experimental evaluation of the impact of euv pellicles on ......transmission at euv, low...
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Experimental evaluation of the impact of EUV pellicles on reticle imaging Iacopo Mochi
a, Marina Timmermans
b, Emily Gallagher
b, Marina Mariano
b, Ivan Pollentier
b,
Rajendran Rajeeva, Patrick Helfenstein
a, Sara Fernandez
a, Dimitrios Kazazis
a and Yasin Ekinci
a
aPaul Scherrer Institut, 5232 Villigen, Switzerland
bIMEC, Kapeldreef 75, 3001 Leuven, Belgium
ABSTRACT
The purpose of EUV pellicles is to protect the surface of EUV lithography masks from particle contamination. Currently
several pellicle prototypes are being developed. It is important to ensure that the optical characteristics of the pellicle
membrane do not critically affect the reticle image quality. We present here a study of the impact of a few selected
EUV pellicle prototypes on the quality and the contrast of the reticle image obtained with an actinic lensless
microscope.
Keywords: EUV, pellicle, actinic pattern inspection, defect inspection, lensless microscopy, coherent diffraction
imaging, carbon nanotubes.
1. INTRODUCTION
Extreme ultraviolet (EUV) masks will eventually be equipped with protective pellicles. The pellicle, installed at a
distance of about 2.5 mm from the mask surface, will hold fall-on particles out of the imaging plane, thereby
minimizing their impact on wafer yield. Although non-pellicle lithography is being considered, this is a risky and non-
ideal solution. The challenge is to find a membrane with suitable optical and mechanical characteristics: high
transmission at EUV, low reflectivity, low scattering, low thermal expansion coefficient, and high mechanical stability.
The pellicle characteristics have an impact on the imaging performance of a scanner or of an inspection system, and for
this reason, an accurate evaluation of their optical properties must be performed.
We tested different pellicle samples on RESCAN (Reflective-mode EUV mask SCANning microscope) to verify their
impact on the aerial image formation. RESCAN is an actinic pattern inspection platform currently under development at
Paul Scherrer Institute (PSI). [1-2] It consists of a lensless scanning microscope based on coherent diffraction imaging
(CDI). In RESCAN, the sample is illuminated with a coherent EUV beam and the reflected diffraction beam is recorded
by a pixel detector. Multiple diffraction patterns are combined together to reconstruct the sample image and phase,
using ptychography. [3] Since RESCAN does not have any imaging optics close to the reticle, there is no physical
obstacle to mount a pellicle on top of the sample and this makes it a flexible and effective tool for prototyping and
investigating the performance of new membranes and materials.
In this work, we present the results of the image reconstruction of an EUV reticle test sample with different carbon
nanotubes (CNT) pellicles with and without coating and we compare the images to a reference obtained with the bare
sample. We also present an estimate of the scattering introduced by different types of EUV-transparent membranes
and its effect on the image formation. Finally, we demonstrate how RESCAN can be used not only as an actinic mask
inspection platform, but also as an efficient tool to inspect the pellicle surface for defects.
2. MEASUREMENT SETUP
RESCAN
The measurements were carried out using RESCAN, a lensless microscope dedicated to EUV mask defect inspection
operating at the XIL-II beamline of the Swiss Light Source. RESCAN’s working principle consists in collecting images of
the sample and comparing them to the image of a reference die or to an aerial image calculated from the reticle’s
design to identify the presence of defects. [1] The images of the sample are obtained through CDI which allows for
retrieval of both the phase and the magnitude of the reticle’s surface. The CDI approach used in RESCAN is based on
Photomask Technology 2018, edited by Emily E. Gallagher, Jed H. Rankin, Proc. of SPIE Vol. 10810, 108100Y · © 2018 SPIE · CCC code: 0277-786X/18/$18 · doi: 10.1117/12.2502480
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Incoming beam
Support Pellicle
EUV reticle sample
utgoing beam
cattered light
ptychography and consists of scanning the sample with an EUV illumination probe, collecting the relative diffraction
patterns and propagate them back to the sample plane with an iterative procedure to retrieve the phase and
magnitude of the reticle. The sample scan is performed moving the sample along x and y axes with a piezo-motor with
about 5 nm accuracy. RESCAN is a research prototype and it is currently located in a small vacuum chamber that limits
the size of the samples to 20×20 mm2 and is equipped with a stage with a travel range of 200 μm.
