[labnetwork] pmma cracking issue

[bryan cord]

Seconding Mark here. Cracking of PMMA during evaporation of high melting 
point metals (like Ta or W) has been a major issue for us, with final 
results that look similar to your "before liftoff" image.

I actually had it randomly happen with gold once too, as metal had 
worked its way out of the crucible during previous runs and gotten 
between the crucible and the block, which increased thermal conductivity 
to the cooling block enough that it took a lot longer to heat the gold 
to melting point. In that case it was very obvious that something was 
wrong (the crucible didn't glow nearly as brightly as it would during a 
"good" run and the deposition rate vs. power was way off), but maybe 
something similar but less severe is happening? Comparing beam currents 
and total run times between the good and bad runs might be able to tell 
you if something similar is up.

-bryan


On 11/18/2015 11:05 AM, Mark K Mondol wrote:
> 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
>

-- 
Bryan Cord
Minnesota Nano Center (MNC)
University of Minnesota
115 Union St SE, Rm 153
Minneapolis, MN 55455
612.626.3287 (work)
857.891.6820 (cell)
bcord at umn.edu
http://wiki.umn.edu/EBPG