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San Jose, California, February 23 – 27, 2020
(The following diary appeared first as a daily blog at life.lithoguru.com and is reproduced here in a slightly edited form.)
SPIE
Advanced Lithography Symposium 2020 – prologue
The SPIE Advanced Lithography conference begins
with one word on everyone’s mind:
coronavirus. I am
fairly certain that the actual impact of coronavirus on the
conference will be zero, but the impact of fear of the coronavirus
is large. Many Asian
companies have either decided not to send anyone, or are sending
very few people. From
Taiwan, I have heard that Winbond is the only semiconductor company
sending people. From
Korea, Samsung is sending maybe only one person.
Last week Intel decided to dramatically reduce the number of
people they are sending.
This has a snowball effect, as many vendors seeing that their
customers will not be there are also reducing their presence.
On Sunday, KLA and Nikon both canceled their major events
(the KLA Litho Users Forum and the Nikon LithoVison).
Several (but not all) Sunday technical meetings have also
been canceled. Most of
the big companies have canceled their hospitality suites (KLA, TEL,
Hitachi, Applied Materials).
Of course, health concerns are the stated reason for the
cancellations, but I think cold, hard cash is the real reason.
Why spend a huge amount of money on an event when hardly any
of your customers are going to be there?
Not to worry – Fractilia’s
Happy Hour will go on as planned!
As the week goes on, I’m sure we’ll all
understand better how the coronavirus scare will impact the
technical events, especially cancelled papers.
As for SPIE, every event of the conference will go on as
planned. While the
conference may be smaller than expected, I am still expecting it to
be a good one.
SPIE
Advanced Lithography Symposium 2020 – day 0
For me
Sunday is always about teaching.
I’ve been teaching at the start of this conference every year
since 1990 (except the year I was too sick to leave my hotel room –
a rotavirus as it turns out).
Alas, my good friend and co-instructor John Petersen was
unable to attend the conference at the last minute (responsibilities
for his new AttoLab at imec have intervened), so I reverted to my
old ways of teaching 8 hours by myself.
That is definitely a young person’s calling, though I
survived with my feet a little tired and my voice mostly intact.
My course’s attendance was about the same as last year, but
the biggest course, Introduction to Microlithography, had only about
50% of the registered students show up.
Early
indications are that conference attendance will be down about 15%
(300 people) compared to last year, with about half of that drop
coming from Asia and most of the other half from Intel (only authors
and conference chairs have been allowed to come from Intel).
I found out that another large company has canceled their
hospitality suite – ASML.
That leaves a few resist companies, Qoniac, Mentor Graphics,
and of course Fractilia carrying on with their evening events (I’m
probably missing some in this list).
That is definitely enough to have fun every night of the
week.
As I
await the beginning of the conference, I am anticipating a few
things. Developments in
Directed Self Assembly (DSA) have been somewhat muted here the last
few years, giving conference attendees the possible impression that
interest has been waning.
But rumors are spreading that several companies are on the
verge of high-volume manufacturing with DSA.
The quiet seems to be due to commercialization, not lack of
interest. I’m not sure
that we’ll hear more about those plans this week, but I’ll listening
for them.
Finally, I have realized that my personal transition is complete.
I no longer call myself a lithographer.
I am a metrologist, and I am proud of it.
My conference of focus will be the metrology conference, and
I find everything about metrology incredibly interesting!
I still know how to think like a lithographer, and I still
work hard to adopt a stochastic mindset – metrologists must steep
themselves in the technology of what they measure.
I’ll be following the stochastic conference track wherever it
leads me, knowing that without good metrology none of us will have
the data needed to make good decisions.
SPIE
Advanced Lithography Symposium 2020 – day 1
The
plenary session began with opening remarks and awards.
We welcomed two new Fellows of SPIE:
Hiroshi Fukuda and Mike Rieger.
Congratulations for that well-deserved recognition.
This year’s
Frits Zernike Award for
Microlithography
was given to Winfried Kaiser of Zeiss for his major contributions to
193nm and EUV optics. He
also gets my nomination for most dapper Zernike award winner!
