Invited Paper

CNT EUV pellicle tunability and performance in a scanner-like

environment
Marina Y. Timmermans*, Ivan Pollentier, Maxim Korytov, Thomas Nuytten, Stefanie Sergeant, Thierry
Conard, Johan Meersschaut, Yide Zhang, Masoud Dialameh, Wilfried Alaerts, Ehsan Jazaeri, Valentina
Spampinato, Alexis Franquet, Steven Brems, Cedric Huyghebaert and Emily E. Gallagher

IMEC, Kapeldreef 75, Leuven, Belgium, 3001

ABSTRACT

The advent of extreme ultraviolet (EUV) lithography to enable smaller circuit dimensions introduced a new application for
carbon nanotubes (CNTs): the EUV-transparent pellicle used during lithography to protect the photomask from fall-on
particles. Diversity of CNT structures and tunability of their configuration within the CNT film (density, bundle size,
composition etc.) allow to tailor the CNT EUV pellicle properties for optimal performance. Thin, free-standing CNT films are
characterized by very high EUV transmission, mechanical and thermal stability, and can be scaled up to a full pellicle size. A
remaining challenge is extending the CNT EUV pellicle lifetime in the scanner environment of EUV-induced plasma. In this
work, optical properties of different uncoated CNT pellicles and their performance under high EUV power exposure in the
hydrogen-based environment are reported. Transmission, spectroscopic and chemical mapping of the exposed CNT membranes
are performed to explore the material modifications under various exposure conditions. These investigations add to the
understanding of lifetime limiters of CNT EUV pellicles under the influence of EUV radiation and plasma. Uncoated CNT
pellicles withstand 600 W source power equivalent in the EUV scanner-like gas environment but exhibit structural changes
with prolonged exposure. Therefore, we anticipate the need for coating the CNT pellicle to protect the CNT material against
plasma damage for the current scanner conditions.

Keywords: EUV pellicle, carbon nanotubes, free-standing film, EUV lithography, scanner environment, lifetime,
mechanical stability, hydrogen plasma, coating

1. INTRODUCTION

Due to its remarkable thermal, electronic and mechanical properties, carbon nanotube (CNT) material offers a wide range of
unique application opportunities. We identified the potential of CNTs to contribute to high yield extreme ultraviolet (EUV)
lithography, enabling advanced chip manufacturing1,2. One of the obstacles in the development of EUV lithography in high
volume manufacturing is mask contamination3. A pellicle is placed at a few millimeters stand-off distance from the EUV mask
so to catch any fall-on particles and not image them onto the wafer by keeping them out of focus. The development of an EUV
pellicle to protect the mask is challenging since most of the materials absorb strongly at the 13.5 nm EUV exposure wavelength.
To be commercially viable, an EUV pellicle must be thin enough to allow more than 90% EUV light transmittance (EUVT),
while durable enough to withstand handling, pressure changes and thermal stresses. A pellicle made of free-standing CNT
films is able to stop particles despite the presence of gaps, while demonstrating superior EUVT, mechanical stability, suitable
EUV uniformity, low EUV scattering and reflectivity, and sufficient light transmission to allow through-pellicle mask
inspection 1,2,4,5. Figure 1 shows a full-size CNT pellicle (~110 mm x 140 mm) similar to one that was successfully exposed

*
contact: marina.timmermans@imec.be

Extreme Ultraviolet (EUV) Lithography XII, edited by Nelson M. Felix,

Anna Lio, Proc. of SPIE Vol. 11609, 116090Y · © 2021 SPIE
CCC code: 0277-786X/21/$21 · doi: 10.1117/12.2584519

Proc. of SPIE Vol. 11609 116090Y-1

 on an NXE:3300 EUV scanner at IMEC6. Although the concept of using CNTs for the EUV pellicle has demonstrated promise,
balancing the CNT material properties for optimal pellicle performance in EUV scanners remains an ongoing research focus7,8.
Depending on the density and morphology of the CNTs within the film and individual CNT parameters, like number of walls,
bundle size, purity etc., the optical and thermal properties of the CNT pellicle can be tuned8. One of the challenges for the EUV
pellicle is its stability in the EUV scanner environment which includes hydrogen plasma and high heat loads associated with
EUV exposure at high powers. The study of the CNT material modifications in a scanner-like environment is important to
understand the CNT pellicle lifetime.

Figure 1: (a) Schematic representation of an EUV mask with pellicle. Image courtesy of KLA Corp. (b) Photo of a full-size CNT EUV
pellicle. (c) SEM image of a random CNT network comprising the pellicle membrane.

2. TAILORING THE CNT PELLICLE PROPERTIES

2.1 CNT pellicle fabrication
A CNT EUV pellicle is made of CNTs randomly intertwined with each other to form a free-standing network structure, which
is supported by the pellicle border on the outer edges. A CNT is a hollow, seamless cylinder consisting of a rolled-up sheet of
single-layer carbon atoms (graphene). It may be composed of a single graphene layer forming a single-walled CNT (SWCNT),
two layers – double-walled CNT (DWCNT) or several graphene sheets rolled up into concentric cylinders – multi-walled CNT
(MWCNT). The diameter of individual SWCNTs can be varied from 0.7 to 2 nm, while MWCNTs are larger with the diameter
up to 100 nm depending on the number of walls. Figure 2 shows scanning electron micrograph (SEM) example images of
pellicles composed of SWCNTs, DWCNTs and MWCNTs used for the EUV pellicle in this work.

(a) (b) (c)

Figure 2: SEM images of CNT pellicle membrane structure made of (a) single-walled CNTs, (b) double-walled CNTs and (c) multi-walled
CNTs. Inset: TEM images of the corresponding CNT types.

