First off, I'd like to ask whether you are using a hotplate or an oven to
perform the pre-bake?  The paametes laid down in the data sheets from MicroChem
are for hotplate baking. If you are using an oven, especially for films that
this, you will have a solvent removal issue; the top layer of resist will dry
first, creating a "crust" of densified, low solvent content material.  This will
act as a diffusion barrier to any solvent attempting to escape from the lower
layers.

Even if you are using a hotplate, I think I may see an issue with the prebake
parameters. Your message implies that you are using the original formulation of
SU-8 50, rather than the faster drying SU-8 2050. With the original formulation,
the recommended bake time is ~25 min. at 65 C, followed by ~ 70 min at 95C. Even
if you are using SU-8 2050, the recommended bake is 5 min at 65C, followed by
~45 min at 95C.

SU-8 has a low glass transition temperature (Tg) of 55 C, so if a sufficient
bake is performed, there should be no reason for the film to remain wrinkled
afterwards, since the material should be able to reflow into a highly uniform
coating.


Best Regards,

Chad Brubaker

EV Group       invent * innovate * implement
Technology - Tel: (602) 437-9492, Fax: (602)437-9435 e-mail:
[email protected], www.EVGroup.com

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 -----Original Message-----
From:   [email protected] [mailto:mems-talk-
[email protected]]  On Behalf Of [email protected]
Sent:   Thursday, December 11, 2003 8:26 PM
To:     [email protected]
Subject:        [mems-talk] SU-8 50 wrinkling issue

Dear all,
i am using SU-8 25 to fabricate a 27 micron feature on a 3 inch
glass wafer (Ti seed layer).After the completion of this process, i am
fabricating a 200 microns thick channel as a second layer using SU-8
50.The process parameters for the 200 microns are the following:
200 process parameters (7ml SU-8 50)

?
Spin: 500rpm for 10sec (100rpm/s ramp)
    1500rpm for 30 sec (300rpm/s ramp)
i.e 500.100.10 and 1500.300.30

?
Leave for 1 min

?
Prebake : 65C for 6 minutes and 95C for 22 minutes
?
Exposure: 160microJ/cm2
?
Postbake: 2min at 65C and 95C for 7 minutes
?
Development: 8-10 min with EC Solvent, Clean with IPA afterwards

The problem is that during the prebake phase,the su-8 wrinkles
irreversibly.as a result the structures are very rough
I would very much appreciate any help and/or recommendations on this
problem.Thank you in advance for your help.I am looking forward to your
reply.


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