I found that an acrylic coating (spray paint enamel) over photoresist works
quite well to keep PDMS from sticking. Also, to release, I dunked the whole
wafer in ice water. The PDMS will swell a bit from absorbing water, while
the cold contracts the mold. I could peel off 10 micron thick PDMS quite
easily then.

Mike

P.s. I never did a 2 layer SU-8 so I can't help you with that part.

Dr. Michael H. Beggans
Naval Surface Warfare Center, Indian Head
101 Strauss Ave. Bldg. 302            Phone: (301) 744-1927
Indian Head, MD 20640                  Fax:    (301) 744-6406

-----Original Message-----
From: Vishwa [mailto:[email protected]]
Sent: Wednesday, February 21, 2007 12:16 PM
To: General MEMS discussion
Subject: [mems-talk] Problem with PDMS removal from the SU-8 mold

Hi,
I use SU-8--2025 to make the mold master. The mold is comprised of two
layers, first one is the base which is a rectangular channel(2.5mm x 50 mm x
60um). The mask that I use is a film which is stuck to the glass plate. Then
I spin the second coating of SU-8 after the PEB of the first layer , but
without developing the photoresist. The second layer is comparable to an
array of small rectangular walls ( 2.5mm x 50um x 30um) and they are present
all along the first layer. The walls are separated by 50um. After SEB and
exposure, I do the PEB, which is then followed by developing.

I plasma treat the silicon wafer before spinning the first layer and I
plasma treat the SU-8 of first layer before spin coating the second layer.
The mold looks good under the microscope. The problem is part of the small
structures comes with the PDMS, when I peel it after baking the PDMS.

Is there some sort of a solvent that I can coat the mold with for easy
removal of PDMS or is there something that I can do in the mold fabrication
process itself to avoid this problem.