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The fabrication of a transparent and uniform free-standing CNT film on a support border of a full pellicle size (~110 mm x 140
mm) is not trivial. In this work free-standing CNT films were produced via two approaches: dry transfer or harvesting from
solution. The dry transfer approach utilized a floating catalyst (aerosol) chemical vapor deposition (CVD) method to synthesize
CNTs that were then collected onto a filter in a random network and dry-transferred onto a border9. In the solvent approach,
CNTs were dispersed in liquid, such as water with an addition of surfactant, and deposited onto a filter via vacuum filtration.
The CNT film is floated off the filtration membrane, and then harvested from the solution onto a support border to create a
free-standing membrane10. In the past, aligned MWCNT films fabricated by pulling CNT sheets from vertically oriented CNT
arrays grown by fixed catalyst CVD method were also explored1, but this approach was terminated due to the high EUV
scattering and non-uniformity7. In this work we focus on random configuration of CNTs within the pellicle membrane.

2.2 EUV transmittance with respect to CNT type and density

All types of CNT pellicles, i.e. SW-, DW- and MWCNT-based, can achieve very high EUVT with acceptable uniformity,
while remaining mechanically strong. Examples of different CNT EUV pellicles with a corresponding EUVT above the 90%,
which is set as the minimum target, and EUVT non-uniformity below 0.4% are shown in Figure 3. The non-uniformity values
are calculated based on the EUVT maps using the scanner relevant area of 330 x 330 µm2, which originates from the EUV
light spot size on a pellicle that is held out of the focal plane during a scanner exposure.

Figure 3: EUV tranmittance (single pass) and EUV transmittance (EUVT) non-uniformity of CNT pellicles made of SWCNTs, DWCNTs
and MWCNTs with the corresponding SEM images of the CNT network. EUV non-uniformity is calculated based on the scanner relevant
area of 330 µm2.

The EUVT of a CNT pellicle can be tuned by varying the density or the amount of CNTs within the membrane. Depending on
the fabrication approach, the CNT areal density can be adjusted by either varying the CNT collection time during the floating
catalyst CVD synthesis or by modifying the amount of CNT material collected during vacuum filtration from the solution.
Spectrometry in the visible spectrum range is a commonly used and non-destructive method to characterize free-standing CNT
films. Absorbance of free-standing CNT films at 550 nm can be experimentally correlated to the CNT film thickness without

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breaking the free-standing CNT film for cross-sectional thickness analysis11. In Figure 4 we illustrate the linear dependence of
the CNT pellicle’s transmittance at 13.5 nm and 550 nm for different CNT types. The coefficient of this linear correlation
depends on the CNT pellicle composition (CNT type, size, purity) and film microstructure (packing density, bundling).
Knowing this relation for a specific CNT type enables tuning of the CNT density during the fabrication process for a specific
target EUVT.

Figure 4: (a) Correlation between the transmittance at 13.5 nm and at 550 nm for different CNT types. (b) SEM images demonstrating an
example of CNT density variation for a DWCNT pellicle.

2.3 CNT film composition

Besides the areal density of CNTs within the membrane, the composition of the CNT pellicle also plays an important role on
its optical properties. A CVD approach, often used for the CNT growth, typically requires a combination of a carbon-containing
reaction gas (e.g. carbon monoxide) with a metal catalyst (e.g. iron (Fe)) in order to form a CNT inside a high-temperature
reactor furnace. If the CNTs are not purified post-growth, then the CNT film will inevitably contain metal nanoparticles. As
shown in the transmission electron microscopy (TEM) image of SWCNTs in the inset of Figure 5, most of the metal catalyst
nanoparticles are embedded into the graphitic carbon shell and are less than 10 nm in diameter. Yet, also slightly larger inactive
catalyst particles as by-product of the synthesis can be present depending on the growth conditions. Figure 5 shows the
elemental composition of four different unpurified SWCNT membranes measured by the Rutherford backscattering
spectrometry (RBS)12. In the RBS characterization of free-standing CNT films the transmitted beam was guided away in the
forward direction not to contribute to the backscattering spectrum, and a multidetector assembly was used to improve the limit-
of-detection and to decrease the measurement time13. All SWCNT samples were prepared using the floating catalyst CVD
technique with different synthesis conditions and collection time. Based on the compositional analysis of these samples, the
corresponding thin film EUV absorbance was calculated using X-ray transmission database from the Center for X-Ray Optics
(CXRO) at Lawrence Berkeley National Laboratory14 (A13.5=-log10T13.5). This composition-based absorbance was compared to
the measured absorbance for the same samples (Fig. 5). We found a good correlation between the experimental and the
calculated absorbances, from which the contribution of Fe (originating from the metal catalyst) to the increase in the absorbance
of the pellicles is clear. Therefore, purification of CNTs is beneficial to increase the EUVT of the CNT pellicles.

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Figure 5: Absorbance calculated based on the measured elemental composition by RBS using CXRO database and measured EUVT for
different types of unpurified SWCNT pellicle samples.

2.4 Role of CNT bundling

Due to their high surface energy, individual CNTs tend to agglomerate during the fabrication process forming bundles that are
held together by van der Waals forces. Such CNT bundles can be tens of nanometers in diameter and tens of micrometers in
length. TEM images in Figure 6 give an example of a SWCNT bundle and average bundle size variation based on changing
the pellicle fabrication parameters.

Figure 6: TEM images demonstrating (a) SWCNT bundle in cross-section consisting of 10-12 individual CNTs, and (b) variation of average
SWCNT bundle size. Note that a SWCNT film is not purified therefore metal catalyst particles are present.

