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

[David Barth]

Thanks, Justin. I think that's a pretty complete and accurate summary. Here
<https://wiki.nano.upenn.edu/wiki/images/8/8b/Dsbarth_pmma_bubbling_maebl2023.pdf>
is a link to the talk Justin mentioned in case anyone is interested.

Best,
David Barth
Senior Manager
QNF, Singh Center for Nanotechnology
University of Pennsylvania

On Thu, Jul 24, 2025 at 11:28 AM Justin C Wirth <jcwirth at purdue.edu> wrote:

> Philipp,
>
>
>
> I want to second Jorg’s suggestion and links here, those papers are
> absolutely worth a read and do a great job explaining the mechanisms
> involved here.
>
>
>
> A couple things stick out to me about this type of behavior, which has
> been seen my many, many people with PMMA under E-beam evaporation of gold
> (although it depends on the particular evaporator, the deposition rate, and
> many factors):
>
>    1. It’s always in an E-beam evaporator, never a thermal evaporator,
>    and it’s **often** gold (although it can happen with other high Z
>    metals as well)
>    2. Heat is often offered as the causal mechanism, but actually PMMA is
>    soft baked at 180 C to evaporate the anisole, and can be left there for
>    very long times (minutes to hours, maybe indefinitely?) without it changing
>    the exposure characteristics (At least if anyone has measured a change in
>    exposure characteristics at 180 C vs. time it’s not something I’ve ever
>    seen). So heating PMMA up to 180 C on reasonable time scales is fine and
>    shouldn’t cause the film itself to bubble (this could be proved if someone
>    had a vacuum annealing chamber and heated a sample with PMMA and a good
>    metal film on top absent any deposition…though I haven’t actually done
>    that).
>
>
>
> So, what this means to me logically is that actually heating is not the
> fundamental culprit. And as explained in the papers that Jorg provided,
> actually the fundamental cause is crosslinking of the PMMA due to charged
> particle irradiation. Crosslinking causes gas molecules to be generated,
> which if there is nothing on top of the PMMA, are simply pumped out of the
> chamber (which is why one would not see any issues if one doesn’t actually
> deposit the gold film). But if there **is** a layer of something on top
> of the PMMA that the gas can’t diffuse through (i.e. gold), the gas is
> trapped and forms bubbles between the PMMA and the gold.
>
>
>
> The solution is to stop the crosslinking from happening, and this can be
> done in a myriad of ways as explained in those papers. Because a number of
> secondary factors are involved, changing those secondary factors can reduce
> or stop the effect as well, and so I assume that all of the suggestions
> you’ve gotten from others on how to stop or reduce the effect have indeed
> worked for them and may well work in your case too. But the crosslinking
> plus gas generation plus gas trapping is kind of the key to understanding
> the whole puzzle (from the sample’s perspective at least).
>
>
>
> It’s also an effect I’ve seen myself in almost real time in an EBL with a
> chamber scope (JEOL JBX-8100FS) when looking at an alignment mark on a
> sample with PMMA and a (probably too thick) gold layer on top for
> anti-charging. After enough SEM dose (seconds to tens of seconds, enough to
> turn the PMMA negative and crosslink), a bubble starts to balloon up where
> the mark is being imaged because of the gas generated. It can either pop
> dramatically or grow or shrink down if microcracks form and let the gas
> out, and it looks exactly like the kind of bubbles you see in your sample
> at varying stages: growing, deflated, popped, etc.
>
>
>
> David Barth at Penn (cc’d) gave a great talk about the practical aspects
> of this at MAEBL
> <https://urldefense.com/v3/__https://maebl.org/__;!!IBzWLUs!RCSqXpqaiP3UUnWSMgay109iY_ivgcdej987ixNUv33AwE6MmlTyEeBUOu1-ZOUDXxxFTDnwgjo5YoAc9GflEm8$>
> a few years back, hopefully he can share his talk with you and add some
> more details on it, he’s experimented with mitigating this effect a good
> amount. But this is something that’s been a repeated topic of discussion
> over the years and is quite confusing because it depends a lot on the
> particular evaporator geometry, the particulars of your deposition, how
> much carbon contamination there is on top of the gold, etc.
>
>
>
> Thanks,
>
> Justin
>
>
>
> Justin C. Wirth, PhD
>
> Senior Research Engineer – EBL Processes & Nanofab Process Consultation
>
> Birck Nanotechnology Center | Room 2287A
> 765.494.8203 | jcwirth at purdue.edu
>
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>
>
>
> *From:* labnetwork <labnetwork-bounces at mtl.mit.edu> *On Behalf Of *Jorg
> Scholvin
> *Sent:* Tuesday, July 22, 2025 7:34 PM
> *To:* Philipp Altpeter <philipp.altpeter at lmu.de>; labnetwork at mtl.mit.edu
> *Subject:* Re: [labnetwork] Bubble formation when e-beam evaporating Gold
> on PMMA
>
>
>
> ---- *External Email*: Use caution with attachments, links, or sharing
> data ----
>
>
>
> Hi Philipp:
>
>
>
> Two papers of interest that may help explain:
>
>
> https://pubs.aip.org/avs/jvb/article/39/5/052601/1070034/Role-of-electron-and-ion-irradiation-in-a-reliable
> <https://urldefense.com/v3/__https://pubs.aip.org/avs/jvb/article/39/5/052601/1070034/Role-of-electron-and-ion-irradiation-in-a-reliable__;!!IBzWLUs!RCSqXpqaiP3UUnWSMgay109iY_ivgcdej987ixNUv33AwE6MmlTyEeBUOu1-ZOUDXxxFTDnwgjo5YoAcMjLAsrU$>
>
> https://iopscience.iop.org/article/10.1088/1361-6463/abe89b
> <https://urldefense.com/v3/__https://iopscience.iop.org/article/10.1088/1361-6463/abe89b__;!!IBzWLUs!RCSqXpqaiP3UUnWSMgay109iY_ivgcdej987ixNUv33AwE6MmlTyEeBUOu1-ZOUDXxxFTDnwgjo5YoAcZ2c2vFY$>
>
>
>
> 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
>
>
>
> --
>
> 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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