The optical layout of RESCAN, shown in Figure 1A, consists of an aperture, a focusing toroidal mirror and a folding
mirror. The XIL-II beamline is equipped with a monochromator that allows RESCAN to operate with a narrow
bandwidth Δ⁄ = 1500, which is an essential requirement for coherent diffraction imaging systems [4]. The beam is
focused by the toroidal mirror and folded onto the reticle sample with a variable numerical aperture (NA) ranging from
0.002 to 0.015. The beam footprint on the sample can be tuned by changing the position of the folding mirror or the
focusing mirror. The beam, reflected and diffracted by the sample, is collected by a 2048×2048-pixel detector at a
distance d of 62 mm. The maximum resolution of RESCAN depends on the NA of the detector defined as: NA = sin(arctan / ), Where s is the detector half size. The detector used for the measurements presented here has a pixel size of 13.5 μm
and its half size s is 13.8 mm. The image resolution is therefore: ≈ 2NA = 35nm. RESCAN is equipped with the adjustable pellicle mount sketched in Figure 1B. It consists of a 35×35 mm
2 support that
can be installed as close as 0.5 mm from the sample surface and can hold a pellicle sample mounted on a 30×30 mm2
wafer.
Figure 1. A. RESCAN optical layout. The EUV beam from the synchrotron is focused and folded onto the sample plane by a
toroidal mirror (M1) and by a plane mirror (M2). Both mirrors are coated with a multilayer to ensure maximum EUV
reflectivity at their respective incidence angle. The reflected and diffracted beam is collected by a CCD camera at a
distance of 62 mm from the sample. B. RESCAN pellicle mount detail.
M1M2
Detector
EUV beam
EUV reticle sample EUV reticle sample
Support Pellicle
A B
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Figure 3. A. Diffraction pattern of a reticle sample covered by a MW-CNT pellicle with support mesh. The picture is cropped
to 1800×1800 pixels and displayed on a logarithmic intensity scale. The diffraction pattern corresponds to a Manhattan-
geometry reticle with a 45°orientation with respect to the CCD axes. The black square delimits a region of the diffraction
pattern where two crossing strands of the support mesh cast a visible shadow. B. Detail of the diffraction pattern
delimited by the black square on A showing the shadow of the two crossing strands. The image is displayed on a
compressed logarithmic scale.
Pellicle scattering
In the absence of the pellicle, the diffraction patterns recorded by RESCAN provide an immediate measure of the
scattering irradiance coming from the sample surface and from the condenser optics. The pellicle generates two
additional diffraction contributions from the interaction with the incoming and outgoing beam. In addition, large
defects on the pellicle surface can cast a shadow on the diffraction pattern generated by the sample. In RESCAN it is
relatively straightforward to evaluate these contributions by comparing the diffraction patterns recorded through
pellicle to a reference obtained from the bare sample. We inspected a region free of absorber patterns on the EUV
mask sample to avoid the strong diffraction signature from the reticle pattern and simplify the data interpretation. The
images in the top row of Figure 4 show the recorded diffraction patterns in log scale. The irradiance intensity is
normalized to the maximum irradiance of the no-pellicle case (A). The images in the bottom row of Figure 4 show the
irradiance intensity of the reflected beam that corresponds to the condenser pupil projection on the detector plane.
From a visual observation of the diffraction patterns, there seems to be little or no difference between the random SW-
CNT pellicle (B) and the reference (A) indicating that the main effect of this membrane is the beam attenuation, visible
from the comparison between the average intensities of the reflected beam. The aligned MW-CNT pellicle (C, D top
images in Figure 4) shows instead a visible diffraction contribution that corresponds to the dominant scattering
direction of the aligned CNTs. Although the individual CNTs are not visible in the diffraction patterns, in the pupil
projection image it is possible to distinguish their alignment direction from the presence of the stripes due to the non-
uniformity of the CNT’s distribution.
