A few of ideas. Hopefully at least one of these is useful.

1) Nitride stress. Stoichiometric LPCVD nitride has very high thin film
stress. Typically, stoichiometric films aren't deposited more than a few
thousand angstroms thick, or they crack (or crack the wafer). So you're
probably using so called "low stress" nitride. These films are silicon rich.
The deposition process is difficult to control, since the thermodynamics
favor stoichiometric films. Your films are probably tensile (10-100 MPa). If
for some reason they are compressive, your membranes could be buckling and
cracking after DRIE. You might be able to do some simple Euler buckling
calculations for square diaphragms. Heck, you can get a ballpark estimate by
using circular diaphragm equations.

2) 7mm?7mm is awfully big. You have to be very careful to not apply any
forces to those diaphragms or they'll crack. Separating the DRIE wafer from
the handle wafer is critical. So is rinsing and drying. You probably already
know this but I include it for completeness. Are the diaphragms cracked
immediately after DRIE (i.e. before any handle wafer removal, etc)?

3) What were the problems associated with DRIE+KOH? That seems like it would
be a good process, though you might have to do some sort of pre dip to get
rid of deposited polymer and a brief HF (or buffered HF or BOE) dip to get
rid of oxides. That would ensure a good start of a KOH etch.

Bill Eaton, Ph.D.
Materials & Analysis Manager
NP Photonics

> -----Original Message-----
> From: [email protected]
> [mailto:[email protected]]On Behalf Of Michael D Martin
> Sent: Friday, June 27, 2003 12:10 PM
> To: [email protected]
> Subject: [mems-talk] Frustrated by 2um Nitride Release!
>
>
> 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.
>
>
>
>
> _______________________________________________
> [email protected] mailing list: to unsubscribe or change your list
> options, visit
http://mail.mems-exchange.org/mailman/listinfo/mems-talk
Hosted by the MEMS Exchange, providers of MEMS processing services.
Visit us at http://www.memsnet.org/