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Plasma-Etched Pattern Transfer of Sub-10 nm Structures Using a
Metal−Organic Resist and Helium Ion Beam Lithography
Scott M. Lewis,*
,†,‡
Matthew S. Hunt,
‡
Guy A. DeRose,
‡
Hayden R. Alty,
†
Jarvis Li,
‡
Alex Wertheim,
‡
Lucia De Rose,
‡
Grigore A. Timco,
†
Axel Scherer,
‡
Stephen G. Yeates,
†
and Richard E. P. Winpenny*
,†
†
School of Chemistry and Photon Science Institute, The University of Manchester, Oxford Road, Manchester M13 9PL, United
Kingdom
‡
Department of Applied Physics and Materials Science and the Kavli Nanoscience Institute, California Institute of Technology, 1200
East California Boulevard, MC 107-81, Pasadena, California 91125, United States
ABSTRACT: Field-emission devices are promising candidates
to replace silicon finfield-effect transistors as next-generation
nanoelectronic components. For these devices to be adopted,
nanoscale field emitters with nanoscale gaps between them need
to be fabricated, requiring the transfer of, for example, sub-10
nm patterns with a sub-20 nm pitch to substrates like silicon and
tungsten. New resist materials must therefore be developed that
exhibit the properties of sub-10 nm resolution and high dry etch
resistance. A negative tone, metal−organic resist is presented
here. It can be patterned to produce sub-10 nm features when
exposed to helium ion beam lithography at line doses on the
order of tens of picocoulombs per centimeter. The resist was
used to create 5 nm wide, continuous, discrete lines spaced on a
16 nm pitch in silicon and 6 nm wide lines on an 18 nm pitch in tungsten, with line edge roughness of 3 nm. After the
lithographic exposure, the resist demonstrates high resistance to silicon and tungsten dry etch conditions (SF6and C4F8
plasma), allowing the pattern to be transferred to the underlying substrates. The resist’s etch selectivity for silicon and tungsten
was measured to be 6.2:1 and 5.6:1, respectively; this allowed 3 to 4 nm thick resist films to yield structures that were 21 and 19
nm tall, respectively, while both maintained a sub-10 nm width on a sub-20 nm pitch.
KEYWORDS: Metal−organic resist, ion beam resist, helium ion beam lithography, high-resolution pattern, high dry etch resistance
The ability of integrated circuit technology to follow
Moore’s law has depended on the continuous reduction
in the size of field-effect transistors (FETs), first in the planar
metal−oxide−semiconductor field-effect transistor (MOS-
FET) architecture and now more recently in the 3D fin
field-effect transistor (FinFET) architecture. This has been
accomplished by reducing the FET’s channel length, width,
and gate oxide thickness and by changing the gate dielectric
material according to Dennard’s scaling rules.
1
Unfortunately,
these scaling rules have begun to break down because as the
gate length is reduced to dimensions of 32 nm or smaller, the
supply voltages need to be scaled down as well, but doing so
does not provide enough voltage to turn on the p−n junction.
Furthermore, the power density in the newest microprocessors
has become so large that powering all transistors simulta-
neously would rapidly exceed the thermal power budget for the
chip, resulting in diminished performance, decreased lifetime
and, eventually, permanent device failure. Overheating can be
addressed by powering 50% of transistors on a single chip on a
single clock cycle,
2
but this presents a significant technical
design challenge. Considering these problems together, it has
been predicted by the International Technology Roadmap for
Semiconductors (ITRS) that it will no longer be economically
feasible to decrease FET device dimensions past the “7nm
node”,
3
thus imbuing a sense of uncertainty on the future
direction of the semiconductor industry.
Field-emission devices are promising candidates to succeed
silicon FinFETs because they can operate in high-power-
density regimes where chip temperatures can reach ≥300 °C.
Solid-state transistors fail in this regime because the p−n
junction’s functionality is lost when electrons in the p-doped
regions are thermally excited to the same conduction electron
concentration as that in the n-doped regions.
4
Conversely,
field-emission devices remain operational because as the
temperature is elevated, the current remains exponentially
dependent on the field until the temperature is sufficient to
initiate thermionic emission, which usually occurs hundreds of
degrees above 300 °C.