6.5 μm
A B
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5
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er diffraction or
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B. Random SW
by 90°.
more quantitat
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o use an estim
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ows the log sca
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pellicle. The im
ntensity and the
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tive evaluatio
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as a function o
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ale diffraction p
ojection of the e
mage was collect
e large blemish
ith metal coatin
n of the pelli
ns along a sp
oes not depen
e diffraction. T
shown in figu( ) =adiance and r
of the spatial
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exit pupil of the
ted on a flat reg
in the lower pa
ng. C. Aligned M
cle scattering
pecific axis. H
nd on the sca
This is done a
re 5. The scat1(2 ∆r + ∆r and Δ are t
frequency of ( ) = .ween the samp
ved in RESCAN.
e condenser op
gion of the sam
art of the image
MW-CNT pellicl
g contribution
However, wh
attering direct
veraging the
ttering intensi
) d . the inner radi
the scattering
ple and the de
The bottom row
tics displayed o
mple, covered w
e come from th
e (uncoated). D
, it is sufficien
hen comparin
tion. In this ca
intensity ove
ty S(r) is then
us and thickn
g features we
etector.
w shows the de
on a linear scale
with HSQ. The ro
he condenser m
D. Same pellicle
nt to extract t
ng different s
ase, we repor
r a circular cr
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oughness
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e shown in
the irradiance
samples, it is
rt the average
own centered
rcular crown C
e
s
e
d
C
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Figure 5. Evaluation of the scattering intensity as a function of the scattering angle is carried out averaging the recorded
irradiance I over a circular crown C centered in the middle of the entrance pupil projection. The radius r of the circular
crown corresponds to the scattering angle. The right panel shows a plot of the scattering intensity as a function of the
spatial frequency of the scattering features for different pellicles.
In Figure 5 we show the average scattering intensity as a function of the spatial frequency of the diffracting features for
the same pellicles shown in Figure 4. While the scattering from the random SW-CNT pellicle has the same behavior of
the no-pellicle reference case, the aligned MW-CNT pellicle shows a higher scattering intensity in the spatial frequency
range between 0.6 and 2 μm-1
. This scattering will superimpose to the diffraction of features with the same spatial
frequency range and reduce their contrast in the aerial image.
Image quality evaluation
Mask defect detection in RESCAN requires the reconstruction of the reticle image. Although this image does not
necessarily correspond to the aerial image generated by a scanner where the illumination is engineered to optimize the
process window, we can still use it to estimate the impact of EUV pellicles on the contrast and on the pattern fidelity.
We performed through-pellicle inspections of the same sample using a constant parameter set, including illumination
NA, scanning area, scanning step size and image reconstruction algorithm. As for the scattering measurements, we
collected a dataset without pellicle as a reference. Figure 6 shows the reconstructed image magnitude for the
reference case (A), for a MW-CNT pellicle with support mesh (B), and for two random SW-CNT pellicles with different
metal coatings (C, D). From a qualitative inspection, it is evident that the two random SW-CNT pellicles have less impact
on the image than the aligned MW-CNT. While images C and D do not show any difference from A, a clear contrast loss
and some artifacts can be observed in image B. To evaluate quantitatively the pellicle’s impact on the reconstructed
image quality, we measured the image contrast and a fidelity metric based on the comparison of the images with a
reference calculated from the reticle design.
The random scattering from the pellicle will contribute to the system flare and reduce the image contrast. To evaluate
the contrast of the reconstructed pattern, we separated the bright regions of the image corresponding to the
multilayer areas from the dark ones corresponding to the absorber. The selection was carried out using an arbitrary
threshold value. We calculated the average and the standard deviation of the intensity in the two regions and we used
the traditional contrast definition: = −+ ± 2 ∆ − ∆( + ) , where and are the average intensities of the bright and the dark regions respectively and ∆ and ∆ are the
corresponding standard deviations. Since there is a focal gradient in the reconstructed image, this calculation was
carried out in a 4×4 μm2 region centered on the best focus spot.
No pellicle
SW CNT (Coating 1)
Aligned MW CNT (support mesh)
Aligned MW CNT (support mesh) 90°
Scatt
ering in
ten
sity [
a. u
.]
10−6
10−5
Spatial frequency [μm-1]
0.5 1 2 5 10
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EJ
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Figure 7. A
the b
EUV
scatt
The values sh
also have a lo
Through-pellicl
. The sample h
d MW-CNT mem
e metal coating
Average contra
bright and dark
multilayer desi
tering show the
hown in Figur
ower contrast
e image recons
as a tilt that ma
mbrane with su
g. D. Image reco
ast of through p
regions in the
igned for maxim
e highest image
re 7 indicate
with respect
structions. A. Re
anifests with a
upport mesh. C
onstruction wit
pellicle reconstr
images. The ma
mum reflectivity
e contrast loss.
that if aligne
to random co
eference image
defocus in the
. Image reconst
h a random SW
ructed images.
