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

[Ryan Rivers]

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>
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
>
>
> --
> 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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