Your process sequence looks fine except I don't like the film mask.
=20
1. If you expose through a film it will absorb some of the UV and thus =
you will get a lower exposure dose at the wafer than what is suggested =
in the SU-8 datasheet. If you underexpose, the sidewalls will have a =
negative slope and the PDMS will thus stick mechanically in the mould. =
Underexposure will also give a poor adhesion between the two layers. =
When you have conservative structures as in your case, it is often a =
good idea to overexpose the SU-8. Just don't go above +50% relative to =
the exposure dose in the datasheet.=20
2. If your film mask is not absolutely clean it will leave residues on =
the first layer, thereby potentially reducing the adhesion between the =
two layers.
=20
I will suggest you measure the actual exposure intensity below your film =
and adjust the exposure time to get the intended exposure dose. If that =
doesn't help I suggest you use Chrome/glass masks instead of the film =
mask.
=20
In my companys dedicated SU-8 lab. we used a process like the one you =
describe, and it worked just fine. We have since then refined it so it =
is different and better, but your process is fundamentally ok.=20
=20
Regards
=20
Jacques Jonsmann
www.smb.dk


________________________________

Fra: [email protected] p=E5 vegne af Vishwa
Sendt: to 22-02-2007 23:25
Til: General MEMS discussion
Emne: RE: [mems-talk] Problem with PDMS removal from the SU-8 mold

Hello,
As a follow up to the earlier posting, I would like to add a couple
more expts and observations that I performed today. I desperately need
some help as I am not getting adhesion

Recipe followed
1) Bare Silicon wafer - Rinse with Ethanol, followed by DI water,
Dried with N2 and then plasma treated with oxygen (1 Torr, 22w, 1
min). When I don't plasma treat the wafer, then the SU 8 moves towards
the center of the wafer during soft bake

2) First Layer spin coat - ramp to 500 at 100rmp/s and hold there for
10 sec, then ramp to 1500 rpm at 300rmp/s and hold for 30 sec. I use
SU 8 -2025 and this rmp is used to fetch 60um thick coating

3) SEB -
4) Exposure - I use a film which is attached to a glass by means of
scotch tape. I expose it according to the application notes provided
by microchem
5) PEB
6)Second layer spin coat - at 3000 rpm to yield 30 um
7)SEB
8) Exposure
9)PEB
10) develop

The SEB and PEB are done according to individual layers.

The mold looks ok. When I pour PDMS( mixed in 10:1 and degassed for 1
hr) and heat on hot plate at 100 degrees for 12 mins, and then peel,
the second layer comes off.

So, I tried to create on the second layer on the wafer and the result
is the same. The second layer is an array of rectangular structures
whose dimensions are 50um x 2.5mm x 30um.

When I was going thru the archives, I did see that heating the wafer
before spinning SU-8 helps. Should I do that in spite of plasma
treating the wafer?

I need some help as have no idea how to address this issue.