What are you using for the exposure?
I would guess either your source isn't well collimated or your mask is
not well contacted with the wafer.

Abhishek Jain wrote:
> I am making SU-8 2025 masters on Si for PDMS replica molding. I use a
> mylar mask and the structures are ~25um tall. I have a problem that I
> do not get good vertical walls of SU-8. A 15um feature comes out as a
> flat length of ~30um and then a linearly decaying thickness that makes
> a total feature size of ~60um.