It has been my experience that positive resists don't hold up as well as
SU-8 for DRIE etching. I would check with Microchem the makers of SU-8 and talk
with Mark Shaw. Mark was able to provide us with a formulation with a
thickness of between 5-8 microns. We did normal processing but did not do the
second
bake for cross linking. In doing so we were able to remove the resist using
normal ashing after DRIE etching. The overall selectivity was approaching
100:1 Si/PR while the positive resists were somewhere around 40:1. The other
disadvantage to the positive resist was a stepper was required to give vertical
profiles and we were able to have exceptional vertical profile with SU-8 using
a contact printing process.MicroChem's phone is 617-965-5511
Bob Henderson