Try this article

Paranjape, J. Vac. Sci. Techn. A 18(2), Mar/Apr 2000

You need low concentration TMAH (5%) to make the method applicable. A
higher TMAH concentration requires very high Si content, which I find
difficult to handle. The problem is that the low TMAH concentration
results in hillock formation. Paranjape solves this by adding ammonium
persulfate, with frequent replenishment due to the consummation of the
oxidiser.

Good luck,

Knut Lian
Process Engineer
Wafer Fab Technology
SensoNor asa
PO Box 196, N-3192 Horten
Norway
http:\\www.sensonor.com
Tel: +47 33 03 50 68
Fax: +47 33 03 51 05

-----Original Message-----
From: BERAUER,FRANK (HP-Singapore,ex7) [mailto:[email protected]]
Sent: 13. desember 2001 04:15
To: '[email protected]'
Subject: RE: [mems-talk] How to protect Aluminium thin film from KOH
solut ion

Michael's statement requires a slight correction: Pure TMAH attacks
Al very rapidly, it is doped (with 0.5% Silicon) TMAH that doesn't.
I have this from literature, but cannot find the reference at the
moment.

Greetings,
        Frank Berauer
        Senior R&D Engineer
        Hewlett-Packard Singapore


-----Original Message-----
From: Michael Pedersen [mailto:[email protected]]
Sent: 12 December 2001 23:32
To: [email protected]
Subject: Re: [mems-talk] How to protect Aluminium thin film from KOH
solution


Hi Soumen,
You may want to consider using TMAH for the silicon etching. This
etchant is known to have limited effect on aluminum.

-Mike Pedersen

Soumen Das wrote:

> Hi all
> We are working on fabrication of various MEMS devices.We are facing
some
> problem regarding how to protect the aluminium layer while doing the
silicon
> etching in KOH solution. Is there any chemical (just like photoresist)
which
> can be covered on aluminium layer and also can be patterned so that
while
> dipping the silicon wafer in KOH solution the chemical layer will not
be
> attacked by KOH solution and it can be used as protecting layer for
underneath
> material.
> Any suggestions will be highly appreciated.
> Bye
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