Did you try Potassium cyanide (I know it etches Au, Ag, and Cu)?
Although I wasn't aware that the KI:I:H2O (did you try it in a 2:1:10
ratio?) etchant would have an affect on Al.  The only other gold etchant
I'm aware of is Aqua Regia and that will definitely etch your Al also...
There is a table of etchants located at
http://grover.mirc.gatech.edu/processing/etchants.pdf
Good Luck

Regards,
Dave Lewis

Sensor R&D
Lake Shore Cryotronics
[email protected]

-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Lisa Gades
Sent: Monday, February 15, 2010 3:59 PM
To: [email protected]
Subject: [mems-talk] Wet-chemical etching of Au without etching Al

Hello,

We have a process in which 20nm Au were deposited as a sacrificial layer

on Al.  We need to remove the Au in a later step.  We have found that
iodine-based Au etchant undercuts the Al.  We are looking for a process
that will selectively etch the Au without etching the Al.

Thank you,

Lisa Gades