You need to put a hard mask like LTO on the nitride. Pattern the resist,
etch the LTO in 5:1 BHF, strip the resist, then etch the nitride in H3PO4.

   --Kirt Williams

-----Original Message-----
From: mems-talk-bounces+kirt_williams=sbcglobal.net@memsnet.org
[mailto:mems-talk-bounces+kirt_williams=sbcglobal.net@memsnet.org] On Behalf
Of jumril yunas
Sent: Tuesday, July 26, 2011 7:52 PM
To: mems-talk@memsnet.org
Subject: [mems-talk] Si3N4 etching using H3PO4

currently I use H3PO4 at 150 degree C to etch nitride layer. The nitride
layer can be etched at etch rate of 3-5 nm/min and standard AZ photoresist
can be used as mask. The problem is that the resist pattern can not protect
the nitride lying underneath the photoresist hence reduce the resolution.
does anyone  know how to improve this etching method ?

thanks

Dr. Jumril Yunas