Taking copper electroplating as an example, metal ions in the plating solution are reduced and deposited on the cathode (wafer). The chemical equation is:
Cu²⁺ + 2e⁻ → Cu(s)
Due to the rapid deposition rate, the Cu²⁺ concentration near the wafer surface quickly drops, while the concentration in areas farther from the wafer remains high. This creates a “diffusion layer,” resulting in: ion diffusion cannot keep up with the deposition rate → local depletion of metal ions at the electrode surface → the rate of the electrochemical reaction is limited by the diffusion speed rather than current intensity.
One-sentence summary:
Concentration polarization refers to the phenomenon where metal ions are rapidly reduced and deposited on the electrode (wafer) surface, but fresh metal ions cannot replenish the cathode (wafer) surface in time, causing the local concentration to drop below that of the bulk solution. The resulting potential difference is called “concentration polarization.”
Why should concentration polarization be minimized?
Concentration polarization leads to:
- Decreased deposition rate → reduced current efficiency, with current wasted on side reactions (e.g., hydrogen evolution)
- Coating defects such as roughness, pinholes, gray or dull appearance, and ineffectiveness of additives
One-sentence summary: Even a skilled cook cannot prepare a meal without rice.
How to eliminate concentration polarization in electroplating?
- Rotate the wafer
- Strong convection
- Circulating agitation, etc.
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