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 studied 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 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.