Hi, I'm making multilayer SU-8 molds and I have a recurring problem
where the bottom layer appears to be blurry or reflowed upon
development.  This bottom layer is a 50um layer of SU-8 2050 (spun at
3000 RPM, soft bake 1min 65C 6min 95C, expose for 45s at 4.2 W/cm2
thru a long pass UV filter, PEB 1min 65C 6min 95C).  The top layer is
a 225um layer of SU-8 2100 (spun at 1350rpm, soft baked for a few
hours, exposed for 82s at 4.2 W/cm2 in roughly 20 second increments to
eliminate resist heating, followed by PEB and then development.
Basically around all of my structures on the bottom 50um layer there
is a blurry line when viewed in the microscope, between 10 and 100 um
wide.  Ashing w/ 02 plasma can remove some of this effect, but I don't
know how that will effect the overall height of the mold and it makes
it harder to de-mold PDMS later.

Has anyone else had this problem?  Testing has yet to indicate one
single parameter that is the cause of the effect, but exposure time
and bake times definitely come into play somehow.  Vastly changing
exposure times results in delamination of the bottom layer, however,
and renders the final mold unusable.

JP Hilton
Graduate Student
Columbia University BioMEMS Laboratory