Three good plenary talks (on machine learning, in-memory
computing, and Flash memory process technology) were full of
interesting technical information (so long as you ignored the
commercial embedded in the Kioxia talk).
The
opening keynote talk for the EUV conference was given by Charlie
Wallace of Intel, where he described not just the current status of
EUV lithography for manufacturing 32 nm pitch lines and spaces, but
the immense challenges of shrinking the pitch to 30 nm or 28 nm.
As pitch shrinks, higher doses are required, but even at
these high doses defect rates are too high.
Some quotes: “It
is execution time for EUV lithography.”
“We need fundamental improvement in EUV materials”.
“Improvement in metrology is required.”
A quick
pause to talk about Intel.
Anyone who has read this blog over the last several years
knows that I have complained about the paucity of
semiconductor-maker talks at this and other lithography conferences,
and especially about how few talks Intel would give.
I must now recognize that this criticism belongs to the past.
Intel has really stepped up
their game recently, and they have seven presentations at AL this
year. Thank you, Intel!
The entire lithography community appreciates your
contribution.
The
Metrology conference opened with an interesting experiment – the
first keynote was given remotely.
Alain Diebold of SUNY Polytechnique called in and spoke while
his slides were advanced onsite.
While not ideal, it was much better than a cancelled talk and
I appreciate the conference chairs thinking experimentally about how
to let the talk go on.
Several afternoon talks covered the important topic of edge
placement errors and how to characterize them using contour-based
metrology rather than the traditional CD-based measurements.
It is clear that this
approach is quickly becoming a standard method.
In the EUV session, Marie Krysak of Intel showed again how
standard “three-sigma” characterization of stochastic contact hole
variations was not good enough to predict chip yield.
She used a combination of non-Gaussian extrapolation and
stress tests (underexposing to make the defect rates high enough to
measure), both of which produced similar results when comparing the
performance of different EUV resists.
Some
news: Canon and (at the
very last minute) Qoniac have cancelled their hospitality events.
I still managed to stay out too late and drink too much beer
(thank you Fractilia and Inpria).
SPIE
Advanced Lithography Symposium 2020 – day 2
Tuesday
was a heavy day of stochastics for me.
Greg Wallraff of IBM got me off to a good start with his
interesting simplified Monte Carlo-like stochastic resist model.
As expected for chemically amplified resists, higher PAG
loading had a big effect on reducing stochastic variability, and
higher amounts of photodecomposable quencher had a smaller but
noticeable impact. Also
as I expected, acid amplifiers only make things worse
stochastically. All of
his simulations used a 15nmx15nmx15nm voxel, but I hope he will look
into the impact of voxel size on his simulation results.
I think that understanding the role of the averaging volume
(voxel size essentially) is one of the biggest gaps in our knowledge
of stochastic behavior.
Andy
Neureuther gave a fantastic talk on the role of dissolution path in
determining missing contact defectivity.
His algebraic model looked very insightful, and dissolution
path plays an underappreciated role in how photon shot noise
manifests itself in stochastic defectivity of contacts.
Dario Goldfarb of IBM and Patrick Theofanis of Intel each
showed wonderfully rigorous experimental and simulation studies
(respectively) of EUV resist exposure mechanisms.
Peter
de Bisschop of imec once again provided the incentive (and the data)
for the industry to look more closely at EUV defectivity versus
dose, this time by adding pitch variation and challenging us to
model the results. Both
Synopsis and Mentor used that same dataset to develop models for
stochastic defectivity (a work still in progress).
I gave
my paper for the week (comparing the noise sensitivity of different
CD-SEM edge detection algorithms), as did two of my coauthors on
separate studies. Jen
Church of IBM compared LER with defectivity for lines and spaces and
LCDU with defectivity for contacts.
While she showed that unbiased LER and low-noise LCDU were
required, these metrics alone were not enough to predict defectivity
or yield. Charlotte
Cutler of DuPont gave the third in a series of papers she has
presented at the Patterning Materials conference on using power
spectral density (PSD) analysis for resist design.
In my completely biased perspective, both of these papers
were highlights of the day.
