Diary of an Advanced Lithographer

SPIE Advanced Lithography Symposium 2019
by Chris Mack

San Jose, California, February 24 – 28, 2019

(The following diary appeared first as a daily blog at http://life.lithoguru.com/ and is reproduced here in a slightly edited form.)

SPIE Advanced Lithography Symposium 2019 – day 0

Let’s call it like it is: I’m a nerd. In fact, I’m an uber nerd. Other nerds say “I want to be nerdy like him.” The easiest way you can tell I’m a nerd? I love being in San Jose the last week in February. It is my favorite week of the year! The weather’s not too great (though I don’t get outside much to see it), and I certainly don’t get around to see the sites (except the new Uproar brewery down the street). I’m here for only one thing – the SPIE Advanced Lithography Symposium. And I love everything about it. The people, the papers, teaching an exhausting course all day Sunday, committee meetings, standards meetings, free beer, the panel discussion (at least this year), last-minute presentation preparation, lots of coffee in the morning, giving talks, listening to talks, asking hard questions, seeing old friends and meeting new ones, hallway conversations, getting that one great idea that changes everything, seeing the next generation of lithographers take over from us old ones, seeing the old ones still hanging on, late nights, early mornings, more beer, and learning learning learning. I absolutely love it all. Let the week begin!

SPIE Advanced Lithography Symposium 2019 – day 1

The conference remains strong, with about 2100 total attendees (nearly constant over the last 10 years). The plenary session Monday morning began as always with awards. We recognized seven new SPIE fellows from our community: Yasin Ekinci (Paul Scherrer Institut), Jo Finders (ASML), Lynford Goddard (University of Illinois), Stephen Hsu (ASML), Hakaru Mizogushi (Gigaphoton), David Ruzic (University of Illinois), and Martha Sanchez (IBM). Congratulations! The first annual Nick Cobb memorial scholarship was awarded to Haoyu Yang of Chinese University of Hong Kong. The scholarship (sponsored by Mentor Graphics) is quite significant ($10,000).

The Zernike Award for Microlithography is an annual award and our community’s highest honor. Last year it was not given as we chose instead to honor Nick Cob after his untimely death. This year, to make up for that gap, two awards were given: Obert Wood (Global Foundries) and Akiyoshi Suzuki (Gigaphoton). Congratulations – two highly deserving honorees. (Full disclosure: I’m on the award selection committee so I am not an unbiased observer.)

The current president of SPIE, Jim Oschmann, was in attendance to give out SPIE’s President’s Award to Bill Arnold for his outstanding service to the society. So far as I recall, this is the first time this award has been given to someone from the lithography community. Since Bill was on the stage anyway, he was further commemorated on the occasion of his impending retirement from ASML (which happens April 1, as I understand). I hope, however, that this will not be the last time we see Bill in our community. Two other notable retirements were also mentioned: Pat Wight (long-time SPIE staff representative to this symposium) and C. Grant Willson of the University of Texas at Austin (more on that later in the week).

We often have some very good plenary speakers at this symposium, but not always. But it rare to have three very good ones the same year, so we were quite fortunate this year. Dario Gil, former lithographer and now Director of IBM Research, gave a great overview of quantum computing and IBM’s efforts in building one. I especially like their open-access model to allow the public to write and run code on their 5-qubit quantum computer. As expected, he confirmed that error rates are the biggest limiter to scaling up quantum computations. I have to admit that I still have a spooky feeling about quantum computing. I’m not sure that I am wishing for its success.

The next two plenary talks dived into the details of 3D NAND manufacturing. I learned much from both (and enjoyed the tag-team presentation format from Lam Research and ASML). I liked learning about the tricks used to make the stair-step patterning practical, with 6 lithography steps plus 42 resist trim steps combined to make 48 stair-step patterns. Cool. The level of vertical integration in 3D NAND is becoming quite staggering. Over 100 layers of transistors! With scaling up continuing into the foreseeable future (possibly to 512 layers)! Wow. Lots of patterning challenges.

I went to the EUV session for the two keynote talks, one by on old lithographer (Bernd Geh of Zeiss) and the other by a young lithographer (Ryoung-Han Kim of imec). Actually, I don’t think of Bernd as old (he is too close in age to me), but that is how he described himself. As for me, I was the grumpy old lithographer portrayed in a cartoon at the end of his talk. I accept the characterization, though I am grumpy only sometimes. The best bit from Bernd’s talk was his definition of “k4”, a scaling constant relating measured LCDU (local critical dimension uniformity) caused by stochastics to NILS (normalized image log-slope) and exposure dose. Alas, he exhibited the standard physicist bias: there is nothing we can do about photon shot noise (since we understand that), but since we don’t understand the ultimate stochastic limits within the resist, we just need a better resist. Ryoung-Han Kim provided my favorite quote of the conference so far: “We used to say EUV would simplify OPC. Now we know that EUV complicates OPC.”

