Hi Shane,

The manufacturer recommends the use of an optical filter to reduce the
wavelengths exposing SU-8.  From the SU-8 2000 series datasheet:

> To obtain vertical sidewalls in the SU-8 2000 resist, we
> recommend the use of a long pass filter to eliminate UV
> radiation below 350 nm. With the recommended filter (PL-
> 360-LP) from Omega Optical (www.omegafilters.com) or
> Asahi Technoglass filters V-42 plus UV-D35 (www.atgc.co.jp),
> an increase in exposure time of
> approximately 40% is required to reach the optimum
> exposure dose.


Note that the resist itself has a sharp knee in its optical response curve
(around 360 nm if I recall correctly), so that the resist itself effectively
functions as a short-pass filter.  Thus, the use of the recommended 350 nm long-
pass filter in conjunction with the resist's own optical properties has the
effect of selecting a narrow wavelength range for exposure.

Microchem also has some other suggestions in its SU-8 FAQ  (
http://microchem.com/su_eight_faq.htm ):

> How do I produce SU-8 structures with near vertical sidewalls in very thick
films?
> SU-8 and SU-8 2000 resists are optically transparent and highly functional,
which permits high aspect ratio structures in very thick films to be imaged with
optimized lithographic processes. Optimization techniques include; spectral
shaping of the exposure bandwidth- remove shorter wavelengths that are absorbed
in the upper portion of the resist film and result in negatively sloped
sidewalls; fine tuning the exposure dose and post exposure bake (PEB) process in
order to obtain uniform cross-link density throughout the resist film and by
optimizing the prebake and develop process. For more information about suggested
baseline processes, please refer to our SU-8 and SU-8 2000 technical data sheets
or your SU-8 technical sales representative.


Best,
Mike Whitson


On Dec 6, 2013, at 1:25, 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
> _______________________________________________
> Hosted by the MEMS and Nanotechnology Exchange, the country's leading
> provider of MEMS and Nanotechnology design and fabrication services.
> Visit us at http://www.mems-exchange.org
>
> Want to advertise to this community?  See http://www.memsnet.org
>
> To unsubscribe:
> http://mail.mems-exchange.org/mailman/listinfo/mems-talk

_______________________________________________
Hosted by the MEMS and Nanotechnology Exchange, the country's leading
provider of MEMS and Nanotechnology design and fabrication services.
Visit us at http://www.mems-exchange.org

Want to advertise to this community?  See http://www.memsnet.org

To unsubscribe:
http://mail.mems-exchange.org/mailman/listinfo/mems-talk