At the
metrology conference I enjoyed a talk by the National Metrology
Institute of Japan on using AFM as a roughness reference metrology,
even though I disagree with some of their conclusions.
Comparing SEM and AFM measurement of the same sample (an
etched silicon line), the two measured edges matched extremely well
except at the high frequencies.
The authors attributed these differences to SEM noise, but
failed to recognize the role of instrument resolution.
With an uncharacterized tip size of about 7nm, their AFM is a
much lower resolution instruments (in terms of high-frequency
roughness measurement) and so was unable to see the high frequency
variations that are visible in a SEM (admittedly contaminated by SEM
noise). I hope the
authors will continue their work be comparing AFM to unbiased SEM
measurements, and that they will work to deconvolve the tip shape
from the AFM measurements (hopefully using different tips with
different shapes).
The
final talk I heard was a fantastic one, by Luc Van Kessel, a student
at the Technical University of Delft.
He studies a subject I have long been fascinated with:
how does the 2D surface roughness of the sidewall of a
feature translate into the 1D edge roughness observed in a top-down
CD-SEM? For his 300V SEM
simulations, the observed top-down edge of an isolated line was
essentially the extreme X-Y points of the 3D feature.
Things were a bit more complicated for a small space because
of the aspect ratio making the bottom of the space less visible in
the SEM. Also, his 500V
simulations were only preliminary and could be somewhat different
due to the greater penetration distance of those higher-energy
electrons. Great work,
Luc!
With
Harry Levinson, I ended the day by hosting an all-conference panel
called “A toast to lithography’s past:
what we learned from technologies not used in HVM”.
Hans Loschner gave us the history of the life (and death) of
ion-beam projection lithography, Reiner Garreis of Zeiss discussed
157-nm lithography, Alexander Liddle recalled his time working on
Scalpel, and I filled in for Tobey Aubrey (who couldn’t make it) to
talk about our lessons learned from proximity x-ray lithography.
While I enjoyed all of the discussion, I didn’t enjoy the
unfortunate logistics.
We made the big mistake of scheduling our panel immediately after
the EUV retrospective panel.
Not only was the EUV panel late to finish (as expected for
EUV), but the time to transition between panels was far too short.
The topics of the two panels were very similar, but nobody
would want to sit through four hours of panel discussions at one
time. Lessons learned
not only about lithography, but about panel discussions as well.
SPIE
Advanced Lithography Symposium 2020 – day 3
Ron
Schuurhuis of ASML began the day with a review of the improvements
they have made to the NXE:3400C, many of them (such as inline tin
refill and reduced collector swap times) resulting in fairly
significant tool productivity enhancements.
But something else in his
presentation has encouraged me to go off on a rant:
calculated throughputs based on unrealistic resist
sensitivity assumptions.
In the very early days of EUV, throughput calculations were based on
the mythical 5mJ/cm2 (dose-to-size) resist.
After source power increased by something like an order of
magnitude, a mythical 10mJ/cm2 resist was introduced for theoretical
throughput calculations.
As the source power increased further, ASML grudgingly acknowledged
that these unrealistic dose targets would never be met and allowed
the theoretical dose for throughput calculations to rise again (to
15 and then 20 mJ/cm2), but always climbing more slowly than source
power so that they could still claim a rising throughput.
In the Schuurhuis presentation I saw what appeared to be the
next transition, to a 30mJ/cm2 mythical resist.
(As an example, their calculated 170 wafer per hour
throughput using a 20 mJ/cm2 resist becomes 135 wph assuming a 30
mJ/cm2 dose-to-size.)
Assuming 30 mJ/cm2 is certainly better than assuming 20, but
line/space patterning requires closer to 40 mJ/cm2 at modest pitches
(and higher for smaller pitches), and contact holes need over
50mJ/cm2 (to print, for example, 40nmx70nm pitch staggered arrays).
Can we just admit reality for once and start using 40 mJ/cm2
for all future throughput calculations on the 0.33 NA tool?
I was
excited by a talk by Rich Wise of Lam Research showing extremely
preliminary results for a dry deposited, dry developed metal-organic
nanocluster resist.
