Andrea,
      LPCVD silicon nitride would better be described by the formula
Si3NXOY. That is it is usually a Silicon OxyNitride. Nitride from a LPCVD
system that doesn't have leaks will make a silicon nitride close to Si3N4,
but a leakier system is going to give Si3NXOY. That is silicon oxynitride.

Silion Oxynitride etches faster in Buffered Oxide Etch (BOE than purer
Si3N4.

In fact one quick quality test of the LPCVD system is to etch the Silicon
nitride in BOE and measure the etch rate. More than one engineer discovered
his or her silicon nitride was mostly silicon dioxide.

So in your case the first wafer had very pure silicon nitride. The etch rate
should be very low for a good silicon nitride. The other wafer had silicon
oxynitride and would, of course, etch much faster.

The etch rate of pure silicon nitride in BOE is about 1 ang./min. and you
are going to wait a long time to etch it.

So it seems your silicon nitride is excellent, but you need an etch process
for real silicon nitride. BOE isn't it.

Plasma etch for silicon nitride is really easy. However, if you need to do
wet etch, than use an oxide layer on the nitride. Mask the oxide with BOE,
strip the resist, and then do a hot phosphoric acid etch, watching that you
have the phosphoric hot, but not dehydrated. It is sort of nasty, so see if
you can plasma etch instead.

Ed
Texas MEMS Secretary

-----Original Message-----
From: mems-talk-bounces@memsnet.org [mailto:mems-talk-bounces@memsnet.org]
On Behalf Of Andrea Mazzolari
Sent: Monday, December 14, 2009 10:23 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