This is a repeat to essentially the same question asked on about 12/15/2009.
This method requires a "pad" oxide underneath the nitride.  20nm of
thermally grown oxide seems to be adequate.  This method was used to etch
hundreds of wafers a day.  BTW, we used to judge the quality of the lpcvd
nitride be how slow it etched in straight HF (the slower, the better).


I have used this method in IC production. The purpose of the nitride was to
prevent silicon oxidation underneath. A layer (100nm) of high quality LPCVD
nitride was deposited. (The etch rate in conc HF was VERY low.) The surface
of the oxide was then oxidized.  The thin layer of oxide was masked and
etched in BOE.  After stripping the resist, the nitride was etched in
phosphoric. We used boiling concentrated phosphoric in a quartz beaker with
a water-cooled reflux condenser as a tightly fitting lid. We monitored the
temperature and added water as needed to maintain the temperature (and
boiling point) at 150 deg C (I may be remembering the temp wrong.)  If you
allow the boiling point to get too high, the phosphoric will etch the oxide
at a much faster rate.  You need a little bit of thermally oxide underneath
the nitride. Once the phosphoric etches through the oxide underneath, it
will attack the silicon and make it look real ugly.

Roger Brennan
Applications Director
Solecon Labs
770 Trademark Drive
Reno, NV 89521


-----Original Message-----
From: mems-talk-bounces@memsnet.org
[mailto:mems-talk-bounces@memsnet.org]On Behalf Of Aleksandar Tomovic
Sent: Monday, December 28, 2009 4:28 AM
To: mems-talk@memsnet.org
Subject: [mems-talk] wet etch of SiN revisited


Dear All,

i need to do a wet etch of SiN (don't have equipment for dry etch),i read
that
it is  possible to do a wet etch with BHF, while using photoresist as etch
mask, but i also read somewhere that the photoresist will last only for a
short period of time (20 min). My SiN layer is 180nm thick. If anyone could
give me more data on this subject i would appreciate it.

Thanks in advance!

A.Tomovic