The bundling of CNTs is an important parameter, which can have a high impact on many membrane properties. An increase
in CNT bundling in the membrane results in an improvement in the mechanical strength of the pellicle as well as its chemical
durability in the etching environment. However, at the same time CNT bundling leads to an increase in the EUV light scattering
in the numerical aperture (NA) which creates flare, and increased gap size which influences the ability of the pellicle to stop
particles. Some of the CNT pellicle properties can be modeled with respect to the CNT bundling degree, as was described in
our earlier work7. Geometrical parameters like gap size can be modeled by considering the CNTs as straight lines. With a
given EUVT, the total CNT length within a specific area, e.g. 2 x 2 µm2, can be calculated based on the geometrical
considerations of how carbon atoms are stacked in the nanotube. This total length can be distributed in random orientation over
that area, providing a visual interpretation similar to SEM images (Fig. 7(a)). In case of bundling, the effective length will be
reduced by the (average) number of CNTs per bundle, which results is a different visual configuration and gap size distribution.

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Optical parameters like EUVT and flare into the NA, can also be modeled. In this model the CNT is simplified by an infinitely
long and straight cylinder with a given diameter and optical constant. In this representation, the absorption and scatter of an
incoming electromagnetic wave can be calculated analytically per unit cylinder length15,16. To include the impact of bundling,
the absorption and scatter of the bundle is simplified to that of one cylinder with a volume equivalent diameter. As an example,
in Figure 7 we compare the modelled properties of a CNT pellicle for constant CNT density but varying the bundle size from
16 CNTs/bundle over 36 CNTs/bundle to 64 CNTs/bundle. The results indicate that the CNT pellicle properties are influenced
by the CNT bundling within the membrane and can be tuned by defining the optimum bundling degree based on the
simulations. Experimentally, controlling of the bundling within a random CNT network is challenging but possible by tuning
the synthesis conditions17 and pellicle fabrication parameters.
(a)

(b)

Figure 7: (a) Simulation of CNT membrane microstructure with CNT bundling, where CNTs are shown as straight lines in random
orientation. (b) Modelling based variation of the CNT pellicle properties, i.e. flair into the numerical aperture (NA), maximum gap size of
the CNT network and specular EUVT with respect to the amount of CNTs per bundle.

2.5 Thermal properties

When the pellicle is heated during exposure caused by the absorption of the EUV radiation, there are limited paths to dissipate
that heat in the scanner conditions. Heating of thin film pellicle at high EUV powers can induce mechanical stress associated
with the thermal expansion of the material18 and facilitate chemical degradation. Radiation (emissivity) is the primary
mechanism to release heat since other pathways are almost negligible, therefore materials with higher emissivity will result in
lower maximum temperatures when exposed with the same power and are expected to withstand higher thermal loads in the
scanner19. According to Kirchoff’s law, the emissivity (ε) is equal to the absorbance at any wavelength. Moreover, since the
heat loss by radiation will be present at infrared (IR) wavelengths, a technique like FTIR (Fourier transform IR spectroscopy)
using a combination of a reflection measurement R(λ) and transmission measurement T(λ), can be applied to calculate the
emissivity where ε(λ) = 1 – R(λ) – T(λ)20. A first order approximation for the wavelength-independent emissivity (εo) is the
average of ε(λ) over the expected IR wavelength range, e.g. 1-10 µm. In thermal equilibrium, the absorbed power in the pellicle
will equal the power lost and an approximation of the pellicle temperature can be calculated. To first order, the convection can
be ignored, and the radiated power, Pradiation, is given by the Stefan-Boltzmann law: 𝑃radiation 𝜀𝜎𝑆 𝑇4max-𝑇4env , where σ is
the Stefan-Boltzmann constant, S is the exposed slit area of the pellicle, Tmax is the maximum temperature of the pellicle in
equilibrium, and Tenv is the environment temperature19. To improve the model, the conduction of heat with the gas environment
is also included. Moreover, Planck’s law for radiation of a black body at a certain temperature is used to create a wavelength-
weighted distribution of emitted radiation, ε(λ). Starting with To and this Planck weighted ε(λ), a second-order correction of

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pellicle emissivity and temperature is possible. This is iterated until convergence, to estimate emissivity and pellicle
temperature.
The relationship between the Planck weighted emissivity and the calculated pellicle temperature is plotted for three CNT
pellicle types in Figure 8. As emissivity increases, the temperature calculated for 600 W source power decreases. The absolute
value of the calculated temperature is dependent on a number of assumptions and measurement errors; however, the relative
temperature trends are expected to remain valid. It is visible that MWCNT pellicles tend to have higher emissivity as compared
to SWCNT pellicles, resulting in the lower expected pellicle temperature. It is worth noting that for a given pellicle type, lower
transmissions associated with denser CNT pellicles, will increase emissivity and reduce pellicle temperature under exposure.

Figure 8: Measured emissivity (columns) vs calculated pellicle temperature at 600 W source power (dots) for SWCNT, DWCNT and
MWCNT pellicles.

3 LIFETIME CONSIDERATIONS: UNCOATED CNT PELLICLE

The mechanical, thermal and chemical stability of the EUV pellicle together determine its lifetime. This section will review
each in turn.

3.1 Mechanical stability

An EUV pellicle must withstand the pressure changes during handling. In order to fulfill the EUV-transparency requirement,
thin film pellicle membranes (e.g. poly-Si, SiN) have a membrane thickness typically less than 30 nm21,18. With the decrease
of the membrane thickness, the maximum pressure load tolerated before rupture is reduced1. Such thickness-dependent
deflection is not that clear for a porous transparent CNT membrane1. In fact, even the concept of a single thickness is challenged
by the network of CNTs. The flow of gas through small gaps in the CNT mesh minimizes the deflection of the CNT pellicle
due to the reduction of the pressure difference across the membrane. This desirable characteristic improves its mechanical
stability1. Recently performed CNT EUV pellicle exposure tests at the NXE EUV scanner demonstrated the ability of the CNT
EUV pellicles to withstand scanner handling and pump/vent cycles6.

