Hi

I'm currently trying to resolve a number of problems with our SU-8
processes in a new cleanroom. One thing I have noticed in particular
is extreme mushroom shaped structures and extreme negative sidewalls
when using a Suss MA/BA6 mask aligner. I don't believe this is
overexposure of the top layer due to deep UV (I believe it should be
filtered but need to check that) since the structures are so much
wider at the top and the effect is very inconsistent across the wafer.
I now suspect the resist is being physically squashed during contact
mode (it occurs in both soft and hard contact modes).
I intend to try the following:

1. Increase my softbake to make the layer more rigid.
2. Try a proximity exposure.

Has anyone encountered similar problems and is there any other tricks
I could try?
I worry about using proximity exposure since with thick layers this
could make things worse. I have always been concerned about using a
rigid mask with thick layers of SU-8 as good conformal contact may be
difficult. This may not be a problem with single layers but I need to
do multiple layers and previous layers which have been post-baked can
cause the wafer to bow slightly.

I would love to hear from anyone with experience of doing thick layers
with a contact mask aligner.
Best regards

Gareth