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Yes, previous litho leaves "ghosts", I have seen it often. </div>
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Howard Northfield
<div class="ContentPasted0">Research Associate</div>
<div class="ContentPasted0 elementToProof">Advanced Research Complex (ARC)</div>
University of Ottawa<br>
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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 Gustavo de Oliveira Luiz <deolivei@ualberta.ca><br>
<b>Sent:</b> Friday, March 24, 2023 1:37 PM<br>
<b>To:</b> labnetwork@mtl.mit.edu <labnetwork@mtl.mit.edu><br>
<b>Subject:</b> [labnetwork] Strange "sample memory" with LOR 5B</font>
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<div style="font-size:9pt; font-family:'calibri'; color:#FF4233"><b>Attention : courriel externe | external email</b></div>
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<div class="x_gmail_default" style="font-family:arial,sans-serif">Hello everyone,</div>
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<div class="x_gmail_default" 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.</div>
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<div class="x_gmail_default" 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).</div>
<div class="x_gmail_default" style="font-family:arial,sans-serif"><img alt="Si_w_Oxide_LOR5b-AZ1512_AsCoated.png" width="542" height="387" data-outlook-trace="F:1|T:1" src="cid:ii_lfmr5qo10"><br>
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<div class="x_gmail_default" 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).</div>
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<div class="x_gmail_default" style="font-family:arial,sans-serif">Here are the steps during my tests:</div>
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<li>Piranha clean</li><li>HMDS prime on a YES oven</li><li>Spin-coat with LOR 5B/AZ 1512 (marks show up on a reused sample)</li><li>Expose using either a mask aligner or DWL</li><li>Auto-develop in our Laurell EDC-650 (resist seems underexposed over the marks)
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<li>AZ Developer 1:1 90 s</li><li>Rinse (DI water) and dry (N2+spin) 60-120 s</li><li>MF-319 5 s</li><li>Rinse (DI water) and dry (N2+spin) 60-120 s</li></ol>
</li><li>Strip resist with Remover PG</li><li>Repeat all steps for every iteration</li></ol>
<div>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 ėm 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.</div>
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<div>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.</div>
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<div>I'm sorry for the long email, and thank you in advance for any comments.</div>
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<div>Best regards,</div>
<div>--<br>
Gustavo de Oliveira Luiz, PhD<br>
Applications/Research Specialist<br>
nanoFAB, University of Alberta<br>
+1 (780) 619-1463<br>
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