To satisfy the requirement on the image placement accuracy, it is very important to consider the stress of the films on the mask substrate. The stress of the EUV mask is much larger than several kinds of optical masks because reflective Mo/Si multilayer (ML) has large compressive stress. In recent years, thinner absorber has been proposed because of better resolution and less shadowing effect. However it results in the leakage of the light to the adjacent chips on wafer. Then the light shield around the pattern area on the mask has been developed. From the viewpoint of manufacturability, etched multilayer black border (BB) is advantageous. Pattern displacement occurs at the edge of the multilayer etched BB. Measured pattern displacement error increased near the BB and it was simulated by using finite element method. The displacement depends strongly on the ML stress and it is successfully described by the release of the compressive stress at the edge of the black border. Most of the deformation near the BB remains even if the mask is chucked to the flat surface. Simulation using various models are discussed and compared with experimental results.
KEYWORDS: Monte Carlo methods, Extreme ultraviolet, Scanning electron microscopy, Photomasks, Sensors, Metrology, Oxides, Signal detection, Multilayers, Electron beams
The required measurement precision for multilayered EUV mask metrology is set below 0.4 nm three
sigma. In addition to limited precision of CD-SEM, there are fundamental physical factors that deteriorate
the accuracy of the measurements, the most important of which is charging. It is widely believed that
EUV masks are conductive. However, experiments have revealed noticeable charging in CD-SEM
measurements of EUV masks that cannot be ignored. In this work, the results of the experiments and
simulations of the SEM signals are presented. It was shown that charging affects the metrology in a few
ways. The SEM signal shifts at each frame, changes with beam current and also depends on the wall
angle of the absorber. The results of the simulations are compared to experimental results.
KEYWORDS: Sensors, Signal detection, 3D metrology, Photomasks, Atomic force microscopy, Lithography, Scanning electron microscopy, Image sensors, Edge detection, Extreme ultraviolet
In next generation lithography (NGL) for the 22nm node and beyond, the three dimensional (3D) shape
measurements of side wall angle (SWA) and height of the photomask pattern will become critical for controlling the
exposure characteristics and wafer printability. Until today, cross-section SEM (X-SEM) and Atomic Force
Microscope (AFM) methods are used to make 3D measurements, however, these techniques require time consuming
preparation and observation.
This paper presents an innovative technology for 3D measurement using a multiple detector CDSEM and reports its
accuracy and precision.
KEYWORDS: Sensors, Signal detection, Photomasks, Algorithm development, 3D metrology, Polonium, Metrology, Scanning electron microscopy, Electron beams, Detection and tracking algorithms
A new metrology method for CD-SEM has been developed to measure the side wall angle of a pattern on photomask. The
height and edge width of pattern can be measured by the analysis of the signal intensity profile of each channel from multiple
detectors in CD-SEM.
The edge width is measured by the peak width of the signal intensity profile. But it is not possible to measure the accurate
edge width of the pattern, if the edge width is smaller than the primary electron beam diameter. Using four detectors, the
edge width can be measured by the peak width which appears on the subtracting signal profile of two detectors in opposition
to each other. Therefore, the side wall angle can be calculated if the pattern height is known.
The shadow of the side wall appears in the signal profile from the detector of the opposite side of the side wall.
Furthermore, we found that there was the proportional relation between pattern height and the shadow length of the signal on
one side.
This paper describes a method of measuring the side wall width of a pattern and experimental results of the side wall angle
measurements.
Influence of the prominent charging effect on the precision of measuring EUV mask features using CD-SEM was studied.
The dimensions of EUV mask features continuously measured by CD-SEM gradually varied because of the charging.
The charging effect on the measured CD variation mainly consists of three factors: 1) shift of the incident points of
primary electrons deflected by the surface charge, 2) distortions of the profiles of secondary electron signal intensity
caused by the deflection of the secondary electrons, 3) deviation of the maximum slope points of the secondary electron
signal intensity due to the variation of the image contrast. For those three factors described above, how the material
constant affect the CD variation measured by CD-SEM is discussed.
KEYWORDS: Sensors, Atomic force microscopy, Signal detection, Photomasks, Etching, 3D metrology, Tantalum, Critical dimension metrology, Scanning electron microscopy, Extreme ultraviolet
The Multiple Detector CD-SEM acquires the secondary electron from pattern surface at each detector. The 3D shape
and height of mask patterns are generated by adding or subtracting signal profile of each detector. In signal profile of the
differential image formed in difference between left and right detector signal, including concavo-convex information of
mask patterns. Therefore, the 3D shape of mask patterns can be obtained by integrating differential signal profile. This
time, we found that proportional relation between pattern height and shadow length on one side of pattern edge. In this
paper, we will report experimental results of pattern height measurement. The accuracy of measurement and side wall
angle dependency are studied. The proposal method is applied to OMOG masks.
