Hi all,

I got some problems with SU-8 PEB. My process is

1st SU-8(50) layer is 100um thick, softbaked from room temperature to 65C, stay
for 15mins, ramp to 95C, hold for 1hr~1.5hrs.

expose w/o a mask and PEB from room temperature to 65, stay for 10mins, ramp to
95C, stay for 15mins. No features on this 1st SU-8 layer.

Then spin coat another 7um SU-8(25) layer on top of the 1st layer,  softbake
from room temperature to 65C, stay for 3mins, ramp to 95C, stay for 5mins.
expose with mask, PEB from room temperature from room temperature to 65, stay
for 1min, ramp to 95 stay for 3mins. ramp down to room temperature.
develop.

The problem is wrinkles appeared.usually during the softbake or PEB for the 2nd
SU-8 layer  For a longer baking time, it even showed some white stuff. Under
microscope, it looks like some streaks ( or more like curled worms). If this
happened in the softbake for the 2nd layer, the thing would extend to everywhere
for a enough long baking time. If it happened in the PEB step, only the 2nd
layer that were not exposed would have this.


I am wondering if you guys found this or had some literatures explaining this.
In my opinion, I believe it was caused by the over baking which dried the 1st
layer completely. However, the lst layer had its own PEB and it was cured, so
would an extra baking further reduce the solvent content and damage the first
layer?

Any suggestions are welcome.

Thank you in advance.