Garrett:

Its not a shift between exposures, because some of the features are on
the bottom layer only and they experience the same blurring effect.  I
had discounted the idea that it was cross-linking between layers due
to, for example, acid generated during exposure of the bottom layer
diffusing to the top layer (which could possibly be fixed by more
baking of the bottom layer), because the blurred portion appears to be
stuck directly onto the silicon wafer.  It is possible that its due to
the second, long soft-bake, although I've not read of that problem in
the literature.  I've attempted to account for this by using a soft
bake that shifts between 65 and 95C - in the hope that by not staying
at 95C for too long it will minimize re-flow.

Also, a single 80s+ exposure tends to introduce warping at the surface
of the mold caused by IR heating.  Thanks for the input.

-JP Hilton

On Fri, Nov 14, 2008 at 2:00 PM, Oakes Garrett  wrote:
> John,
>
> We observed when working with double coating process to make hyperthick (>250
µm) films of SU-8 that crosslinking of the SU-8 can begin thermally.  This
appeared to be a bottom up phenomena and presented itself as a thin layer of
SU-8 that would not develop away in the unexposed regions.
>
> I am left wondering a few things...
>
> ...is this a chemical effect of casting a new layer of SU-8 onto an exposed
layer of SU-8?  The more I think about this the more I believe that you are
inducing some lateral cross-linking between the first exposed layer and the
second unexposed layer during the long second soft bake.
>
> ...if the blurry line you observe is a resist foot.  It could be a response to
the long soft bake for the second coating layer.  Perhaps more optimization of
the second soft bake will help?
>
> ...is a shift between exposures occurring?  You didn't indicate what effect a
single exposure of 80+ seconds has on the SU-8 process.
>
> Good luck.
>
> Best Regards,
> Garrett Oakes
>