[labnetwork] Blistering of metal film stack on PMMA

[David S Barth]

Hi Savitha,

As others have mentioned, this is mainly caused by electron damage to the PMMA. Here is another paper that addresses the issue with an interesting solution: https://csmantech.org/OldSite/Digests/1999/PDF/54.pdf.

Depending on how your chamber is set up, I have one other suggestion that has not been mentioned yet. In our evaporator, we have a short throw distance and a shutter only near the sample, not at the crucible. We found that damage was occurring before the shutter was even opened. Electron scattering from solid metal is much more than from liquid, so we believe that the flux of scattered electrons during the initial melt steps was high enough that electrons would go around the shutter and still damage the sample. We put a metal shielding ring in place around the samples to prevent this (shown in the attached pictures), and you can see the difference between samples inside and outside of the shield.

Best,
David

From: labnetwork-bounces at mtl.mit.edu <labnetwork-bounces at mtl.mit.edu> On Behalf Of Savitha P
Sent: Friday, February 1, 2019 6:54 AM
To: labnetwork at mtl.mit.edu
Subject: [labnetwork] Blistering of metal film stack on PMMA


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<http://www.cense.iisc.ac.in>
*Please note the change in my e-mail id*
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