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

[Natalia Pankratova]

Hello Philipp,

Another thing you can try is to bake PMMA longer to make sure it's not
going to outgas during Au evaporation. For example, for Josephson Junctions
fabrication when double layer MMA/PMMA is used, the baking time is 2 min +
30 min at 180C. If longer baking is not an option, you can bake for
standard 2-5 min and leave the sample under the vacuum overnight (before
exposure development).


Best,
Natalia.

On Wed, Jul 23, 2025, 1:07 AM Demis D. John <demis at ucsb.edu> wrote:

> Perhaps try a higher dep rate - the theory being that heating (from
> optical/other emission) is causing the PMMA to outgas, and long slow deps
> may cause more heating.
> Eg. Keep the Au dep to <1min total (you may need to adjust the soak steps
> w/ shutter closed so it opens the shutter with ~0.5Å/sec and fairly fast
> feedback loop).
>
> Also, measure the thin-film stress of your Au - maybe it is high,
> causing/exacerbating delamination.
> However since your image also shows exploded bubbles on the PMMA only, I
> suspect outgassing/bubbles more than just regular delamination due to
> stress.
>
> -- Demis (contact info <https://wiki.nanotech.ucsb.edu/wiki/Demis_D._John>
> )
> *Reminder*: The NanoFab has a publications policy
> <https://wiki.nanotech.ucsb.edu/wiki/Frequently_Asked_Questions#Publications_acknowledging_the_Nanofab>
>
>
> On Tue, Jul 22, 2025 at 07:23 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. EfetovGeschwister-Scholl-Platz 1
>> D-80539 München <https://www.google.com/maps/search/Geschwister-Scholl-Platz+1%0D%0AD-80539+M%C3%BCnchen?entry=gmail&source=g>
>> T. +49 (0)89 2180-3733
>>
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