I shifted to the Metrology session to hear Ofer Adan talk about AMAT’s new SEM. Or actually I heard him not talk about AMAT’s new SEM since he didn’t really say anything about it. I gather that it is a higher electron voltage tool that collects and combines signals from new backscatter and secondary electron detectors. But I’m not sure. The goal was to see multiple layers of the device at once in order to measure within-device overlay. Back at the EUV conference I heard the papers in the high-NA session. But since high-NA EUV lithography is many years away, it was mostly plans and simulations. The exception was Chris Anderson’s talk on the newly operational MET-5, a microfield EUV lithography tool with NA = 0.5 built by Lawrence Berkeley National Labs. I’m sure this will serve as a very valuable resist testing platform, just as the MET-3 did.

I ended the conference day by giving a keynote talk at the Novel Patterning conference. My talk was entitled “Will stochastics be the ultimate limiter for nanopatterning?” If you missed the talk, I’ll give the abridged version: Yes.

The evening involved a customer dinner for my day job at Fractilia, followed by staying out too late and drinking too much beer. The first of many similar nights this week I am sure.

SPIE Advanced Lithography Symposium 2019 – day 2

Tuesday was stochastics day for me, but before I start talking about papers let me talk about the conference as a whole. My technical area of interest – stochastics – is a cross-cutting technology, meaning that papers on this topic can be found in almost every conference at the Advanced Lithography Symposium. That is great, but it is also problematic when two or more papers on this same topic are being presented simultaneously in different conferences. Last year I pushed for better coordination among conferences for cross-cutting technologies and the conference organizers listened! They came up with a really good solution: define three cross-cutting technologies (stochastics, overlay, and machine learning), assign various papers in various conferences to these technologies as appropriate, then create “tracks” (sessions within various conferences with no overlap). It is working great! Of course, it is not perfect, but the number of conflicts for me so far have been far reduced. Kudos to Will Conley and the rest of the conference organizers for making this happen. I certainly hope it will be a permanent feature of future symposia.

The two morning stochastics sessions in the EUV conference were great. There was a nice combination of theoretical studies and experimental work, with both wafer printing studies and more fundamental measurements. The industry is (finally) putting serious scientific attention to this fundamental problem, including the announcement of imec’s new AttoLab facility to probe the fundamental mechanisms of EUV resist exposure. Peter Di Bisschop’s paper (delivered expertly by Eric Hendrix) added more depth to his important stochastic defect studies. Anuja De Silva of IBM also gave a talk full of interesting results. I liked her decision to use a 30 nm pitch as a “canary in the coal mine”, an easy way to make stochastic defects and thus to try out ideas for reducing them. Steven Grzeskowiak of SUNY CNSE used flood exposure of resist with 80eV electrons as a model for EUV exposures – a nice approach. Roberto Fallica of imec showed off some great fundamental studies (as he always does), this time using photoelectron spectroscopy. I was intrigued by Mark Maslow’s idea of correlating stochastic defects with what he called “Tail CD”, mean plus three sigma, rather than mean CD. It is a simple idea (after you have thought of it!) that seems to have value. It is on my list of things to investigate carefully when I get back to the office.

In the afternoon I stayed on the stochastics track as it switched to the metrology conference. I gave a talk about taking the SEM out of SEM measurements, and heard several other good talks on roughness metrology. A full day of nothing but roughness and stochastics – just about heaven for me.

The evening ended with the all-conference panel discussion. Regular readers of these conference diaries will know that I sometimes (OK, almost always) complain about panels being boring. There are three things that make a panel boring: a topic that is too narrow, a topic that is not controversial, and powerpoint. Especially powerpoint. So when Will Conley (Symposium Chair) asked Harry Levinson and I to put a panel together, we knew what we had to do. We agreed on an all-conference panel with a major theme (Is this the end of scaling?). We decided to use questions that might elicit some controversy, and then tried to make it fun so that it would be interesting. Here are those questions

1) The technology for which conference at this Symposium is most responsible for bringing about this end of scaling?
2) How is the technology of your conference saving Moore’s Law?
3) If lithographically-driven scaling does come to an end, what topics will you list in your conference’s call for papers?
4) How will we redefine the meaning of Moore's Law in order to keep its spirit alive?

