[labnetwork] Strange "sample memory" with LOR 5B

[Gustavo de Oliveira Luiz]

Hello everyone,

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.

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).
[image: Si_w_Oxide_LOR5b-AZ1512_AsCoated.png]
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).

Here are the steps during my tests:

   1. Piranha clean
   2. HMDS prime on a YES oven
   3. Spin-coat with LOR 5B/AZ 1512 (marks show up on a reused sample)
   4. Expose using either a mask aligner or DWL
   5. Auto-develop in our Laurell EDC-650 (resist seems underexposed over
   the marks)
      1. AZ Developer 1:1 – 90 s
      2. Rinse (DI water) and dry (N2+spin) – 60-120 s
      3. MF-319 – 5 s
      4. Rinse (DI water) and dry (N2+spin) – 60-120 s
   6. Strip resist with Remover PG
   7. Repeat all steps for every iteration

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.

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.

I'm sorry for the long email, and thank you in advance for any comments.

Best regards,
--
Gustavo de Oliveira Luiz, PhD
Applications/Research Specialist
nanoFAB, University of Alberta
+1 (780) 619-1463
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://mtl.mit.edu/pipermail/labnetwork/attachments/20230324/248511a0/attachment.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: Si_w_Oxide_LOR5b-AZ1512_AsCoated.png
Type: image/png
Size: 1204197 bytes
Desc: not available
URL: <https://mtl.mit.edu/pipermail/labnetwork/attachments/20230324/248511a0/attachment.png>