[labnetwork] pmma cracking issue

[Mark K Mondol]

I too wonder if heating during metallization is the issue.

We normally do not "sweep" the beam during evaporation, we do this to 
reduce the spot size of the hot metal, which is important for small 
features. But I think this also means less total heat generation.  I 
also evaporate metals at at least 1nm/sec. As the evaporation rate tends 
to be non-linear with power I think this decreases the heating of the 
substrate.  An increased distance from source to substrate will also 
minimize heating of the substrate. Excess soak times may also generate 
excess heat at the substrate. The most important technique, which we 
use, to reduce heating is to have a "second" shutter very close to the 
substrate, this shields the substrate from heating during the ramp up 
and soak times.  On the other hand I have no clue why this would 
suddenly become an issue for you.

I suppose you could try a test of evaporating more Au, so heating the 
substrate more. Or evaporating Pt or W, which in my experience, requires 
much more heat. Or, conversely evaporating a  thinner layer of Au to see 
if the cracking decreases.

Regards,

Mark K Mondol

On 11/17/2015 4:50 PM, Espen Rogstad wrote:
> Hi John,
> We've also had issues with cracking and even bubbling of pmma during 
> metallization. The problem was most severe for Pt deposition. We 
> haven't gotten totally rid of the problem but the following helped in 
> limiting it:
> - Increasing the deposition rate from 5Å/s normally used to ~10Å/s.
> - Increasing the source-sample distance. We have around 40cm distance 
> in one of our e-beams and it mostly works fine.
> - Attaching samples to a 1cm thick aluminum disc (will act as a heat 
> sink) using Mung II vacuum grease (we normally attach samples to a 4" 
> Si wafer using kapton tape).
> - Putting the source material directly in the Cu pocket without using 
> a crucible liner

-- 
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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