I began the last day of the conference at the EUV talks.  Jo Finders of ASML described how non-idealities of the scanner can effect single-layer edge placement errors (a combination of CD errors, both global and local, and local pattern placement errors).  This was the first time I had seen a numerical breakdown of mechanisms of “NILS loss”, the reduction of the Normalized Image Log-Slope from its theoretical value caused by mask topography, aberrations, flare, focus variation across the slit, etc.  The NILS loss totaled to 15%, and Jo described various approaches to get some of that back.

Suk-Koo Hong of Samsung provided “speculations” on why pushing k1 below 0.4 in EUV is problematic (specifically, for printing contact holes).  The reason is stochastics, and there seems to be a scaling with pitch and CD that is worse than the famous z-factor predicts:  z-factor = feature size^3 * LCDU^2 * Dose-to-size.  Plotting LCDU (local CD uniformity) versus dose produces higher than expected LCDU at the lowest doses and seems to be following a different iso-z-factor limit.  The specifics of CD and pitch and anything else that made up the data points in his graphs, however, were not revealed.  Still, it was an interesting way to look at the problem, one still without a solution.

As an aside, Samsung gave many papers this week, and some of those papers were some of the best in the conference.  Case in point: Hyungju Ryu presented work by Sangjim Kim (who couldn’t make it here) on process control for EUV metal oxide resists (MOR, read Inpria).  The two MOR challenges are CD variation due to sensitivity to humidity, and poor etch resistance.  Apparently, these problems are well known to the users of Inpria resists, but they weren’t being discussed publicly until now.  For example, CD was shown to vary by 2 – 8% depending on the post-coating delay.  Samsung showed, however, that careful optimization of every resist processing step reduced the CD variation to 35% of its original value.  Etch resistance of the MOR was not as good as expected since partially exposed resist does not have a well-connected network of core-to-core bonds necessary for best etch resistance.  Samsung’s solution was a UV flood exposure.  They said that more work was required to make the MOR ready for high volume manufacturing, but these were good steps in that direction.

On a completely different topic, Etienne Poortere of ASML showed how a carefully designed test mask coupled with voltage contrast metrology could be used to establish design rules for via connections to metal 1 in a dual damascene process of 28 nm pitch.  The technique made evaluation of a matrix of tip-to-tip spacings and M1 via overlap rules quite easy.

Back in the SEM world, Ofer Adan gave a good marketing talk on Applied Materials’ new cold field emission (CFE) electron source for their inspection SEMs (and maybe for the CD SEM?  It wasn’t clear.).  I’m convinced this new CFE source is better, but I don’t know why.

As is usually the case, by Thursday afternoon my brain had reached its absorption limit.  I continued going to talks, but my notes became brief as my attention strayed.  Summing up, this week lived up to my expectations – it was the Advanced Lithography and Patterning conference back to its full glory.  There were many solid talks – nothing earth shattering (but there rarely is), just good incremental progress towards the harder to reach goal of keeping Moore’s Law alive.  It’s a shame that the memory sector downturn kept many Micron and SK Hynix lithographers away, and that Intel’s troubles kept their attendance to a minimum.  It was gratifying to see so many good papers from Samsung, and decent attendance from TSMC.  I go away looking forward to next year.

Bright and early, the talks begin at 8:00 am.  From Mark van de Kerkhof of ASML, and then Joost Bekart of imec, we got two updates on EUV pellicles.  The composite (metal-silicide) pellicles are the second-generation pellicle after polysilicon, but they are stalling out with transmission near 90%.  Higher transmission requires thinner pellicles, which is quite difficult.  The low transmission also limits the highest power that can be used (absorption heats up the pellicle, and if the heat cannot be removed fast enough the temperature rise will ultimately destroy it).  Slow progress can be expected, but it seems that increases in source power will outpace increases in transmission.  That is why the carbon nanotube (CNT) pellicle, first proposed by imec in 2015, is so interesting.  It has extremely high transmission (> 97% after being annealed to remove contaminants) and so can withstand very high source power (> 1000W).  It’s drawback is the gradual thinning of the pellicle with use as the carbon (activated by all the EUV light) reacts with hydrogen gas, which ASML puts in the system specifically to remove carbon deposits that might occur inside the tool!  An interesting dilemma, and I will enjoy watching progress towards a solution.

