You might try resist casting.

Calculate the volume of the desired SU-8 layer (substrate surface *
SU-8 layer thickness).

Consult the SU-8 data sheet for the percentage of solvent in
your SU-8 formulation and calculate the amount of SU-8 resist
(SU-8 + solvent) you need to get the right SU-8 thickness.
Place the substrate on a level hot plate and pour the calculated
amount of SU-8 resist onto the substrate so that it is fully covered.
Long soft-bake ...

- key is to get the hot plate level, else you get a SU-8 wedge on top
- more liquid SU-8 variants flow better and the higher amount of
   solvent makes measuring the volume easier
- when using a liquid SU-8 you should place the hot plate below an
   extraction hood as you will evaporate a lot of solvent

Good luck!

Michi

On 12/01/2010 06:30 PM, Nirmal punjabi wrote:
> Hi,
>
> I need a thickness of around 400um of SU-8, i am spin coating SU-8 2100 at
> 800rpm for 60 sec.
>
> But the final structure formed has uneven surface.
>
> Can anyone help me on this matter?
>
> Thank you.
>
> Nirmal.