SPIE Advanced Lithography and Patterning Symposium 2023 – day 4

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.

SPIE Advanced Lithography and Patterning Symposium 2023 – day 3

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!

SPIE Advanced Lithography and Patterning Symposium 2023 – day 2

(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 50th 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.

Micralign 300 by Perkin-Elmer
Micralign 300 by Perkin-Elmer on display at the SPIE Advanced Lithography and Patterning Symposium

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!

SPIE Advanced Lithography and Patterning Symposium 2023 – day 1

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.

SPIE Advanced Lithography and Patterning Symposium 2023 – day 0

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…

Pay Attention to the Bark

My dog Minion is a barker.  He barks all the time, so much that I have gotten to know his barks.  There’s the “saw a squirrel” bark, the “UPS man on the front porch and I’m going to kill him” bark, and the “what, you’re leaving the house without me, don’t you know the world is a dangerous place” bark.  So when I was awoken by my dog barking at 5am one Saturday morning, I was not surprised.

What did surprise me was the smell. An awful, putrid odor.  By wife sat bolt upright – “what is that?”  My first thought was a natural gas leak, but that wasn’t quite right.  More like the sewer had backed up, but not quite that either.  I was confused, and not just because I woke up at 5am.  I began walking around the house to find the source, but the smell was everywhere – it couldn’t be localized.  Could it be coming from outside?  I quickly got dressed and went out the front door.  It was even stronger outside.  Then I saw my neighbor out in his front yard.  He said the smell had woken him up and he had called 311.  The fire department was on its way in case it really was a gas leak.

When the fire truck showed up, the first words out of the fireman’s mouth was “that’s a skunk”.  Of course, I had smelled skunk before, but it was always the dead-skunk-in-the-middle-of-the-road variety.  Fresh skunk smells different.  I don’t know how to describe it.  Fresher?  They brought out a gas sniffer just in case, but a few minutes later a flashlight-toting fireman said he had found where the skunk had sprayed the back of my house.  Sure enough, on the windows and wall of my house facing the back yard was a six-foot diameter spray pattern of dime-sized droplets, sticky and yellow.  Did you know that skunk spray was yellow?  Neither did I. 

I learned a lot about skunks that day.  For example, no double-paned window has yet been invented that can block that smell from getting into your house. From Google I found out that tomato juice might be good for use on sprayed humans and dogs, but for walls a bleach mixture was best.  Indoors I boiled vinegar on the stove after opening the back door and windows to air out the house.  By this time my kids were awake and they announced that they were not, under any circumstances, going back into that house until the smell was under control.  My wife took them to Starbucks.

I learned that skunk spray is VERY hard to clean off.  Even today some stains remain on my outside wall.  Nonetheless, after several hours of work, the house started getting closer to normal.  By this time it was late in the morning, and I was exhausted.  I decided to relax up in my office, not coincidentally the farthest point in my house away from the source.  A while later I heard my dog barking, loud and insistent.  This was his “I’m stressed to the max and I will not be ignored” bark.  I went downstairs to check it out, not thinking too much about it.  After all, my dog is a barker.

I found him in the living room, tense, straight as an arrow, his nose pointing under a piece of furniture, barking like mad.  Oh no.  Please, no.  I felt my shoulders stiffen and my throat tightening up.  I slowly lowered my head to be able to see under the armoire and there it was:  a mass of black fur with a white stripe.  When I had left the back door open to air out the house, it never occurred to me that a skunk might wonder in.  Just to be clear, I don’t live in the country.  This is a neighborhood near downtown.  These kinds of things don’t happen in the city, right?  My heart started to race and a minor panic began, but my first worry was that my dog was about to get sprayed.  I grabbed him and ran out the back door, shutting it behind me.  I felt relieved that we had avoid a major catastrophe.

