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EUV Lithography: State-of-the-Art Review

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Although several years delayed than its initial plan, extreme UV lithography (EUVL) with 13.5nm wavelength has been finally implemented into high volume manufacture (HVM) of mainstream semiconductor industry since 2018. With the delivery and installation of ASML EUV scanners in those giant Fab players like Samsung, TSMC and Intel, EUV lithography is becoming a sort of industry standard exposure metrology for those critical layers of advanced technology nodes beyond 7nm. Although ASML NXE EUVL scanner is the only commercialized EUV exposure system available on the market, its development is the concentration of all essence from worldwide industrial and academic collaboration. It is becoming more and more important not only for fab runners but also for main stream fabless design houses to understand and participate the progress of EUVL. In this review, working principles, module structures and technical challenges have been briefly discussed regarding each EUV subsystem, including light source, reflection mirrors and system, reticle module as well as photoresist development. EUV specific issues of light intensity, defectivity within reflection system, line edge roughness (LER) and mask 3D effects have been focused respectively and promising solutions have been summarized as well.
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J. Microelectron. Manuf. 2, 19020202 (2019)
doi: 10.33079/jomm.19020202
1
EUV Lithography: State-of-the-Art Review
Nan Fu1,*, Yanxiang Liu1, Xiaolong Ma1, Zanfeng Chen1
1 HiSilicon Technologies Co. Ltd, Shanghai, China, 201206
Abstract: Although several years delayed than its initial plan, extreme UV lithography (EUVL)
with 13.5nm wavelength has been finally implemented into high volume manufacture (HVM) of
mainstream semiconductor industry since 2018. With the delivery and installation of ASML EUV
scanners in those giant Fab players like Samsung, TSMC and Intel, EUV lithography is becoming
a sort of industry standard exposure metrology for those critical layers of advanced technology
nodes beyond 7nm. Although ASML NXE EUVL scanner is the only commercialized EUV
exposure system available on the market, its development is the concentration of all essence from
worldwide industrial and academic collaboration. It is becoming more and more important not
only for fab runners but also for main stream fabless design houses to understand and participate
the progress of EUVL. In this review, working principles, module structures and technical
challenges have been briefly discussed regarding each EUV subsystem, including light source,
reflection mirrors and system, reticle module as well as photoresist development. EUV specific
issues of light intensity, defectivity within reflection system, line edge roughness (LER) and mask
3D effects have been focused respectively and promising solutions have been summarized as well.
Keywords: EUV lithography, EUV review, mask 3D, line edge roughness, EUV light source.
* Address all correspondence to: Nan Fu, E-mail: funan1@hisilicon.com
1. Introduction
The development and growth of semiconductor
industry has been following Moor’s low in recent 50
years, and it is fair to say that the continuous
shrinking of transistors in microchips has been
mainly driven by the development of
photolithography technology[1-3]. Since the
appearance of the first G-line (436nm wavelength)
step-and-repeat system, namely wafer stepper, in late
1970s, this exposure system quickly has become
mainstream and dramatically promoted the capability
and efficiency of photolithography[4,5]. Due to the
Rayleigh diffraction effect, smaller wavelength of
light has to been adopted in order to get a reduced
feature size with high resolution through a lens
system, which makes a semiconductor device smaller.
After jumping onto the fast lane of step-and-repeat
system, the light source adopted in photolithography
has been continuing switching from excimer laser for
i-line (365nm)[6], to krypton fluoride (KrF) excimer
laser for 248nm deep ultraviolet[7] and to argon
fluoride (ArF) for 193nm wavelength in order to get
even smaller feature size[8,9]. On this track, 157nm
wavelength of F2 excimer laser was supposed to be
produced to reach smaller half pitch than 65nm[10,11].
However, wavelength is not the only factor to
improve the resolution of smaller pitches. Numerical
aperture, NA as the Acronym, plays another role by
the Rayleigh criterion:
R (smallest feature size) = k1 * λ / NA
where λ is the wavelength of the light used, NA is
the numerical aperture of the system’s lens and k1 is
known as the resolution factor and accounts for all
other process variables. Furthermore, since
NA=n*sinα, where α is the light incident angle and n
is the refractive index of the medium surrounding the
lens, and liquid has a larger n than air, an immersion
ArF scanner was developed to reach a higher
resolution for smaller features, in which immersion
fluid (mainly water) is well controlled to stay
between last lens and wafer surface in order to get
higher NA[12] (Figure 1). The first commercial tool
with this innovative technology was built and
shipped by ASML in 2006 which started to dominate
the market since then. The cost-effective
implementation of immersion ArF scanner in the
industry killed further development of tools with
157nm wavelength.
