Perhaps a photosensitive polyimide could be patterned and then cured and
might withstand phosphoric etch.

Another method would be to spin polyimide and then use resist to
photomask the poolyimide. Developer etches polyimide. Then remove the
resist and then cure the polyimide and then use the polyimide pattern
for phosphoric etch.

However, polyimide is a mess to use. Once cured, you will need a strong
O2 plasma to remove it. The phosphoric acid might be absorbed into the
polyimide and complicate the O2 plasma strip. The first two polyimide
proposals are if you absolutely can't pattern your nitride any other
way.

I would deposit SiO2 over the Silicon nitride. Then do the photo mask.
Then BOE etch the oxide, then use the oxide mask in the phosphoric, then
strip the oxide afterwards. Make the oxide thin and just enough to mask
the phosphoric etch. That way the BOE strip step won't etch too much
oxide in the openings of the nitride pattern.

I am presuming that you can't do a plasma etch, which is the easiest
way.

Ed



-----Original Message-----
From: mems-talk-bounces@memsnet.org
[mailto:mems-talk-bounces@memsnet.org] On Behalf Of Sandip Agarwal
Sent: Tuesday, January 08, 2008 9:49 AM
To: mems-talk@memsnet.org
Subject: [mems-talk] Etching silicon nitride

I want to pattern silicon nitride by etching with hot phosphoric acid.
What are the photoresists that would withstand these etching conditions?