ask sample use
y at 6° incidenc
d MW-CNT p
oated SW-CNT
e reconstructed
lower part of th
truction with a
W-CNT membran
The contrast is
ed is generated
ce angle. The m
pellicles have
T pellicles. The
d without pellic
he image. B. Im
random SW-CN
ne with a differ
calculated from
patterning a 14
membranes exhi
a stronger sc
e overall cont
le. The CD of th
mage reconstruc
NT membrane w
rent protective
m the average v
40 nm HSQ film
ibiting the stron
cattering cont
rast level is re
he pattern
ction with
with a
metal
values of
m on an
ngest
tribution, they
emarkably low
y
w
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for all these cases. This is partly due to the fact that this is an average contrast and it is not based on the absolute
maximum and minimum values in the images, but also to the fact that the absorber of the mask sample under
investigation consists of a 140 nm layer of HSQ. The absorption coefficient of HSQ ranges from 4.7 e-3
to 7.4 e-3
nm-1
.
Considering a total thickness of 2⋅140/cos(6°)nm, the EUV intensity transmitted through the absorber ranges from
12% to 27% of the intensity reflected by the bare multilayer leading to a contrast cap that goes from 73% to 88%.
Figure 8. Reconstructed image fidelity is estimated as the standard deviation of the difference between the reconstructed
image and a simulated aerial image calculated from the sample design. The smaller the value of the standard deviation,
the higher the reconstructed image fidelity. The reconstruction obtained through the random SW-CNT membrane with
metal coating 1 showed little or no difference in image fidelity with respect to the no pellicle case.
To evaluate the reconstructed image quality, we defined a fidelity metric based on an ideal reference image. We
calculated the reference aerial image by propagating the binary GDS design of the sample through a perfect lens with
an aperture equal to the one defined by RESCAN's detector. The reference aerial image does not include mask 3D
effects or imperfections in the mask pattern. The reconstructed image and the reference image are aligned using a
least square registration algorithm, normalized for best contrast matching and cropped to a 4×4 μm2 area centered in
the best focus spot as done for the contrast evaluation case. The image fidelity is defined as the standard deviation of
the difference between the two images. The lower the standard deviation, the closer the reconstructed image is
assumed to be to the ideal case. The results of the image fidelity evaluation are shown in Figure 8, where we observed
that one of the random SW-CNT pellicles with metal coating (corresponding to image C in figure 6) exhibits little or no
difference from the reference image reconstructed without the pellicle.
4. CONCLUSIONS AND OUTLOOK
We used the RESCAN defect inspection platform to investigate the effects of different CNT-based pellicles on the
actinic image of EUV mask samples. In particular, we demonstrated a method to measure the pellicle-induced
scattering in double-pass mode, we evaluated the contrast loss caused induced by the pellicle membrane and we
defined an image fidelity metric to compare the effects of the pellicles on the reconstructed image quality.
We observed that the metal-coated random SW-CNT membranes show a scattering behavior similar to the bare sample
case, in line with the results described by previous studies [7]. Aligned MW-CNT membranes show instead a higher
scattering contribution in the spatial frequency region between 0.6 and 2 μm-1
, suggesting that reticle structures in this
spatial frequency range may suffer a contrast loss in the aerial image.
The image contrast and the fidelity metric we defined showed that metal-coated random SW-CNT pellicles perform
better than the aligned MW-CNT ones. In particular, one of the two metal coated pellicles, shows little or no
performance difference from the no-pellicle reference case.
We also demonstrated that RESCAN can detect pellicle defects with size below 6.5 μm which is approximately the
smallest dimension that can cause a critical CD error in the reticle aerial image. This is an inherent advantage of CDI-
based inspection tools that directly record the diffraction patterns of the sample under investigation. Thanks to this
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capability, RESCAN can be used not only as an actinic mask inspection platform, but also as an efficient tool to inspect
the pellicle surface for defects, to characterize its scattering properties and to evaluate the impact of pellicles on the
reticle aerial image contrast and quality.
ACKNOWLEDGMENTS
The authors wish to thank the pellicle suppliers, Canatu Oy and the Nano-Science & Technology Center of Lintec of
America Inc., who agreed to share the samples used in this work, and the PSI ALM technical team, Markus Kropf,
Michaela Vockenuber and José Gabadinho for their contribution to the experiments at the XIL-II beamline.
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