The implementation of immersion ArF scanner,
enhanced by RET (Resolution Enhancement
Technology) and multi-patterning technologies
enabled the industry to reach 10nm/7nm FinFET
tech node with acceptable cost, efficiency and die
yield. But what is next? To resolve lines (half-pitch)
smaller than 7nm, quadruple patterning or even more
masks need to be used only for one functional layer,
which will dramatically increase the cost and process
Fu et al.: EUV Lithography: State-of-the-Art Review
J. Microelectron. Manuf. 2, 19020202 (2019) 2
Figure 1. Illustration of Rayleigh criterion. Larger NA benefits smaller pitches. Courtesy of Vu Luong, IMEC[14].
Figure 2. Comparison of an ASML ArF immersion scanner and an ASML EUV scanner. Courtesy of Vu
Luong, IMEC[14].
variation. The industry has to follow Rayleigh
criterion again to push the exposure wavelength to
the limit.
The industrialization of EUV lithography
(EUVL) has been turned on since the turn of the 21st
century and even earlier. Almost all key players from
the industry and academic institutes have been
involved in EUV light source development, Bragg
mirrors with multi-coated layer, reflection system in
vacuum, reflection reticle design and material
development, EUV photoresist synthesis and
optimization, contamination and defects reduction,
DTCO (Design Technology Co-Optimization) and
EDA tool development, as well as many other related
engineering fields[13]. Most of the accomplishments
of research and development have been concentrated
to the ASML NXE EUVL module, which began its
journey of High Volume Manufacture (HVM)[1,14]
since 2017. Modern EUV scanner is equipped with a
high power EUV light source which can generate
13.5nm wavelength radiation at a high power up to
250W (above 400W light source is also under
development), a full refection mirror system in a
vacuum chamber which prevent high adsorption on
EUV optical path and maximize reflectivity, as well
as redesigned reflection mirror and reticle system
with 0.33 NA from Zeiss SMT to enable up to 140
wph (Wafer Per Hour) throughput. Figure 2 shows
visual structures of a deep UV immersion module
TWINSCAN NXT:1980i and a latest TWINSCAN
NXE-3400B EUV module which is clearly
distinguishable from the deep UV module due to the
light reflection system. With the help of EUV
lithography system, together with economic
affordable multi-patterning technologies
semiconductor technology node can be pushed
further to beyond 3nm, and extend Moore’s Law to
next decades (Figure 3). In the following chapter,
main sub-modules of a EUV scanner will be briefly
reviewed.
2. Light Source
Unlike the direct deep UV generation from
excimer lasers, extreme UV light with 13.5nm
wavelength can only be produced by excitation and
Fu et al.: EUV Lithography: State-of-the-Art Review
J. Microelectron. Manuf. 2, 19020202 (2019) 3
Figure 3. Evolution of patterning resolution and lithographic wavelength. Courtesy of Vu Luong, IMEC[14].
Figure 4. (A) Illustration of EUV light source vessel with hydrogen gas as buffer. (B) An image of laser
produced plasma around a tin droplet[21]. Courtesy of Igor Fomenkov, ASML.
relaxation of relatively inner electrons of atoms (e.g.
Sn20+), which could only be realized by atom
ionization to form hot dense plasma[15-17]. In a
commercial ASML EUV system, high power CO2
laser with 40~60kW has been applied to bombard
liquid tin (Sn) droplet to generate dense plasma at
high temperature around the droplet, where EUV
radiation takes place[18,19]. EUV photons are
collected by a multilayer coated collector in an
ellipsoidal shape (Figure 4A) and conducted to the
reflection optics in a scanner. As a standalone
component of ASML EUV scanner, the LPP (laser
produced plasma) EUV light source is composed of
high power laser generator, laser beam delivery
system and EUV source vessel, which is one of the
most distinguish parts of a EUV system compared
with deep UV scanners[15]. Due to the high gas
absorption of 13.5nm wavelength UV light, the EUV
vessel has to be kept in high vacuum which may
suffer the system with particle contamination in
contrast with deep UV chamber filled by clean
nitrogen. In addition, high energy laser beam will
evaporate liquid tin and produce tiny microparticles
of tin vapor. These tin deposit contamination in the
vacuum chamber was one of the biggest challenges
in EUV system[20]. ASML is implementing hydrogen
buffer gas with pressure about 100 Pa around the
collector mirror to decelerate tin ions and reduce
atomic tin deposition by a chemical reaction: Sn (s)
+ 4H (g) --> SnH4 (g), these SnH4 gas product can be
pumped away from the chamber, as shown in Figure
4[21].
