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Does gold induce galvanic coupling in a copper etching system?

October 21, 2008

Q. Please help me settle a discussion I'm having with a colleague. We're etching nickel and copper with a hard gold mask in a copper chloride solution.

He says that the presence of gold increases the copper etch rate due to galvanic coupling. The gold, as a very effective sink for the copper electrons, strongly shifts the copper equilibrium toward dissolution because of the high work function. The higher work function of the gold drives the gold to plate, and drives the copper to etch.

I say that the presence of gold has no effect on copper etching rate. The equilibrium concentration of gold in the cupric etch solution is essentially zero, and the gold and gold ions are essentially in equilibrium. No voltage is generated by the gold electrochemical reaction, so there's no change to the EMF of the copper CuCl2 reaction.

I respect my colleague and he has many more years in the electrochemical industry. I'm a lowly chemical engineer from Berkeley. Can you help set us straight?

Chris Jerbic
Plating engineer. - Sunnyvale, California

Fundamentals of Electrochemical Science

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November 4, 2008

A. Your colleague is absolutely correct and quite brilliant, I might add.
Std Electrode Potentials:
Au+ + e- > Au +1.692
Cu2+ + 2e- > Cu +0.342
I recommend John Albery's "Electrode Kinetics".
as a starting point for reading about the phenomena of galvanic coupling. There are numerous corrosion science references that also have good treatments of galvanic coupling.

David Light
- Los Gatos, California

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