In this paper, we will report on our experimental and simulation results on the impact of EUVL mask absorber
structure and of inspection system optics on mask defect detection sensitivity. We employed a commercial simulator
EM-Suite (Panoramic Technology, Inc.) which calculated rigorously using FDTD (Finite-difference time-domain)
method. By using various optical constants of absorber stacks, we calculated image contrasts and defect image signals as
obtained from the mask defect inspection system. We evaluated the image contrast and the capability of detecting
defects on the EUVL masks by using a new inspection tool made by NuFlare Technology, Inc. (NFT) and Advanced
Mask Inspection Technology, Inc. (AMiT). This tool is based on NPI-5000 which is the leading-edge photomask defect
inspection system using 199nm wavelength inspection optics. The programmed defect masks with LR-TaBN and LRTaSi
absorbers were used which had various sized opaque and clear extension defects on hp-160nm, hp-225nm, and hp-
325nm line and space patterns. According to the analysis, reflectivity of EUVL mask absorber structures and the
inspection optics have large influence on image contrast and defect sensitivity. It is very important to optimize absorber
structure and inspection optics for the development of EUVL mask inspection technology, and for the improvement of
performance of EUV lithographic systems.
We evaluated a FIB-CVD (Focused Ion Beam-Chemical Vapor Deposition) process for repairing clear defects on EUV
masks. For the CVD film, we selected Carbon material. Our simulation result showed that the properties of wafer-prints
depended on the density of the carbon films deposited for repairing the clear defects. Especially, when the density of
carbon film was higher than that of graphite the properties of the wafer-prints came out to be almost same as obtained
from Ta-based absorbers. For CVD, in this work we employed typical carbon based precursor that has been routinely
used for repairing photomask patterns. The defects created for our evaluation were line-cut defects in a hp225nm L/S
pattern. The performance of defect repair was evaluated by SFET (Small Field Exposure Tool) printability test. The
study showed that the focus characteristic of repaired region deteriorated as the thickness of the deposition film
decreased, especially when the thickness went below the thickness of the absorber. However, when the deposition film
thickness was same as that of the absorber film, focus characteristic was found to be excellent. The study also revealed
that wafer-print CDs could be controlled by controlling the CDs of the deposition films. The durability of deposition
films against the buffer layer etching process and hydrogen radical cleaning process is also discussed.
EUV mask damage caused by Ga focused ion beam irradiation during the mask defect repair was studied. The
concentration of Ga atom implanted in the multilayer through the buffer layer and distributions of recoil atoms were
calculated by SRIM. The reflectivity of the multilayer was calculated from the Ga distribution below the capping layer
surface. To validate the calculation, Ga focused ion beam was irradiated on the buffer layer. The EUV reflectivity was
measured after the buffer layer etching process. The measured reflectivity change was considerably larger than the one
predicted from the absorption of light by the implanted Ga. The large reflectivity loss was primarily due to the absorption
of light by chromium silicide residue which was generated by the intermixing of the buffer and the capping layer. Both
lowering of the acceleration voltage and using thicker buffer layer were found to be effective in reducing this intermixing.
The reduction of the reflectivity loss by using thicker buffer layer was confirmed by our experiments. An aerial image of
patterns with etching residue formed by the intermixing was simulated. When the thickness of the intermixed layer
happened to be 8 nm and the size of the resulting residue was larger than 100 nm, then the impact of the estimated
absorption by the residue on the linewidth of 32 nm hp line pattern became more than 5 %.
In this paper, we will report on our experimental and simulation results on the impact of EUVL mask absorber
structure and of inspection system optics on mask defect detection sensitivity. We employed a commercial simulator
EM-Suite (Panoramic Technology, Inc.) which calculated rigorously using FDTD (Finite-difference time-domain)
method. By using various optical constants of absorber stacks, we calculated image contrasts and defect image signals as
obtained from the mask defect inspection system. We evaluated the image contrast and the capability of detecting
defects on the EUVL masks by using a new inspection tool made by NuFlare Technology, Inc. (NFT) and Advanced
Mask Inspection Technology, Inc. (AMiT). This tool is based on NPI-5000 which is the leading-edge photomask defect
inspection system using 199nm wavelength inspection optics. The programmed defect masks with LR-TaBN and LRTaSi
absorbers were used which had various sized opaque and clear extension defects on hp-160nm, hp-225nm, and hp-
325nm line and space patterns. According to the analysis, reflectivity of EUVL mask absorber structures and the
inspection optics have large influence on image contrast and defect sensitivity. It is very important to optimize absorber
structure and inspection optics for the development of EUVL mask inspection technology, and for the improvement of
performance of EUV lithographic systems.
