If it helps, I have only seeing "balling up" occur on two occasions. Once
was an ill-fated attempt to spin onto untreated COC, and the other was onto
glass wafers which had been contaminated by polymeric substances during the
dehydration bake (someone had spilled PDMS in the oven which was used for
dehydration and this nicely screwed up the surface).
I am not sure about overbaking unless it is getting really well crosslinked
and cured. Maybe this could happen with an unfiltered UV source heating the
SU-8. Filtering is always a good idea anyway.

On 5 July 2012 22:16, Eric Johnston  wrote:

> I've had this exact same thing happen myself.  I never did figure out the
> problem but the comment regarding baking the first layer too long sounds
> plausible.
> On Jul 3, 2012 8:52 AM, "Yingtao Tian"  wrote:
>
> > Hi Darren,
> >
> > Could you remind how thick is the first layer and second layer? Also,
> when
> > you say 'the resist flow away from the edges into the centre', did you
> mean
> > the resist at the edge shows poor wettability and completely flows off
> the
> > first layer surface? Or, did you mean the resist at the edge just flows
> > back a bit and the edge is thinner than the centre? If it is the latter
> > case, I think that's normal situation for thick SU8 due to the high
> surface
> > tension.
> >
> > But, it would be strange to see the SU8 to 'ball up'. I would suspect the
> > first layer was baked too long.
> >
> > Yingtao
> >
> > -----Original Message-----
> > From: [email protected] [mailto:
> > [email protected]] On Behalf Of Darren
> > Alvares
> > Sent: 03 July 2012 03:07
> > To: [email protected]
> > Subject: [mems-talk] Su-8 2075 Process
> >
> >  Hi All,
> >
> > SU-8 is a pretty well studied and commonly used resist and it seems in
> > most instances I can achieve the types of results I want. However,
> recently
> > I have encountered a few problems with multilayer structures. The main
> > issue I think stems from the uniformity of the thickness. However, I
> could
> > be wrong on this.
> >
> > I am after making a freestanding su-8 mould for use in pdms moulding. The
> > su-8 is applied to a silicon wafer.  I would like to have a thin first
> > layer followed by my patterned top layer. The first layer seems to work
> ok.
> > I use a high spin speed of 4000RPM and a long softbake (gradual ramp up
> and
> > down). The exposure is done in steps to avoid any over heating and a
> > shortened prebake is done after. The problem occurs when I spin on the
> > second layer. During the softbake (which is designed to be around 2hrs)
> the
> > resist flows away from the edges into the center. This is not an issue I
> > think that stems from the hotplate incline as that would cause the the
> > resist to flow to one edge.
> >
> > The only explainations I had were
> >
> >   *   The edge effect of the first layer means it is thicker on the
> > outside then on the inside which means that the second layer when heated
> > flows inward. I tried increasing the spin speed of layer 1 but this still
> > happened.
> >   *   The surface tension of the SU-8 uncured is very high causing it to
> > ball up.
> >
> > I had a search online for multilayer papers and found quite a few but
> none
> > mentioned this issue. Anyone experience this or heard of it in the
> > literature? Would be great to get your thoughts.
> >
> >
> > Regards,
> >
> > Darren Alvares
> > _______________________________________________
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