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

[Rob Belan]

Hi Philipp,

For electron beam evaporation, the slower the rate the hotter the substrate will get.  Also, the more power you use, gold usually requires a fair amount of power to evaporate, the hotter the substrate gets.  PMMA will bubble at temps of 80C or higher.  One way to prove this out is to double the rate (which decreases the exposure time in half) and see if that cures the bubble issue.  There are alternate ways to run at lower powers and keep the same rate through the use of spacers under the crucible liners.  But I would for now just see if increasing the rate a bit will keep the PMMA from bubbling.

Let me know if you have further questions.

Rob


Rob Belan
Technical Director PVD Products
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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

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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 01DBFAF5.FA722E00]

--

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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