3.2 Thermal and chemical stability

Modern lithography tools operate in an environment where a low-pressure hydrogen gas is introduced22,23,24. The hydrogen-
containing gas is ionized by the absorption of high energy EUV photons (92 eV) and plasma is created as a by-product, which

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consists of energetic photoelectrons, ions and radicals25,24. EUV-induced plasma in hydrogen was shown to effectively clean
carbon contamination26. Investigating the influence of hydrogen-based plasma and high energy EUV photons on the transparent
CNT membranes offline (outside of the scanner) is challenging. The main challenge is mimicking the scanner environment for
the pellicle lifetime studies. One approach is to isolate non-plasma related processes, i.e. pellicle heating caused by the
absorption of the EUV radiation, and the chemical stability of CNTs in hydrogen plasma environment.
In vacuum, a 97% EUV transparent CNT membrane was shown to withstand high thermal loads up to >1000 W EUV power
equivalent exposures (>20 W/cm2)27.
To investigate the chemical stability, the CNT material on a silicon substrate was exposed to hydrogen radicals (H*) at room
temperature. H* were generated by the catalytic decomposition of H2 gas (15 Pa) using a tungsten hot-wire catalyzer. The
exposure time was varied and Raman spectroscopic measurements of the sample after each exposure step were performed (Fig.
9). Raman spectroscopy is an effective technique to probe structural modifications of the CNT sidewalls and crystalline quality,
allowing to detect small changes in the CNT crystal structure28. Raman spectra were collected using a Horiba Jobin-Yvon
LabRAM HR800 Raman spectrometer with 2.33 eV laser excitation energy through a 10X objective. Raman spectra of CNTs
typically consist of the G-band, related to the vibration of sp2-bonded carbon atoms in a two-dimensional hexagonal lattice,
and the D-band, related to defects on the nanotube walls and non-CNT impurities29. The ratio between the two bands (ID/IG) is
a sensitive measure of the CNT structural quality (or defect density) and the relative amount of amorphous carbon.

Figure 9: (a) Raman spectra before and after exposing SWCNTs to hydrogen radicals (H*) at room temperature. Inset: ID/IG ratio variation
as a function of the exposure time. (b) TEM images of SWCNTs before and after exposure to H* at room temperature.

Here we focus on the analysis of SWCNTs but note that a similar effect was observed for MWCNTs. The as-grown SWCNT
material possesses good initial structural quality (ID/IG~0.02). When exposing CNTs to H* at room temperature, the G peak
intensity decreases and the D peak intensity increases, and the most pronounced change occurs after the 1st hour of exposure
(Fig. 9). The ID/IG ratio increase reflects the introduction of defects and etching of the CNTs. The damage and etching of the
CNTs can be recognized in the TEM images of the exposed samples (Fig. 9b) and is in agreement with literature 30.
The relevance of these results for understanding the CNT pellicle lifetime in the scanner environment depends on the chosen
experimental conditions. In the scanner, the CNT pellicle is exposed to an EUV-induced hydrogen-based plasma and at the
same time is heated due to the absorption of EUV radiation. In order to probe the synergistic effect of EUV radiation and
plasma on the CNT membrane, uncoated CNT pellicles were dynamically exposed to EUV light at 600 W EUV scanner source
power equivalent in the scanner representative gas environment. These exposures were performed at the beamline operated by
the Physikalisch-Technische Bundesanstalt (PTB) at the BESSY II synchrotron facility 31. To emulate the EUV source powers,
apertures were used to select a specific part of the beamlines’ spot and the distance from the focus was adjusted31.

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3.3 EUV scanner-like environment at 600 W equivalent source power
Non-scanner (offline) exposures of CNT pellicles enable the study of CNT material changes under concurrent exposure to
EUV radiation and EUV-induced plasma in the scanner-like gas environment although it is important to note that the exposure
conditions remain an approximation of the scanner conditions and gases.

3.3.1 SWCNT pellicle case: short exposure

In this experiment, we exposed an uncoated SWCNT pellicle to 600 W EUV scanner source power equivalent (EUV power
density ~12 W/cm2) in a hydrogen-based gas environment and varied the exposure time. For these exposures, test pellicle
samples with the free-standing CNT membrane of 10x10 mm2 were used. In order to ensure that the pellicle remains
mechanically strong for post-exposure analysis, we started with exposing the sample to EUV beam for 15 min. Changes in
EUVT before and after the exposure were monitored (Fig. 10). The EUV beam was scanned across the middle part of the
sample, the so-called IN-beam region. Since plasma, formed by the ionization of the background gas, is expected to be larger
in dimensions than the EUV beam itself24, the OUT-beam region of the sample is also analyzed. The CNT material changes of
IN-beam and OUT-beam regions were compared. Figure 10 shows EUVT mapping for SWCNTs with two different CNT
densities, i.e. SWCNT_d1 with ~97% EUVT and SWCNT_d2 with 95.5% EUVT (d1<d2), before and after their exposure. A
small EUVT increase is observed for the IN-beam region for both sample types, with a slightly higher increase for the higher
density sample (SWCNT_d1) than the lower density one (SWCNT_d2). No change (for SWCNT_d1) or a slight decrease (for
SWCNT_d2) in EUVT is observed in the OUT-beam region.

Figure 10: (a) EUV transmittance maps for SWCNT pellicles with two different densities (d1<d2) before and after exposure to the EUV
scanner-like environment at 600 W equivalent source power for 15 min. EUV beam was scanned across the middle region of the sample (IN-
beam). (b) Corresponding EUV transmittance before and after the exposure of SWCNT with two different densities (d1<d2). IN-beam and
OUT-beam regions after exposure are analyzed separately. Inset: photo of the EUV beam scanning the CNT pellicle sample as shown with
the arrow.