5
These devices are also attractive
because they are capable of operating at frequencies of
hundreds of gigahertz; this has been achieved by fabricating
150 nm vacuum gaps using optical lithography and resist
trimming.
6
Other researchers recently demonstrated that when
Received: May 9, 2019
Revised: July 20, 2019
Published: August 19, 2019
Letter
pubs.acs.org/NanoLett
Cite This: Nano Lett. 2019, 19, 6043−6048
© 2019 American Chemical Society 6043 DOI: 10.1021/acs.nanolett.9b01911
Nano Lett. 2019, 19, 6043−6048
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sub-50 nm emitter−collector gaps were fabricated, electric
fields high enough for field emission could be achieved at <10
V; the devices were complementary metal−oxide−semi-
conductor (CMOS)-compatible, functional at atmospheric
pressure, and able to be independently gated on a single
integrated chip.
4
Turn-on voltages can be further reduced by
fabricating sharper emitters with smaller emitter−collector
gaps, incentivizing the creation of new fabrication techniques
that yield tightly spaced, sub-10 nm structures.
Whereas electron beam lithography (EBL) offers high-
resolution patterning to create sub-10 nm structures in the
resist,
7
it is difficult to pattern these with high density, for
example, with sub-10 nm wide lines spaced <20 nm apart. This
is because secondary, Auger, and backscattered electrons (SEs,
AEs, and BSEs) scatter in between the nanostructures during
patterning, which exposes the resist in that space, resulting in
bridges between lines following the development process.
8
This “proximity effect”limits the resolution of the pattern that
can be produced. To alleviate this issue, a technique that uses a
focused helium ion beam instead of an electron beam has been
explored over the past decade. Previous helium ion beam
lithography (HIBL) studies have demonstrated a reduced
proximity effect
9
owing to less backscattering, the smaller
interaction volume with the substrate, and the subnanometer
beam diameter,
10
resulting in sub-10 nm resolution.
11
This is
accompanied by orders of magnitude higher resist sensitivity
than can be achieved with EBL
12
due to a higher SE yield per
incident helium ion compared with each incident electron.
13
Once the pattern has been defined in the resist by
lithography, it must be transferred to the underlying material,
which is often done using inductively coupled plasma reactive-
ion etching (ICP−RIE). The most common metal used to
produce field-emission devices is tungsten, which exhibits a low
work function and has a high thermal conductivity, preventing
the device from being destroyed via Joule heating.
4
Trans-
ferring the desired nanoscale pattern (e.g., sub-10 nm
structures with sub-10 nm gaps in between) to tungsten is a
challenge because the probability of landing ions in ever
smaller gaps becomes ever lower. This leads to a decrease in
etch efficiency, which inherently decreases the etch rate and
selectivity. To increase the etching efficiency, the ICP forward
power must be increased, but this also increases the etch rate
of the resist. The thickness of the resist would then need to be
increased to achieve the desired etch depth, which would
require a higher dose, which, in turn, would reduce the
resolution of the pattern. To avoid this problem, one may use a
hard mask to withstand the aggressive nature of the plasma
etch,
4
but doing this introduces more processing steps and
leads to higher production costs. Another route is to enhance
the etch selectivity of the resist by introducing into the
molecular chemistry a metal species that effectively oxidizes
upon lithographic exposure to become the hard mask. This has
previously been demonstrated by our group using supra-
molecular Ni- and Cr-containing assemblies while maintaining
a sub-10 nm patterning capability,
14
albeit at a relatively low
pattern density compared with what is needed for modern
nanoelectronics.
In this Letter, a metal−organic, negative tone resist
candidate, Cr8F8(O2CtBu)16 (Figure 1), first introduced by
our group in ref 15 and henceforth denoted as
Cr8F8(pivalate)16, is presented. It is formed by the binding of
eight chromium atoms (in green in Figure 1) in a ring-like
structure, with an exterior composed entirely of tert-butyl
groups (pivalates).
15
The pivalates provide a high solubility in
nonpolar solvents, which allows the resist molecule to be
dissolved in hexane and spun onto substrates (e.g., Si and W).
