Hello. This fall I have been having trouble with my PMMA EBL processing. After
development, the patterns are filled in randomly and also there are waves of
PMMA deposited with macroscale features in-plane. I believe the issue is
developed PMMA redepositing everywhere. I found some hints online about PMMA
redepositing or scumming, but nothing to suggest a solution for me. I cannot
figure out why this is happening as my process has been the same for two years
and never had this issue until recently. Please see two photos:

https://www.dropbox.com/s/rpmuv6heh9zfauj/ebl%20scum%20image%20width%201mm.jpg?d
l=0
https://www.dropbox.com/s/wl3jgpo3tc66d5b/ebl%20scum%20image%20width%202mm.jpg?d
l=0

I tried getting new resist from the main bottle, used different resists,
inspected the resist optically before exposure and there is no thickness
variation, have tried “developing” unexposed resist and optically everything
looked fine.

Process:
SiO2 on Si, thickness 90nm or 300nm or 1um.
PMMA 950 A4 or 950 A6.
Spin 3k 30s, bake 180C hot plate for 60s.
Sometimes I expose right away, sometimes the chips rest overnight - same
results.
Exposure is with a Jeol6300 at 2nA.
Development is MIBK:IPA 1:3 for 55s.
Then dunk in a large bath of DI water to quench development.

Thanks,
Kyle Godin
kgodin@stevens.edu

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