What is Concentration Polarization in Wafer Plating?

Taking copper plating as an example, the metal ions in the plating solution are reduced and deposited on the cathode (wafer), and the chemical equation is:

Cu(2+) + 2e --> Cu(s)
Due to the faster deposition rate, the concentration of Cu²⁺ near the wafer surface decreases rapidly, while the concentration in the area of solution away from the wafer remains high. This creates a "diffusion layer", resulting in ion diffusion not keeping up with the deposition rate→ local lack of metal ions on the electrode surface→ electrochemical reaction rate limited by diffusion velocity rather than current intensity.

0010-20351 6 INCH DEGAS LAMP MODULE 350C PVD

In a word: concentration polarization refers to the potential difference caused by the phenomenon of "concentration polarization" on the surface of the electrode (wafer) due to the rapid reduction and deposition of metal ions, and the fresh metal ions have no time to replenish the surface of the cathode (wafer), and its concentration is gradually lower than the concentration of the main body of the solution.

Why do we need to try to reduce concentration polarization?
Concentration polarization can cause:
1,The deposition rate decreases → the current efficiency decreases, and the current is wasted on side reactions such as hydrogen evolution.

The coating is rough, pinholes, the coating is gray and dark, etc., and the additives cannot play their due role.

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How to eliminate plating concentration polarization?
1,Rotate the wafer
2,Strong convection
3,Stir on a cycle