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You evaporator folks are kind in trying to work out the source of these problems and coming up with solutions, but from a lithography perspective the problem is the resist performance. You might consider tossing the problem back to your litho people to find
a better performing resist or at least inform users of this known issue and advise them on the alternatives. I know CSAR and ZEP are much more expensive, but as far as I know they don't have this problem. Have the litho people experimented with different
molecular weights of PMMA or the MAA copolymer? (I admit I haven't kept up and read all the literature provided). </div>
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Other thoughts: 1) those molecules outgassing from the PMMA are probably contributing to the carbon contamination of your gold source. 2) going to another resist should allow you to use your SEM for imaging the resist patterns.</div>
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It's clear from this research that PMMA is the source of this problem. You can replace it rather than experimenting with evaporator specific solutions.</div>
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-Cliff</div>
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<p style="margin: 0px;"><span style="font-family: Aptos, sans-serif; font-size: 11pt; color: rgb(0, 0, 0);"><b>Cliff Knollenberg</b></span></p>
<p style="margin: 0px;"><span style="font-family: Aptos, sans-serif; font-size: 11pt; color: rgb(0, 0, 0);">Processing Engineer - Lithography</span></p>
<p style="margin: 0px;"><span style="font-family: Aptos, sans-serif; font-size: 11pt; color: rgb(0, 0, 0);">Stanford Nanofabrication Facility</span></p>
<p style="margin: 0px;"><span style="font-family: Aptos, sans-serif; font-size: 11pt; color: rgb(0, 0, 0);">650-721-1274</span></p>
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" style="font-size:11pt" color="#000000"><b>From:</b> labnetwork <labnetwork-bounces@mtl.mit.edu> on behalf of David Barth <dsbarth@seas.upenn.edu><br>
<b>Sent:</b> Thursday, July 24, 2025 2:09 PM<br>
<b>To:</b> Justin C Wirth <jcwirth@purdue.edu><br>
<b>Cc:</b> labnetwork@mtl.mit.edu <labnetwork@mtl.mit.edu><br>
<b>Subject:</b> Re: [labnetwork] Bubble formation when e-beam evaporating Gold on PMMA</font>
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<div dir="ltr">Thanks, Justin. I think that's a pretty complete and accurate summary.
<a href="https://wiki.nano.upenn.edu/wiki/images/8/8b/Dsbarth_pmma_bubbling_maebl2023.pdf">
Here</a> is a link to the talk Justin mentioned in case anyone is interested.
<div><br>
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<div>Best,</div>
<div>David Barth</div>
<div>Senior Manager</div>
<div>QNF, Singh Center for Nanotechnology</div>
<div>University of Pennsylvania</div>
</div>
<br>
<div class="x_gmail_quote x_gmail_quote_container">
<div dir="ltr" class="x_gmail_attr">On Thu, Jul 24, 2025 at 11:28 AM Justin C Wirth <<a href="mailto:jcwirth@purdue.edu">jcwirth@purdue.edu</a>> wrote:<br>
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<p class="x_MsoNormal"><span style="font-size:11pt; font-family:Calibri,sans-serif">Philipp,<u></u><u></u></span></p>
<p class="x_MsoNormal"><span style="font-size:11pt; font-family:Calibri,sans-serif"><u></u> <u></u></span></p>
<p class="x_MsoNormal"><span style="font-size:11pt; font-family:Calibri,sans-serif">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.
<u></u><u></u></span></p>
<p class="x_MsoNormal"><span style="font-size:11pt; font-family:Calibri,sans-serif"><u></u> <u></u></span></p>
<p class="x_MsoNormal"><span style="font-size:11pt; font-family:Calibri,sans-serif">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):<u></u><u></u></span></p>
<ol start="1" type="1" style="margin-top:0in">
<li class="x_m_489391253500884669MsoListParagraph" style="margin-left:0in"><span style="font-size:11pt; font-family:Calibri,sans-serif">It’s always in an E-beam evaporator, never a thermal evaporator, and it’s *<b>often</b>* gold (although it can happen with
other high Z metals as well)<u></u><u></u></span></li><li class="x_m_489391253500884669MsoListParagraph" style="margin-left:0in"><span style="font-size:11pt; font-family:Calibri,sans-serif">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).<u></u><u></u></span></li></ol>
<p class="x_MsoNormal"><span style="font-size:11pt; font-family:Calibri,sans-serif"><u></u> <u></u></span></p>
<p class="x_MsoNormal"><span style="font-size:11pt; font-family:Calibri,sans-serif">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 *<b>is</b>* 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.