This is when I realized that there was a skunk, loose in my house.  And I was trapped outside.  I couldn’t go back in!  There was a skunk loose in my house!  He could be anywhere, and I certainly didn’t want to be sprayed.  Fortunately, I had my phone in my pocket.  I started dialing.  I told my wife not to bother coming home.  I called animal control – “there’s a skunk, loose in my house,” I said.  They were not very helpful.  Once they determined that no one had been bitten and so there was not a rabies risk, they said they could have someone out there in the next 48 hours.  “But there’s a skunk loose in my house”, I repeated.  They did not seem too concerned.  I was concerned.  I started running down the list of Austin pest removal companies that Google provided, getting one answering machine after another.  I was beginning to get desperate – what was I going to do?  Finally, at the end of the list, Urban Jungle Wildlife Specialists answered their phone.  They could have someone out there in an hour.

When the technician arrived he gave off a strong sense of competence.  He had a uniform, and a truck full of equipment.  He had a cage, with a black cloth to through over it, and a long pole with a loop at the end.  The pole with a loop at the end looked very official.  And he had a gun – a pellet rifle.  He had done this before, though he admitted that it was not common to have a skunk loose in your house.  “Skunks are shy,” he explained.  “If one is in your house, it’s probably sick or dying.”  The risk of a rabid skunk was not small.  The first order of business was finding it.  As expected, he did not stay under the armoire.  But 20 minutes later the technician came out.  The good news was he had found the skunk.  The bad news was the skunk could not be approached and coaxed into the cage.  It had found its way into an unfinished storage space underneath the front stairs, backed tightly into a corner, low under that first stair.  Extraction would not be easy.  It would not go well for the skunk.

Reluctantly I gave permission for the use of deadly force.  I have to admit, by this point in time I did not have much sympathy for that skunk.  It was removed, but not before I paid the price of my decision:  he sprayed inside my house.  And I thought it was bad when had sprayed the outside of my house!  The next few days was a blur of hotel rooms, attempts to get skunk smell out of our kids’ clothes so they could go to school, and me cleaning, gaging, bleaching, gaging, deodorizing, gaging, painting, gaging, rinse, repeat, and finally, after two exhausting days, deciding it was safe for the family to go home.  It has now been 18 months since the great skunk disaster, and getting a whiff of that lingering odor happens only rarely now.  I continue to be aware, though, of my dog’s expanded vocabulary.  I will not soon forget the “there’s a skunk loose in my house” bark.

“There’s a skunk loose in my house!”

SPIE Advanced Lithography and Patterning Symposium 2022 – day 4

The mood at the conference this week can be summed up in one word:  happy.  We were all just happy to be here, with smiles visible everywhere, even under masks.

Thursday morning began with a quite philosophical keynote talk in the metrology session on the role of MI (metrology and inspection) in semiconductor manufacturing by Younghoon Sohn of Samsung.  He touched on broad subjects like the role of sampling (depends on the failure rate), the dilemma between resolution and speed in inspection (and the wide gulf in both between optical and e-beam inspection), and the basic roles of MI (define a process window, identify cause and effect, and process monitoring and control).

A joint session between Optical/EUV and Etch provided several nice papers.  Angelique Raley gave an overview of three techniques being promoted by TEL:  a spin-on SiC underlayer for EUV to prevent pattern collapse, a development process (not really explained) called ESPERT for Inpria resists that also prevents pattern collapse by improving the sidewall profile, and a cryogenic etch for lower LCDU (local critical dimension uniformity) and defectivity.  Roberto Fallica of imec gave a quote that I like (and often say myself), “Stochastics is the major roadblock for EUV Lithography”.  He then talked about a “healing” etch process that reduces contact hole LCDU through an aspect ratio dependent etch rate (high aspect ratio resist patterns etch faster, causing narrow holes to widen, while low aspect ratio resist patterns etch mode slowly, causing wide holes to narrow).  One interesting (and confusing) result was that the dose that provided smallest LCDU was not the dose that gave the lowest defectivity.  Finally, Qinghuang Lin of Lam talked about the application of Lam’s new dry-deposited and dry-developed resist to contact holes (I was not able to catch Rich Wise’s earlier paper on its application to lines and spaces).

