Dear Soojin,

The dimpling or reticulation of the resist you describe is often attributable to
excess solvent evolution through the the resist film. In your case I believe
that solvent retained from underbaking is being evaporated during exposure and
causing your dimples. I would first check to be sure that your SU-8 was
sufficiently softbaked. Secondly I would adjust your softbake of the S1813 to
the recommended 115C for one minute. This means that the resist must "see" 115C
for one minute. Since you have a layer of SU-8 underneath, this will take longer
than 60 seconds. Next I would address your exposure. I am not sure what type of
source you are using that produces light at 330 nm, but I suspect it is also
producing the high heat which is evaporating the retained solvent from the
S1813, possibly from the SU-8 and casing the reticulation. S1800 is a broadband
resist which is optimized for g-line (436 nm) exposure so you are not exactly in
it's "sweet" spot. Surprisingly S1800 is very fast at i-line (365 nm) but
absorbs strongly at the top of the resist film causing degradation of the
sidewall angle, especially in films thicker than one micron. Using light at 330
nm will exacerbate this problem. The sidewall angle of SU-8 is also affected at
this wavelength. If possible I would move to a broadband exposure tool. For
superior sidewall angles in your SU-8, I would also use a filter to remove
wavelengths below 350 nm.

Very Best Regards,
Rob Hardman
617-965-5511 ext.313

rom: Soojin Oh 

To: [email protected]

Subject: [mems-talk] Shipley1813 on su-8

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Message: 8

I spin-coated su8 25 on my ITO wafer and after soft

bake (as instructed by MicroChem) I spin-coated S1813

on it and baked it at 95C briefly. At this stage the

two layers of photoresist look fine. But after UV

exposure (330 um 20 sec), it was apparent that

something's happened on the second PR or interface

btwn S1813 and su8. To describe the apperance it looks

like the surface of the Moon under microscope! Some

bumpy bubble-like stuffs all over the surface. Can

anybody explain what I did? Thank you.