Furthermore, various material analysis techniques were applied to study the CNT material changes with exposure.
Interestingly, Raman spectroscopic measurements of the free-standing CNT membranes after 15 min exposure reveal a
noticeable effect on the CNT material. The entire CNT membrane was raster-scanned, creating a full-map Raman spectrum
with data collected at every 200 µm step. The maps of the ID/IG ratio in Figure 11 for two SWCNT pellicles with different
EUVT show a clear signature of the CNT pellicle exposure pattern, which correlates strongly with the corresponding EUVT
mapping results of Figure 10. An increase in the ID/IG ratio (or defect density) is observed in the OUT-beam region for both
samples.

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 (a) (b)

Figure 11: Raman mapping of ID/IG ratio for samples with (a) 97% EUVT (SWCNT_d1) and (b) 95.5% EUVT (SWCNT_d2), after their
exposure to the EUV scanner-like environment at 600 W equivalent source power for 15 min. EUVT maps of the corresponding samples are
shown in Figure 10.

Further Raman spectroscopy at different excitation energies was performed and the results were compared with the reference
(unexposed) sample (Fig. 12). In addition to the ID/IG ratio, the ratio of the G´-band to D-band intensities (IG´/ID) was used as
an indication of sample purity, increase of which points to the reduction of contaminants32. G´-band spectra arise from a two-
phonon, second-order Raman scattering process and is not sensitive to tube defects28,32. A decrease in ID/IG ratio, complemented
by the increase in IG´/ID, in the IN-beam sample region, where CNT pellicle is expected to heat up with the absorption of the
EUV radiation, as compared to the reference suggest sample purification, cleaning and possibly defect healing or defect
etching. SWCNT purification via removal of amorphous carbon and defect healing using laser exposures at different
wavelength and environments was described in the literature33,34,32. In these experiments the use of an optimum irradiance
power density before the onset of the CNT damage and the presence of an oxidizing environment were critical35,34,32. In the
OUT-beam region of the sample an increase in the ID/IG ratio and concurrent decrease in IG´/ID, reveal CNT damage and
contamination. To have a better view of the CNT membranes changes, Raman analysis was supplemented with the chemical
composition analysis of the exposed pellicles using Elastic Backscattering Spectroscopy (EBS); X-ray photoelectron
spectroscopy (XPS) and Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS).

(a)

(b)

Figure 12: (a) Examples of Raman spectura for SWCNT_d1 (97% EUVT) measured with different laser energies (i.e. 1.96 eV, 2.33 eV and
3.06 eV) after exposure to the EUV scanner-like environment at 600 W equivalent source power for 15 min, compared with a reference CNT
sample of the same density. (b) Corresponding ID/IG and IG´/ID ratio analysis. An error bar for the reference sample takes into account possible
variations between similar samples.

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EBS measurements, using proton beam as probe projectiles at 2 MeV, was applied to quantify the elemental composition of
the SWCNT pellicle (SWCNT_d2) in the two regions. This approach enables accurate quantification of such light elements as
carbon36, dominant element in CNT films. Figure 13 (a) shows EBS results for carbon in thin film units (TFU or 1E15 at/cm2)
displayed on top of the Raman ID/IG map of the corresponding sample. A higher carbon amount OUT-beam as compared to
the IN-beam was measured.

Figure 13: (a) Raman mapping of ID/IG ratio and corresponding initial carbon (C) amount in thin film units (TFU) measured by EBS for a
SWCNT pellicle exposed to the EUV scanner-like environment at 600 W equivalent source power for 15 min. The statistical error on C
measurements was 1.1%. (b) XPS spectra of IN-beam and OUT-beam regions of the SWCNT pellicle sample for the different energy regions,
i.e. C1s, Fe2p and O1s. The XPS spectra were normalised in order to achieve an easier comparison. Relative amounts can be retrieved from
Table 1.
XPS measurements were performed to investigate the surface chemistry of the SWCNT pellicle IN-beam vs OUT-beam after
exposure (Fig. 13(b)). XPS measurements were carried out in Angle Integrated mode using a Theta300 system from Thermo
Instruments. A monochromatized Al Kα X-ray source (1486.6 eV) and a spot size of 100 µm was used. The XPS survey
spectra reveals the presence of C, O and Fe, in line with the EBS analysis. A small amount of surface contaminants like F, N
and Si was also detected by XPS. Analysis of the C1s peak of the XPS spectrum distinguishes sp2-hybridized graphite-like
carbon of the CNT walls, sp3-hybridized diamond-like carbon associated with defect sites, and carbon atoms with C=O and C-
O groups (Table 1). To achieve a repeatable fitting procedure, the main C1s peak was set at 284.8 eV and fixed binding energy
differences relative to the main peak were set for the other components of the spectra (0.9 eV for sp3, 2.1 eV for C-O and 4.7
eV for C=O). It should be noted that the identification of the peak at ~285.7 eV as sp3 is not unique and some oxygen bounded
carbon could also contribute to the intensities at that energy. The intensity of the components attributed to C-O groups on the
C1s spectra were higher in the OUT-beam vs IN-beam regions. This was also accompanied by an increase of the measured
atomic percentage of O in the OUT-beam region (Table 1). Additionally, a lower intensity of unoxidized Fe contribution on
the Fe2p spectra (Fe-Fe at ~707 eV) OUT-beam compared to IN-beam was observed. This change in the chemical state of Fe
could be explained by damage of the carbon coating surrounding the catalyst nanoparticles OUT-beam, likely to be etched
sooner than the nanotube wall under the exposure to hydrogen-based plasma37. Once exposed to air, the nanoparticles OUT-
beam are further oxidized. The higher intensity of metallic Fe in the IN-beam region could be the result of iron oxide reduction
and carbon shell repair around the particles. No clear change in the amount of Fe IN-beam vs OUT-beam could be measured
by EBS, nor observed in SEM.