We evaluated a new FIB-GAE (Focused Ion Beam-Gas Assisted Etching) repairing process for the absorber defects on
EUVL mask. XeF2 gas and H2O gas were used as etching assist agent and etching stop agent respectively. The H2O gas
was used to oxidize Ta-nitride side-wall and to inactivate the remaining XeF2 gas after the completion of defect repair.
At the Photomask Japan 2008 we had reported that side-etching of Ta-nitride caused CD degradation in EUVL. In the
present paper we report on the performance of defect repair by FIB, and of printability using SFET (Small Field
Exposure Tool). The samples evaluated, were in form of bridge defects in hp225nm L/S pattern. The cross sectional
SEM images certified that the newly developed H2O gas process prevented side-etching damage to TaBN layer and
made the side-wall close to vertical. The printability also showed excellent results. There were no significant CD
changes in the defocus characterization of the defect repaired region. In its defect repair process, the FIB method showed
no signs of scan damage on Cr buffered EUV mask. The repair accuracy and the application to narrow pitched pattern
are also discussed.
EUV mask damage caused by Ga focused ion beam irradiation during the mask defect repair was studied. The
concentration of Ga atom implanted in the multilayer through the buffer layer was calculated by SRIM. The reflectivity
of the multilayer was calculated from the Ga distribution below the capping layer surface. To validate the calculation, a
multilayer sample was irradiated with Ga FIB, and then EUV reflectivity was measured. The measured reflectivity
change was in good agreement with the calculated value. An aerial image of patterns with Ga implanted region was
simulated. The impact of the estimated Ga absorption on the linewidth of 32 nm hp line pattern was found to be less than
1 nm.
We utilized a newly developed low acceleration voltage FIB (Focused Ion Beam) system and evaluated the process for
repairing the absorber layer on EUVL mask.
During the etching of the absorber layer, which is a step in conventional repair technique, a phenomenon of side-etching
of Ta-nitride layer with XeF2 gas was observed. This phenomenon was considered to be caused by the isotropic
etching of the Ta-nitride layer with XeF2 gas. We then added another gas for etching and evaluated the new process to
prevent the side-etching of Ta-nitride layer.
In this paper, we will report four evaluation results of EUVL mask pattern defect repair using FIB-GAE (Gas Assisted
Etching). The first one is the problem of pattern topography after conventional repairing process and the reaction
mechanism of gas assisted etching of Ta based absorber. The second evaluation result is addressed in two parts. One is
the evaluation of a new gas assisted etching process that employs an additional gas that has an ability to control the
etching rate of absorber layer. The second part addresses the repairing accuracy of EUVL mask pattern. The third is the
basic etching performance e.g. etching rate of Ta based absorber, Cr based buffer, and Si based capping layer. The fourth
and the last evaluation is the application of the newly developed gas assisted etching process on programmed bridge
defect in narrow pitched L/S patterns.
"Reticle protection during storage, handling and use" is one of the critical issues of EUV lithography because no
practical pellicle has been found for EUV reticles as yet. The front surface of an EUV reticle has to be protected from
particles larger than 20-30 nm to maintain the image quality on the wafer plane, and the backside also has to be protected
to maintain the flatness of the reticle chucked on an electrostatic chuck (ESC). In this paper, we are focusing on particles
on the backside of the reticle. If a particle lies between the reticle and the chuck, it has a strong impact on the flatness of
the reticle, and the wafer overlay is degraded by out-of-plane distortion (OPD) and in-plane distortion (IPD) due to the
particle1-5. From this point of view, we need to know the maximum permissible size of particles on the backside of the
reticle. MIRAI-Selete introduced an experimental setup that can measure the flatness of the chucked reticle in a vacuum.
An electrostatic chuck is installed in the vacuum chamber of Mask Protection Engineering Tool (MPE Tool)6, a reticle is
automatically carried from a reticle pod to the chuck in the tool. The flatness of the reticle can be measured by an
interferometer through a viewport underneath the chamber. We can measure the reticle flatness with 3-nm@rms
reproducibility using this setup. We report results of experimental evaluation about the relationship between the reticle
OPD, the size of particle and the chucking force of ESC.
The deposition characteristics of carbon film on EUV mask surface, the impact of carbon deposition on lithography
performance, and cleaning of deposited carbon film on EUV mask are studied. The density of the carbon film was found
to be nearly half of that of graphite by X-ray reflectivity measurement. The impact of carbon deposition on the
lithography performance was simulated by SOLID-EUV. The CD variation by carbon deposition on the mask depends
on the deposition profile on the absorber pattern. Intentionally created contaminated masks were treated by a cleaning
process using atomic hydrogen. The cleaning efficiency and durability of film materials are discussed.