ASML subsidiary Cymer and Japan-based
Company Gigaphoton are long-term rivals, both are
struggling to win the battle to develop high power,
high conversion efficiency light sources for EUV
lithography applications[22].
The following paragraph describe main parts of
a typical LPP system in a market-available ASML
NXE 3x00 EUV system.
2.1. CO2 Laser Generator and Power Amplifier
High power of input CO2 infrared laser is
crucial to EUV final power output, which gives a
Fu et al.: EUV Lithography: State-of-the-Art Review
J. Microelectron. Manuf. 2, 19020202 (2019) 4
Figure 5. Illustration of EUV generator. Incident CO2 and Nd:YAD laser beams hit tin droplet to generate
dense plasma around the droplet, with the relaxation of plasma, EUV radiation takes place and focused by the
collector mirror. Courtesy of Vu Luong, IMEC[14].
Figure 6. (A) Illustration of Sn droplet generator. (B) Snapshot of Sn droplets with different size and interval.
Courtesy of Vu Luong, IMEC[14].
second life of this mature laser technology[18,19]. It
was proven that tin plasma EUV could be driven by
CO2 laser pulse at 10.6 micron wavelength for the
reasons of higher conversion efficiency, lower
production of debris and higher average power levels
without serious problems of beam distortions and
nonlinear effects which occur normally in solid state
lasers at high intensity. Practically a seed beam from
Nd:YAG solid state laser at about 1 micron
wavelength is preferred to be coupled together with
the tin plasma as shown in Figure 4[14]. An optimal
timescale for an energy coupling was found to be a
few nanoseconds for the Nd:YAG lasers and around
ten nanoseconds for the CO2 lasers.
The high power demand of a EUV source could
be satisfied by a Master-Oscillator-Power-Amplifier
system configuration. This new type of hybrid
pulsed laser system combines pre-pulse seed laser,
master oscillator, power amplifier (PA), fast flow
systems, beam transport system and diffusion cooled
planar waveguide lasers. The system can provide up
to 40kW laser pulse, which could trigger 250W tin
droplet EUV light source for latest ASML NXE
3400B EUV system with conversion efficiency (CE)
up to 6%, so as to enable 140 wph (Wafer Per Hour)
scanner throughput in HVM (High Volume
Manufacturing)[21]. New generation of 60kW modern
is under development in labs.
2.2. Droplet Generator and Metrology
Inside the EUV source vessel, individual Sn
droplets are continuously released with modulated
size, speed and frequency from a nozzle of droplet
generator (DG) located on top. As illustrated in
Figure 5, tin raw material with high purity is melt in
a container connected to the nozzle and pressed by
an inert gas into the nozzle. Before spraying out of
the nozzle, the tin jet is cut into droplets by
mechanical vibration applied on in the nozzle under
well-control. Separation space and size of droplets
could be well-controlled by generator pressure and
vibration frequency. Taking the nozzle outlet as a
position reference, with the increasing distance from
the outlet neighbor droplets coalesce together to
from bigger droplets at larger separation space. The
droplet size and timing must be precisely calculated
Fu et al.: EUV Lithography: State-of-the-Art Review
J. Microelectron. Manuf. 2, 19020202 (2019) 5
Figure 7. (A) Illustration of multilayer coated on collector mirror[14]. (B) Photo of a real collector mirror from
ASML[21]. Courtesy of Igor Fomenkov from ASML and Vu Luong from IMEC.
and measured at the bombard position where laser
pulse will hit the droplet to excite dense Sn+ plasma.
This angry-bird-like process play a critical role in
EUV light source generation, and it has been found
the EUV power is proportional to droplets speed and
space between droplets. All components must be
modulated with highest precision.
ASML third generation droplet generator
produces with 27µm diameter of droplet with about
2700 hours average lifetime[1].
2.3. Droplet Generator and Metrology
Plasma generated EUV light will be collected
by an ellipsoidal MLM (Multi-Layer Mirror)
collector with high reflectivity (Figure 7). The radius
of the collector is well-designed to have its EUV
focus on the vessel outlet called intermediate focus
(IF) where EUV light is measured and conducted
into the scanner vacuum chamber. In order to filter
only 13.5nm EUV from the plasma halo and improve
the reflective rate, collector surface is coated with
multiple Mo/Si layers. By Bragg’s law n λ=2d*sin(θ),
refection wavelength λ could be precisely selected
by adjusting layer thickness d and refection angle θ.
