Hi Andrea,

I am not sure if I understand your problem completely, but when using BHF it may
be best to use a hard mask. Depending upon the thickness of your silicon nitride
that you would like to etch, you may want to use Trilogy etch or a mixture of
80ml 10%HF to 120ml 85% Nitric acid. Obviously both of these etchants are
serious stuff, but I have had good experience etching sputtered Si3N4 using the
HF-Nitric etchant and photoresist. Remember, trilogy etch also creates HF from
NH4F from dissolution in water, so treat it the same way that you would treat
HF.

Sincerely,
David Casale

-----Original Message-----
From: Andrea Mazzolari [mailto:mazzolari@fe.infn.it]
Sent: Tuesday, September 07, 2010 10:58 AM
To: mems-talk@memsnet.org
Subject: [mems-talk] cleaning problem

Hi all,

here i have (110) silicon wafers coated with silicon nitride.
I patterned the silicon nitride and etched the wafers in KOH and obtained
silicon pieces of lateral size 20x70mm.

I need to pattern the silicon nitride on such silicon pieces.
I cleaned the silicon pieces with acetone/IPA then RCA1 and RCA2 to remove
any residue. I coated the silicon pieces with S1813 photoresist and tried
to pattern the silicon nitride with BHF. After sometime the PR is removed
from the silicon pieces.

I suppose the starting silicon pieces were not correctly cleaned. Any
suggestion on how to improve the cleaning ?

Best regards,
Andrea