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Table 1: Relative atomic concentrations (at%) as calculated using the fits on the C1s, O1s, Fe2p, N1s, F1s and Si2p XPS curves for SWCNT
pellicle exposed to the EUV scanner-like environment at 600 W equivalent source power for 15 min.
C sp2 C sp3 C-O C=O O Fe N F Si
IN-beam 65.1 15.3 5.9 7.3 4.0 1.6 0.2 0.0 0.7
OUT-beam 61.0 15.4 7.9 6.3 6.6 1.4 0.2 0.1 1.1

Moreover, a ToF-SIMS analysis was performed with a ToF-SIMS V system (ION-TOF GmbH, Münster, Germany) using a
30kV Bi1+ beam as a primary ion source. Another SWCNT pellicle sample exposed under the same conditions was analyzed.
ToF-SIMs results indicate a lower intensity of C, O and surface contaminants in the IN-beam compared to OUT-beam regions
of SWCNT samples (Fig. 14). SEM observations did not reveal significant changes IN-beam vs OUT-beam, although for
certain SWCNT pellicle types several broken CNT bundles OUT-beam were observed. CNT etching IN-beam does not seem
to occur for short exposure times.

Figure 14: Carbon (C) and oxygen (O) ion mapping obtained by ToF-SIMS for SWCNT pellicle with 96% EUVT after exposure to the EUV
scanner-like environment at 600 W equivalent source power for 15 min.

In summary, transmission, spectroscopic and chemical mapping of the SWCNT pellicle exposed for 15 min to 600 W
equivalent EUV scanner source power in the scanner-like gas environment demonstrate an initial CNT purification, likely
removal of non-nanotube impurities IN-beam causing a minor EUVT increase. Hydrogen etching is expected to start on carbon-
based contamination, which can include amorphous carbon, fullerene-like particles, carbon atoms on the defective SWCNTs
and pentagonal carbon at the end of the CNTs or around catalyst particles37. Under EUV radiation in the presence of H2 gas in
EUV lithography, carbon contamination is known to be removed via physical and chemical sputtering by ionized species22.
The onset of CNT damage is clearly observed OUT-beam together with an increase of carbon and oxygen content as compared
to the IN-beam, suggesting a possible cause of the measured slight EUVT decrease OUT-beam. This contamination OUT-
beam could be a result of in-situ re-deposition due to the formation of volatile hydrides and redeposit of the reaction products
on other surfaces24, or ex-situ with the exposure to air where oxygen and other species could be adsorbed at the newly-generated
defect sites. It is important to note that EUVT and other measurements of the SWCNT pellicles after EUV radiation were
performed after the sample was exposed to air, therefore in-situ measurements would be important to better understand the
effect of EUV+H2-based gas environment on the CNT pellicles.
Overall short 15 min exposure of the uncoated SWCNT pellicle to the representative pellicle environmental conditions at 600
W source power equivalent yielded only minor structural changes to the CNT pellicle membrane. The CNT pellicles remained
mechanically strong and were able to withstand multiple cases of pressure changes when loading the samples to various
material analysis tools as well as pellicle shipments.

3.3.2 SWCNT pellicle case: longer exposures

Keeping the EUV exposure conditions the same, i.e. 600 W EUV scanner source power equivalent in the scanner-like gas
atmosphere, we increased the time that the EUV beam is scanning across the middle area of the uncoated SWCNT pellicle

Proc. of SPIE Vol. 11609 116090Y-12

sample. In a similar manner as shown above, the EUVT of the pellicle was mapped before and after the exposure, where in the
latter case two regions were analyzed, IN-beam and OUT-beam. Note that with this exposure the beam size was a few
millimeters wider as compared to the shorter exposures. The mapping results are shown in Figure 15.

Figure 15: (a) EUV transmittance maps for SWCNT pellicle before and after exposure to the EUV scanner-like environment at 600 W
equivalent source power for 30 min. EUV beam was scanned across the middle region of the sample (IN-beam). (b) Corresponding EUV
transmittance before and after the exposure. IN-beam and OUT-beam regions after the exposure were analyzed separately and compared to
a similar SWCNT pellicle sample exposed to the same conditions for 15 min.

Even though the starting EUVT of this SWCNT pellicle sample was slightly lower, i.e. 93.5%, a similar trend of EUVT increase
IN-beam and no significant change of EUVT OUT-beam was observed after 30 min exposure. It is clear that with the increased
exposure time, a larger EUVT drift IN-beam was observed, i.e. 1.4% EUVT increase (Fig. 15). Further, Raman and XPS
mapping of the exposed sample were performed. Figure 16(a) shows the ID/IG ratio map of the sample where a similar trend of
higher ID/IG ratio (or defect density) was observed in the OUT-beam region as compared to the IN-beam region. At the same
time, the IN-beam region showed an increase in ID/IG ratio and a decrease of the IG´/ID ratio (or purity index) as compared to
the reference sample. The results of the Raman quality and purity indicators together with an overall decrease in the Raman
signal intensity suggest that the uncoated SWCNT pellicle experienced significant damage and CNT removal under the
influence of EUV+scanner gases after 30 min. The radial breathing mode (RBM) of the Raman spectra, which corresponds to
the coherent vibrations of carbon atoms in the radial direction, enables an estimate of the CNT diameter, dt (ωRBM=234/dt)28,32.
A slight shift towards larger average diameter SWCNTs after the exposure was seen as compared to the reference sample
(Fig.16(d)). These findings indicate that larger diameter CNTs are more stable when exposed to plasma which is in line with
literature observations30,38. Higher hydrogenation reactivity of smaller diameter CNTs is explained by their higher curvature
and strain in the structure30.

