no other organic additives, an excessively high Cl⁻ concentration can promote the deposition of insoluble complexes like CuCl on the copper surface, hindering further deposition and resulting in a rough coating with increased stress.
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