I encountered a similar problem.  I had a gold seed layer, an electroplated
nickel layer, and a gold top layer.  When I put it in an I2-KI type etching
solution, the nickel etched at an insanely high rate in from the edges.  The
thin top gold would peel off because all the nickel was etched away.

The only though I had at the time is that it might be somehow related to the
various metal work functions, and that the Nickel was acting as an anode and
plating out into the solution.

I never found a solution to the problem. Perhaps putting an electrical bias
on the device being etched (relative to some inert metal plate in the bath)
might help.

But it's very odd, and if anyone figures it out, I'd love to hear about it.

David Nemeth


-----Original Message-----
From: [email protected] [mailto:[email protected]]
On Behalf Of Sven Holmström
Sent: Monday, May 21, 2007 7:04 AM
To: General MEMS discussion
Subject: [mems-talk] Au-etchant attacking plated nickel

Dear all,

We selectively plate nickel onto gold coated silicon wafers. Later we etch
the gold using standard gold etch (Aldrich, Potassium Iodide etchant). We
have sometimes problem with the gold etchant etching also the nickel.
According to data sheets and literature KI is not supposed to etch nickel
(for example Etch Rates for Micromachining Processing—Part II by
Williams,
Gupta and Wasilik.).

The keyword is 'sometimes'. The behaviour is very erratic. We have produced
several nice pieces without any problems, but then at some times the nickel
is etched even faster than the gold! (the nickel etch also in general seem
uneven, etching from the sides in concentric half circles rather than
evenly). It seems to us like the nickel etching is mainly depending on the
individual nickel baths. We use all the time standard Nickel Sulphamate
baths, but since we mix them by hand slightly individual differences are
unavoidable. The concentric half circles also to tend to point  to
contaminations being a problem.

I wonder if anyone else has encountered this problem and in which direction
we should go to get control of the process.