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<p class="MsoNormal"><span style="color:#1F497D">Hi John,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">We had a “cracking” issue with PMMA in two different time frames and labs here at Ohio State. Many years ago the first problem was also sporadic and we did a similar analysis to your analysis below. We even
sent some samples (and a student) to another university for spinning and/or ebeam lithography. In the end, it was found that certain areas of our evaporator were hotter than others. That problem was a gross problem however and was visible immediately after
removal from the evaporator. The metal was Cr/Au.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">Do you have any images of the metal before the liftoff? You are welcome to email me directly if you wish.
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">Our most recent instance was also a heating issue in a completely different, and much newer evaporator, but with Ni.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">In both cases they were temperature related. We solved them by either slowing down the deposition rate, so using lower power in the electron beam evaporator, or changing the soak times.
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">Just looking at your process, I would also take a look at the actual temperature of your hotplate(s). We have several of one type and one attached to a coater. The one type has the thermocouple under the stage
and the actual temperature varies as much as 10deg Celsius from the setpoint. Some of them also vary across the stage itself. The actual temperature may be an issue, since you want the resist to actually see 175C not 160C etc.
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">Also, I’d take a look at the resist under SEM after all of the steps post develop if you have time/enough samples. If you are concerned about the spinner verify the exhaust has not and is not changing. We’ve
had issues in the past with the exhaust changing and causing trouble with other resist systems.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">Best of luck to you. <o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">Regards,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">Aimee<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> labnetwork-bounces@mtl.mit.edu [mailto:labnetwork-bounces@mtl.mit.edu]
<b>On Behalf Of </b>John Watson - TNW<br>
<b>Sent:</b> Tuesday, November 17, 2015 12:47 PM<br>
<b>To:</b> labnetwork@mtl.mit.edu<br>
<b>Cc:</b> Jakob Kammhuber - TNW<br>
<b>Subject:</b> [labnetwork] pmma cracking issue<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span lang="EN-GB">Dear colleagues,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB">Our group has been having a troublesome and seemingly random problem with pmma cracking during metalization steps, and I am hoping to get the community’s ideas on what might be causing the problem. Our typical process
(used to make ohmic contacts to III-V nanowire devices on Si/SiO2 substrates) is as follows:<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB">PMMA 950K A4 spun at 4000 RPM (~200nm thickness)<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB">Bake 175C 10 min<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB">Expose 1100-1300 uC/cm^2 at 100kV<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB">Develop 60s MIBK:IPA I:3, rinse in IPA 30s<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB">60s O2 ashing (sample sits in Faraday cage, pmma etch rate ~1-3 nm/min)<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB">Ammonium polysulfide passivation of III-V surface (sample sits in heavily diluted, super-saturated polysulfide solution at 60C for 30 min)<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB">Quick load into evaporator followed by short in-situ He ion milling to remove residual sulfur layer<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB">Evaporation of 10/120nm Cr/Au at 0.5/1.5 A/s. Vacuum level typically high 10^-8 Torr throughout deposition<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB">Liftoff in acetone ~50C<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB">The problem that we see is illustrated in the attached SEM image – thin filaments of metal extend from the contacts (typically starting at sharp corners) and short out devices. The fact that the cracks originate at sharp
corners suggests the issue is stress-related, but we have been unable to track down the cause of this excess stress. Many users (8-10) have seen this problem using different bottles of resist, different viscosities and molecular weights of pmma, different
baking times and temperatures (as short/cold as 5 min/150C and as hot/long as 190C/30min), different hotplates, 3 different e-beam lithography systems, a number of substrates/gate dielectrics (SiO2, SiNx, BN, InSb, sapphire), different metalization systems
(two different e-beam evaporators and one sputtering system – the helium etch is only done in one evaporator), different metalizations (Ti/Au, Cr/Au, NbTiN), and varying waiting times between spinning/exposure/developing/metalization (varying from 5 hours
to 3 days for the whole process). The process I described above had been stable for over a year until about a month ago when the cracking problem came up. Since then it has been sporadic, ruining a few (out of many) processing runs per week, but it has not
been repeatable enough to allow for really systematic testing. <o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB">Does anyone have any insight into what could increase stress in the resist? For instance, is temperature/humidity during spinning and baking particularly critical? One theory we are currently testing is that a recently
instituted rule of cleaning the spinners with a metal-ion-free photoresist developer following pmgi spinning could be causing the problem (e.g. this would help explain the random nature of the problem since it would depend on what the previous user did with
the spinner). Thanks,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB">John<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB"><o:p> </o:p></span></p>
<p class="MsoNormal" style="margin-bottom:12.0pt"><span lang="DE" style="font-size:10.0pt;mso-fareast-language:EN-GB">-----------------<br>
John Watson<br>
Postdoctoral Fellow - Kavli Institute of Nanoscience<br>
Delft University of Technology<br>
Lorentzweg 1, 2628 CJ Delft<br>
The Netherlands</span><span lang="EN-GB" style="mso-fareast-language:EN-GB"><o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB"><o:p> </o:p></span></p>
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