Try KOH.

I use 0.1 M KOH to remove 100A Al overcoat on my PMMA before
developing--it takes about 30-40 sec.  You may want to try a
higher KOH concentration for your thickness of Al, or it
will take a long time!

-Bill

---- Original message ----
>Date: Fri, 2 Apr 2004 00:23:13 -0500
>From: "Zannitto, Peter J."

>Subject: [mems-talk] wet etch of Al
>To: 
>
>
>My structure consists of 250nm PMMA / 700nm of Al / 1um
SiO2.
>
>I have done e-beam lith on the PMMA layer and developed.  I
now want to etch the Al down to the SiO2.
>
>Can anyone recommend an Al etchant that should be used in
these circumstances?  The selectivity of the etch between
the Al and the PMMA should be as high as possible.
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