Normally one can etch Nitride in RIE....the etch rate is much better and
isotropic. But if you decide to etch in BHF....then the etch rate as i know is
60nm/1hr at a temperature of 31°C..... the etch rate may change depending of the
ration of Silicon and Nitrogen in the Nitride.....at any define temperature.....

If you have to option to etch with RIE then it will be your best option....or at
least if you fear that it will etch your other structures...you can etch about
95% of the nitride thickness in RIE and etch the rest of the 5% in BHF..

I hope this helps,
Nimo


--- On Mon, 12/14/09, Andrea Mazzolari  wrote:

From: Andrea Mazzolari 
Subject: [mems-talk] silicon nitride HF resistant
To: "General MEMS discussion" 
Date: Monday, December 14, 2009, 8:23 AM

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