[labnetwork] [EXTERNAL] [BULK] Re: Bubble formation when e-beam evaporating Gold on PMMA

Wed Jul 23 13:16:37 EDT 2025

Hi Philipp,
I second Ryan’s suggestion about depositing faster!

Best,

Larry

Larry A. Hess, PhD
NASA
Goddard Space Flight Center
8800 Greenbelt Road
Code 553, Detector Systems Branch
Building 11, Room E011
Greenbelt, MD 20771
301-286-0259 (P)
301-286-1672 (F)

From: labnetwork <labnetwork-bounces at mtl.mit.edu> On Behalf Of Ryan Rivers
Sent: Tuesday, July 22, 2025 3:21 PM
To: Philipp Altpeter <philipp.altpeter at lmu.de>
Cc: labnetwork at mtl.mit.edu
Subject: [EXTERNAL] [BULK] Re: [labnetwork] Bubble formation when e-beam evaporating Gold on PMMA

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Hi Philipp,

Looks like a heat transfer problem. If you've installed a new tool, it's likely the heat transfer is different than your old one. Here's some basics that can drive it:

Source to substrate distance - you probably can't do anything about this parameter but if this changed from one tool to the other it can be the driving cause of thermal failure. A closer throw distance makes for less wasted material but larger transfer of radiant energy.
Fixturing of the wafer (a wafer with no fixturing behind it heats much faster than one on a thermally conductive surface, even in vacuum) - if your wafer's only being held up by an edge ring you can usually mount a 1/8" copper or other high specific heat disc behind it and help alleviate this kind of problem.

Otherwise it's just pure process development and keeping your total energy emitted from the melt as low as possible. PMMA is fragile stuff and it tends to react strongly to minor heat loads. One of the more counterintuitive realities is that going *faster* may help. Faster depositions finish faster and give the PMMA less time to heat up and react. Bump your dep rate to 4A/s (and make sure to recalibrate your rise and soak before opening the shutter) and see if the problem gets better.

-Ryan

On Tue, Jul 22, 2025 at 7:20 AM Philipp Altpeter <philipp.altpeter at lmu.de<mailto:philipp.altpeter at lmu.de>> wrote:

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 01DBFBD4.03F26F60]

--

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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