The molecule achieves high-resolution patterning because it is
simultaneously low density (ρ= 1.212 g cm−3), meaning that it
does not have many lateral scattering centers for the
lithography beam to interact with as it travels through, and
has a high molecular weight (2192 g mol−1), meaning that the
number of resist molecules that are required to produce a thin
film is significantly reduced, leading to a high-resolution
pattern. Upon exposure, SEs and AEs break carbon bonds in
the resist, liberating some C and O atoms while permitting
other O and Cr atoms to react to form a chromium−oxide
hard mask that is particularly resistant to the ICP−RIE
chemistry used to etch both silicon and tungsten.
14
Prior to the spin-on application of the Cr8F8(pivalate)16
resist, atomic force microscopy (AFM) was used to evaluate
the surface morphology of silicon and tungsten substrates
(Figure 2a,b). The root-mean-square (RMS) roughness was
measured to be 0.29 nm for silicon. For tungsten, which was
sputter-deposited onto silicon as a 100 nm thick film on top of
a 5 nm sputter-deposited titanium adhesion layer, the RMS
roughness was 0.42 nm. The tungsten was 45% rougher than
silicon; topographical contrast revealed that the film was
composed of nanograins that individually were ∼5 nm wide
and as long as 50 nm. For all sputter processes, wafers were
first cleaned inside the chamber with argon plasma, and targets
were presputtered for 60 s to remove surface oxides.
The exact nature of the resist film in this stage is uncertain.
Previous studies of similar compounds sublimed onto gold
show that an ordered monolayer forms,
17
but subsequent
layers are not ordered because there are only weak van der
Waals interactions between the molecules of metal rings. The
films formed here, shown here by AFM (Figure 2c,d) to be
∼3.5 nm thick (approximately two layers), are therefore
amorphous. The resist is monodispersed and in some ways
resembles the molecules studied by Ober and coworkers
18
that
form molecular glasses rather than conventional polymeric
resists. We have not observed a glass-transition temperature
because Cr8F8(pivalate)16 sublimes before such a transition is
observed. This low sublimation temperature is again due to the
very weak intermolecular forces within the resist films.
Samples were created by dicing wafers into 20 mm ×20 mm
pieces. Both substrate types, bare silicon and tungsten-coated
silicon, provided a smooth enough surface upon which sub-10
Figure 1. Structure of the Cr8F8(pivalate)16 molecule in a ball-and-
stick representation. Chromium atoms are green and fluorine atoms
are yellow. Hydrogen atoms are omitted for clarity.
Nano Letters Letter
DOI: 10.1021/acs.nanolett.9b01911
Nano Lett. 2019, 19, 6043−6048
6044
nm features could be clearly resolved. This is evident in Figure
3, which shows plan-view helium ion microscope (HIM)
images of Cr8F8(pivalate)16 resist nanostructures following
HIBL and the subsequent development in hexane. The
Cr8F8(pivalate)16 resist (30 mg) was dissolved in hexane (3
g); then, the solution was filtered using a 0.2 μm
polytetrafluoroethylene syringe filter before being spun onto
substrates with a spin rate of 6000 rpm for 40 s, followed by a
100 °C soft bake for 2 min to evaporate the solvent. The
spacing of adjacent lines (i.e., the pitch) was set to be 16−22
nm using a Raith ELPHY MultiBeam pattern generator, which
controlled the helium focused ion beam (35 keV, 0.50 pA) on
a Zeiss ORION NanoFab apparatus. The exposure clearing
dose of the resist on each substrate was determined using a 1D
matrix of single-pixel-wide lines that were 5 μm long. The
width of the line was therefore the width of the ion beam,
which is estimated at 0.5 nm;
10
the beam step size was 1 nm.
At any pitch, patterns were exposed in sets of 20 lines with one
pass of the beam per line, and the line dose of each set ranged
from 10 to 100 pC/cm with incremental steps of 1 pC/cm.
Following lithography, the resist was developed in hexane for
10 s to dissolve away the unexposed resist, then blown dry with
nitrogen.
