This sounds like "T-topping" (overexposure of the top layer due to
strong absorption below 350nm).
Try filtering your UV source with a long pass filter to cut out <350nm.


Marhefka, Joie wrote:
> Hi!
>
> I am fabricating microchannels in PDMS from an SU-8 on silicon master.
> I am having a hard time getting a rectangular cross section.  My cross
> section has both "ears" at the PDMS side, and a bit of a "lip" at the
> open side.  From talking with some others, I believe that the "ears" on
> the PDMS side are due to overexposure of the SU-8.  Does anyone have any
> ideas what may be causing a "lip" on the open side and how to get rid of
> this and get a more rectangular cross section?
> Thanks,
> Joie
>