Finally, we refused to let any panelist get even near powerpoint. And it worked! We started out with 400 people in the room, and a majority of them stayed through the biggest part of the time. I’d be interested in getting more feedback, but the comments so far have all been positive. Thanks to our great panelists for making it work so well:

Tony Yen (EUV Lithography)
John Petersen (Optical Microlithography)
Rob Aitken (Design-Process-Technology Co-optimization)
Ryan Callahan (Patterning Materials)
Rich Wise (Etch Technology)
Erik Hosler (Novel Patterning Technologies)
Phillipe Leray (Metrology, Inspection and Process Control)

We agreed to disagree on almost every point, except that progress in lithography and patterning will require cooperation by us all.

SPIE Advanced Lithography Symposium 2019 – day 3

Finally, a day where my only responsibility was to go to papers! Dan Sobieski of Lam Research described combined etch and hardening techniques that could reduce microbridging and microbreaks in lines and spaces, just not at the same time. It would be interesting to try these techniques combined with Peter De Bisschop’s defect analysis to see how much the defect-free process window could be opened up. Toshiharu Wada of TEL described an area-selective deposition to reduce low-frequency roughness. A resist line is coated with some material such that the deposition rate is slower in a narrow space than a wider one. If roughness made a space wider, higher deposition rates would tend to fill it in more. If a space were narrower, slower deposition rates would fill it in less. The result would be a space that varied less after deposition than before, even at low frequencies. His unbiased PSD (power spectral density) data showed that it actually worked. An important caveat is that it may work properly only at one pitch (Wada-san’s results were at 36nm pitch). More work is required, and I look forward to seeing it.

Switching to the metrology session I heard several SEM talks. Hitachi explained that one of their biggest priorities is tool matching among a fleet of 10 to 100 CD-SEM tools, where tool matching specs can be extremely challenging to meet. Sample variation, tool variation, environmental variation, and tool calibration errors all contribute about equally to this overall budget. Another Hitachi paper described progress in in-situ aberration measurement and adjustment in a prototype CD-SEM to shrink the spot size by about 2X, with subsequent resolution improvements. They demonstrated the technology with a 100eV beam energy (since the spot size is pretty large at that voltage), but I hope to see results at 500V soon.

It was good to see STMicro back at the conference, and Bertrand Le-Gratiet gave a great talk on monitoring and controlling SRAM contact holes using massive CD measurements. It is interesting that at the 28-nm node stochastic variations (local CDU) are by far the dominant source of variation. Stochastics have been with us for a long time, but we are just now putting effort into seeing that. Finally, I enjoyed Vassilios Constantoudis comparing an edge placement error (EPE) metric to a line-edge roughness metric for synthetic rough features, showing the important role of correlation length in EPE of short features.

For me, Wednesday always ends with a beer (or two) with my friends at the KLA PROLITH party. Cheers!

SPIE Advanced Lithography Symposium 2019 – day 4

Thursday, the final day of the conference, is always full of mixed emotions. I’m exhausted and glad the end is coming since I don’t think I can absorb anything else and need time to process all that I have seen and heard. But like a sugar rush that turns into a sugar crash, when the last paper is over and the halls start emptying out, I feel almost depressed. The energy and pace of the week is entirely unsustainable, but it is intoxicating.

In the morning I saw another good paper by Bertrand Le-Gratiet of STMicro, this time on the uses of contour-based metrology. Both of his talks were perfect illustrations of the power of the visual representation of data. Sufficiently complex data sets can contain many important lessons, but pulling out the new and useful information from the background of the known and expected can be difficult. The right graph or presentation of the data can make the important points excessively clear. I also liked two imec papers showing PSD analysis of AFM data. By running an AFM tip in a 1D stripe along the top of a photoresist line we can get important information not available in a top-down SEM image. PSD analysis of the data proved a very useful representation of that top surface roughness.

I spent the entire afternoon in the Grant Willson Tribute Session. On the occasion of Grant’s retirement, the Patterning Materials conference devoted three hours to honoring the most famous and most beloved resist chemist in the world. The talks walked us through Grant’s career with heartwarming stories of how he has profoundly changed each of us. Here is the list of speakers, covering many (but not all) of the important phases of Grant’s life in science and engineering:

Bob Allen (IBM Almaden)
Yan Borodovsky (retired from Intel)
Ralph Dammel (EMD)
Cliff Henderson (Univ. of South Florida)
Chris Mack (Fractilia)
Dave Medeiros (Globalfoundries)
Doug Resnick (Canon)
Mark Somervell (Tokyo Electron)

As almost every speaker emphasized, Grant’s massively important technical accomplishments are dwarfed by the personal influences he has had on so many people, but especially the 274 (+1) students that have graduated from his University of Texas research group. This tribute was definitely the highlight of the week for me, with the love for Grant pouring out from each speaker, and palpably present throughout the room. We will miss you, Grant. (But not me, since I live in Austin and hope to continue seeing Grant just as often as before!)

Chris Mack is a writer and lithographer in Austin, Texas.

© Copyright 2019, Chris Mack.

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