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

Mon Mar 27 04:27:51 EDT 2023

Hi All,

We’ve seen ghosting before when using the old Shipley SJR series resist. We replaced those with Megaposit SPR and had the same and have since seen it with Microresist Ma-P. Its happened on both silicon and fused silica (SiO2), though is only visible on a bare substrate when we get condensation on it (eg acetone evaporating or from breath). We tried measuring the surface using white light interferometers and not found anything so generally ignored it. This is using pieces for lithography test exposures, not chemical etching them.

I’d be interested to know where they come from though!

Daniel Lloyd
Development Engineer,
Laser Optical Engineering Ltd.
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From: labnetwork <labnetwork-bounces at mtl.mit.edu> On Behalf Of Hollingshead, Dave
Sent: Friday, March 24, 2023 9:13 PM
To: Gustavo de Oliveira Luiz <deolivei at ualberta.ca>; labnetwork at mtl.mit.edu
Subject: Re: [labnetwork] Strange "sample memory" with LOR 5B

Hi Gustavo,

We’ve seen ghosting before, although as you mention it has usually been caused by mild etching of the substrate in the developer. Based on your comments I wouldn’t count out something else in this case.

This reminds me of some strange behavior we have seen in some of our nLOF processes done on our MLA. We are still trying to investigate what is happening, but we get odd residue or material adhesion issues on reworked samples. One working theory is that the high power density of the MLA laser exposure is either locally heating the resist to the point of excessive cross-linking or somehow modifying the surface during exposure. Microchem has a white paper that seems to lend some credence to this theory (https://www.microchemicals.com/technical_information/exposure_photoresist.pdf, see page 10).

Any chance you have tried (or could try) repeating the process using a contact aligner on the same substrate and see if you still get the same exposure? If it is an MLA-exclusive issue that would be very interested to know and investigate further.

-Dave
Dave Hollingshead
Manager, Research Operations

The Ohio State University
Nanotech West Labs
Suite 100, 1381 Kinnear Rd, Columbus, OH 43212
614.292.1355 Office
hollingshead.19 at osu.edu<mailto:hollingshead.19 at osu.edu> / nanotech.osu.edu<http://nanotech.osu.edu>

Pronouns: he/him/his

From: labnetwork <labnetwork-bounces at mtl.mit.edu<mailto:labnetwork-bounces at mtl.mit.edu>> On Behalf Of Howard Northfield
Sent: Friday, March 24, 2023 14:10
To: Gustavo de Oliveira Luiz <deolivei at ualberta.ca<mailto:deolivei at ualberta.ca>>; labnetwork at mtl.mit.edu<mailto:labnetwork at mtl.mit.edu>
Subject: Re: [labnetwork] Strange "sample memory" with LOR 5B

Yes, previous litho leaves "ghosts", I have seen it often. Howard Northfield Research Associate Advanced Research Complex (ARC) University of Ottawa From: labnetwork <labnetwork-bounces@ mtl. mit. edu> on behalf of Gustavo de Oliveira

Yes, previous litho leaves "ghosts", I have seen it often.

Howard Northfield
Research Associate
Advanced Research Complex (ARC)
University of Ottawa
________________________________
From: labnetwork <labnetwork-bounces at mtl.mit.edu<mailto:labnetwork-bounces at mtl.mit.edu>> on behalf of Gustavo de Oliveira Luiz <deolivei at ualberta.ca<mailto:deolivei at ualberta.ca>>
Sent: Friday, March 24, 2023 1:37 PM
To: labnetwork at mtl.mit.edu<mailto:labnetwork at mtl.mit.edu> <labnetwork at mtl.mit.edu<mailto:labnetwork at mtl.mit.edu>>
Subject: [labnetwork] Strange "sample memory" with LOR 5B

Attention : courriel externe | external email
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).
[cid:image001.png at 01D9608D.77CE6310]
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)

     *   AZ Developer 1:1 – 90 s
     *   Rinse (DI water) and dry (N2+spin) – 60-120 s
     *   MF-319 – 5 s
     *   Rinse (DI water) and dry (N2+spin) – 60-120 s

  1.  Strip resist with Remover PG
  2.  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
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