Main technical challenges include sputtering
thickness and uniformity control of MLM layer,
sputter target material purity, super fine polishing for
surface roughness control, deposition of debris and
contaminants, material decomposition and oxidation
under EUV. The most advanced Bragg reflector of
Mo/Si system can reach peak reflectance up to 70%
(theoretically) of 13.5nm EUV light.
2.4. System Power Efficiency
EUV light source power is critical for high
volume manufacture. To get an acceptable
throughput of over 100 wph, EUV source above
200W @ IF (Intermediate Focus) is needed. Key
factors for high source power are high input CO2
laser power, high conversion efficiency (CE), high
collection efficiency (reflectivity and lifetime) and
advanced controls to minimize dose overhead, which
follows below formula[1,23]:
EUV Power = (CO2 laser power * conversion
efficiency [%]* transmission) * (1 dose overhead
[%])
Latest ASML NXE3400B system adopts a
40kW CO2 laser with system CE up to 6% and 10%
dose overhead, which could generate 250W EUV
light source. This enables wafer throughput up to
140 wph already acceptable for HVM. 400W EUV
light source with 0.55 NA is under development at
ASML, which will enable 185 wph throughput[1].
3. Reflection System and High NA
Due to the strong absorption of 13.5nm EUV to
any gases, the whole optical path must be fully
reflective and enclosed in a vacuum environment,
which is different from that of a deep UV scanner
system. Moreover, in order to further reduce
absorption and boost reflectivity, metal-based
reflection mirrors is coated with alternating
molybdenum and silicon (MoSi) layers with several
nanometer in thickness (called Bragg mirrors),
similar as the collector in EUV light source vessel.
High reflectance is achieved with careful control of
mirror quality, layer thicknesses, multilayer
materials, interface quality, and surface termination.
After several years of research and development,
reflectance and film properties are relatively stable
and can satisfy the requirements of an advanced
EUV lithography system. The reproducibility of the
reflectance peak was characterized to be as small as
0.2 percent[24]. The coating uniformity has been
improved to be better than 0.5 percent across a 150
mm diameter substrate[25].
Fu et al.: EUV Lithography: State-of-the-Art Review
J. Microelectron. Manuf. 2, 19020202 (2019) 6
Figure 8. Internal structure of ASML NXE:3400B scanner. Source: ASML.
Figure 9. (A) Anamorphic half field (HF) exposure with high NA compared with full field (FF) with 0.33 NA.
(C) shows aerial image intensity mapping beteen FF and HF A from a test mask feature (B), where HF presents
much higher contrast[28]. Courtesy of Jan van Schoot, ASML.
Figure 10. EUV reflection comparison between NA=0.33 and high NA mirrors[26]. Courtesy of Anthony Yen,
ASML.
Reflection optics is also critical to next
generation EUV scanner with high NA (> 0.5). High
NA system will improve the wafer throughput to
above 160 wph with dramatically reduced multi-
pattern masks (number of LE), as shown in Figure 9.
However, to reach high NA, reflection mirrors have
to be redesigned to be asphere and the size of the
mirror will be dramatically enlarged, accompany
with EUV power over 400W [26]. Besides,
anamorphic half filed reticle and exposure system
has been introduced with 8x Y-magnification instead
of traditional 4x magnification in both X and Y
direction (Figure 9). Accordingly mask stage and
wafer stage acceleration have to be adjusted to
support twice number of scans/shots on wafer.
Figure 10 illustrates mask comparison between 0.33
NA and 0.5 NA and wafer shot difference.
High NA EUV will benefit wafer throughput
and reduce mask layers, but at a cost of super high
EUV light power, very large mirror, asymmetry and
Fu et al.: EUV Lithography: State-of-the-Art Review
J. Microelectron. Manuf. 2, 19020202 (2019) 7
Figure 11. (A) An ASML presentation slide illustrates pellicle application on an EUV reticle and how particle
can be isolated from reticle surface by a pellicle. (B) Defects decreased with the application of pellicle[26].
Courtesy of Anthony Yen, ASML.
complicate reticle and wafer stage, as well as a
special wafer cooling system. ASML future EXE
module is supposed to be equipped with high NA
optics[26-28].
