If I remember correctly, Pyralin 2721 is one of the photoactive polyimides that
you can pattern like photoresist, but use a solvent to develop.

Since I didn't have spray development equipment available, I also had to work
with immersion developing. My experience was that I had no big problems
developing out big features (>50 microns), but features from 5-10 microns were
not very reliably developed. From talking to other people, I found that alot of
precautions had to be taken, as far as keeping the polyimide stored in a
freezer, but only opening the bottle when it was warmed to room temp. to keep
water vapor from condensing into the product. Since I was not using alot of
polyimide at a time, I tended to get good results from a fresh bottle, but
increasingly poorer results as I got to the end of a bottle.

In the end, the hassle wasn't worth it to me. I ended up using regular
polyimide, and patterning it with photoresist, in order to open up my features
with a CF4/O2 RIE.

Brian Cunningham
Draper Laboratory
Cambridge, MA