Jordan,
The surface state of the glass surface is changing.  You need to modify it by
cleaning in a sulfuric -nitric acid combination or possibly precoating with the
SU8 solvent just prior to the coating step.  If you have no SU8 solvent try
acetone just prior to apply.  If you are doing an edge bead removal try using an
alternate solvent.

>>> "Jordan M. Berg"  08/10/01 07:52PM >>>
We have been using MicroChem SU-8 25 to create negative molds about 100 um deep
on glass wafers. We spin it on at 750 rpm. We began to have a problem several
months ago with the SU-8 shrinking back from the edge of the glass, and leaving
an irregular annular gap around the circumference of the wafer. This worsened
during the 25 minute, 95 C, pre-exposure softbake, but some gap was noticable
immediately after spin coating. The problem became so bad that only a 1.5 inch
spot in the center of the 2.5 inch wafer was covered. We finally just bought a
new batch of resist, and this seemed to solve the problem.

That was a month ago, and now it seems to be starting to happen again. A 1/8 to
1/4 inch gap is appearing around the wafer edge. Can anyone suggest why this is
occuring? I am concerned that we are storing the resist incorrectly, but we also
recently switched to a cheaper type of glass wafer. Is this a common problem?
Are there processing steps to correct it?

Thank you for any help you can offer.

Jordan Berg
Texas Tech University