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<p class="MsoNormal">Hi Gustavo,<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I don’t have a definitive answer for you, and I’m certainly no chemist, but also consider the role of AZ Developer (another base) and reactions of NMP with residual water or alkaline solutions.
<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">First, the pair of alkaline developers <i>may</i> actually be enough to break through the relatively thin oxide created by the piranha, at which point the bases will start attacking the silicon. Second, if this is only happening with LOR,
it’s also possible that residual liquid (likely alkaline) is being trapped under the AZ 1512 then reacting with the NMP. Spinning may not do a great job of removing this liquid trapped beneath the resist overhang. I suspect a bulk attack, though, since the
residual patterns in the wafer reflect the resist pattern itself rather than the perimeters of the resist patterns. I haven’t noticed this before on SiO2, but NMP alone – and especially water-contaminated NMP – can attack some metals (Al, Cu, etc.). This
paper suggests that acidic or alkaline contaminants in NMP may exacerbate the problem. I see these ghosts of previous patterns all the time in aluminum-coated wafers I pattern and reuse repeatedly for process development/characterization, and I’ve recently
started seeing it on Ti as well – no LOR, just an assortment of positive resists.
<o:p></o:p></p>
<p class="MsoNormal"><a href="https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=9211805">https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=9211805</a><o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Best,<o:p></o:p></p>
<p class="MsoNormal">Travis Massey<o:p></o:p></p>
<p class="MsoNormal">Center for Micro and Nanotechnology<o:p></o:p></p>
<p class="MsoNormal">Lawrence Livermore National Laboratory<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><b>From:</b> labnetwork <labnetwork-bounces@mtl.mit.edu> <b>
On Behalf Of </b>Gustavo de Oliveira Luiz<br>
<b>Sent:</b> Friday, March 24, 2023 10:37 AM<br>
<b>To:</b> labnetwork@mtl.mit.edu<br>
<b>Subject:</b> [labnetwork] Strange "sample memory" with LOR 5B<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">Hello everyone,<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">While working on a recipe for LOR 5B/AZ 1512 in our automatic development system, I encountered some intriguing effects when reusing wafers for my tests. This could be a problem for our users
when developing their own process, so we'd appreciate it if anyone could help us to understand what is going on.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">Below is a picture of a sample right before exposure, taken using our MLA150. The dark/bright features you see are NOT etched on the wafer (these wafers were never etched). The marks are from
a previous lithography test. They become apparent after coating the sample with LOR 5B and even more after adding AZ 1512. And I don't see them when coating only with AZ 1512 (I reused wafers for that process development without any issues).<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><img border="0" width="542" height="387" style="width:5.6458in;height:4.0312in" id="Picture_x0020_1" src="cid:image001.png@01D95E44.038894D0"></span><span style="font-family:"Arial",sans-serif"><o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">And what is more intriguing is that these features affect exposure/development of my test mask. For instance, on a virgin sample I can expose and auto-develop with the same recipe (dose and development
time) I use for the manual process. On a reused sample, the reisst stack behaves as if it were underexposed (a dose test made this very obvious).<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">Here are the steps during my tests:<o:p></o:p></span></p>
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<li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l0 level1 lfo1">
<span style="font-family:"Arial",sans-serif">Piranha clean<o:p></o:p></span></li><li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l0 level1 lfo1">
<span style="font-family:"Arial",sans-serif">HMDS prime on a YES oven<o:p></o:p></span></li><li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l0 level1 lfo1">
<span style="font-family:"Arial",sans-serif">Spin-coat with LOR 5B/AZ 1512 (marks show up on a reused sample)<o:p></o:p></span></li><li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l0 level1 lfo1">
<span style="font-family:"Arial",sans-serif">Expose using either a mask aligner or DWL<o:p></o:p></span></li><li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l0 level1 lfo1">
<span style="font-family:"Arial",sans-serif">Auto-develop in our Laurell EDC-650 (resist seems underexposed over the marks)<o:p></o:p></span></li></ol>
<ol start="5" type="1">
<ol start="1" type="1">
<li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l0 level2 lfo1">
<span style="font-family:"Arial",sans-serif">AZ Developer 1:1 – 90 s<o:p></o:p></span></li><li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l0 level2 lfo1">
<span style="font-family:"Arial",sans-serif">Rinse (DI water) and dry (N2+spin) – 60-120 s<o:p></o:p></span></li><li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l0 level2 lfo1">
<span style="font-family:"Arial",sans-serif">MF-319 – 5 s<o:p></o:p></span></li><li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l0 level2 lfo1">
<span style="font-family:"Arial",sans-serif">Rinse (DI water) and dry (N2+spin) – 60-120 s<o:p></o:p></span></li></ol>
</ol>
<ol start="6" type="1">
<li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l0 level1 lfo1">
<span style="font-family:"Arial",sans-serif">Strip resist with Remover PG<o:p></o:p></span></li><li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l0 level1 lfo1">
<span style="font-family:"Arial",sans-serif">Repeat all steps for every iteration<o:p></o:p></span></li></ol>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">At first I thought that this could actually be some etching of my Si wafers by MF-319, even though unlikely given the low TMAH concentration (and I'm not sure why that would affect exposure/development).
But the sample in the image above has 2 μm thermal oxide, so practically impervious to TMAH. Not to mention that the brightest crossing marks come from testing a recipe where TMAH was not used at all. This must be some strange interaction between LOR 5B and
the sample surface, which I'd expect to be practically reset after piranha and HMDS priming.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">My search for more information regarding LOR 5B and it's sensitivity to surface conditions has proven fruitless so far. And requiring a brand new sample for every iteration can get expensive
quite quickly. We'd appreciate it if you could point us to some references where this was discussed in any form, or if you know of a method to avoid this from happening.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">I'm sorry for the long email, and thank you in advance for any comments.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">Best regards,<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">--<br>
Gustavo de Oliveira Luiz, PhD<br>
Applications/Research Specialist<br>
nanoFAB, University of Alberta<br>
+1 (780) 619-1463<o:p></o:p></span></p>
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