All,

Re: Cracks in SU-8

As my friend Chad Brubaker at EV Group has stated there are some ways of
reducing and potentially removing any of the stress cracks created when
working with SU-8.

        > Try to incorporate stress relief patterns in the resist film,
especially when operating at thick films (100um +)...These patterns can
take the shape of fences around devices/layouts or even as Chad points
out design with rounded corners instead of sharp, square internal
corners.

        > As these cracks are only very shallow they can easily be
annealed out by performing a Post Development Bake (PDP) or hardbake at
a temperature in the range 140-160C for only a minute or so...This
resist doesn't really reflow but this bake step removes any evidence of
surface crazing or cracking..

        > In addition to the suggestions above another method of
minimising the cracking is to switch to an alternative developer such as
Ethyl Lactate or Di-Acetone Alcohol...These are milder solvents that
works well with exposed SU-8...The IPA rinse is still necessary to
'chase' out the spent developer and also to act as a developer endpoint
indicator.

        > Another method that is used at very thick films is to use
relaxation time-outs...These can be as long as 12 hours between
processing steps and theoretically allow the resist to rest before
proceeding to the next processing step.

        > Contrary to theory the exposure dose effect on SU-8 is
significant and these cracks can be reduced, not fully removed, by
actually increasing the exposure dose...If the film is not optimally
softbaked and then underexposed it will be more susceptible to stress
cracks....By increasing, or optimising the exposure dose these stresses
can be controlled, however, there is a trade off....With this increase
in exposure the resolution can decrease, as with most negative resists
and so a balancing act between optimal resolution and crack free
features is required...

Re: Thermal issues with SU-8

In the uncured state the maximum temperature that SU-8 should be exposed
to is ~120C...If this temperature is exceeded then optimal processing of
SU-8 will be difficult and subsequent development could be a problem...

Acetone will develop unexposed SU-8 but it is very aggressive and will
generate many stress cracks, I would only use this solvent as a
developer for very deep, high aspect ratio work with several final
development steps with SU-8 Developer.

If there are any other specific issues with SU-8 and the processing of
the product please contact me directly at the number and email address
below.


Mark Shaw
[email protected]
Cell: 617-407-9490