Mike:

I am the MEMS Exchange process manager here at UCB, and would like to address
the problems you have been experiencing with your process. Our low stress
nitride does not have a very high selectivity in our DRIE tool, therefore most
membranes of the type you describe are made here at UCB via a wet etch. The
handle wafer you are using in the DRIE process is causing me a little concern,
it could actually be a stress inducing component to your membrane. Our LSN is
rated at 300MPa or less, so the membrane you describe should not be breaking.
Please feel free to contact me directly, and I will be happy to discuss this
issue with you further.


thank you,

Matthew Wasilik
Senior Development Engineer
Berkeley Sensor & Actuator Center
University of California at Berkeley


> -----Original Message-----
 > From: mems-talk-bounces@memsnet.org
 > [mailto:mems-talk-bounces@memsnet.org]On Behalf Of Dave Kharas
 > Sent: Monday, June 30, 2003 7:14 AM
 > To: mems-talk@memsnet.org
 > Subject: RE: [mems-talk] Frustrated by 2um Nitride
 >
 > 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
 >
 > Plain Text Attachment [ Save to my Yahoo! Briefcase  |
 >  Download File ]
 >
 > 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.