Dear Frank,
I'm not sure that I entirely understand your explanation of the problem, but
my experience with SU-8 and UV exposure systems may apply here. I was only
working with 20 micron tall structures but I found that the sidewall angle of
the developed SU-8 geometry was very heavily dependent on exposure time. Too
much exposure time could result in the fingers of your capacitor being
undercut. Not enough exposure time will not allow you to expose all the way
through the SU-8. Your geometries may not allow you to satisfy both
criterion.
You may have to go to a deep UV or an X-ray exposure technique. I don't know
the sensitivity of the SU-8 to these wavelengths, however. Perhaps you could
deposit a chrome layer on top of the SU-8, pattern the chrome, and then
plasma etch to create the micromold. (They might kill you for putting a wafer
with SU-8 into the sputterer.) If you try the third technique, or some
variation, you can use straight epoxy without the photosensitizer. All of
these techniques will give you better sidewall profiles.
Best regards,
Mike Mattes
MicroMechatronics