One of the key issues in extreme ultraviolet lithography (EUVL) is the influence of defects on a mask because of the
high printing resolution of EUVL. In order to address this issue, it is necessary to estimate the critical size of an absorber
pattern defect and that of a repaired defect. The repair of an opaque defect by milling or of a clear defect by deposition
might not be perfect; so the area, height, and optical constant of the repair material must be taken into consideration. By
estimating the threshold of calculated aerial images, the critical dimension (CD) that can be printed was found to equal
the square root of the defect area. For the repair of opaque defects, residual Ta was found to be more likely to cause poor
printing than the etching of the multilayer by excessive milling. Since a clear defect is repaired with Ta with the same
optical properties as the absorber material, the CD error in printing is mainly caused by the repair of a CD error and is
not caused by an error in height that is less than ±25% of the height of the Ta absorber. The optimal optical constant of
the repair material was estimated by varying the refraction coefficient from 0.9199 to 0.9999 and the extinction
coefficient form 0.0001 to -0.0451. We found that carbon is a useful repair material that provides a CD error of at most
±0.5 nm around a defect with an area of 64 nm because the maximum refraction should be below 0.97.
In this paper, we will report two evaluation results. One is the relationship between EUVL mask structure and image
contrast values captured by 199nm inspection optics. The other is the influence of mask structure on defect inspection
sensitivity.
We utilized a commercially available DUV inspection system that has the shortest inspection wavelength at 199nm.
Using the 199nm inspection optics, enough image contrast values on hp32nm 1:1 lines and spaces using ArF-half tone
(HT) mask were obtained. On the other hand, image contrast values were not sufficient for conventional EUVL mask
that have a 70nm absorber layer thickness. To improve the contrast values of mask pattern image, we evaluated the
effect of absorber layer thickness on inspection image contrasts. As a result, reducing the thickness of the absorber layer
to 44nm, enough image contrast values of hp32nm 1:1 lines and spaces patterns were obtained.
In this paper, the influence of the thickness of absorber layer on inspection sensitivities for opaque and clear extension
defects are also discussed.
We evaluated the capability of a commercially available DUV system equipped with
reflective inspection optics with the shortest inspection wavelength of 199nm in detecting
pattern defect on EUVL mask of hp45nm programmed defect pattern. The sensitivity of the
system for opaque extension defects for hp45nm node was quite acceptable but for clear
extension defects the sensitivity of the system was rather poor. In this paper, the influence of
base pattern size on inspection sensitivities for opaque and clear extension defects is discussed.
Temperature of EUV mask surface under inspection laser beam irradiation is modeled and simulated. Various conditions
including beam power, beam size, irradiation time, and wavelength are considered. Calculation program for this study
has two components: at first, average power passing through the film is calculated from optical properties of materials,
and then heat transfer equations are solved using finite difference method. Temperature of multilayer below the absorber
depends on the optical properties of absorber film surface. At the wavelength of deep ultraviolet region, temperature of
multilayer below the absorber rises higher than in the temperature of multilayer directly exposed to the beam.
We evaluated TaSix-based bi-layer absorber on ZrSi-based buffer for EUV mask, especially
considering the possibility of ZrSi-based film as a combined buffer and capping layer. Since
ZrSi-based film has both high dry-etching resistance and EUV transparency, it has potentiality to
work as a combined capping and buffer layer. AFM machining repair of bi-layer TaSix absorber
on ZrSi-based buffer can be performed to good profile. Printing evaluation showed that
over-repair into buffer layer did not affect significantly to wafer CD. FIB (10keV) repair of
bi-layer TaSix absorber on ZrSi-based buffer needs improvement for side-wall profile and
distinguishable evaluation from implanted Ga+ effect in more detail. Effect of FIB (10keV) scan
with ordinary repair process seems to be at least smaller than 10%.
We evaluated and optimized Ta-based absorber added by Si for EUV mask. Consequently, we
confirmed TaSix based bi-layer absorber stack has the following performances; It has amorphous
morphology without columnar structure advantageous to fabricate fine pattern with smaller line edge
roughness. In order to realize better position accuracy, it has low internal stress capable to control.
As an optical property, it has low DUV reflectance at 257nm which facilitates to perform defect
inspection. As it can be etched anisotropcally by conventional halogen gases without using hard
mask, we achieved almost vertical sidewall profile of 120nm lines and spaces pattern and promising
CD control accuracy.
We investigated the film property and the lithographic performance of five commercialized NCARs. This report shows the relationship between chemical structure and EB lithographic performance, such as resolution, sensitivity and environmental stability. In this study, we found the good matching the matrix polymer, the cross linker and the photo acid generator(PAG) against NCARs issues. Furthermore, we could demonstrate the trade-off relation for lithographic factor and stabilized factor by chemical characteristics. This report suggests the strategy that was design of chemical structure for the next generation NCARs.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.