Proc. of SPIE Vol. 11609 116090Y-13

Figure 16: (a) Raman mapping of the ID/IG ratio for a SWCNT pellicle with 93.5% EUVT after exposure to the EUV scanner-like environment
at 600 W equivalent source power for 30 min. (b) Analysis results of Raman quality (ID/IG) and purity (IG´/ID) indicators for a reference and
exposed samples IN-beam and OUT-beam. (c) Example Raman spectrum for a reference and exposed sample IN-beam and OUT-beam. (d)
Distribution of SWCNT diameter for a reference and exposed samples IN-beam and OUT-beam based on the RBM mode analysis of the
Raman spectrum. 2.33 eV laser energy was used for all the measurements.

Figure 17 presents the comparison of the C1s, O1s and Fe2p XPS spectra for IN-beam and OUT-beam regions of the sample
corresponding to the Raman mapping shown in Figure 16(a). Similar observations as for the shorter exposure time were made:
a larger relative C-O component on the C1s spectra and more oxygen detected OUT-beam (Table 2). It is interesting that almost
no metallic Fe remained present in the OUT-beam region of the SWCNT sample with prolonged plasma exposure indicating
that all carbon coatings enclosing metallic particles were damaged or removed resulting in air oxidation of the catalyst particles.
It is possible that in the IN-beam region some of the Fe particles were removed after etching of the surrounding carbon shell
which could also explain the higher intensity of metallic Fe as measured by XPS and contribute to an EUVT increase IN-beam.
However, due to the pellicle breakage EBS or SEM measurements could not be performed to check this hypothesis.

Proc. of SPIE Vol. 11609 116090Y-14

Figure 17: Comparison of the C1s, Fe2p and O1s XPS spectra IN- and OUT-beam for the SWCNT pellicle sample exposed to the EUV
scanner-like environment at 600 W equivalent source power for 30 min. Spectra were normalized in order to allow a better chemical
comparison. Relative amounts can be retrieved from Table 2.

Table 2: Relative atomic concentrations (at%) as calculated using the fits on the C1s, O1s, Fe2p, N1s, F1s and Si2p XPS curves for SWCNT
pellicle exposed to the EUV scanner-like environment at 600 W equivalent source power for 30 min.
C sp2 C sp3 C-O C=O O Fe N F Si
IN-beam 55.5 12.3 13.0 6.8 8.6 1.9 0.4 0.3 1.2
OUT-beam 51.7 11.8 16.1 4.5 12.6 1.4 0.4 0.4 1.2
This SWCNT material was further examined in TEM. Figure 18 shows a comparison of the unexposed reference sample with
the exposed SWCNT material likely originating from the OUT-beam region of the sample. After prolonged exposure, uncoated
SWCNTs show deformation and amorphization, contamination, local collapse of the CNTs with the disappearance of the
central hollow. fragmentation and removal of the CNTs. Individual CNTs and smaller bundles are damaged and etched first.

Figure 18: TEM images comparing unexposed reference SWCNT sample with the one exposed to the EUV scanner-like environment at 600
W equivalent source power for 30 min.

Proc. of SPIE Vol. 11609 116090Y-15

In summary, 30 min exposure of the uncoated SWCNT pellicle sample to the EUV scanner representative pellicle
environmental conditions at 600 W equivalent source power yielded major structural changes to the CNT pellicle membrane.
The CNT pellicles survived the exposure, sample shipment and a number of measurements; however due to the material
modifications with exposure the pellicle became more fragile and broke with handling. Transmission, spectroscopic and
chemical mapping revealed significant CNT damage, deformation and etching.

3.3.3 MWCNT pellicle case: longer exposure

MWCNT pellicles were also exposed in a similar manner as SWCNT pellicles to the EUV scanner-like environment at 600 W
equivalent source power for 30 min. EUVT before and after the exposure was mapped (Fig. 19(a)). The observed trend of
EUVT increase IN-beam and slight EUVT decrease OUT-beam for MWCNT is similar to the trend reported above for SWCNT
pellicle samples. However, it is interesting to note that for the same exposure time under the same exposure conditions a smaller
EUVT drift IN-beam for MWCNT pellicles was observed, i.e. 0.7% EUVT increase for a MWCNT pellicle vs 1.4% increase
for a SWCNT pellicle (Fig. 19(b)).

Figure 19: (a) EUV transmittance maps for MWCNT pellicle before and after exposure to the EUV scanner-like environment at 600 W
equivalent source power for 30 min. EUV beam was scanned across the middle region of the sample (IN-beam). (b) Corresponding EUV
transmittance before and after the exposure. IN-beam and OUT-beam regions after exposure were analyzed separately and compared to a
SWCNT pellicle sample exposed to the same conditions for the same time (30 min).

Raman spectroscopy was performed to study the MWCNT pellicle material modifications after exposure. The starting ID/IG
ratio of MWCNTs was higher compared to SWCNTs, i.e. 0.7 for MWCNTs vs 0.02-0.05 for SWCNTs, suggesting more
disorder in the MWCNT structure. The MWCNT quality is significantly decreased (ID/IG increases) after 30 min exposure in
both IN-beam and OUT-beam regions (Fig. 20(a)). XPS measurements also show a noticeable increase in the O level and C-
O bonds in the OUT-beam region compared to the IN-beam region (see Fig. 20(b) and Table 3). The MWCNT sample was
purified to remove catalyst particles and consequently did not contain a measurable amount of Fe.

Proc. of SPIE Vol. 11609 116090Y-16

Figure 20: (a) Example Raman spectra for a reference MWCNT pellicle and the one exposed to the EUV scanner-like environment at 600
W equivalent source power for 30 min, IN-beam and OUT-beam. 2.33 eV laser energy was used for the measurements. (b) Comparison of
the C1s and O1s spectra IN-beam and OUT-beam. Spectra were normalized in order to allow a better chemical comparison.