It can be seen in Figure 3 that discrete, continuous lines
were successfully patterned at all pitches on silicon, with no
bridging between any adjacent lines. On tungsten, patterning
was likewise successful at 18, 20, and 22 nm pitches; at a 16
nm pitch (Figure 3h), the line uniformity was poor and
bridging had occurred, a hallmark of being just beyond the
lithographic resolving limit. The line width, on average, was
measured to be 5.5 nm (standard deviation, σ= 0.9 nm) on
silicon at a 16 nm pitch and 5.6 nm (σ= 0.9 nm) on tungsten
at an 18 nm pitch. The line edge roughness (LER), defined as
3σ, was ∼3 nm for both sets of Si and W lines. Tungsten
performed slightly worse than silicon in both the minimum
achievable pitch and the minimum line width because tungsten
has a significantly larger atomic number (Z= 74 for W, Z=14
for Si) and therefore leads to a larger number of SEs and AEs
generated by the primary ion beam; this effect is triggered by
both incident electrons in EBL
16
and incident He ions in
HIBL.
13
The ejected SEs can be calculated using the Joy
model
19
to have a scattering angle of 80°relative to the
incident beam vector,
20
which leads to the exposure of the
resist material adjacent to the beam’s entry point. A similar
mechanism is at play with low -energy ion recoil events
initiated by incident ions, which scatter SEs at the same high
angle in addition to physically displacing atoms.
21
The more
SEs and AEs that are generated, the wider the exposure radius
is that surrounds the beam entry point, leading to wider lines
and, when the pitch is too small, bridging between them.
Whereas this proximity effect diminishes the smallest
achievable line width and pitch, the generation of more SEs
and AEs also has the benefit of decreasing the necessary
exposure dose, which was as much as 1.9 times lower at an 18
nm pitch for tungsten (11 pC/cm) compared with silicon (21
pC/cm). The necessary exposure dose also decreased on
tungsten as a function of decreasing pitch, whereas it did not
for silicon due to the intensity of the proximity effect when
lines are written ever closer to each other on a high-Zmaterial.
On the basis of these results, the outlook for patterning sub-10
nm wide lines on tungsten is that the achievable pitch may be
slightly higher compared with silicon (18 versus 17 nm), in
exchange for nearly half of the exposure dose. It must also be
noted that these HIBL doses are an order of magnitude below
the threshold dose at which He implantation has been shown
to induce dislocation damage in Si.
22
Figure 2. AFM images of substrates prior to spin-on application of the
resist: (a) silicon and (b) 100 nm tungsten film (on silicon). (c)
Roughness and (d) thickness of the patterned resist are also
demonstrated by AFM.
Nano Letters Letter
DOI: 10.1021/acs.nanolett.9b01911
Nano Lett. 2019, 19, 6043−6048
6045
In Figure 4, the resist is shown at its smallest successfully
etched pitch for silicon (17 nm) and tungsten (18 nm), both in
plan view (Figure 4a,d), and when tilted to 87°to better show
its thickness (Figure 4b,e). The same spin settings were used
to apply the resist to both silicon and tungsten. The resist
thickness was measured at the front edge of the tilted lines (the
higher tungsten roughness perhaps accounts for the thinner
measurement) and confirmed by AFM (Figure 2c,d). It should
be noted that the resist had been changed to a chromium−
oxide material by the time the lines were imaged as a result of
the lithographic exposure. It has been observed that this resist
can shrink under the exposure of an electron or helium ion
beam; as bonds are broken and carbon and oxygen are
volatilized, the resist film volume consolidates slightly into the
oxide material. The initial resist thickness, which was not
measured, is therefore necessarily larger than depicted here.
Regardless, the tilted view images in Figure 4 show more
clearly than the plan-view images that the resist structures are
resolvable against the roughness of the substrates beneath
them. The ability to spin the resist into a sub-5 nm thick film
also helps to reduce the smallest feature size and dose; a
thinner resist yields fewer lateral scattering sites for the
traversing beam and also means that fewer ions are needed to
generate enough SEs and AEs to change the small volume of
resist material into the chromium−oxide material.
It is important to note that when characterizing these
nanostructures, each HIM image was captured via a single scan
of a 600 nm ×600 nm area, meaning that sputtering of the
Figure 3. Plan-view HIM images of lines spaced with pitches of 22, 20, 18, and 16 nm on silicon substrate (a−d, respectively) and on a 100 nm
thick tungsten film that was sputter-deposited onto a silicon substrate (e−h, respectively). Average width (w), standard deviation (σ) and line edge
roughness (LER) (3σ) to the nearest 0.1 nm were determined using GenISys ProSEM software.