4. EUV Mask
In the EUVL reflective optical system, a
reflective reticle is crucial to imaging quality and
lithography process window on wafer. Similar as
multiple coating on reflective mirrors in the vacuum
system, 40 pairs of Si/Mo layers are deposited on the
substrate of an EUV mask in order to get maximum
reflection, which are capped by a thin film of
Ruthenium (Ru) as thermal emission and reinforcing
layer[25,29]. On top of the thermal emission layer,
absorber layers will be patterned to form “clear” or
“dark” features. Material selection of absorber is
critical to maximize absorption and minimize
reflection. Shadowing effect and mask 3D effects are
also related to absorber layer thickness and side-wall
profile[30]. Moreover, UV, thermal and mechanical
stability of the reticle also need to be considered.
4.1. Pellicle Application
Usage of pellicle can prevent reticle from
contamination and particles in the vacuum system,
and greatly improve the lifetime of a reticle. But
pellicle membranes on top of a reticle will reduce the
EUV transmission and add extra cost to a reticle.
ASML is struggling in both approaches: improve
vacuum system cleanliness to avoid particle
generation, and develop high performance pellicles
membranes with high EUV transmission and low
defect[26,28]. Progress has been made in recent years
which makes the 1st generation pellicle ready for
HVM which has been firmly tracked by Intel and
other main EUV customers.
Advanced materials has been developed for
pellicle membranes which should be durable under
high power (> 300W) EUV radiation. SiNx is found
to be a good backbone rigid materials for mechanical
stability, Ru-capped SiN and Graphene sheet have
been investigated at ASML and IBM, Graphite film
is being developed at Samsung, Carbon Nanotube
(CNTs) thin film composite with SiNx as a pellicle
membrane has been developed and evaluated deeply
at Hanyang University and IMEC. CNT enhanced
SiNx membrane was found to have 2.5 times
decrease of deflection when pressure applied,
without scarify EUV transmission[31].
4.2. Mask Substrate and Bragg Mirror Layers
Physical properties of quartz glass based
backbone influence aerial image deformation, and
resist line edge roughness (LER) as well. Substrate
macro properties like bow, material stress, surface
roughness and flatness is determined by
manufacturing process, e.g. lapping, cleaning, and
fine CMP polishing. According to analysis from
Applied Material, around 75% blank defects are
introduced during manufacturing process of the
substrate, compare to 25% of that from the
deposition of multiple layers[25]. On the other hand,
heat transmission and high temperature mechanical
stability of materials are also concerned. Applied
Materials has taken a lot of efforts on reticle blank
material investigation and development and already
Fu et al.: EUV Lithography: State-of-the-Art Review
J. Microelectron. Manuf. 2, 19020202 (2019) 8
Figure 12. EUV reticle structure illustration from Applied Materials[29]. Courtesy of Vibhu Jindal, Applied
Materials.
made great progresses to improve substrate flatness,
surface roughness, bow and defectivity to HVM
level during last two years[25,29].
Si/Mo switching Multilayers are sputtered on
top of the substrate, which generate stress within
multilayers and substrate interface as shown in
Figure 12. One method is to selectively cut
continuous mirror layer on uncritical area to release
stress. Other critical properties which need to be
preciously controlled are layer uniformity, thickness,
interfacial roughness and defects within multilayers.
It was observed that silicide formed within Si-Mo
interface which could deteriorate EUV reflectivity,
an optimized deposition process has been developed
by Applied Materials to limit the silicide within sub-
nm region, and such maximize reflectivity of EUV
light[32].
A capping layer on top of mirror layer is needed
to enhance mechanical durability and reliability of
the reticle, and improve the adhesion between mirror
layers and the absorber. Ruthenium was
implemented as a capping layer in state-of-the-art
EUV reticles, other advanced materials or
composites have been investigated as well within
major plays like Applied Materials and Veeco[33].
4.3. Absorber and Mask 3D Effect
Mask 3D effect in EUVL is more predominant
for wafer image quality, compared with that in DUV
lithography. Thickness of absorber and Chief-ray
Angle (CRA) introduce asymmetry EUV reflection,
mask shadowing and different printing behavior
between horizontal and vertical features, as shown in
Figure 13 where horizontal patterns A suffers much
higher shadowing effect than vertical pattern B due
to off-axis incidence angle only in Y direction. Mask
3D effect is also a result of multilayer mirror
reflectance changes over incidence and wavefront or
phase deformation of the material. In current EUV
scanner modules, horizontal and vertical lines show
different CD variation and contrast[14].
