A flying particle in a vacuum tool necessarily has a higher velocity than the critical velocity determined by the particle material and the surface material of chamber walls or tool structures. A flying particle loses kinetic energy at every collision with a surface and the velocity is reduced by about 10% a collision. When the velocity of the particle is below the critical velocity, the particle is captured by the surface at the next collision. We must catch the particles before they reach the reticle surface not protected by pellicle. It is a well-known fact that silica aerogels can capture stardust in space. Typical size of the stardust is a few tens of microns and the estimated velocity is a few tens of kilometers per second. On the other hand, typical size of particles in EUV tools is about 100 nm and the typical velocity is about a few hundreds of meters per second. The ratio of kinetic energy of the two is about 1:300,000,000,000. Can silica aerogels capture such low energy particles in the same way as stardust without bouncing the particles or generating debris? The question motivated us to evaluate the properties of silica aerogels in a vacuum; we evaluated the outgassing rate and the particle trapping performance of several aerogels with different densities, 0.012 - 0.19 g/cm3. We found that some aerogels captured more than 90% incident particles and the outgassing rate is sufficiently low for EUV tools.
In extreme ultraviolet lithography, particle-free mask handling is a critical issue because the use of pellicles is impractical. We measured the long-term change in the number of particle adders on a mask blank during transfer processes using a reticle SMIF pod (RSP) and a dual pod, which consists of an outer pod and an inner pod that holds the mask. In the RSP, the number of particle adders during the transfer test of a load port in air to an electrostatic chuck chamber in vacuum decreased from 0.053/cycle to 0.032/cycle because of a clean-up during the pumping down and purging operations. However, the number of particle adders during vacuum transfer did not change with long-term use. Moreover, we found that particles were added by mask blank sliding on a robot hand during vacuum transfer. In contrast, for the dual pod, no accident was observed during the 2000-cycle transfer test, and the number of particle adders was 0.004/cycle. We confirmed that the filter effect and gap effect for protecting the mask from particles were effective. We concluded that the dual pod was a reliable mask carrier for vacuum transfer.
A new SEMI standard E152-0709 "Mechanical Specification of EUV Pod for 150 mm EUVL Reticles" has been
published in July 2009. In the standard, reticle grounding requirements are mentioned as related information: an
electrical connection between the front and back sides of EUVL reticles as well as the electrical connection to the reticle
backside from outside the outer pod may be needed and specified in future. Reticle grounding is very important for
reticle protection not only from electrostatic discharge (ESD) damage but also from particle contamination due to
electrostatic attraction (ESA). Many past data suggested that EUV masks have to be grounded during shipping, storage
and tool handling to prevent particle adhesion. Canon, Nikon and Entegris have jointly developed a new ESD-free EUV
pod "cnPod-ESD" which has electrical connections to the reticle from outside the outer pod by modifying a SEMI
compliant EUV pod "cnPod". In order to have an electrical connection between the reticle backside and the outer pod, a
cantilever is installed inside the inner pod cover. The cantilever touches the reticle backside just inside 146mm x 146mm
which is specified as the minimum conductive layer area in SEMI P37 "Specification for Extreme Ultraviolet
Lithography Substrates and Blanks". In order to have an electrical connection between the reticle frontside and the outer
pod, though it is not required in E152, an electrical conductive material is used for the reticle supports on the inner pod
baseplate. We will show various evaluation data of the new ESD-free pods from particle contamination point of view and
will discuss the necessity of the reticle grounding in this paper. We will also mention the necessity of modification of the
SEMI standard P37 to make a universal EUV ESD-free pod.
Two EUVL masks were made using the compensation method for nonflatness of a mask; and the EUV1 was used to
evaluate the resulting overlay accuracy. For the same mask, the reproducibility of the intra-field overlay errors was better
than 1 nm (3σ) without linear components; and that of the flatness was better than 20 nm PV. In contrast, the overlay
errors were about 3 nm (3σ) for the two masks. The main cause of this degradation in overlay accuracy might be the
difference in mask flatness (~260 nm PV). Using overlay patterns corrected by the compensation method reduced the
overlay errors to about 2.5 nm (3σ). Although the compensation method produced only a small change, it definitely
improved the intra-field overlay of the EUV1. Furthermore, the EUV1 was used to evaluate the intra-wafer overlay for
23 shots. The single-machine overlay (SMO) was found to be better than 4.5 nm (Mean + 3σnonlinear), and the
mix-and-match overlay (MMO) between the EUV1 and an ArF immersion scanner (NSR-S610C) was about 20 nm
(Mean + 3σnonlinear). The main cause of the MMO errors might be the nonflatness of the mask and wafer chucks of the
EUV1. Thus, the chucks must be made flatter to reduce MMO errors. This work was supported in part by NEDO.
