[labnetwork] Seeking advice regarding developer/unexposed resist interaction

[Gustavo de Oliveira Luiz]

Hello everyone,

I am having some trouble developing recipes or testing performance on our
Heidelberg MLA150. Especially when testing performance, it is required for
us to expose the test pattern once, develop, inspect and repeat. We usually
coat a larger sample (e.g. a 4" wafer) with resist and expose a small test
design, allowing us to use the same sample many times. The problem that I
encounter is that the development time increases after each iteration, and
it is not due to the developer getting weaker (I tried using a new
developer to make sure). This behavior is observed on AZ 1500 and AZ 4000
series resists, with the latter presenting a more drastic change.

It seems to me that the developer changes something on the unexposed
resist, making it harder to develop. This is made obvious by the fact that
we don't see this problem when processing multiple samples and developing
them with the same developer bath. In that case the only problem we
encounter is the developer getting weaker depending on the exposed area and
number of samples, as expected.

I thought that it could be over-hydration, since the developer is a water
solution and we rinse with water as well. But high water content tends to
make development faster, not slower. Does anyone have more information on
the interaction between developer and unexposed resist? I'd appreciate any
input you may have.

Best regards,
--
Gustavo de Oliveira Luiz, PhD
Applications/Research Specialist
nanoFAB, University of Alberta
+1 (780) 619-1463
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