Figure 13. Feature orientation dependent mask 3D effect,
horizontal features suffer more since they are
perpendicular to the incident light plane.
Figure 14. High k absorber can reduced mask 3D effect
and get less phase deformation compared with low k
absorber[14]. Courtesy of Vu Luong, IMEC.
Fu et al.: EUV Lithography: State-of-the-Art Review
J. Microelectron. Manuf. 2, 19020202 (2019) 9
Ideal EUV absorber material should have a high
extinction coefficient (k) and extremely low
reflectivity, as well as small phase deformation after
longtime EUV radiation at the same time, so that a
reduced thickness and diminished 3D effect could be
obtained. Boron doped TaN with about 70nm
thickness is currently adopted for HVM EUV
application with satisfactory results. But alternative
absorber materials is also under evaluation. Today
high-k absorbers such as Ni, Al, Co and their alloys
have been evaluated by Veeco, IMEC[34], ASML
together with mask shops like Toppan and Hoya[35],
amorphous single phase materials as absorber is
under development by Applied Materials[29], which
shows low surface roughness and stress, good
chemical durability, good adhesion and etch
selectivity with capping layer and less than 2%
reflectivity for less than 45nm thickness. Pt-CrN
Multilayer absorbers used for phase-shift mask
(PSM) has been investigated by Hanyang
University[36]. On the other hand, EDA vendor
Synopsys has evaluated absorber side-wall angle
impact on optical parameters like NILS and intensity
latitude by simulation[30].
4.4. Actinic Inspection and Defect of An EUV
Mask
Mask defect is one of the major challenges
limiting the high volume manufacture of EUVL
system. Since EUV light will penetrate into
multilayer structure on the mask and reflected back,
both internal and surface defects have to be taken
into account of amplitude and phase shift of the
reflected EUV light. Each component of a mask
multilayer can contribute to final defect formation.
As mentioned in previous paragraph, the quality of a
mask substrate is decided by its bow, surface flatness
and roughness. Impurities or surface roughness can
generate dislocations and lattice boundary on the
substrate surface which can be easily extended to
upper layers of the reticle and eventually reticle
surface. Uniformity of the multilayer Bragg mirror
may suffer from pits or bumps buried within 80x
multilayers during sputtering deposition due to any
contamination inside the chamber, gas or target
impurities, thickness also vary on tool variation, so
as the absorber, capping layers and pellicles.
Contrary to deep UV masks, the surface roughness
of an EUV mask causes phase variations of the
incoming wave fronts, which lead to intensity
variations in the aerial image i.e. speckles which are
local intensity inhomogeneities[37,38]. Because of the
reflective nature and much smaller wavelength, the
speckles contrast is significantly higher than that of
the deep UV masks and may increase Line Width
Roughness (LWR) at wafer level.
Fortunately not all defects or impurities in
multilayers (ML) will result in disaster of reticle
scrap, some substrate defects can be de-magnified
when they spread onto the reticle surface through
ML of the mirror[39]. Some defects from multilayers
can be locally repaired by E-beam similar as that for
deep UV masks, as illustrated in Figure 15.
On the other hand, EUV mask characterization
and actinic review of defects is also critical for HVM
of EUVL since traditional SEM direct
characterization cannot detect light phase changes
and intensity variation due to mask internal defects
or surface roughness. EUVL infrastructure
consortium among ZEISS, SUNY Polytechnic
Institute and SEMATECH has developed an actinic
EUV mask Aerial Image Monitor System called
AIMS™ EUV which is trying to close the gap[37].
With the help of AIMS system, aerial image of the
mask could be obtained before wafer exposure, any
phase changes or surface roughness from will be
captured and their impact on wafer and line edge
roughness will be evaluated automatically.
5. EUV Photo Resist
Stochastic effect of photo resist is another major
concern for EUVL application[40-42]. Due to limited
light source power, short wavelength and high
photon energy of EUV light, Auger electrons or 2nd
electrons will be generated after photon absorption
by the resist, resulting in uncontrolled free electron
path and photo acid spread after electronic relaxation,
which is considered to be the main root cause of line
edge roughness and CD variation[43,44].
Interdisciplinary teams worldwide have been
working on understanding fundamental mechanism
of photo chemical reaction, new material
development, and process control.
