Hi M. Aslan,

The answer to your question is: It depends.
Both KOH and TMAH are very suitable for anisotropically
etching 500um Si. KOH is more common in "traditional"
applications (e.g. pressure sensors) and is cheaper.
TMAH is less aggressive (doped with Si it can even be
used on wafers with Al or other metals), less toxic and
more compatible with other IC processes.
KOH's selectivity Si/SiO2 is approx. 300, so you need
>2um SiO2 for your purpose (or use SiN). TMAH does
practically not affect SiO2, but etches faster than KOH
in the <111> direction (approx. 5% of the <100> rate,
but very variable with process conditions) resulting
in more mask undercut.
You can get a good introduction and some recipes from
Marc Madou: "Fundamentals of Microfabrication", a book
that every MEMS engineer should have read.

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


-----Original Message-----
From: aslam muhammad [mailto:m_aslammalik@yahoo.com]
Sent: Thursday, April 11, 2002 4:48 PM
To: mems-talk@memsnet.org
Subject: [mems-talk] Wet Etching


Hi Friends

              I want to use TMAH and KOH as etchant to
etch bulk silicon ( 500 micron).  which one is more
suitable.  my etch mask is SiO2.  what is the recipe
of a good etching rate KOH and TMAH please.

Thankx


M. Aslam
Yahoo! Tax Center - online filing with TurboTax
http://taxes.yahoo.com/
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