<u></u><u></u></span></p>
<p class="x_MsoNormal"><span style="font-size:11pt; font-family:Calibri,sans-serif"><u></u> <u></u></span></p>
<p class="x_MsoNormal"><span style="font-size:11pt; font-family:Calibri,sans-serif">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).<u></u><u></u></span></p>
<p class="x_MsoNormal"><span style="font-size:11pt; font-family:Calibri,sans-serif"><u></u> <u></u></span></p>
<p class="x_MsoNormal"><span style="font-size:11pt; font-family:Calibri,sans-serif">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.<u></u><u></u></span></p>
<p class="x_MsoNormal"><span style="font-size:11pt; font-family:Calibri,sans-serif"><u></u> <u></u></span></p>
<p class="x_MsoNormal"><span style="font-size:11pt; font-family:Calibri,sans-serif">David Barth at Penn (cc’d) gave a great talk about the practical aspects of this at
<a href="https://urldefense.com/v3/__https://maebl.org/__;!!IBzWLUs!RCSqXpqaiP3UUnWSMgay109iY_ivgcdej987ixNUv33AwE6MmlTyEeBUOu1-ZOUDXxxFTDnwgjo5YoAc9GflEm8$" target="_blank">
MAEBL</a> 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.<u></u><u></u></span></p>
<p class="x_MsoNormal"><span style="font-size:11pt; font-family:Calibri,sans-serif"><u></u> <u></u></span></p>
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<p class="x_MsoNormal" style="background:white"><span style="font-size:11pt; font-family:Calibri,sans-serif; color:black; border:1pt none windowtext; padding:0in">Thanks,<u></u><u></u></span></p>
<p class="x_MsoNormal" style="background:white"><span style="font-size:11pt; font-family:Calibri,sans-serif; color:black; border:1pt none windowtext; padding:0in">Justin<u></u><u></u></span></p>
<p class="x_MsoNormal" style="background:white"><span style="font-size:11pt; font-family:Calibri,sans-serif; color:black; border:1pt none windowtext; padding:0in"><u></u> <u></u></span></p>
<p class="x_MsoNormal" style="background:white"><span style="font-size:11pt; font-family:Calibri,sans-serif; color:black; border:1pt none windowtext; padding:0in">Justin C. Wirth, PhD </span><span style="font-size:11pt; font-family:Calibri,sans-serif; color:black"><u></u><u></u></span></p>
<p class="x_MsoNormal" style="background:white"><span style="font-size:11pt; font-family:Calibri,sans-serif; color:black; border:1pt none windowtext; padding:0in">Senior Research Engineer – EBL Processes & Nanofab Process Consultation </span><span style="font-size:11pt; font-family:Calibri,sans-serif; color:black"><u></u><u></u></span></p>
<p class="x_MsoNormal" style="background:white"><span style="font-size:11pt; font-family:Calibri,sans-serif; color:black; border:1pt none windowtext; padding:0in">Birck Nanotechnology Center | Room 2287A<br>
765.494.8203 | </span><span style="color:black"><a href="mailto:jcwirth@purdue.edu" target="_blank"><span style="font-size:11pt; font-family:Calibri,sans-serif; color:rgb(5,99,193); border:1pt none windowtext; padding:0in">jcwirth@purdue.edu</span></a></span><span style="font-size:11pt; font-family:Calibri,sans-serif; color:black; border:1pt none windowtext; padding:0in"> </span><span style="font-size:11pt; font-family:Calibri,sans-serif; color:black"><u></u><u></u></span></p>
<p class="x_MsoNormal" style="background:white"><span style="color:black"><a href="https://urldefense.com/v3/__https://purdue.atlassian.net/wiki/spaces/BNCWiki__;!!IBzWLUs!RCSqXpqaiP3UUnWSMgay109iY_ivgcdej987ixNUv33AwE6MmlTyEeBUOu1-ZOUDXxxFTDnwgjo5YoAc-hdVzOg$" target="_blank"><span style="font-size:11pt; font-family:Calibri,sans-serif; color:rgb(5,99,193); border:1pt none windowtext; padding:0in">BNCWiki</span></a></span><span style="font-size:11pt; font-family:Calibri,sans-serif; color:black">
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iLab</span></a></span><span style="font-size:11pt; font-family:Calibri,sans-serif; color:black"> |
</span><span style="color:black"><a href="https://urldefense.com/v3/__https://outlook.office.com/bookwithme/user/5ad94572a9c04a4e8aa9159479128868@purdue.edu?anonymous&ep=plink__;!!IBzWLUs!RCSqXpqaiP3UUnWSMgay109iY_ivgcdej987ixNUv33AwE6MmlTyEeBUOu1-ZOUDXxxFTDnwgjo5YoAcTXdaA_c$" target="_blank"><span style="font-size:11pt; font-family:Calibri,sans-serif; color:rgb(5,99,193)">Book
time on my calendar</span></a></span><span style="font-size:11pt; font-family:Calibri,sans-serif; color:black"><u></u><u></u></span></p>
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<p class="x_MsoNormal"><span style="font-size:11pt; font-family:Calibri,sans-serif"><u></u> <u></u></span></p>