Gupta team from UCLA focus on LER analysis
and impact on FEoL and BEoL of advanced FinFET
technologies, LER mathematic model and LER
impact on device reliability have been
investigated[45]. Resist half-pitch (HP) resolution,
dose sensitivity and their relationship (Figure 16) to
LER have been evaluated by Paul Scherrer Institut
(PSI) to get a comprehensive solution[46]. It was
found that amount of photons, quencher and acid
rank top three LER contributor, LER could be
Fu et al.: EUV Lithography: State-of-the-Art Review
J. Microelectron. Manuf. 2, 19020202 (2019) 10
Figure 15. Defect on EUV mask could be repaired. From left to right, it shows two dot defects on line-space
features, defects removed, and an aerial image after reparation[37]. Courtesy of Dirk Hellweg, Zeiss SMT.
Figure 16. (A) LER is strongly dependent on exposure dose. (B) Resist sensitivity, resolution and line edge
roughness (LER) are correlated each other, an optimization of smaller LER with good resolution and sensitivity
is needed for EUVL.
Figure 17. CDSEM images show LER is decreasing with increasing dose and resist quencher concentration[48].
Courtesy of C. Popescu, University of Birmingham.
reduced by optimizing multi-trigger component ratio
and quencher amount of the resist chemicals[47,48].
Metal-contained photoresist, chemical
amplified resists (CARs), photo acids (PAs) together
with their backbone polymers have been investigated,
material quantum yield and absorption coefficient
have been measured and correlated to electron mean
free path within resist, providing a theoretical guide
for photoresist selection. Resist chemical processes
after EUV radiation are widely investigated by teams
from Berkeley lab, Molecular Foundry, LBNL,
Columbia Hill Technical Consulting and IBM
Almaden Research Center[49,50]. Numeric model of
imaging mechanism for sol-gel prepared
Fu et al.: EUV Lithography: State-of-the-Art Review
J. Microelectron. Manuf. 2, 19020202 (2019) 11
organometallic polymer resists have been created by
Inpria Corp., stochastic diffusion of secondary
electrons has been simulated with changing dose[51].
Innovative Inorganic-organic composite photoresist
with a metal oxide core and polymer ligands has
been synthesized as a non-CAR resist which is
supposed to have smaller LWR (3σ <3nm)[52-54].
Moreover, lithography process control and
optimization are also essential to LER reduction.
Toshiro Itani and Takahiro Kozawa from Japan have
conducted experiments to improve line edge
roughness by alternating development, bake, rinse
solution as well as resist top and bottom coating[55].
Outgassing is another issue during resist
exposure, hydrogen-contained molecules can
contaminate the vacuum chamber, mirrors and
reticles. ASML has developed a Dynamic Gas Lock
(DGL) membrane to prevent outgassing spread and
suppress DUV and IR during the wafer exposure[1].
6. Summary and Outlook
In this paper, we had a broad but not specialized
overview of extreme ultraviolet lithography
technology and its application in mainstream
semiconductor wafer manufacturing. Major EUVL
modules include light source and vessel, reflection
mirror system under vacuum, reflective reticle and
aerial inspection system. Multilayer material
fabrication and photoresist characterization are also
critical in terms of EUV patterning performance,
defectivity and edge roughness control. As EUVL
scanner is an integrated system, final wafer printing
quality is determined by every component especially
the weakest ones. Thanks to worldwide collaboration
and big investments in recent years, critical issues
related to light source, substrate material and process,
multilayer defects, mask absorber and defects, as
well as resist edge roughness have been overcome in
an ASML EUV scanner system, and wafer HVM
using these EUV tools are already realized in those
leading fabs worldwide.
Major progresses and challenges that are related
to wafer throughput and pattern printing quality are
summarized below.
EUV light source power
250W light power boosts wafer throughput to
140wph, which makes ASML EUV scanner ready
for HMV. Next generation EUV source with further
increased EUV energy power, improved conversion
efficiency and better dose control is under
development, not only to raise wafer throughput but
also to further minimize LER which is closely
related to dose on wafer. It was found 40mJ/cm2
dose (compared with 20mJ/cm2 from current module)
can dramatically reduce LER to smaller than 3nm.
400W EUV source will be implemented in next
module of ASML NXE scanner soon. High power
EUV is also critical to next generation high NA
scanner.
