I think I understand you fabrication process. You have arc shaped
channels in SU-8 which then become the arc shaped sieves in PDMS after
casting.

What may be happening is a problem I have encountered recently. A film
may be forming on top of the SU-8 over the unexposed regions which
prevents efficient development. In my case, I was able to solve it
using vacuum contact mode in my mask aligner which prevented any gap
between the mask and SU-8 which can lead to this problem. Another
cause of such a film could be deep UV in your exposure source. This
can be eliminated by using a filter to cut out wavelengths below
350nm.

If you can look at the side profile of your PDMS structures, this may
help see what the problem is (i.e. cut a slice from one of your chips
and look at it under a microscope).

Gareth


On Wed, Oct 29, 2008 at 10:00, Liang Zhao  wrote:
> thanks,
> The channel was transparent and patterns of seives was opaque arcs which
> located in the channel. That is to say, by using SU-8, the channel patterns
> was a positive mold and with a very small holes (the seives). Our obstacle
> was how to make the holes thoughly from top to bottom? The unefficient
> development or unefficient exposure? I am not sure if I explain it clear.
> Now, we still working on it. We tried different exposure time but cannot
> solve such a problem.
> the exposure dosage we tried was: 400mJ/cm2, 200mJ/cm2, 100mJ/cm2, 50mJ/cm2,
> but no one works. could any one help me? Thanks.