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