Hello All,

During the soft bake, my SU-8 film tends to "spontaneously" form a number of
divots or craters that are not initially present (after spin-coating).

I am using SU-8 2050 (MicroChem) on a 1.1mm 4-inch square borofloat glass
substrate. I am spreading at 500rpm (100rpm/s, hold 20sec) and spinning at
1300rpm (300rpm/s, hold 30sec) with a resultant film thickness of ~100
microns. I dispense SU-8 by pouring a small puddle of SU-8 in the center of
the wafer. After spinning, there are no (or very few bubbles) but I do
notice a faint ring where the original SU-8 puddle was poured. The soft bake
is performed on a hotplate at 65 C for 5 min and 95 C for 30 min using a two
step hotplate process.

In order to characterize my problem, I have tried slowly ramping the soft
bake temp in 10 C increments (starting at 45 C) and holding for 30 min.
Problems are not present after 30 min @ 65 C but appear when the temp is
increased to 75 C. The SU-8 does indeed reflow at temperatures above 65 C,
but the craters are still quite severe after ~2 hours at 95 C.

Has anyone else encountered this problem? Any potential solutions or process
modifications would be greatly appreciated!

Kind Regards,
Chris