Hey Gareth,

Air gap between the mask and the resist could also cause such T profile in
SU-8 due to diffraction. If you have large edge beads or non-uniform
surface, air gap may exist when you exposing your wafer.

Peng


> I am using both 100 micron and 20 micron thick films with around 200
> micron sized features.
> The dimension of the top portion is much greater than the mask (in
> both film thicknesses) leading me to believe that the exposure dose is
> not the cause although I certainly do agree that a much higher dose is
> generally required than specified in the datasheets.
> Cheers
>
> Gareth
>
> On Mon, Jul 28, 2008 at 14:57, Andrew Sarangan 
> wrote:
>> How thick is the SU-8 you are using, and what are the sizes of your
>> features?
>> I have encountered this problem in SU-8 due to under-exposing the
>> resist. One way to check this is to verify the dimension of the top
>> part of the exposed structure. If it is the same as the mask size,
>> then you have an under-exposed lower portion. I have also found the
>> dose specified on the spec sheet to be on the low side. If your
>> features are large, then it doesn't hurt to over-expose.

--
Peng Li
Research Assistant
Department of Mechanical Engineering & Applied Mechanics
University of Rhode Island
133D Kirk, 92 Upper College Road,
Kingston, RI  02881
Phone: (401) 874-2247