Hi Mike,

I have checked that there was no cracking in PMMA film after UV stripping. I
don't do post-develop baking either.

The original intention of using UV Ozone stripper is to clean the remaining PMMA
after developing. Perhaps I should try to skip this step in order to make sure
whether the UV exposure cause the film cracking.

Given your experience of depositing Ti with 1.5Å/s, then 0.6Å/s should not be a
problem. My E-beam Evaporation is Edwards Auto 306. The distance from the source
to the target is rather short (perhaps ~30-40 cm). 20nm of Ti deposition was
accomplished with indicated temperature of 40 degree C. I am not sure whether
such temperature is sufficient to crosslink PMMA.

Btw, after spin-coating I baked the sample on hot-plate at 180 degree C for 90s
(following the manual from MicroChem). I understand that in some of
nanofabrication facility websites, PMMA is usually baked at 170 degree C for 15
minutes.

Could this be the reason?

Thanks for your help,

Lando

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That sounds like a textbook case of your PMMA getting unintentionally cross-
linked; high temperature or UV exposure could be a culprit.  You say that the
cracks appeared after evaporation.  Did you also inspect between the stripping
step and evaporation step?  Are you doing any post-develop baking, and if so,
what are the parameters?

I'm not personally familiar with that type of stripper, but the manufacturer
states that it heats the sample as part of its descum process.  What temperature
are you using in that step?  Is anyone else in your facility successfully using
that tool for lift-off descumming of PMMA, and if so, can you compare process
recipes?  As a test, you can always try your process without the descum step;
your metal adhesion won't be as good, but if the cross-linking goes away, you've
tracked down the problem.

It's possible the sample is getting too hot during the evaporation step, but
0.06 nm/s (0.6 Å/s) is a low enough rate that I'd be somewhat surprised if that
were the problem (unless your belljar is small so that the sample is relatively
near the crucible).  I used to evaporate up to 1.5 Å/s of Ti onto PMMA with no
problems, though the heating really depends on the details of your evaporator,
and the deposition rate which won't cause heating problems is pretty much
something you have to figure out empirically.