I don't think it's bending from the temperature. It's bending from the
solvent in the SU8. The same thing happened to me when I tried to spin
coat 1813 on PDMS. PDMS swells in solvents. If you drop a block of
PDMS into acetone for a couple hours the whole thing will swell up.
Harsher solvents like toluene will make it balloon up to 2x its
original size. People take advantage of this to remove unbound
monomers in the PDMS so that hydrophilic surface treatments last
longer. The bad news for you is that if you spin coat photoresist on
one surface, that surface will swell up while the other surfaces
remain the same, and the result is a bowed substrate. Unfortunately, I
don't have a good solution for you because I didn't pursue it any
further. Two possible suggestions, though, would be to either:

a) coat the surface of the PDMS with something to prevent the
photoresist solvent from contacting the PDMS material. Maybe sputter
or evaporate a metal layer.

b) oxygen plasma treat the heck out of the surface. It's my
understanding that extended oxygen plasma creates a glassy layer on
the surface of the PDMS, which is why you shouldn't over do the plasma
treatment when bonding pdms. But maybe you could do it on purpose and
form a surface layer to protect the rest of the material.

good luck!
Joe Grogan



Quoting Shao Guocheng :

> hi, friends:
>
>      I'm working on some PDMS molding process and encountering a
> problem. I can successfully got PDMS replica from SU-8 master mold.
> But then I need to use the PDMS replica as mold and try to SU-8
> replica from that PDMS mold. and that's when problem happens. The
> PDMS mold with SU-8 poured into it needs to undergo a 100C baking
> process to evaporate SU-8 solvent, since my SU-8 layer is thick,
> after about 10 hours baking, the SU-8 layer seems fine, but the PDMS
> mold itself bended. Does any one have faced similar problems? or any
> one know what is the highest temperature that PDMS can undergo
> without causing bending or other deformation? I thought people
> sometimes cure PDMS under much higher temperature withou any
> problem.... Thanks in advance.
>
> Guocheng Shao