One of the critical issues for extreme ultraviolet lithography masks is particle-free mask handling. We report that the number of particle adders on the front side of a mask in a dual pod can be reduced to less than 0.01 particles/cycle (>46-nm polystyrene latex) during the process of starting from the load port to placing an electrostatic chuck (ESC) in vacuum. In addition, we find that chucking the mask on the ESC causes serious issues. One of these issues is whether the masks will be electrically charged by chucking the ESC and whether some particles will be added on the front side. We measure the electric potential of the back and front sides of the mask and examine the particle adders. We find that when the mask is electrically floated, potential on the front side of the mask increases during ESC chucking; when the mask is released from the ESC, it is electrically charged. This electrification causes adhesion of the particles. Our experiments show that to protect the mask from particles, the mask must be grounded throughout the entire process. For electrification, we confirm that a dual-pod system is effective in protecting the mask from particles.
In EUV lithography (EUVL) it is important to protect a mask from the adhesion of particles because it is difficult to use
a pellicle. At Selete, we evaluated a dual-pod carrier and reported on its ability to protect a mask from particles. In the
evaluation the average number of particles added to the mask during several hundred handling-cycles was 0.4.
Therefore, it is very important to precisely count the number of particle adders. However, according to the specification
of the inspection tool, the counting error was greater than the average number of particle adders in the evaluation. In
addition, it is known that the error increases for particles with a size near the detection limit. In the evaluation, we
inspected a mask substrate four times and regarded signals detected multiple times as real particles. We studied the
counting error by assuming that the detection probability followed a static statistical fluctuation. We found that the
expected value of counting error was represented with the equation by the number of initial particles, particle adders,
capture rate, and inspection times. Under our evaluation condition, even if no quasi-particles existed, the counting error
by a single inspection was estimated to be approximately 4. However, the counting error by our evaluation (four
inspections) was estimated to be approximately 0.05. Therefore, we found that the reliability by multiple inspections was
much higher than that by a single inspection and that the number of particles near the detection limit could be found
precisely by multiple inspections. * This work was supported by NEDO.
Since 2005, Canon, Nikon, and Entegris have been jointly developing an EUV mask carrier based on the "Dual Pod"
concept in place of a pellicle. By using our MIRAI-Selete Mask Protection Engineering (MPE) tool, a few prototypes
were tested for performance of particle protection in the case of both mask shipping and its handling in vacuum. As a
result, the fundamental mechanical specifications of the Dual pod were registered as those of SEMI "E152" of an EUV
Pod used in EUV mask. It is found that the latest pod named "cnPod", which is based on the SEMI E152, performs
almost satisfactorily. Although superior protection performance with respect to external particles has been confirmed, the
performance with respect to internal particles laid on the base plate of an inner pod is still under investigation. Therefore,
we evaluate the influence of the internal particles laid on the base-plate surface for the first time. In order to confirm
whether the particles on the base plate are transferred to the mask-patterned surface, well-characterized particles are
dispersed on the base-plate surface. By using this contaminated base plate and the MPE tool, mask handling experiments
are conducted. Under our experimental conditions, it is found that the number of test particles transferred to the mask
surface is very low compared to the total number of particles on the base-plate surface.
To protect the reticle during shipping, storage and tool handling, various reticle pod concepts have been proposed and
evaluated in the last 10 years. MIRAI-Selete has been developing EUV reticle handling technology and evaluating EUV
reticle pods designed using "Dual Pod Concept" for four years. The concept was jointly proposed by Canon and Nikon
at the EUV mask technology and standards workshop at Miyazaki in November 2004; a mask is doubly protected by an
inner pod and an outer pod and the mask is carried into an exposure tool with the inner pod. Canon, Nikon and Entegris
have started collaboration in 2005 and developed three types of EUV pod prototypes, alpha, beta and gamma. The
gamma pods were evaluated by MIRAI-Selete and the superiority of the dual pod concept has been verified with many
experimental data on shipping, storage and tool handling. The dual pod concept was standardized as SEMI E152-0709
"Mechanical Specification of EUV Pods for 150mm EUVL Reticles" in 2009. Canon, Nikon and Entegris have
developed a new pod design compatible with SEMI E152; it has a Type A inner baseplate for uses with EUV exposure
tools. The baseplate has two alignment windows, a window for a data matrix symbol and five pockets as the front edge
grip exclusion volumes. In addition to the new features, there are some differences between the new SEMI compliant
pod design and the former design "CNE-gamma", e.g. the material of the inner cover was changed to metal to reduce
outgassing rate and the gap between the reticle and the side supports were widened to satisfy a requirement of the
standard. MIRAI-Selete has evaluated the particle protective capability of the new SEMI compliant pods "cnPod" during
shipping, storage and tool handling in vacuum and found the "cnPod" has the excellent particle protective capability and
the dual pod concept can be used not only for EUVL pilot line but also for EUVL high volume manufacturing.
