[labnetwork] pmma cracking issue

[Michael Rooks]

I have seen PMMA form stress cracks like this, but not in 200nm thick 
films. It usually happens with > 1 um thick films developed in MIBK/IPA, 
since MIBK swells the resist film. A simple solution is to develop in 
IPA/water (3:1), preferably at a low temperature (we use 7C). You can 
find a paper about IPA/water development here. 
<http://nano.yale.edu/sites/default/files/files/pmma_develop_ipa_water.pdf> 
This is just a guess, but maybe the pmma is a lot thicker than you think.


(Also, I suggest using hot NMP instead of hot acetone. It's a lot safer.)

--------------------------------
Michael Rooks
Yale Institute of Nanoscience and Quantum Engineering
nano.yale.edu <http://nano.yale.edu>



On 11/17/2015 12:46 PM, John Watson - TNW wrote:
>
> Dear colleagues,
>
> Our group has been having a troublesome and seemingly random problem 
> with pmma cracking during metalization steps, and I am hoping to get 
> the community’s ideas on what might be causing the problem.  Our 
> typical process (used to make ohmic contacts to III-V nanowire devices 
> on Si/SiO2 substrates) is as follows:
>
> PMMA 950K A4 spun at 4000 RPM (~200nm thickness)
>
> Bake 175C 10 min
>
> Expose 1100-1300 uC/cm^2 at 100kV
>
> Develop 60s MIBK:IPA I:3, rinse in IPA 30s
>
> 60s O2 ashing (sample sits in Faraday cage, pmma etch rate ~1-3 nm/min)
>
> Ammonium polysulfide passivation of III-V surface (sample sits in 
> heavily diluted, super-saturated polysulfide solution at 60C for 30 min)
>
> Quick load into evaporator followed by short in-situ He ion milling to 
> remove residual sulfur layer
>
> Evaporation of 10/120nm Cr/Au at 0.5/1.5 A/s. Vacuum level typically 
> high 10^-8 Torr throughout deposition
>
> Liftoff in acetone ~50C
>
> The problem that we see is illustrated in the attached SEM image – 
> thin filaments of metal extend from the contacts (typically starting 
> at sharp corners) and short out devices.  The fact that the cracks 
> originate at sharp corners suggests the issue is stress-related, but 
> we have been unable to track down the cause of this excess stress.  
> Many users (8-10) have seen this problem using different bottles of 
> resist, different viscosities and molecular weights of pmma, different 
> baking times and temperatures (as short/cold as 5 min/150C and as 
> hot/long as 190C/30min), different hotplates, 3 different e-beam 
> lithography systems, a number of substrates/gate dielectrics (SiO2, 
> SiNx, BN, InSb, sapphire), different metalization systems (two 
> different e-beam evaporators and one sputtering system – the helium 
> etch is only done in one evaporator), different metalizations (Ti/Au, 
> Cr/Au, NbTiN), and varying waiting times between 
> spinning/exposure/developing/metalization (varying from 5 hours to 3 
> days for the whole process).  The process I described above had been 
> stable for over a year until about a month ago when the cracking 
> problem came up.  Since then it has been sporadic, ruining a few (out 
> of many) processing runs per week, but it has not been repeatable 
> enough to allow for really systematic testing.
>
> Does anyone have any insight into what could increase stress in the 
> resist?  For instance, is temperature/humidity during spinning and 
> baking particularly critical?  One theory we are currently testing is 
> that a recently instituted rule of cleaning the spinners with a 
> metal-ion-free photoresist developer following pmgi spinning could be 
> causing the problem (e.g. this would help explain the random nature of 
> the problem since it would depend on what the previous user did with 
> the spinner).  Thanks,
>
> John
>
> -----------------
> John Watson
> Postdoctoral Fellow - Kavli Institute of Nanoscience
> Delft University of Technology
> Lorentzweg 1, 2628 CJ Delft
> The Netherlands
>
>
>
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