Shorter UV wavelengths are strongly absorbed by the SU8, so the bottom
will get a lower dose than the top, resulting in a more pronounced
sidewall taper. Since you have large features simply increasing the
overall dose may resolve the problem. For smaller geometries, you
could filter out the shorter wavelengths with a plexiglas or similar
plastic (but check the UV transmittance first). I doubt the beam
collimation has anything to do with this, unless you have a large gap
between the mask and the SU8 film.



On Fri, Dec 6, 2013 at 1:25 AM, SHANE GUO  wrote:
> Hi all,
>
> We have a UV light source which is being used for exposing SU-8. It is not a
mask aligner with an advanced UV source but it still works for patterning SU-8
features that are more than 100um wide with an aspect ratio of 0.5. However, I
found that the side walls are not very vertical for narrow(smaller than 30um)
and tall (over 200um) features.
>
> I believe the UV light is the culprit, which is not very directional so the
side walls are tapered, especially for those tall and narrow features.
>
> Does anyone know if there is a way to improve the directionality of the light
source?
>
>
> Best
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