Due to potential applications of Extreme Ultraviolet Lithography (EUVL) to 22 nm half-pitch (hp) generations, EUVL
is well researched. However, current SEMI standards about the chuck are based on only the local slope of roughness.
Herein chuck standards, which consider the spatial frequency of the chuck surface roughness as well as the local slope of
the shape, are proposed by examining the chuck roughness. To prevent a mask pattern shift when an EUVL mask is
clamped by an electrostatic chuck, the roughness height must be limited. Thus, the in-plane distortion (IPD) and
out-of-plane distortion (OPD) are introduced to evaluate the mask pattern shift. This research utilizes ANSYS to evaluate
the relationship between the spatial frequency of chuck roughness and IPD/OPD induced on the mask surface after an
EUVL mask is clamped by the chuck.
The IPD depends on the local slope of the surface roughness shape of the electrostatic chuck (ESC) as well as the
spatial frequency of the roughness. Therefore, re-polishing the chuck surface can decrease IPD. Moreover, the spatial
frequency of roughness must be considered when a mask pattern shift correction is performed according to the surface
roughness shape of the EUVL mask and ESC.
One of the critical issues for EUVL masks is clean and particle-free mask handling. We reported that the number of
particle adders on the front side of a mask in the dual pod during the process from the load port to putting on the
Electrostatic chuck (ESC) in vacuum could be reduce to less than 0.01 particle/cycle (≥46 nm). In addition, we found
that chucking the mask on the ESC caused two serious issues. The first is that many particles stick to on the backside of
the mask after chucking on the ESC, raising the question of whether the particle adders on the backside will travel to the
front side. We examined the travel of these particles using the substrates after chucking and polystyrene latex (PSL)
substrates that were dispersed on the backside. These experiments show that there is very little probability that particles
on the backside will travel to the front side. The second issue is whether the mask blanks will charge up by chucking on
the ESC and some particles will add on the front side. We measured the electric potential of the back and front sides of
the mask and examined the particle adders. Our experiments revealed that to protect the mask from the particles, the
mask must be grounded from the beginning to the end. For these two issues, we confirmed that a dual pod system works
effectively to protect the mask from particles. This work is supported by NEDO as a part of the EUV mask program.
"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 projected 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 a reticle. If a particle lies between a reticle and a 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)
caused by the particle. From this point of view, we need to know the maximum allowable size of particles on the
backside of a reticle. MIRAI-Selete introduced an experimental setup that can measure the flatness of the chucked reticle
in a vacuum. Two electrostatic chucks were alternately installed in the vacuum chamber of Mask Protection Engineering
Tool (MPE Tool), 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 the viewport underneath the chamber. We report results of experimental
evaluation about the relationship between the reticle OPD and the initial size of particles and mention the maximum
allowable size of particles between a reticle and a chuck.
To protect the reticle during shipping and storage, several reticle pod concepts have been proposed and evaluated in the
last 10 years. MIRAI-Selete has been developing EUV reticle handling technology and evaluating EUV reticle pods for
two years. In this paper, we report results of shipping tests and storage tests using CNE pods; the CNE pod is
designed by Entegris using "Dual Pod Concept" which Canon and Nikon jointly proposed in 2004. The pod consists of
an inner pod and an outer pod. The inner pod has two components, a baseplate and a cover; the base plate protects the
reticle front surface and the cover protects the back surface from particle contamination in shipping, storage and loading
to a reticle chuck in an exposure tool. The outer pod is a RSP-200 slightly modified to contain the inner pod in it. We
carried out thirty shipping tests and several storage tests and found the CNE pods had very promising protecting
performance during shipping and storage.
