[labnetwork] Blistering of metal film stack on PMMA

[Nathan Nelson - Fitzpatrick]

Hi Savitha,

I have seen and experienced this issue before.  My understanding is that the PMMA is blistering due to electrons impinging on your sample, over-dosing the PMMA.  When I experienced this issue on my tool I did some searching online and found this report that correlated carbon in the Au melt to an increased current of electrons scattering off of the melt and into the sample.

http://www.skyworksinc.com/downloads/press_room/published_articles/GaAs_Mantech_052010_6.pdf

In my case I supposed that the likely source of the carbon was from the FABMATE crucible liner our lab was using for our Gold evaporation process.  The liner’s glassy finish appeared to be wearing down due to handling and was likely leaving a small amount of carbon dust on the melt every time it was added or removed from our evaporator.

I was able to fix our issue by taking the gold melt out of our old FABMATE liner, cleaning the carbon dust off of the ingot with some IPA and putting it back in a new FABMATE liner.  Once the ingot was cleaned and put into a new liner the surface of the melt looked appreciably different.  When contaminated with carbon it appeared to have a “skin” on the top of the melt, while the clean melt has a uniform “liquid-like” appearance.

FYI:  I have recently moved on to a Molybdenum liner as described in this Materion report, the downside with this is that the Gold wets the liner surface and will likely be difficult to reclaim if/when I have to do so.

https://materion.com/-/media/files/advanced-materials-group/me/technicalpapers/developingafundamentalunderstandingofgoldspitting.pdf?la=en&hash=502E7BF53ECF2EE5923F4B5F7488A78B3B2ED05F

Feel free to reach out if you want to discuss this problem further.

Best regards,
  -Nathan

--
Nathan Nelson-Fitzpatrick  PhD
Nanofabrication Process & Characterization Engineering Manager
Quantum-Nano Fabrication and Characterization Facility (QNFCF)
Office of Research
University of Waterloo
200 University Avenue West, Waterloo, ON  N2L 3G1
P: 519-888-4567 ext. 31796
C: 226-218-3206
https://fab.qnc.uwaterloo.ca

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From: <labnetwork-bounces at mtl.mit.edu> on behalf of Savitha P <savithap at iisc.ac.in>
Date: Friday, February 1, 2019 at 8:58 AM
To: "labnetwork at mtl.mit.edu" <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
*Please note the change in my e-mail id*
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