[labnetwork] Bubble formation when e-beam evaporating Gold on PMMA

[Jorg Scholvin]

Hi Philipp:

Two papers of interest that may help explain:
https://pubs.aip.org/avs/jvb/article/39/5/052601/1070034/Role-of-electron-and-ion-irradiation-in-a-reliable
https://iopscience.iop.org/article/10.1088/1361-6463/abe89b

An insightful experiment, inspired by Sun’s paper, to have a plain wafer with PMMA spend time in the evaporator, with the power level just below the point where you get deposition.  Maybe 15-30 mins is sufficient. That will expose your sample to all the things you don’t want (temperature, electrons, …) without putting down a film. After that treatment, develop the PMMA (assuming there was no deposition) and you may find some interesting patterns that may suggest electron exposure. See attached image of a simple experiment we ran a few years ago (these were quick qualitative tests only).  Using a shim under the crucible helped fix things for our users (less cooling = less current = less electron radiation).

Best,

-Jorg




From: labnetwork <labnetwork-bounces at mtl.mit.edu> On Behalf Of Philipp Altpeter
Sent: Tuesday, July 22, 2025 4:23 AM
To: labnetwork at mtl.mit.edu
Subject: [labnetwork] Bubble formation when e-beam evaporating Gold on PMMA


Dear all,

In recent months, we installed a new UHV e-beam evaporator and have since encountered significant issues with our standard lift-off process using 3 nm Chromium followed by 50 nm Gold. Specifically, we are observing the formation of large bubbles beneath the PMMA, which severely damage the PMMA layer (see image below). The rate is decently low, 0.6 A/s at around 40mA emission current, 10 kV. After around 30 nm of thickness, bubbles become clearly visible.

Interestingly, deposition on bare silicon, silicon dioxide, or other (photo)resists appears to proceed without problems.

We have already tried adjusting various parameters — including cooling conditions, beam wobbling, throw distance, and acceleration voltage. We also modified the PMMA baking protocol and tested PMMA dissolved in different solvents — all without success.

If anyone has experienced similar issues or has suggestions for troubleshooting, your input would be greatly appreciated.

Thank you in advance for your help!

Best regards,
Philipp



[cid:image001.png at 01DBFB3B.43EE8930]

--

Philipp Altpeter

Fakultät für Physik der LMU

LS Prof. Efetov

Geschwister-Scholl-Platz 1

D-80539 München

T. +49 (0)89 2180-3733
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