Hi, Herman, I think IPA or PAG are not the problem, in my opinion it is due to the solvent, and probably you have the bubbles since the prebake... well, I had it, but my layers were thicker than yours, with so thin layers the problem should be not so important, because the main part of the solvent evaporates during the spining process. I guess you pour some SU-8 on the glass wafer and then put it in the spinning machine???. If not, you can try that, and use an extra time or a new spin cicle ( or two or three...) to eliminate the main part of the solvent. To follow the film evolution, you can use an interferometer, if there are bubbles you will see them. With a microscope, I use an upper light and bent a bit the waffer to see by reflection surface roughness or bubles, but interferometer is much better. I hope this help Regards and Merry Xmas!! Eowin