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<p class="x_MsoNormal"><b><span style="font-size:11pt; font-family:Calibri,sans-serif">From:</span></b><span style="font-size:11pt; font-family:Calibri,sans-serif"> labnetwork <<a href="mailto:labnetwork-bounces@mtl.mit.edu" target="_blank">labnetwork-bounces@mtl.mit.edu</a>>
<b>On Behalf Of </b>Jorg Scholvin<br>
<b>Sent:</b> Tuesday, July 22, 2025 7:34 PM<br>
<b>To:</b> Philipp Altpeter <<a href="mailto:philipp.altpeter@lmu.de" target="_blank">philipp.altpeter@lmu.de</a>>;
<a href="mailto:labnetwork@mtl.mit.edu" target="_blank">labnetwork@mtl.mit.edu</a><br>
<b>Subject:</b> Re: [labnetwork] Bubble formation when e-beam evaporating Gold on PMMA<u></u><u></u></span></p>
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<b>External Email</b>: Use caution with attachments, links, or sharing data ----<u></u><u></u></span></p>
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<p class="x_MsoNormal">Hi Philipp:<u></u><u></u></p>
<p class="x_MsoNormal"><u></u> <u></u></p>
<p class="x_MsoNormal">Two papers of interest that may help explain:<u></u><u></u></p>
<p class="x_MsoNormal"><a href="https://urldefense.com/v3/__https://pubs.aip.org/avs/jvb/article/39/5/052601/1070034/Role-of-electron-and-ion-irradiation-in-a-reliable__;!!IBzWLUs!RCSqXpqaiP3UUnWSMgay109iY_ivgcdej987ixNUv33AwE6MmlTyEeBUOu1-ZOUDXxxFTDnwgjo5YoAcMjLAsrU$" target="_blank">https://pubs.aip.org/avs/jvb/article/39/5/052601/1070034/Role-of-electron-and-ion-irradiation-in-a-reliable</a><u></u><u></u></p>
<p class="x_MsoNormal"><a href="https://urldefense.com/v3/__https://iopscience.iop.org/article/10.1088/1361-6463/abe89b__;!!IBzWLUs!RCSqXpqaiP3UUnWSMgay109iY_ivgcdej987ixNUv33AwE6MmlTyEeBUOu1-ZOUDXxxFTDnwgjo5YoAcZ2c2vFY$" target="_blank">https://iopscience.iop.org/article/10.1088/1361-6463/abe89b</a><u></u><u></u></p>
<p class="x_MsoNormal"><u></u> <u></u></p>
<p class="x_MsoNormal">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).<u></u><u></u></p>
<p class="x_MsoNormal"><u></u> <u></u></p>
<p class="x_MsoNormal">Best,<u></u><u></u></p>
<p class="x_MsoNormal"><u></u> <u></u></p>
<p class="x_MsoNormal">-Jorg<u></u><u></u></p>
<p class="x_MsoNormal"><u></u> <u></u></p>
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<p class="x_MsoNormal"><b><span style="font-size:11pt; font-family:Calibri,sans-serif">From:</span></b><span style="font-size:11pt; font-family:Calibri,sans-serif"> labnetwork <</span><a href="mailto:labnetwork-bounces@mtl.mit.edu" target="_blank"><span style="font-size:11pt; font-family:Calibri,sans-serif">labnetwork-bounces@mtl.mit.edu</span></a><span style="font-size:11pt; font-family:Calibri,sans-serif">>
<b>On Behalf Of </b>Philipp Altpeter<br>
<b>Sent:</b> Tuesday, July 22, 2025 4:23 AM<br>
<b>To:</b> </span><a href="mailto:labnetwork@mtl.mit.edu" target="_blank"><span style="font-size:11pt; font-family:Calibri,sans-serif">labnetwork@mtl.mit.edu</span></a><span style="font-size:11pt; font-family:Calibri,sans-serif"><br>
<b>Subject:</b> [labnetwork] Bubble formation when e-beam evaporating Gold on PMMA<u></u><u></u></span></p>
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<p class="x_MsoNormal"><u></u> <u></u></p>
<p>Dear all,<u></u><u></u></p>
<p>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<span style="font-family:Arial,sans-serif"> </span>nm Chromium followed by 50<span style="font-family:Arial,sans-serif"> </span>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.<u></u><u></u></p>
<p>Interestingly, deposition on bare silicon, silicon dioxide, or other (photo)resists appears to proceed without problems.<u></u><u></u></p>
<p>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.<u></u><u></u></p>
<p>If anyone has experienced similar issues or has suggestions for troubleshooting, your input would be greatly appreciated.<u></u><u></u></p>
<p>Thank you in advance for your help!<u></u><u></u></p>
<p>Best regards,<br>
Philipp<u></u><u></u></p>
<p><u></u> <u></u></p>
<p><img border="0" width="526" height="394" id="x_m_489391253500884669Picture_x0020_1" style="width:5.4791in; height:4.1041in" data-outlook-trace="F:1|T:1" src="cid:ii_1983e4386764cff311"><u></u><u></u></p>
<pre>-- <u></u><u></u></pre>
<pre>Philipp Altpeter<u></u><u></u></pre>
<pre>Fakultät für Physik der LMU<u></u><u></u></pre>
<pre>LS Prof. Efetov<u></u><u></u></pre>
<pre>Geschwister-Scholl-Platz 1<u></u><u></u></pre>
<pre>D-80539 München<u></u><u></u></pre>
<pre>T. +49 (0)89 2180-3733<u></u><u></u></pre>
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