Back in the metrology session Yoshihiko Fujimori of Nikon introduced a new tool for high-density CD measurements on the wafer.  The paper was sparse on some important details, but the tool reminded me of a crystallography setup, measuring the diffraction pattern of the whole patterned wafer with variable wafer and detector angles.  (The name AMI-5700, along with a marketing picture, indicated at least close to commercial readiness.)  A full wafer, up to 100,000 points, can be measured in a couple of minutes.  That means about a 1 mm spatial resolution, and it is obviously limited to periodic structures (and so memory chips).  The key idea appears to be correlating various “signals” coming from the detected diffraction pattern with SEM-based measurements of CD in a previous calibration step.  Then a measurement of that signal across the wafer is used to infer CD variation across the wafer.  Like all such techniques, the two key questions are sensitivity to noise, and sensitivity to other systematic variations (such as an underlying film thickness) that might also impact the signal being monitored.  These problems have bedeviled other similar attempts in the past.

Gian Lorusso of imec gave a fast-paced overview of the metrology of thin versions of the dry Lam Research resist.  As Gian has shown in the past, low signal-to-noise ratio CD-SEM images that often result from very thin resist patterns can make accurate metrology difficult (even when using state-of-the art metrology software from Fractilia!).  The main message from this paper is the happy fact that the Lam dry resist offers fairly high material contrast in the SEM, making measurement of even 10 nm thick resist patterns acceptable at normal SEM settings (such as 16 frames of averaging).

On the topic of resist shrinkage in the CD-SEM (a problem that never goes away, no matter how much we ignore it), Musaki Sugie of Hitachi offered an interesting combination of ideas.  As many SEM vendors have demonstrated, going to 100 V reduces shrinkage, though it increases the noise in the image.  (Ran Alkoken of AMAT showed their 100 V CD-SEM in the following talk.)  This voltage was coupled with an optimized scan speed to minimize charging and to prevent CD measurement repeatability from suffering.  An interesting additional idea in the talk was to average together 32 sub-sections from a large 1-frame image of lines and spaces to measure CD with the precision of a 32-frame image and the shrinkage of a 1-frame image.  Information such as linewidth roughness or local CDU cannot be obtained, but for the one goal of finding the CD with minimal shrinkage, the approach may work.  As an aside, he noted that metal oxide resist (that is, Inpria) has 3X less shrinkage than chemically amplified resists for EUV.

After lunch I saw a few papers from imec, updating prior efforts.  Andreas Frommhold used Mark Maslow’s concept of Tail CD (measured mean +/- 3sigma of the contact hole distribution) as a predictor of merged contact hole rates.  In this case, the CD being measured was the gap between the holes, and the mean – 3sigma value is correlated with rates of merged holes.  I know from experience, however, that exactly how one defines and measures this hole-to-hole gap CD is quite critical.  Lieve Van Look of imec continued her very detailed work on local MEEF (mask error enhancement factor) and its impact on LCDU, bringing the concept of MEEF into the era of stochastics.  Her collection of papers on this topic so far represents the definitive work on the topic.

The poster session in the evening was, thank goodness, in a massive hall with plenty of room to work your way among the posters without being jammed in by the crowds (which frequently happens at other conferences).  Alas, being a long-time participant in this conference meant that my mean free path (the average distance traveled before running into someone to talk to) was very small.  I didn’t see many posters.  For all the poster presenters out there – please turn in a manuscript of your paper so I can read them!

(This post is certified ChatGPT-free.)

The day began with a second plenary, starting with ASML/SVGL/Perkin-Elmer veteran Chip Mason describing the history of the first commercial projection lithography tool, the Micralign, on the occasion of its 50^th birthday.  I am very sorry to say that another commitment prevented me from attending Chip’s talk, but numerous people (young and old) told me that it was fantastic.  My first job as a lithographer (exactly 40 years ago this month!) involved supporting a fab with both a Micralign 300 and an Optimetrix 10X stepper, so the importance of the Micralign is also a bit personal for me.  (Steppers were introduced five years later, in 1978, but the Micralign remained the workhorse of the industry well into the 1980s.)  But not only did we hear about the Micralign, we get to see one.  ASML now owns the former Perkin-Elmer site in Wilton, Connecticut where the Micralign was developed and manufactured, and they managed to find a Micralign in a garage somewhere (Vermont, I believe).  The owner donated the tool and ASML had it shipped to San Jose where it is now on display (for one more day) in the Exhibitors’ Hall.  Seeing a Micralign 300 in person is a wonderful opportunity.

Attending papers on Tuesday morning had me bouncing between sessions, as usual.  Rudy Peeters of ASML gave an update on the readiness of high-NA EUV tools, including the status of the the four major components of the tool (source, reticle stage, wafer stage, and projection optics).  Three EXE:5000 sources have been qualified (benefiting from commonality with sources from the low-NA tool), four wafer stages have been built (and are working towards testing at full acceleration), and one reticle module was built and is being tested.  Multiple mirrors have been made, and so the next major milestone to watch for will be the first working projection optics box.  It is fun to hear ASML express their optimism as if it were established fact:  the goal of the high-NA EUV tool introduction in 2025 is to “replace EUV double patterning.”  It is also interesting to infer the market ambiguity of this tool from statements like “high contrast imaging can be used for better images or better dose.”

