SPIE Advanced Lithography Symposium 2021 – day 2

Attending a virtual conference is obviously much different than in-person.  When it comes to Advanced Lithography, one of the biggest differences is the lack of parallel sessions.  A typical Tuesday at the San Jose Convention Center would involve dashing between sessions to catch talks, sometimes cursing that the two talks I wanted to see most were at the same time, and sometimes realizing that there are no talks I want to see for the next two hours.  There is a lot of “task switching”, where my mind alternates among the physics of shot noise, the chemistry of resist development, and the usefulness of the latest metrology tool advance.  This week on Tuesday, I binge-watched all the metrology talks (or rather, the 50% or so of them I was most interested in).  It was both fun and exhausting.

As one might expect, the highlights for me were the topics that most closely related to my work.  I watched the presentations on the latest SEM tools, though there was essentially nothing related to the physics of SEM image formation, my special interest.  There were many papers on how to employ SEM contours in metrology use cases rather than just traditional CD values, a topic that I have been seeing at this conference for 25 years.  It seems we have still not solved all the many issued required to make that happen.  Yosuke Okamoto’s talk helps explain why – contours can change significantly depending on the scan direction of the CD-SEM.

Of course, there were many papers on roughness measurement, with most of them related to machine learning in some way.  I have to admit that I am not a big fan of image denoising.  Maybe I’m just an old guy who prefers understanding the physics rather than letting a neural network make connections we can never understand.  I also think that many people working on image denoising are not carefully defining metrics of success that a metrologist would appreciate, things like accuracy and precision, repeatability and the size of the error bars around your answer.  Getting an image that looks less noisy is not success.

I liked George Orji’s talk on wavelet analysis of roughness.  Someday I’ll have to do the work to really understand wavelets, beyond the surface level I have today.  My hat’s off to George Papavieros for trying to measure LER with a SEM pixel size (in the direction perpendicular to the line edge) that is greater than the 3sigma LER.  That is not something I want to try.  I am a big fan of the stochastic process window (something that both Fractilia and ASML have been promoting lately), and there were a few ASML talks with some interesting results.  In a stochastic process window, one includes stochastics measures (such as defectivity, LCDU, or unbiased LWR) in the focus-exposure process window determination in addition to CD.  From Mary Breton’s talk I got a good sense of the nanosheet gate fabrication process and what metrology needs exist at each step.

I also watched the EUV conference keynote, a “live” event on Zoom.  Jos Benschop gave a very nice (prerecorded) talked, followed by live Q&A.  And sure enough, it was the Q&A that was the most interesting part.  I also will have a live event Wednesday afternoon (3pm Pacific Time), a tutorial and networking event.  This is a new presentation format for this conference, so we are all anxious to see how it will go.  The 35-minute prerecorded tutorial on the power spectral density will be followed by 25 minutes for Q&A and networking.

With my metrology binge-watching over, it is time to move on to the other conferences as the next day begins.

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