The fracture strength of a 30 um membrane which is 5x5 mm^2 should be
high enough that you don't have to be ultra-careful (but, still, be
careful!); it should be over 100 psid.  I don't have the equations right
at my desk, but feel free to email me if you'd like to know more exact
estimates of fracture strength from my experience with similar
membranes.

Kirt is right about depletion, but there could also be a stress effect
during the initial etching; although I did not experience this effect.
Did you measure the stress in your nitride film?

Al

---
Albert K. Henning, PhD
Director of MEMS Technology
NanoInk, Inc.
215 E. Hacienda Avenue
Campbell, CA  95008
408-379-9069  ext 101
[email protected]

-----Original Message-----
From: Andrea Mazzolari [mailto:[email protected]]
Sent: Friday, December 25, 2009 1:58 AM
To: [email protected]
Cc: 'General MEMS discussion'
Subject: Re: [mems-talk] silicon membranes

Hi Kirt,

i already tried adding Sb2O3 as surfactant, surfaces were mirror like,
so surfactant was working well.

Agitation was realized with a magnetic stirrer, but was not so strong.
I'm afraid that strong agitation may destroy membranes.
Ok, if etching at room temperature will not work i will try with
ultrasound.

In any case i need to realize just 4-5 membranes, and i have a lot of
time to realize them, so i can accept to realize etching at room temperature.

Best regards,
Andrea