Hello Barbara,

The waves will appear if the surface of the SU8 bakes faster than the
inner regions. This will always happen in any baking regime of course,
and is the reason why SU8 users opt for hotplates rather than
convection ovens (the latter encouraging surface drying and skin
formation, preventing solvent evaporation from deeper planes within
the film). If by 'waves' you mean 'wrinkles' then it looks like the
surface is in fact drying much faster than inner regions; preventing
the required levels of soft baking / film hardening. My advice is
twofold. First, try to spin a layer with half the thickness and cure
with the same parameters. If there are fewer or no waves, then it
would suggest that solvent evaporation is responsible for the problems
you are seeing. Then (second) extend your softbaking time at BOTH 65
and 95 deg C for your desired SU8 layer thickness. For instance, try
30 mins, and 1hr at 65 before ramping slowly to 95 deg C and hold at
this temperature for a longer time. Another thing you could try is to
place a petri dish over (and not touching) your coated wafer on the
hotplate, standing on glass slides to allow solvent escape. This will
help to retain some solvent over the surface layer, slowing its drying
rate and homogenising the overall drying rate of the film. This is
after all what you are aiming to do by having a two step baking
process. The general rule to follow if your film is not uniform is to
take measures that reduce temperature and evaporation differentials
between surface and inner regions of the SU8 film. The more you can
homogenise your curing process (including exposure) the more uniform
your film properties / thickness will be.

Anyhow, try these things and let us all know if you have any success.
The results will be useful to share so we can add to the body of
knowledge.

Best wishes,

Michael

>
> -----Message d'origine-----
> De : Barbara Cortese [mailto:KORTESEB@libero.it]
> Envoyé : Thursday, December 14, 2006 8:41 AM
> À : mems-talk
> Objet : [mems-talk] Su8-2100 soft bake
>
>
> Dear all
>
> Recently we met a problem using SU-8 2100. When spinning (at 1000rpm),after
softbake treatment, waves appear on the layer. The hotplate I use is levelled,
and I am using as a substrate silicon. The soft bake lasts 15 minutes at 65°and
60 min at 95°. I tried longer times of soft bake but the problem is always the
same, and the layer is always soft.
> Can you help me to solve this problem? What do you think the problem is caused
by?
>
>
> Barbara Cortese
>