I am having a problem with uncured PDMS residue after casting over an SU-8
patterned substrate. I am using a 10 to 1 mixture (generally 25g of PDMS base
with 2.5g of curing agent), and mixing by hand. The residue sometimes does not
show up at all for a particular batch of castings, but other times shows up on
as many as 75% of them. No amount of additional curing time or heat has any
effect. I have tried more mixing more thoroughly, but it does not seem to have
any effect either. The residue most commonly appears at the interface of the
PDMS with the substrate (with either glass or silicon substrates), but
occasionally it appears on the top (exposed) surface of the cured PDMS. Changes
in the mixture ratio both higher and lower also have no effect on the occurence
of this residue problem. I have also tried a 10 minute hard bake of the SU-8
patterned masters at 150 C in the hopes of removing any leftover solvents, but
this had no effect either. Others in this lab have had this problem from time to
time, but we can not clearly identify the cause.

Has anyone else experienced this problem, and has a cause been identified?

Thank you,

-Lawrence Sasso (lsasso@eden.rutgers.edu)