Nimo:

To obtain high selectivity between Si3N4 and oxide the hot phosphoric acid
method is probably the best. To be done properly the water concentration
needs to be maintained so a closed system with a water cooled lid works
well. An open bath must have water injected periodically and if not done
safely bad things can happen. Caution: Using new Teflon boats can actually
etch oxide so be sure to run your boats without wafers for several hours
before attempting the process. The outgassing of fluorine from new boats
makes a good oxide etch. Bob Henderson

-----Original Message-----
From: mems-talk-bounces+bob.henderson=etchedintimeinc.com@memsnet.org
[mailto:mems-talk-bounces+bob.henderson=etchedintimeinc.com@memsnet.org] On
Behalf Of antwi nimo
Sent: Thursday, May 27, 2010 4:33 AM
To: mems-talk@memsnet.org
Subject: [mems-talk] SiliconNitride etch with selectivity to Oxide

Hi everyone,

I was wondering what is the best way to etch both stoichiometric(Si3N4) and
high concentration silicon Nitride with selectivity to oxide(thermal oxide).
I have been reading the LOCOS process and have come accross Phosphorus
solution and so on but would like to ask some practical experience in
etching Nitride with selectivity to oxide as i have no experience. May be
the methods used and the etch rate between Si3N4 and oxide if any.

Thanks in advance for any help.
Nimo