I believe you should try to oxidize Ni (e.g. on hot plate) prior to etching.
This should reduce the Au driving force, as a nobel metal, in short circuit
with Ni.

Igor Kadija
www.fibrotools.com


-----Original Message-----
From: mems-talk-bounces@memsnet.org [mailto:mems-talk-bounces@memsnet.org]
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.