As others have suggested, intrinsic thin film stress is a likely culprit in
your troubles. If your film was highly compressive, it could buckle upon
release and you would notice wrinkles. Tensile films shouldn't show
wrinkles.

Here's the puzzling part: nitride films are generally tensile.
Stoichiometric nitride films, Si3N4, are usually *very* tensile - on the
order of 1 GPa or more stress. This is so much stress that it limits the
thickness of the films that can be deposited -- too thick can cause the
silicon substrate to shatter.

Because of this, many micromachinists use low stress nitride. This is
silicon rich, and the stresses are often in the range of 1-100 MPa tensile.
I believe (but am not sure) that most LPCVD films will be tensile. It may be
possible to deposit a compressive film by PECVD. If you know the thickness
of the film and the span of the finished diaphragm, you might be able to
estimate what levels of compressive stress would cause Euler buckling. One
place to start might be Roark's Formulas for Stress and Strain.

A couple of other thoughts: could it be that the films are really tensile,
and you're observing cracks in the nitride (that look like wrinkles)? Or is
your diaphragm a composite of oxide and nitride? If so, the oxide stress
could be compressive and causing buckling. Finally, the drying process can
be traumatic on diaphragms (though it sounds like you've been very careful).
It might be instructive to keep the diaphragms immersed in the rinse water
and look at them under the microscope. Drying wouldn't cause buckling, but
it could cause cracking.

Good luck

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

>
> Has anyone had significant experience etching nitride windows
> on silicon using
> a KOH etch? I am trying to create very thin (4200 Angstroms)
> Silicon Nitride
> windows, but my yield goes below 50%. My KOH bath is composed
> of KOH pellets
> and water at 80 degrees C. Are there any ways to reduce
> wrinkles in windows? I
> currently dip my wafers in cool DI water for 10 minutes and
> then dry them in a
> 120 degree oven with my pits opening upward (i.e. window
> surface downward).
>
> Any help would be great. Thanks.
>
> -- Kishan Gupta
> UC Berkeley
>
>
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