These early results looked promising.
I always worry that nanocluster resists will not have high
enough development contrast (best measured using a focus-exposure
process window and mask linearity compared to a standard resist),
but I look forward to seeing more from Lam on this material in the
future.
Gurpreet Singh of Intel gave a pair of talks on complementing EUV
with directed self-assembly (DSA).
(I have to be careful with my spelling – I started to say
that DSA was “complimenting” EUV, but in fact the opposite is true).
The first application of DSA was in rectification:
print lousy EUV patterns at a tight pitch (say, 30 nm or 28
nm) and low dose, etch them into an underlayer, then fix the
terribly rough features using DSA guided by the underlayer pattern,
without pitch division.
This works very well for line/space patterning and could replace an
SAQP flow, but of course Intel said nothing about design rule
constraints. Their goal
was clear: improve edge
placement error by reducing the pitch walking endemic to SADP and
SAQP. With the low EUV
doses possible using this approach, it might even be cost effective.
They used the very mature PS-b-PMMA system since it has the
possibility of sufficiently low defectivity for practical
manufacturing. But
pushing to smaller pitches (below about 24 nm) will likely require a
new material, and he proposed the development of a “modified”
PS-b-PMMA system as the best path forward.
From
Charlie Lin of IBM I heard my new acronym of the week:
PB&S (print big and shrink).
Hyo
Seon Suh of imec updated us on their continuing progress in making
DSA practical for high-volume manufacturing (full disclosure – I was
a coauthor on this talk).
Through a number of optimizations they were able to shrink
the unbiased LER from 3.0 nm to 2.5 nm, while keeping defectivity
near the 2/cm2 level.
Customer meetings kept me away from much of the afternoon talks, and
as a substitute for the canceled KLA PROLITH party many of us met up
in the evening at my new favorite San Jose brewpub, Uproar, where we
toasted another successful day advancing lithography.
SPIE
Advanced Lithography Symposium 2020 – day 4
The
final day of the conference!
Zhingang Wang of Hitachi talked about CD-SEM tool matching,
describing all of the sources of variation that affect matching.
This year he added a new error source to his list:
detection/image level variation.
The variation of SEM image quality across the SEM image field
is something that I have been discussing (related to Fractilia) for
the past few years, and I am glad to see Hitachi start talking about
it as well.
Jara
Garcia Santaclara gets my vote for best paper title of the
conference - “One metric to rule them all:
new k4 definition for photoresist characterization”.
I’m a sucker for Lord of the Rings references.
Jara and Bernd Geh have made some good progress on the k4
factor introduced by Bernd last year (essentially trying to create a
predictive scaling relationship that is more detailed than Gregg
Gallatin’s original RLS formulation).
Their work seems to be converging with my (still unfinished)
approach to simple LER modeling that I discussed at the last two
EUVL Symposiums. With
some more effort, we all might get these scaling rules to a very
useful place, so I hope we continue to work this topic.
There
were several useful papers on measuring and modeling secondary
electron blur radius in EUV resists, an important but difficult
topic. But most of the
Thursday papers were not as on-target to my interests as the earlier
days. I did end the day
with a fun paper on “Sub-Wavelength Holographic Lithography” (SWHL)
by a Swiss startup of that name.
Holographic lithography is an old approach with many very
interesting characteristics (no projection lens, masks that are hard
to make but insensitive to defects).
There were other attempts to make this approach work 15 years
ago and 25 years ago, but improvements in lasers, mask making, and
computational capabilities seem to be enabling a renewed interest.
I’ll be watching Nanotech SWHL to see how they do.
Looking
back over the week I have two closing thoughts.
This is, I believe, the first time I have been to SPIE
Advanced Lithography without seeing Grant Willson, who retired last
year. I saw him present
at my first SPIE in 1985, met him at my second conference in 1986,
and have been friends with him ever since.
I’m glad he is enjoying his retirement, but we certainly miss
him here. 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.
Chris Mack is a writer and lithographer in Austin, Texas.
© Copyright 2020, Chris Mack.
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