Polyimides are problematic depending on the formulation of the Polyimide.
Some contain fillers like carbon like materials that only seem to etch in the
presence of fluorine. It also helps to keep the substrate cooled and use a
powerful ICP source to generate the density of oxygen radicals used to etch the
imide. We use a system that has both a powerful ICP and rie for directionality
in
removing polyimides. During the etch phase we also include 1-3% of the total
flow of CF4 or CHF3 to remove any filler materials that can slow or stop the
etch. Since you are trying to free an aluminum feature you should be able to
release it by removing the polyimide from under the feature up to a point. Bob
Henderson