Mirror and reticle defectivity
Impurities, thickness inhomogeneity of
multilayer mirrors and reticles can change EUV light
intensity or phase which will generate defects on
wafer. Reticle surface roughness also contributes to
LER. Besides superb quality fabrication of
multilayer mirrors and reticles, the control of
contamination and particles in vacuum chamber is
also a big concern since there is no clean pressure
gap protection in the reflection chamber and high
energy natural of EUV. With the introduction of
pellicles and DGL membrane, chamber defects have
been reduced to a very low level. In order to detect
not only surface impurities but also light phase
changes due to internal defects, AIMS inspection
system has been developed by ZEISS together with
SEMATECH. This tool could check wafer aerial
image and detect potential pattern failure on wafer.
LER and CD variation
Compared with deep UV lithography, line edge
roughness is much worse for EUVL due to relatively
low dose and secondary electron diffusion
(stochastic effect). New photo resists such as metal-
oxide with polymer ligands are being developed,
lithography process flow has been optimized, and
reticle flatness is well-controlled, all efforts have
successfully reduced the LER to an acceptable level
for an ASML EUV system. Further investigation is
trying to reduce LER to below 3nm for 3σ from
current 3.5nm range from inorganic resist and 5nm
range from CAR.
Mask 3D effect
Due to absorber thickness and the chief-ray
angle of incident EUV light onto the mask,
horizontal and vertical features show different
imaging behavior and critical dimension change on
wafer, this is called shadowing effect. Mask 3D is
also related to reticle blank flatness and bow,
multilayer EUV reflectivity, and absorber properties.
These effects will decrease the margin of process
window with a reduced or shifted depth of focus
(DOF). By introducing phase control SMO and OPC
Fu et al.: EUV Lithography: State-of-the-Art Review
J. Microelectron. Manuf. 2, 19020202 (2019) 12
Figure 18. Technology node scaling is driven by the development of lithography and DTCO. High NA EUV
will lead the industry in next decade[14]. Courtesy of Vu Luong, IMEC.
correction, orientation-dependent mask CD bias
could be compensated. High-k absorbers are also
under development to help further reduce shadowing
effect. In next generation of high-NA EUV scanner,
anamorphic half filed reticle will be introduced to
overcome x-y asymmetry and improve throughput[56],
but with the cost of design, OPC and mask
complexity.
EUVL has achieved great progress since 2015,
most of breakthroughs take place after the formation
of EUV industry alliances among big player like
ASML, INTEL, TSMC and SUMSANG. Industry
requirements combined with strong financial support
finally drive this technology into HVM. As the only
EUVL scanner commercial provider, ASML is going
to sell over 30 EUV modules in 2019. The market is
ramping up and this revolutionary technology will
lead the industry to the next decade[57] (Figure 18).
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... In Fig. 1 Technology trend of the integrated circuit process and device beyond the 5 nm node (source: Refs. [2][3][4][5][6][7][8][9]). 2D represents two-dimensional and IC represents integrated circuit. ...
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As more aggressive EUV imaging techniques and resists with lower intrinsic roughness are developed for patterning at 7- and 5-nm technology nodes, EUV mask roughness will contribute an increasing portion of the total printed line-width roughness (LWR). We perform a comprehensive characterization of the EUV mask impacts on wafer LWR using actinic aerial images and wafer SEM images. Analytical methods are developed to properly separate and compare the LWR effects from EUV masks, photon shot noise, and resist stochastics. The use of EUV AIMS™ to emulate and measure incident photon shot noise effects is explored and demonstrated. A sub 10-nm EUV mask is qualified using EUV AIMS™ with scanner equivalent dose settings that are required for patterning 16- and 18-nm half-pitch L/S features. Typical chemically amplified EUV resists with low and high-dose sensitivities are patterned and characterized with SEM metrology. The variance and spectral components contributing to wafer LWR are quantified and compared. Our analysis shows that speckle-induced aerial LWR is not a significant factor at the experimental imaging conditions when ML roughness is 50-pm rms. At the current scanner dose levels, mask absorber pattern roughness is a major factor in aerial LWR, but not as significant a contributor to wafer LWR where resist stochastics still dominate.
Article
Continuous ongoing development of dense integrated circuits requires significant advancements in nanoscale patterning technology. As a key process in semiconductor high volume manufacturing (HVM), high resolution lithography is crucial in keeping with Moore's law. Currently, lithography technology for the sub-7 nm node and beyond has been actively investigated approaching atomic level patterning. EUV technology is now considered to be a potential alternative to HVM for replacing in some cases ArF immersion technology combined with multi-patterning. Development of innovative resist materials will be required to improve advanced fabrication strategies. In this article, advancements in novel resist materials are reviewed to identify design criteria for establishment of a next generation resist platform. Development strategies and the challenges in next generation resist materials are summarized and discussed.