Rajkumar,

you are right, the stress is the cause of the difficulty. Si3N4 has
tensile stress (membranes are stretched after release). A typical
solution is to avoid high stress areas, which typically happen near sharp
corners, by rounding the corners slightly. You also want to make sure
your wafer is rigid enough to support this stress (avoid many membranes
close together, etc).

As for SiO2, it is in general very difficult to have membranes made out
of it, because it high compressive stress of the films. Typically
anything thicker than a fraction of a micron will "crumple" and you will
see a wave-like pattern on your membranes after release, if the don't
break even earlier. I would recommend avoiding SiO2 films if possible. If
you absolutely have to have them, then play with deposition parameters
trying to minimize stress, and use very thin membranes.

Anatoli Olkhovets,
Lucent - Bell Labs.
[email protected]

-----Original Message-----
From: [email protected]
[mailto:[email protected]]On Behalf Of Raj Kumar
Sent: Monday, November 25, 2002 5:00 AM
To: [email protected]
Subject: [mems-talk] Diaphragm breakage


Dear MEMS Experts
        Hello! I am trying to make diaphragms of SiO2 and Si3N4. The SiO2
is dry grown on Si and is 8000A. Si3N4 ( on a different sample)is LPCVD
deposited (1500A) on a grown buffer oxide layer(500A). I am releasing
these diaphragms by opening trenches along four sides of the diaphragm
(diaphragm remain hinged to four corners) and subsequently bulk
micromachining in KOH/TMAH.
        The diaphragm breaks from the corners (where it is hinged) in
case
of Si3N4. Whereas in case of SiO2 in breakage take place in the
diaphragm,
in front of hinge.
        This happens when sufficient release has taken place.

I am suspecting the stress is making it. Any suggestion is welcomed.

Sincerely yours

   RAJKUMAR
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