Table 3: Relative atomic concentrations (at%) as calculated using the fits on the C1s, O1s, Fe2p, N1s, F1s and Si2p XPS curves for
MWCNT pellicle exposed at 600W EUV power equivalent for 30 min in the scanner-like gas environment.
C sp2 C sp3 C-O C=O O Fe N F Si
IN-beam 56.6 14.7 12.1 8.3 7.1 0.1 0.4 0.2 0.5
OUT-beam 51.4 12.5 16.9 6.0 11.8 0.0 0.7 0.2 0.5

TEM analysis of the as-exposed MWCNT pellicle sample as compared to the unexposed reference shows significant damage
to the MWCNT structure including wall deformations and formation of small ripples. However, the MWCNT cylindrical
structure is preserved and few removed or broken MWCNTs are observed (see Fig. 21). This indicates that MWCNTs possess
a comparatively higher structural stability and radiation tolerance than SWCNTs. Similar effects were reported in the literature
for electron beam-induced surface modifications of SWCNT and MWCNTs, where a higher stability of MWCNTs towards
irradiation was associated with the presence of the inner tubes that prevent the outer tubes from collapsing 39,40,41.

Proc. of SPIE Vol. 11609 116090Y-17

Figure 21: TEM images comparing unexposed reference MWCNT sample with the one exposed to the EUV scanner-like environment at
600 W equivalent source power for 30 min.

In summary, despite a significant structural damage of the uncoated MWCNT pellicle sample revealed by Raman and TEM
after 30 min exposure at 600 W equivalent source power in the EUV scanner-like gas environment, a lower EUVT drift was
measured as compared to a SWCNT pellicle under the same exposure conditions explained by less CNT material removal as
observed in TEM.

4 COATING CNT PELLICLE

Coating the CNT pellicle is a potential path to reduce the CNT structural damage by hydrogen-containing plasma and to extend
the pellicle lifetime in the EUV scanner environment. Our preliminary results showed the ability of the coating to preserve the
CNT structure when exposing a coated SWCNT material (~90% EUVT) to H* (15 Pa) at room temperature for 65 hrs. As
revealed by TEM observations of the same area (before and after the exposure), coated CNT material remained intact after a
65 hrs exposure to H* (Fig. 22(a) and (b)), while an uncoated SWCNT reference was significantly damaged and largely
removed already after 56 hrs of the same exposure as discussed in section 3.2. Chemical composition of the two regions of
coated CNTs before and after exposure was evaluated by Energy-Dispersive X-ray Spectroscopy (EDS). EDS done at the
thicker bundle showed higher carbon (C) percentage than at the thinner ones, but in both regions no measurable change in the
chemical composition of the coated CNTs due to H* exposure was observed (Fig. 22(c)). Coated CNT pellicles were also
exposed at PTB to the scanner-like gas environmental at 300 W (EUV power density ~6 W/cm2) EUV source power equivalent
for 30 min. Figure 22(d) shows ID/IG ratio analysis from Raman mapping for the coated SWCNT pellicle before and after its
exposure. Coating preserved the structural quality of a SWCNT pellicles relative to the uncoated case. This demonstration is
important, and further exploration and optimization of possible coating options to fulfil the EUV pellicle requirements for the
scanner environment are the subject of our ongoing research.

Proc. of SPIE Vol. 11609 116090Y-18

Figure 22: (a) TEM and (b) scanning TEM images of coated SWCNTs and bundles before (left) and after (right) exposure to H* (15 Pa) for
65 hrs at room temperature. (c) Chemical composition locally measured by EDS before and after H* exposure for two regions as marked in
(b). (d) Raman ID/IG ratio analysis of coated SWCNT pellice before and after exposure to the scanner-like gas environment at 300W for 30
min IN-beam and OUT-beam.

5 CONCLUSIONS
CNTs are promising building blocks for a highly configurable EUV pellicle. Different CNT types were tested for the pellicle
application, i.e. SWCNTs, DWCNTs and MWCNTs. All CNT types allow to achieve high EUVT with acceptable uniformity.
CNT material type, density, bundle size, and composition (purity) are parameters examined to tailor the CNT EUV pellicle
properties for optimal performance.
Uncoated CNT pellicles survive 600 W equivalent source power (~12 W/cm2 EUV power density) exposure in the EUV
scanner-like gas environment. However, material analysis after exposure reveals CNT structural modifications leading to the
degradation of the mechanical strength of the pellicle with prolonged exposures. The degree of these changes is modulated by
various CNT material parameters including initial structural quality, density, diameter, bundling, purity, and surface
contamination. MWCNT pellicles have lower EUVT drift and maintain their cylindrical structure better than SWCNT pellicles
under the same exposure conditions. However, improvements are still needed in terms of the mechanical strength and structural
quality of current MWCNT pellicles.
The CNT pellicle performance in a scanner depends not only the CNT type and material parameters, but largely on the exposure
conditions and gas environment. The influence of EUV radiation and hydrogen-based gas environment on the uncoated CNT
pellicle observed in this study are a combination of chemical effects governed by the reactive species produced in the plasma,
and photothermal processes involving the CNT pellicle heating with the absorption of the EUV radiation. More in-depth
investigation of the mechanisms involved in CNT material modification is necessary.
In addition to CNT membrane optimization, coating of the CNT pellicle is a potential route towards CNT pellicle lifetime
extension in the scanner environment. The protection of CNT material from structural degradation by means of coating was
shown. Further optimization of both the CNT pellicle material and the coatings are in progress to fulfill the EUV pellicle
requirements for high volume manufacturing with EUV lithography.

Proc. of SPIE Vol. 11609 116090Y-19

 ACKNOWLEDGEMENTS

We would like to thank CNT suppliers: Lintec NSTC, Canatu Oy, and Institute of Metal Research (IMR). We appreciate the
cooperation and assistance of the ASML pellicle research team. The authors acknowledge Frank Scholze and Christian Laubis
from PTB for useful discussions and Wim Arnold Bik from Detect99 for EBS measurements.

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