[labnetwork] Blistering of metal film stack on PMMA

[Mary Tang]

Hi Savitha --


I agree with Norman and Nathan, as controlling the temperature and 
bombardment of your substrate should address the blistering problem.  
I'd like to add that your question reminds me of some really cool tricks 
with PMMA that I've learned from one of our most esteemed researchers, 
Jim Kruger (who is retired for the second time, but still comes around 
to share valuable nuggets.)


Optical resists generally harden and carbonize at higher temperatures 
and with ion bombardment.  However, PMMA undergoes thermal decomposition 
starting around 300 C, attributed to depolymerization from chain ends.  
And at around 400 C, thermal decomposition happens regardless of chain 
length, due to random scission.  Interestingly, good quality PMMA leaves 
no residue on when heated briefly to 400-450 C.  This makes it a really 
handy method for bonding chips, if your samples can tolerate the heat.


Also, it turns out that xrays from Ebeam evaporation will fully expose 
PMMA.  This means you can use ordinary PMMA developer for lift off.  
This opens up LOADS of possibilities for processing because you can 
pattern with optical resist and PMMA and selectively develop or lift-off 
one versus the other, using the appropriate developer.


If anyone else has done this or has cool tricks to add, I'd love to hear 
about it.


Cheers -


Mary

-- 
Mary X. Tang, Ph.D.
Managing Director
Stanford Nanofabrication Facility
Paul G. Allen Building, Rm 141
420 Via Palou Mall
Stanford, CA  94305
(650)723-9980
mtang at stanford.edu
https://snf.stanford.edu

On 2/1/2019 3:53 AM, Savitha P wrote:
>
> Hello!
>
>
> We need inputs deposition of metal film stack on PMMA coated 
> substrates. We see blisters on the PMMA which creates problems during 
> lifting off process.
>
>
> We are using a Leybold UNIVEX400 E-Beam Evaporator.
>
>
> For a stack deposition of metals, example Cr/Au (10/50 nm)
>
> the film on Si and SiO2 substrate is smooth with good resistivity 
> value.The film deposited on optical lithography patterned samples 
> also  give good lift off and the appearance of metal film on resist is 
> smooth and usual.
>
> Whereas, for a film stack deposition on e-beam lithography patterned 
> samples, the metal on the PMMA resist becomes completely blistered. 
> The lift off is not clean, leaving behind random patches of metal over 
> resist on the substrate even after longer duration in solvents.
>
>
> Few tool and process details:
>
> Source to substrate distance- Minimum 20 cm and Maximum 35 cms. 
> Blisters observed for both the distances.
>
> Deposition rates of Cr 0.3 nm/s and Au 0.3 nm/s
>
> Base Pressure around 2 E-06 Torr
>
> Deposition at room temperature
>
> OPL resists used (AZ 5214E, S 1813, AZ 4562)
>
> EBL resists(PMMA 950 A4, PMMA 495 A4, EL-9)
>
>
> We have already tried different baking times for PMMA, different 
> deposition rates,  wait times between depositions and also grounding 
> the substrate.
>
>
> Please let us know if anyone has an idea on how to solve this.
>
>
> With best wishes,
>
>
> Savitha
>
>
>
> Dr. Savitha P
> Chief Operating Officer
> National Nanofabrication Centre
> Centre for Nanoscience and Engineering
> Indian Institute of Science
> Bangalore - 560012
> India.
> Ph. +91 80 2293 3319
> www.cense.iisc.ac.in
> /**Please note the change in my e-mail id**/
>
> _______________________________________________
> labnetwork mailing list
> labnetwork at mtl.mit.edu
> https://mtl.mit.edu/mailman/listinfo.cgi/labnetwork

-- 
Mary X. Tang, Ph.D.
Stanford Nanofabrication Facility
Paul G. Allen Building, Rm 141
420 Via Palou Mall
Stanford, CA  94305
(650)723-9980
mtang at stanford.edu
https://snf.stanford.edu

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