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.
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.
"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.
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.
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.
An EUV reflectometer, based on a laser-produced plasma (LPP) light source, has been developed for characterization of EUV lithography systems. The reflectometer consists of the LPP light source, a prefocusing toroidal mirror, a grating monochromator, a polarizer, a beam intensity monitor, a refocusing toroidal mirror and a sample stage. The LPP light source is driven by a Nd:YAG laser; the laser beam is focused onto a copper tape target. A debris mitigation system that uses a rotating shutter was developed. Higher-orders formthe grating monochromator were suppressed to less than 0.2% of incident beam intensity by total reflection of three grazing incidence mirros. In order to compensate for beam intensity instability, a beam intensity monitor using a grating beamsplitter was installed between the refocusing mirror and the sample. Beam intensity instability can be corrected to less than 0.1% by using the beam intensity monitor.
We describe some results of exposure experiments using the present prototype SR stepper which Canon has developed and also describe the novel technology development which is necessary to establish the next generation SR stepper for volume production. In the evaluation of the prototype machine, alignment performance, stage accuracy, and printing performance were examined, and we found the SR lithography can be applied to manufacturing devices beyond 0.15 micrometer level. In the technology development for the production machine, we have examined methods related to masks; they are reduction of thermal expansion, suppression out-of-plane displacement of mask membrane, and magnification correction. As a result of the examinations, we have a good perspective in development of a high-throughput SR stepper which is suitable for the production beyond 1 G-bit DRAM.
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.