Do you mean SiliconNitride (Si3N4)?  Then you can use hot phosphorus (H3PO4)
for a thickness of 200nm Nitride for 1hr at about 180°C.  This etch rate is for
Stoichiometric.  For rich siliconnitride you will have a different etch rate.
This is when selectivity to other materials is important.

regards

________________________________
From: kapil kumar jain 
To: mems-talk@memsnet.org
Sent: Thu, March 24, 2011 6:23:41 AM
Subject: [mems-talk] Etching of SiN

What is the optimum concentration of SiN etchant ? The thickness is of
the order of 1000 A.