Hi Lando,

1. Don't take my anecdote of 1.5 Å/s as "permission" to do it in your tool;
every evaporator/material combination is different.

2. What you need to do is figure out with 100% certainty which process step is
cross-linking the PMMA.  I would use acetone (or NMP) solubility rather than
visual inspection for cracking as a test - if the PMMA is dissolved by acetone
or NMP, it's not cross-linked.  This is a bit more reliable, since it's possible
for PMMA to be cross-linked without showing visible cracking.  So if you can,
simply run through your process again with a dummy wafer, but try to dissolve
the PMMA after the UV strip.  If it dissolves, you know the steps leading up to
that point did not cross-link the PMMA.  And so on.  Once you know that for
certain, you can focus on fixing the problem in that process step.

3. The difference between the PMMA bake recipes sounds like the difference
between a hotplate bake (90 s) and an oven bake (15 min).  Don't worry about
that.

4. You say that your temperature monitor in the evaporator registered only 40 C.
Where is the temperature probe in the evaporator?  Is it physically connected to
the sample?  To the wall of the belljar?  Free-floating in the vacuum chamber?
Is it a thermocouple or some kind of remote IR-based thermometer?  It's very
possible for a thermocouple in one position to register quite different thermal
conditions than the sample - in some cases it's meant more as a rough guideline
to heating in the evaporator chamber than any true measurement of sample
temperature.

On Jun 2, 2010, at 13:57, landobasa wrote:

> 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