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 89521-5926

-----Original Message-----
From: mems-talk-bounces@memsnet.org
[mailto:mems-talk-bounces@memsnet.org]On Behalf Of Andrea Mazzolari
Sent: Monday, December 14, 2009 9:23 AM
To: General MEMS discussion
Subject: [mems-talk] silicon nitride HF resistant


Hi all,

i have some silicon wafers coated with 100nm LPCVD silicon nitide.
I need to patter the silicon nitride. I realized photolitograhy and
immersed the wafer in BHF. Usually removal of silicon nitride takes about
4 hours, but this time after 7 hours silicon nitride was still present.
I removed photoresist and immersed the wafer in concentrated HF. I can see
that the color of silicon nitride slowly changes with time, so probably
there is some etching, but after 1 hour silicon nitride was still
present!!

After that i immersed in the same HF a second wafers coated with 100nm
LPCVD silicon nitide (this second wafer is coming from another source).
Silicon nitride was removed in some minutes.

Where could be the problem with the silicon nitride on the first wafer ??

Best regards,
Andrea