Hi, Lawrence,

   I had silimar problem before. and from what I have seen, longer curing time
helps. I do curing at 70c for overnight, this, for the most time, eliminate the
residue. And PDMS thickness also seems playing a roll here, I have more residue
problem with thicker PDMS layer (I think it also relates to unsufficient
curing).  I also find that once a certain master mold had residue problem, it's
really hard to clean the master. Any thought on this one?  thx

Guocheng Shao

--- On Thu, 7/9/09, Lawrence Sasso  wrote:


From: Lawrence Sasso 
Subject: [mems-talk] Uncured PDMS residue after curing
To: mems-talk@memsnet.org
Date: Thursday, July 9, 2009, 2:54 AM


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)