Chris Bliss wrote:
> 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.
>
I've read other reports of this happening, and it's happened to me at
times as well on glass substrates.
We have overcome this somewhat with longer pre-exposure bakes at a lower
temperature (85 C).  I've also found a prebake of my glass substrates is
essential for good adhesion.
Good luck,
Jon

--

Dr. Jon R. Fox
Principal Scientist, Research Support Instruments, Inc.
Visiting Researcher, Princeton University
fox@researchsupport.com
tel: 732-329-3700 x 103
fax: 732-329-9200