Figure 4. HIM images of lines spaced with pitches of 17 and 18 nm on silicon substrate (a−c) and on a 100 nm thick tungsten film that was
sputter-deposited onto a silicon substrate (d−f), respectively. In the top row of images (a,d), developed resist structures are shown in plan view
prior to an ICP−RIE etch. In the middle row (b,e), developed resist structures are shown when tilted to 87°prior to the etch. In the bottom row
(c,f), fin-like structures are shown following the etch. Measurements to the nearest 0.1 nm were made using GenISys ProSEM software. The LER of
etched Si lines was determined via the plan-view image (not shown); the LER of etched W lines was not determined because the triangular shape of
the cross-section does not lend itself to the LER calculation.
Nano Letters Letter
DOI: 10.1021/acs.nanolett.9b01911
Nano Lett. 2019, 19, 6043−6048
6046
nanostructure material by the low-current He ion beam (1.0
pA, 30 keV) was negligible; tests were done to show that even
multiple scans of the same area at these settings did not alter
the size of structures. This ensured that HIM imaging could be
used as a nondestructive technique while offering higher
resolution (and higher depth of field, important for imaging
tilted structures) than, for example, a scanning electron
microscope operated with an immersion lens. Furthermore,
measurements made on AFM and HIM micrographs were
calibrated against images taken of a NIST traceable standard
(50 nm wide gold lines spaced on a 100 nm pitch); the same
microscope settings were used on both the experimental
samples and the standard to ensure measurement accuracy.
After characterizing the resist on substrate, samples for both
silicon and tungsten were subjected to the same 30 s ICP−RIE
process (forward power: 20 W RIE, 1200 W ICP) using SF6
and C4F8gases flowing at 22 and 35 sccm, respectively. Figure
4c shows that the average width of the resultant silicon fins was
6.4 nm, with an average height of 21 nm; the resist has been
completely etched away. The effective etch rates, based on a
3.4 nm resist thickness (Figure 2d), were therefore determined
to be 0.11 and 0.70 nm/s for the resist and silicon, respectively.
This indicates that silicon etches ∼6.2 times faster than the
resist when subjected to these etch conditions (i.e., the
selectivity is 6.2:1). For tungsten (Figure 4f), the structures
etched with less of a straight-sidewall fin shape and more of an
angled-sidewall triangular shape, suitable for a sharp-tipped
field emitter. The average width at the top and bottom of the
triangle was 5 and 16 nm, respectively, with an average height
of 19 nm. The resist was completely etched away from the top
of these W structures in 30 s, resulting in etch rates of 0.11 and
0.63 nm/s for the resist and tungsten, respectively (i.e., the
selectivity is 5.6:1). It is impossible to compare these results
directly with other common resists because the etch
selectivities of those resists have not been reported for sub-
20 nm pitches. At a 100 nm pitch, the etch selectivities of
common resists on silicon are 2.0:1 for poly(methyl
methacrylate) (PMMA), 2.9:1 for ZEP520A, and 4.2:1 for
hydrogen silsesquioxane (HSQ).
23
Etch selectivity is expected
to decrease at a smaller pitch due to the decreasing probability
of landing ions between the features, so the 6.2:1 selectivity for
Si reported here at a 17 nm pitch is especially notable, by
comparison. The improvement in etch performance demon-
strated here by Cr8F8(pivalate)16 on silicon has also been
previously demonstrated for related metal−organic resists,
where selectivities greater than 100:1 could be achieved at
larger pitches.
14
It is also notable that whereas lines narrower
than those shown in Figures 3 and 4have been patterned by
other groups in the resist (e.g., 4 nm lines on a 8 nm pitch by a
combination of HIBL and nanoimprint lithography
24
), the
etched structures reported here are both the narrowest and the
tallest to be transferred to substrate on a sub-20 nm pitch. The
next smallest transferred patterns found in the literature are on
a 22 nm pitch via thermal scanning probe lithography.
25
In comparison with a previous study of Cr8F8(pivalate)16
with EBL (100 keV, 300 pA),
15
HIBL required a dose three
orders of magnitude smaller to achieve its smallest pitched
lines. (EBL achieved 40 nm pitch lines at a 30 500 pC/cm line
dose compared with a 16 nm pitch at 22 pC/cm here.) It must
be mentioned that an orders-of-magnitude smaller dose with
HIBL is accompanied by an orders-of-magnitude smaller
current as well (e.g., 0.5 pA He ions versus 300 pA electrons).
