Hi Michael,
The 2um thick nitride is quite thick. The thickest
I've grown is 1.2um. Do you know what the stress is?
Did you use the Berkeley Low stress nitride or regular
nitride? I also saw they have a variable stress
nitride, which i am guessing is even lower stress with
higher DCS:NH3 ratios. According to their web site,
even the low stress nitride has a stress of around
300MPa (Is this info correct?), this could still be
too high for your application. With the variable
stress nitride you can proably get stress lower to
100MPa but maintaining film thickness and stress
uniformity from wafer to wafer becomes more
difficult. An oxide layer under the nitride could
introduce some compressive stress and may reduce some
of the cracking that your seeing.

good luck

dave kharas Ph.D.
Sarcon Microsystems
dave_Kharas@yahoo.com

Dave


Forwarded Message
From: "Michael D Martin"

Subject: [mems-talk] Frustrated by 2um Nitride
Release!
Date: Fri, 27 Jun 2003 15:09:36 -0400
To: mems-talk@memsnet.org



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Hi,
  I've been trying to use a 2 micron thick LPCVD
silicon nitride layer
(from the Berkley Fab) for release to produce a
suspended membrane over
an area of 7mm x 7 mm. The wafers are (100) and the
only layer on them
is the silicon nitride. That is, there is no oxide
between the wafer
and
nitride.
  Techniques I've tried for release:
   DRIE
     Samples are diced and adhered to a handle wafer.
The nitride is
the
stop layer. Incidentally, the nitride works as a great
stop layer
contrary to what STS claims. The adhesives between die
and handle I've
tried are super glue, resist, thermal grease, and
more. No mater what
I've tried the samples are fractured when they come
out of the DRIE.
   DRIE+KOH
     In this approach I DRIE until I'm about to reach
the nitride
(about
10-20 microns of silicon left) then I put the die into
KOH. I've tried
this with 200 rpm stirring, no stirring and very small
solution volumes
to reduce convection all to no avail!

  I'm begining to think there is too much tension
although I think
these
exact films were used to make X-ray masks that were
released to form
membranes as large a 1 cmm in diameter. Is there some
technique to
introduce compressive stress in the film to
compensate?

Any input is welcome,
  Mike Martin
  U of Louisville Fab.






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