Since I left after lunch to catch a plane home, I was not able to see what I’m sure were some good papers on the last afternoon of the symposium.  After a valuable and rewarding week here in San Jose, I was still anxious to get home.  Looking back two years, here is how I ended my Advanced Lithography Diary in February of 2020:

“The week has also seen an escalating concern over the new coronavirus, COVID-19.  Like everyone else I am monitoring developments with morbid fascination, but also to see how it will impact my immediate future.  And it has.  If there is any positive to the spreading fear over the spreading virus, it is that I will soon be traveling far less.  I have started asking customers if we could schedule our meetings, demos, and courses using video conferencing rather than in-person, and they are readily agreeing.  Maybe such accommodations will be a permanent trend, with the significant savings in time and resources that come with less travel (not to mention a better quality of life when I spend more time with my family).  I will look to this thought as a small consolation.”

That prediction proved true.  Like everyone, I have spent much of the last two years living my life on Zoom.  But since my life before the pandemic involved far too much travel, I am grateful for the respite that the pandemic forced upon me.  I am very glad to be back at ALP live and in person, and am glad that I can start visiting customers again (most of them, anyway).  But the much-accelerated use of video meeting technology has permanently changed the way I do business, and I am happy for the family time it will enable.  Like most of us, this pandemic has triggered a reckoning in my life/work balance, and I am happy for the result.

So, for those of you who wanted to but could not come to San Jose this week, I hope to see you next year.  But if not, maybe I’ll see you on Zoom.

SPIE Advanced Lithography and Patterning Symposium 2022 – day 3

In the morning, the optical and EUV session included two very good talks on EUV pellicles.  Mark van de Kerkhof described ASML’s latest material, a composite made of metal silicide crystals (if I got that right) that performs just a little bit better than the prior polysilicon-based stack.  At almost 92% transmission (one pass), it is a few percent better than the previous best and survives up to 400W source power.  Is it good enough to be adopted in manufacturing?  I’m not sure.  The next talk by Lintec described a 95% transmitting carbon nanotube pellicle, quite a promising result.  Their pellicle is making progress but did not seem manufacturing ready, requiring a bit more time to mature.

In the metrology session, my colleagues Gian Lorusso and Mohamed Zidan from imec gave a pair of good talks on the metrology challenges for measuring very thin resists.  (Full disclosure – I was a coauthor on both papers.)  When the as-coated resist thickness reaches 10 nm, line/space patterns have almost no contrast in a SEM, making measurement of CD and LWR extremely difficult.  Lowering the SEM voltage to 300V, and even lower for some materials, improved things.  It looks like 15 or 20 nm thickness and above is manageable with the right SEM measurement conditions.

I was also very impressed by Nearfield Instruments and their high throughput AFM, described by Cornel Bozdog.  Using four AFM heads running in parallel they could measure 64 0.5micronX0.5micron regions per wafer and get a throughput of 12 wafers per hour.  While I’m sure the typical “your results may vary” caveat applies, it is still an order of magnitude faster than I would have expected.

Quite a few students are attending the conferences this year, and I’ve been able to meet some of them.  Seeing the look of these eager young people, drinking from the firehose of information pouring out in each of the sessions, makes me hopeful for the future of our industry.

SPIE Advanced Lithography and Patterning Symposium 2022 – day 2

The first talk of the metrology conference on Tuesday was by Andras Vladac of NIST on a topic I am very interested in – characterizing the non-ideal behavior of scanning electron microscopes.  His presentation style was somewhat unique:  taking the material from what appeared to be a half-day short course and presenting it in 20 minutes.  This is a definitely a talk where viewing and studying it later (thanks to SPIE’s recording) is a must.  The other talks in the SEM session were good as well, but more digestible.

Tuesday was packed with customer meetings for me – a mixed blessing.  I missed many good talks, but got to have facetime with people I had not been able to visit for at least two years.  I managed to catch the end of Erik Hosler’s plenary talk in the afternoon on “The path to a useful quantum computer”.  One of the more interesting insights was his need to use state-of-the-art immersion lithography for the fabrication of his devices, not for the resolution but for the precision of the manufacturing.  For an optical device, feature sizes are in the hundreds of nanometers or microns.  But quantum optical devices require on the order of 1 nm line-edge roughness from those features, which definitely pushes state-of-the-art capabilities and makes fabrication quite challenging.

Musings of a Gentleman Scientist