In EUV lithography, particle-free handling is one of the critical issues because a pellicle is impractical due to its high
absorption. To investigate this subject, we have developed a mask protection engineering tool that allows various types
of tests to be carried out during the transfer of a mask or blank in air and in vacuum. We measured the number of particle
adders during the transfer of a mask blank in a dual-pod carrier and in an RSP200 carrier. We found that the number of
particle adders (>=46 nm PSL) to a mask blank in a dual pod is less than 0.01 over the whole process from taking the
blank out of the load port in air to putting it in the electrostatic chuck chamber in vacuum. Through various experiments,
the number of particle adders during any process using a dual pod was found to be very few and very stable. In contrast,
for a naked mask, many particle adders were found in large variations. Below one particle were added in over 80% of
experiments on a dual pod and in about 20% of experiments on a naked mask. Based on the test results, we can conclude
that the use of dual pod is an excellent particle-free transfer technique.
"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.
We have developed a mask protection engineering tool (MPE Tool) that simulates various types of tests during the
transfer of a mask or blank in air and in vacuum. We performed mask transfer experiments to investigate particle-free
mask handling techniques using the MPE and mask inspection tools. We measured the number of particles accumulated
during the transfer of the mask blanks. Less than 0.3 particles were added over a path from a load port (in air) to an ESC
chamber (in vacuum) and more than half the particles accumulated appeared during the pumping down and purging steps
in the load-lock chamber. Consequently, we consider that pumping down and purging are the most important steps for
particle-free mask handling.
We, MIRAI-Selete, started a new EUV mask program in April, 2006. Development of EUV mask handling technology is
one of the key areas of the program. We plan to develop mask handling technology and to evaluate EUV mask carriers
using Lasertec M3350, a particle inspection tool with the defect sensitivity less than 50nm PSL, and Mask Protection
Engineering Tool (named "MPE Tool"). M3350 is a newly developed tool based on a conventional M1350 for EUV
blanks inspection. Since our M3350 has a blank flipping mechanism in it, we can inspect the front and the back surface
of the blank automatically. We plan to use the M3350 for evaluating particle adders during mask shipping, storage and
handling. MPE Tool is a special tool exclusively developed for demonstration of pellicleless mask handling. It can
handle a mask within a protective enclosure, which Canon and Nikon have been jointly proposing1, and also, can be
modified to handle other type of carrier as the need arises.
The concept of Extreme Ultra-Violet Lithography (EUVL) mask dual pods is proposed for use in both mask shipping and handling in exposure tools. The inner pod was specially designed to protect masks from particle contamination during shipping from mask houses to wafer factories. It can be installed in a load-lock chamber of
exposure tools and evacuated while holding the mask inside. The inner pod upper cover is removed just before the
mask is installed to a mask stage. Prototypes were manufactured and tested for shipping and for vacuum cycling.
We counted particle adders through these actions with a detectable level of 54 nm and up. The adder count was
close to zero, or we can say that the obtained result is within the noise level of our present evaluation environment.
This indicates that the present concept is highly feasible for EUVL mask shipping and handling in exposure tools.
We have developed a high numerical aperture (NA) small-field exposure system (HiNA) for EUV exposure process development. NA of projection optics of EUV exposure tools for 45-nm node lithography is expected to be around 0.25, which is higher than that previously expected (0.1). HiNA has compatible illumination system, which can be switched to partial coherent illumination and coherent illumination by changing some optical elements. Coherent illumination system was prepared for a high contrast imaging but the uniformity of intensity is less than that of partial coherent illumination. A reflected-type fly*fs-eye element was adopted for partial coherent illumination, which can provide uniformity of both coherency and intensity simultaneously. The coherency of the partial coherent illumination is 0.8. HiNA projection optics consists of two aspheric mirrors, with the NA and the imaging field of 0.3 and 0.3×0.5mm2, respectively. We fabricated two sets of projection-optics. Although the wavefront error of set-1 optics was 7nmRMS, that of set-2 optics was improved to 1.9nmRMS, which was measured with a point diffraction interferometer (PDI) using He-Ne laser. The wavefront error of the set-2 optics was improved by using a new mirror mount mechanism. The mount system consists of several board springs made of super invar in order to minimize the deformation of mirrors by mounting stress. The projection optics of the set-2 has a remote controlled mirror adjustment mechanism which has five degrees of freedom (X,Y,Z,X-Tilt and Y-Tilt). The position of the concave secondary mirror was adjusted precisely with measuring the wavefront error using PDI.
