I would recommend SU-8 resist by MicroChem. I have been using it for deep
silicon etch and it has higher selectivity properties than any of the positive
resists. They make a special formulation for us that is about 5 microns thick.
At this thickness we have been able to produce anisotropic profiles easily which
translates into steep sidewalls in single crystal silicon. Call Mark Shaw at
617-965-5511 x 308 and tell him Bob Henderson recommended him. Bob