Whereas at first glance that might indicate that HIBL writing
speeds are approximately equivalent to EBL writing speeds, it
is important to also consider how the resist thickness impacts
doses. In the EBL study, the resist was 10 times thicker (30
nm) than in this HIBL study. If the resist thickness were to
increase here, then we might expect the HIBL dose to actually
decrease because we would be taking advantage of a cascade of
scattering events that cannot similarly take place when the
thickness is confined to something as small as 3 nm. (That
decrease in dose with increasing thickness, it must be noted,
would come at the expense of resolution.) Additionally, it is
well known that the clearing dose increases as a function of the
decreasing pitch, as was the case in the comparative EBL study,
where the smallest pitch was 40 nm. If we were able to
compare the 17 nm pitch EBL lines with the 17 nm pitch
HIBL lines shown here, then we would expect the HIBL dose
to be even more favorable than the three orders of magnitude
difference noted above.
For mass manufacturing, the high exposure doses inherent to
EBL, which translate into long writing times, have always
outweighed the allure of EBL’s small-probe, high-resolution
capability. Much work has been put into developing EBL tools
that split one primary beam into many beamlets to decrease
writing times by exposing many patterns in parallel.
26
Here we
see a demonstration of HIBL yielding both better resolution
and an orders-of-magnitude smaller dose than EBL. Whereas
single-beam HIBL, with the same pixel-by-pixel exposure
mechanism as EBL, may still not offer the lithographic speed
desired by the industry, perhaps this study indicates that if any
beam is to be split and operated in parallel, then it is a beam of
helium ions and not electrons.
In conclusion, it has been demonstrated that the molecule
Cr8F8(pivalate)16, when used as a resist, is capable of
producing sub-10 nm structures in silicon and tungsten,
spaced on a sub-20 nm pitch, following pattern transfer with an
ICP−RIE. This result is due to several interrelated factors
associated with the resist material and the method of
lithography, HIBL. First, the ability to spin the resist into
sub-5 nm thick films reduces the lateral scattering as the beam
travels through the resist, resulting in high resolution. Second,
the material’s high molecular weight and low density limit the
number of scattering sites that the beam encounters, which
also improves resolution. Third, the nature of helium ion beam
interactions yields more SEs and AEs per incident beam
species than is achievable by the more traditional EBL; the
HIBL dose can therefore be orders of magnitude lower, which
allows for a low current to be selected, which results in a
subnanometer probe diameter that further improves the
patterning resolution. Finally, because exposing the resist
changes it from a metal−organic compound to a chromium−
oxide material, the material exhibits extremely high etch
selectivity to both silicon and tungsten in the presence of an
SF6/C4F8etch, allowing for the transfer of 6 nm wide lines into
the substrates, even when the etch efficiency is reduced by
tightly spacing the lines on a sub-20 nm pitch. It is therefore
possible to fabricate sub-10 nm wide, 19 nm tall silicon and
tungsten structures in a single lithography-and-etch step,
opening new possibilities for future nanoelectronics. The role
of HIBL in the future should also not be discounted.
■AUTHOR INFORMATION
Corresponding Authors
*E-mail: scott.lewis@manchester.ac.uk;slewis2@caltech.edu.
*E-mail: richard.winpenny@manchester.ac.uk.
Nano Letters Letter
DOI: 10.1021/acs.nanolett.9b01911
Nano Lett. 2019, 19, 6043−6048
6047
ORCID
Scott M. Lewis: 0000-0002-4183-1906
Richard E. P. Winpenny: 0000-0002-7101-3963
Notes
The authors declare no competing financial interest.
■ACKNOWLEDGMENTS
We acknowledge the EPSRC (U.K.) for funding (grant EP/
R023158/1). The University of Manchester also supported
this work. We gratefully acknowledge the critical support and
infrastructure provided for this work by the Kavli Nanoscience
Institute at Caltech.
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Nano Letters Letter
DOI: 10.1021/acs.nanolett.9b01911
Nano Lett. 2019, 19, 6043−6048
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