I have the same problem. I tried a speed of spin coating about 1000 RPM for 30
sec. and followed by a 4500 RPM for 60 sec. In fact, a thickness of 10 um was
obtained for Su-8 2010. I just use Su-8 for Si etching, so I think the thickness
is not very important (just enough for Si etching, but I don't know how enough
it is, I want to etch a depth of Si about 10-30um).

The edges are really thicker than inner after soft baking. I have not followed
the data sheet for softbaking, because I want to avoid the problem of bubble, so
soft baking was includes 2 stages with a low temp (65 deg) for 60 min, and a
higher temp. (100) for another hour. However, the Su-8 surfaces were not very
uniform and smooth. It cause to some error of pattern profile when exposure.

If you know the solution, please share with us! Thanks!