Greg Denbeaux of SUNY Polytechnic (Albany) gave a very nice talk on attempts to move the idea of polymer aggregation as a source of resist stochastic variability from speculation to experimental measurement.  By printing an open-frame EUV exposure at the dose to clear, residual resist at the substrate can be measured with an AFM to get some feeling of the size of the clumps of resist there.  I wouldn’t call the method exactly quantitative, but it is a start.

Yaniv Abramovitz of AMAT looked at using in-device overlay measurement by a SEM at ADI (after develop inspect) rather than the traditional AEI (after etch inspect) measurements, and compared them to optical scribe-line measurements.  His results from the classical skew experiment (where the scanner is directed to purposely offset the stage position of the second layer in increments, and then the measured overlay is plotted versus this scanner input skew) yielded unexpected slopes far less than 1.  How could this be?  Scanners have incredibly accurate stage positioning.  AMAT has some more work to do.

Overlay and the role of stochastics in edge placement error (EPE) is my new special interest, so I spent the rest of the morning and much of the afternoon in sessions on those topics.  Myungjun Lee of Samsung gave a fascinating talk on their development of hyper spectral imaging reflectometry for massive overlay and CD measurements.  The traditional OCD (optical CD) approach collects reflected spectrum (reflectivity versus wavelength at a fixed angle) using a modest spot size (20 – 100 microns) aimed at a target of regular patterns (usually lines and spaces of fixed pitch).  Analysis of the spectrum yields measurement of the CD, and possibly other information about the features.  Spectral “imaging” reflectometry shrinks the spot size (to about 5 microns in this case) and uses that spot as one pixel in a larger “image” of many pixels.  It is only an image in the sense that imaging optics are uses to collect data from each pixel in parallel, so that this technique is essentially massively parallel OCD.  How massive?  3200 x 3200 pixels covering a 20.8 mm x 20.8 mm field.  The massive data that can result from this tool opens up many interesting use cases.  It is my understanding that Samsung is looking for an equipment partner to commercialize the technology.

Several authors (starting with myself the day before) described how EPE measurements are best used as an input to a calculation of yield (or number of good die).  Inho Kwak of Samsung showed that using a prediction of number of good die (generically called a KPI = key process index, in the jargon of the fab) during advanced process control (APC) resulted in a 5% improvement of dies in spec.  The control strategy first suggests an adjustment of the dose of the second layer, then calculates the overlay correctables to achieve lowest EPE (rather than individually trying to match CD with dose, then overlay with the correctables).  Harm Dillen of ASML and Franz Zach of KLA proposed similar approaches.

In the SEM measurements session I especially like John Villarrubia’s talk on three new SEM-based fundamental research projects at NIST.  John is the author of a standard Monte Carlo simulator for SEM behavior called JMONSEL, and these new projects aim to fill in some gaps and improve the accuracy of this simulator.  The first experimental piece will compare a top-down SEM image at low voltage to a STEM (scanning transmission electron microscopy) image at high voltage for the same sample and in the same instrument.  The second experiment measures secondary electron yield of materials, with the benefit that the materials are deposited and then measured without exposing the films to the atmosphere.  The third project seeks to improve the models in JMONSEL using the data from the first two projects.  This is a very worthwhile activity, and I commend John and NIST for taking a leadership role in these investigations.

The last talk I attended was also one for which I was a coauthor.  Genevieve Kane of IBM gave her first SPIE presentation – congratulations on a nice job!

The plenary session Monday morning began with awards.  I was glad to hear that 27 students received grants from SPIE to cover their registration and travel to the conference (Fractilia is one of the corporate sponsors of these student grants and I was happy to meet some of those students later that day at a student-mentor lunch).  Tony Yen of ASML was this year’s winner of the Frits Zernike Award for Microlithography – congratulations!  The Kingslake Award for Optical Design is not usually presented at this conference, but this year’s winner Wilhelm Ulrich chose to receive that honor here since his work at Zeiss in designing lithographic lenses was the reason for this prestigious reward.

Was saw eight new SPIE fellows from our community inducted this year, seven of whom were on hand to be recognized for this achievement: Martin Burkhardt of IBM, Debbie Gustafson of Energetiq, Larry Melvin of Synopsis, Warren Montgomery of EMD Electronics, Linyong Pang of D2S, Takashi Sato of KIOXIA, Geert Vandenberghe of imec, and Yayi Wei of the Chinese Institute of Microelectronics.  A great group of fellows, indeed.

