[labnetwork] Direct write + plasma etch residue

[Hollingshead, Dave]

Hi Joon,

Thank you for the feedback and thank you to everyone else that has responded. I responded to many of you directly, but this topic has come up a few times so I wanted to address it with the whole group.

We historically have not noticed this phenomenon on the contact aligner, although we have not run that SPR220 process with an etch in a few years. That is another experiment we are planning but have not done so yet.

We will have some SEMs of these particular sidewalls this week, but we do have SEM x-sections of other recent samples and I can confirm that the side walls are directionally angled usually with a slope differential of ~5°. We have worked with Heidelberg a lot on this issue and have it mostly solved, but it is still somewhat visible on thicker resists. The problem was actually much, much worse with negative resists (nLOF2020 or nLOF2070) where the slope variation was so extreme that one side of the pattern would be undercut, while the other positive. Attached is an example of the "poor" profile prior to the fix, along with the more recent "improved" profile.

Heidelberg was able to fix the alignment well enough that we were happy with our negative resist profiles but we have not been able to get it perfectly orthogonal. So we know that the light is slightly angled coming in, but I am still unsure why this would lead to the etch issues that we are seeing.

Thanks,
-Dave

-----Original Message-----
From: Joonhyeong Park <telemann0 at yahoo.com> 
Sent: Friday, April 29, 2022 20:16
To: labnetwork at mtl.mit.edu; Hollingshead, Dave <hollingshead.19 at osu.edu>
Subject: Re: [labnetwork] Direct write + plasma etch residue

Hello Mr. Hollingshead,

When you did optical lithography with contact mask aligner, SPR220 and S1827, did you get the similar result such as only one side residue?
Have you checked the cross-section in SEM after develop before etching? If there is a directional problem in MLA150, you may observed the difference in side angle in SEM. You may also check sidewall corner residue under darkfield microscope, too.
I think that you had better break down and narrow down the cause and result step by step. 

Regards,

Joon Park
Research Engineer
Birck Nanotechnology Center
Purdue University


On Friday, April 29, 2022, 03:40:02 PM EDT, Hollingshead, Dave <hollingshead.19 at osu.edu> wrote: 





  


Good afternoon everyone,

 

We have recently run into some very ‘interesting’ (completely perplexing) issues while trying to etch some III-V samples in our ICP. Essentially we are seeing some sort of residue on the protected regions of the devices after stripping the resist. Whatever it is, the material seems to also be affecting our etch profiles. The odd part is that the residue has a clear directionality to it and in the present case only appears on two edges. This has led us to believe that is must in some way be a result of some non-uniform / “angled” exposure from our MLA150. While I can completely understand differing etched sidewall slopes or skewed features if the resist sidewalls are not uniform on all sides, I am having difficulty understanding how this might cause the residue we are seeing (especially in a positive resist). 

 

Any thoughts or possible explanations would be greatly appreciated.

 

Attached are some microscope and SEM images of what we are seeing. I have plenty more I’d be happy to share with those that may be interested!

 

A few relevant processing notes:

     
    * All etches were done using a BCl3/Ar chemistry in our Plasmatherm 770 system
    * ~18-20 minutes of etch time, total depth of ~2.5-3.0µm
    * We have repeated these issues on InGaAs device samples as well as bulk GaAs test samples
    * Issue repeated with both SPR220 and S1827 resist masks (both were un-hardbaked, but this is also something we are going to look into)
    * Exposures seem to be properly dosed, but we are planning an etch test on a dose/defocus array to see if exposure has any effect
    * All resist removal was done in 80C NMP
    * We have attempted some follow-on O­2 plasma etches, that seem to remove the material, but it is very slow

 

Thanks,

-Dave

 

Dave Hollingshead Manager of Research Operations – Nanotech West LabThe Ohio State University

Suite 100, 1381 Kinnear Road, Columbus, OH 43212
614.292.1355 Office
hollingshead.19 at osu.edu osu.edu

 



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