My guess is you are getting exposure due to scattered light from the
back side of the wafer (since you are using pyrex).  SU8 is very
transparent to UV. I use a second wafer on the back side painted with
a mat black paint.

On Mon, Jul 7, 2014 at 1:25 PM, Dimitar Dimitrov
 wrote:
>
> Hello,
>
> I am graduate student in Brooklyn College and I am trying to reinvent the
whole MEMS procedure as I do not have any professional help. My science adviser
is from another field.
>
> I am having difficulties with the developing of SU-8 2025. Here is what I am
doing:
>
> I am spin - coating 4 inch Si or Pyrex wafer to 50 - 60 um thin film
> Pre - baking at 96 degrees for 15 min
> Sometimes instead I am putting the wafer in the vacuum chamber of my UV
imprinter. The vacuum is taking away the solvent and clear most of the bubbles.
> UV exposure with a hand made Imprinter: Xenon lamp 22kV at 10 microsec.
pulses. Three 30 seconds exposures with couple of seconds pause.
> UV lamp creates heat so the wafer is let for 2 - 3 min to relax.
> Post bake of 15 min at 95 degrees.
>
> Developing by tenderly shaking the wafer in a vessel with Propylene glycol
monomethyl ether forever.
>
> The result is that I clearly can see my pattern as exposed SU-8 is yelowish
but the unexposed parts are not removed and form thin film with the exposed
part.
> Less exposure leads to removing all of the polymer.
> (This is happening even without any mask but the pattern is formed by metal
structure that divides 4 inch wafer into several smaller regions.)
>
> Can anybody figure out what is my mistake?
>
> Thank you
> Dimitar Dimitrov
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