[labnetwork] pmma cracking issue

[Mark K Mondol]

John:

Thanks for documenting the process so well.

I am not sure that I would put this down to resist cracking, although it 
might be. My first thought was a metal evaporation issue, but you say it 
happens with different evaporators and sputtering systems (impressive to 
liftoff what look to be 100nm lines with a sputtering system putting 
down the metal).  Stress in the metal seems more likely than stress in 
the resist. Almost any hotplate bake >160C should relieve stress in the 
PMMA. My second thought, uniformed by any real knowledge of chemistry, 
is that your Ammonium polysulfide is too base. In my experience base 
solutions attack PMMA and ruin it as a resist.

The other, obvious, culprit is the He Ion Milling as it is common to all 
your failures; though I can't think of a particular mechanism that would 
create this fault.

In many years of ebeam lithography I have never found PMMA resist to be 
the problem (i.e. it is extraordinarily stable) other than when it was 
inadvertently exposed to x-rays. So I would not be looking at the 
initial resist.  Baking PMMA is also a very tolerant procedure. On the 
other hand your resist profiles must be suffering, to some extent, by 
the exposure to heat (plasma ash and possibly He ion milling and 
possibly by the metalization.  The doses you mention and development 
times and developer all seem very reasonable.

Have any users examined the exposed and developed PMMA prior to 
metalization?  I think that would be my first step.

Regards,

Mark K Mondol


-- 
Mark K Mondol
Assistant Director NanoStructures Laboratory
And
Facility Manager
Scanning Electron Beam Lithography Facility
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mondol at mit.edu
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