Dear Richard,

SU-8 absorbs MUCH more at 320 nm than at 365 nm. Look at the SU-8 datasheet.
A rough estimate is that the absorption is 4 orders of magnitude higher.
If you are aiming for an SU-8 thickness of 100 micrometer, it will be
practically impossible for you to get enough light at the lower parts of the
SU-8 layer. During postbake you will end up with a few micrometer thick
structures floating on an unexposed  "sea" of SU-8.
Your structures will curve because of a stress gradient across the exposed
layer. This is due to the exposure dose gradient across the layer. The top part
of your SU-8 will be massively overexposed, and I think you consequently will
see very poor 2D structure definition.

320 nm lasers should not be used for SU-8 exposure, unless it is for very thin
layers.

Jacques Jonsmann



________________________________

Fra: [email protected] på vegne af Richard Chang
Sendt: ti 17-01-2006 19:19
Til: General MEMS discussion
Emne: [mems-talk] SU-8 layer is curved up during postbake



Hi, All,

   I expose a thick SU-8 2100 layer with a 320nm laser. After postbake, I
found out the layer is curved up.
   The picture is shown in the link below.

   http://base.google.com/base/items?oid=15423750006658009412

   We used a broadband UV  before for exposure and we never have the problem
like this with the same layer of SU-8.
   Any suggestion would be appreciated.
   Thank you.

Richard