Your right - this should work. I can think of only two possibilities:
1) are you using the 2000 series for both layers? Different series will not
work.
2) contamination from your mask. It could be leaving some residue and
changing the surface.
I suspect the second as something is changing the surface from what should
be highly compatible one to one with high surface energy and low adhesion.
I don't think uniformity is the problem.

On 2 July 2012 19:07, Darren Alvares  wrote:

>  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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