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