As I glanced back across the room before the start of the first plenary talk, I was gratified to see what once was a familiar site:  the largest lecture hall at the San Jose convention center was full to overcrowding, standing room only, with an overflow room set up to accommodate the rest.  We are back!  And our first plenary speaker of the day, Martin van den Brink of ASML, did not disappoint in a talk densely packed with interesting information.  Martin gave a plenary speech 15 years earlier where he projected scaling for the coming 15 years, so it was fun to hear him compare those projections to reality (he did better than most prognosticators) and then try it again, providing ASML’s vision of Moore’s Law in the coming decade.  Of course, he predicted lithography scaling would continue – it is in ASML’s DNA (and I suspect in their corporate bylaws as well).

Martin described the well-known evolution of Moore’s Law away from transistor scaling towards system scaling, especially for energy efficient performance (EEP).  Still, he remains hopeful that DRAM scaling will continue down to 15 nm pitch, and vertical NAND flash will grow to 1000 transistors tall!  ASML is pushing massive e-beam inspection to approach optical defect inspection in terms of productivity, a tough road, but I am sure they will be at least partially successful.  I was almost numbed to see projections of DUV scanner throughput to 400 wafers per hour, and even 500 wph!  My first thought was “that’s impossible”, but I’ve come to understand that these kinds of things are impossible only until ASML does them.  Of course sustainability is on everyone’s mind nowadays, so Martin addressed the 100 kW-hour per finished wafer energy expenditure for lithography.  The goal is for that number to at least remain the same, if not go down over time.  As for EUV source power, 500W is said to be on the way, 600W is being demonstrated, and 800W is on the roadmap.  Each increase is EUV source power is of course very challenging.

Maybe the most interesting part of his talk (at least for me) was the new optical designs that removed one mirror from both the 0.33 and 0.55 NA systems (from the illuminator, it appeared).  Since each mirror has less than 70% reflectivity, the removal of one mirror represents a significant throughput advantage for these EUV scanners. He also described ASML’s plans for EUV “hyper” NA, a 0.75 NA design.  But without polarization (which Martin admitted would not happen), the image contrast benefit of the higher NA is reduced, so that a cost/benefit analysis of hyper-NA EUV seems clearly against it ever becoming a reality.  Time will tell, but of course ASML must have a roadmap.

I did not stick around for the second plenary talk on the Chips Act (I’m not going to apply for any of that money), so it was off to the regular conference papers.  Martin Weiss of Intel talked about modeling the matching of high-NA EUV two-field to low-NA one field difficulties.  It seems Intel is still hoping to jump back into the lead in scaling by being the first to implement high-NA EUV into production.  Good luck to them – that is going to be very hard, and probably late.  Zhigang Wang and B.H. Lee of Hitachi High Technology talked on the future of CD-SEM metrology, emphasizing the need for 0.01 nm tool-to-tool matching (or at least of that order) without giving much of a hint how they might achieve that.  I agree with them that SEM tool matching needs significantly more attention.  Ryosuke Kizu of the National Metrology Institute of Japan gave another good paper this year on their metrological tilt-AFM tool.  This year they looked at how SEM exposure of resist results in sidewall roughness smoothing – an important topic for those of us trying to measure resist feature roughness using top-down CD-SEMs.  I hope Kizu can partner with a CD-SEM owner to turn this interesting experimental technique into results useful to semiconductor metrologists.

In the EUV+Optical/Resist joint session in the afternoon there were a pair of papers by FujiFILM followed by Samsung looking at developer and rinse effects on line-edge roughness.  Both seemed to indicate that NTD (negative tone develop) using solvent developer for EUV held some promise for roughness and defect reduction, but I would consider these results preliminary at best.  I hope they work with imec to validate this behavior.

While there were a few other good papers late in the afternoon, my business obligations prevented me from attending them.  I had to go to the Fractilia Happy Hour at Uproar Brewing.  Ah, the burdens of entrepreneurship.

It looks like we are back to normal.  After three years where Covid 19 was on everyone’s minds and tongues (if not yet in the upper respiratory tracts), today more people seem interested in snow on the mountains surrounding San Jose, the massive downtown in the memory business, the nasty weather outside, and who survived the layoffs at company xyz.  Different is the new normal.  It is good to be back!

The conference itself is back to its former vigor.  Compared to 2019, submissions are up (over 450), attendance is about the same (over 2000 seems likely), and the number of exhibitors is about the same (a low 54).  I had 11 people in my all-day short course on Sunday (lower than hoped, but a great group!).  And the reconnecting with friends has already begun. From a technology perspective, what will the week be like?  I have to admit that I am completely clueless.  I haven’t yet dived into the agenda of talks (I miss the printed programs), and only know that I need to be at the plenary program at 8am on Monday.  My one talk is Monday at 2:30pm, and the Fractilia Happy Hour is also Monday, so that day is my current focus (I’ll think about Tuesday on Tuesday).  And so another week at ALP begins…