An experimental extreme UV (EUV) interferometer (EEI) using an undulator light source was designed and constructed for the purpose of developing wavefront measurement technology with the exposure wavelength of the projection optics of EUV lithography systems. EEI has the capability of performing five different EUV wavefront metrology methods.
In-plane displacement (IPD) of an extreme ultraviolet lithography (EUVL) mask in a flat state during the electrostatic chucking stage without friction is examined through simulations. For predicting IPD of an EUVL mask, a simulation model based on two-dimensional plane stress theory is developed. With regard to the absorber patterns both square and rectangle, film stress and absorber coverage dependency of IPD is investigated. Mitigation of IPD to the 1-nm level is possible by reducing absorber stress to ±100 MPa. The change in surface height caused by absorber film stress of ±500 MPa is less than 1 nm. The influence of change in surface height on image placement shift was found to be negligible because the image placement shift is 0.03 nm.
The in-plane distortion (IPD) arising from the stress in the films on an EUVL mask in a flattened state was calculated using a newly devised simulation model based on 2D planar stress theory. One cause of IPD is the stress gradients in the multilayer and absorber films. Since there is no stress gradient in a large absorber pattern with uniform internal stress, the placement error was found to be extremely small in this case. Furthermore, concave or convex regions occur int the free space areas not covered with absorber film. The magnitude of the change in surface height in the detph-wise direction is 0.4 nm for an absorber stress of +/- 500 Mpa. It was found that the change in surface height could be suppressed by reducing the absorber stress. In order to improve the placement accuracy of EUVL masks, it is desirable to deposit films with a uniform internal stress during mask fabrication.
An point diffraction interferometry (PDI) system is used for measurement of EUV aspherical mirrors, because diffracted light by a small aperture has a nearly ideal spherical wavefront and EUV projection systems is designed with mild aspheres so that the mirrors can be tested at the center curvature without null optics. An advanced point diffraction interferometer has been developed and its precision and accuracy performance tested with a spherical mirror have been reported in last year1. After that, the diameter of the pinhole employed in the PDI system is switched from 1.0mm to 0.5mm in anticipation of measurement accuracy improvement. An aspherical mirror is measured, and the system error is estimated from the aspherical measurement data. In this system error estimation, an aspherical mirror designed for a four-mirrors EUV projection optics is used.
EUV lithography is a successor to DUV/VUV lithography, and is the final photon base lithography technology. The concept of EUV scanners for 50nm node and below is considered by clarifying the similarities and differences between EUV scanners and DUV scanners. Illumination optics, projection optics, wafer alignment sensors and wafer focus sensors are examined. And the throughput model, overlay budget and focus budget are introduced. The concrete design of illumination optics and the requirements for sources are described. Numerical aperture, magnification and field size are discussed. EUV scanners for 50nm node and below are realized.
An advanced point diffraction interferometer for measuring EUV aspherical mirrors with high accuracy has been developed. It is designed for measuring various EUV mirrors with high accuracy and high precision. It can measure the surface figure of all mirrors that will be used in high numerical aperture systems. Using this interferometer, 0.1nm rms precision and 0.2nm rms accuracy are expected.
Two new alignment sensors for wafer steppers are developed to attain high alignment accuracy on all layers by targeting layers that are difficult to align using the existing alignment sensor, which is based on a laser beam scanning system. The Field Image Alignment (FIA) is a bright- field TV image processing alignment system using broadband illumination. The major advantage of FIA is that due to the broadband light source used for illumination, the edges of the alignment mark can be detected without being influenced by the interference fringes formed by the photoresist. Additionally, even if the cross section of the alignment mark is asymmetrical, the asymmetry can be accurately captured and alignment at the proper position can be achieved. The Laser Interferometric Alignment (LIA) is a grating alignment system based on an optical heterodyne interferometry technique. The advantage of this sensor is that is not affected by surface irregularities such as grains because it will process only specific spatial frequency components diffracted from the alignment mark. Therefore, the spatial frequency components which are diffracted from the metallic grains will be disregarded and will not influence alignment. This allows the alignment to be successful even for low step height or deformed marks. With the development of FIA and LIA, the authors have successfully complemented the existing sensor, so that a high alignment accuracy for the mass production of VLSI with 0.5-0.35 micron rules can be achieved on most layers.
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