Defect inspection of EUV substrates and mask blanks must be controlled consistently to ensure repeatable and accurate defect counts. Initial sensitivity must be maintained without producing false counts. Various constructed and native defect monitors are created on substrates to track inspection tool performance. Remedies are applied to an inspection tool when monitors go out of control.
Extreme ultraviolet lithography (EUVL) is the leading next-generation lithography (NGL) technology to succeed optical
lithography at the 22 nm node and beyond. EUVL requires a low defect density reflective mask blank, which is
considered to be one of the top two critical technology gaps for commercialization of the technology. At the
SEMATECH Mask Blank Development Center (MBDC), research on defect reduction in EUV mask blanks is being
pursued using the Veeco Nexus deposition tool. The defect performance of this tool is one of the factors limiting the
availability of defect-free EUVL mask blanks. SEMATECH identified the key components in the ion beam deposition
system that is currently impeding the reduction of defect density and the yield of EUV mask blanks. SEMATECH's
current research is focused on in-house tool components to reduce their contributions to mask blank defects.
SEMATECH is also working closely with the supplier to incorporate this learning into a next-generation deposition tool.
This paper will describe requirements for the next-generation tool that are essential to realize low defect density EUV
mask blanks. The goal of our work is to enable model-based predictions of defect performance and defect improvement
for targeted process improvement and component learning to feed into the new deposition tool design. This paper will
also highlight the defect reduction resulting from process improvements and the restrictions inherent in the current tool
geometry and components that are an impediment to meeting HVM quality EUV mask blanks will be outlined.
EUV lithography is considered the most promising lithography solution for the 16 nm node and beyond. As EUV
light is strongly absorbed by all known materials, reflective optics are used instead of conventional transmittance optics
applied to ArF and KrF lithography. The EUV mask must also need be reflective. It typically consists of a Ta-based
absorber layer, Ru capping layer, Si/Mo multilayer on a low thermal expansion material (LTEM) substrate with a
backside Cr-based metal coating. Because of the strong absorbance of the EUV light, a pellicle is not practical. Therefore,
EUV masks must be cleaned more frequently to maintain the necessary cleanliness. This poses numerous unique
challenges in cleaning processes. For example, the EUV mask integrity, including critical dimension (CD), EUV
reflectivity, and absorber thickness must be kept intact during multiple cleanings throughout the mask's lifetime.
Requirements of defect size for the cleaning, furthermore, are becoming tighter as semiconductor circuit design rules get
smaller. According to the International Technology Roadmap For Semiconductors (ITRS), the smallest defect size that
must be removed is 23 nm for the 18 nm NAND Flash node in 2013. In addition to defects on the frontside, defects
on a backside also need to be minimized since they might lead overlay error due to local distortions of EUV masks on an
This paper focuses on evaluations of cleaning performances using the Lasertec M1350 and M7360 blank
inspection system, which has a 71 nm and 43 nm sensitivity. The 43nm is the current best sensitivity while keeping a
>90% defect capture rate. First, the cleaning performance using the standard process has been investigated. We found a
mitigation of adders was a key challenge for the EUV mask cleaning. The primary source of the adders was also
identified as pits. Secondly, the megasonic cleaning process has been optimized to mitigate the adders. We could
successfully reduce the adders by 30%. Thirdly, to confirm the entire cleaning process, a backside cleaning process
combined with frontside cleaning was investigated, demonstrating that the backsides of the EUV mask blanks could be
cleaned without additional impact on frontside defectivity.
The majority of extreme ultraviolet (EUV) lithography mask blank defects originate from chemical mechanical polishing
(CMP) of the substrate. The fact that CMP has not yet been able to yield EUV substrates with low defect counts
highlights the challenges of polishing doped fused silica surfaces. Here we investigate alternative techniques based on
processing either the substrate or coatings of amorphous silicon thin films and inorganic metal oxides. In particular, we
evaluate a novel polymer-based non-abrasive a-Si CMP process, a photo-induced dry etching of substrate protrusions, a
smoothing coat of spin-on or capillary coated Inpria metal oxide solution, CO2 laser polishing of the substrate surface,
and annealing an a-Si thin film surface in reducing atmospheres. Although CMP still remains the best process with
respect to overall process integration, these techniques have the potential to support CMP in solving the substrate
defectivity issue and help pave the way to commercializing EUV mask blanks.
Photo-induced defects (or haze defects) on 193nm optic masks (haze defects) have been a serious problem not only to reticle engineers
working for mask manufacturing and handling but also to photo-lithography engineers. The most widely accepted explanation of the
root causes of haze defects is the cleaning chemical residues remaining on the mask surface and unavoidable outgassed molecules that
outgas from pellicle materials when exposed to 193nm radiation. These have been significant challenges for reticle cleaning engineers
who need to use cleaning chemicals whose residues do not lead to progressive defect formation on the mask and to find improved
materials to minimize pellicle outgassing.
It is assumed that contamination generation on EUV masks would have a higher probability than on optic masks, primarily since EUV
masks are not protected by a pellicle and amorphous carbon films can accumulate during exposure to EUV light. While there is
potential to mitigate the generation of carbon contamination by improving the exposure tool environment and removing carbon films
using in-situ atomic hydrogen cleaning, it is not yet clear whether the reaction of mask cleaning chemicals to EUV radiation will lead
to creation of progressive defects on EUV mask surfaces.
With the work to being done it has been observed that carbon contamination on EUV masks dominates any effects of solvent
chemicals under normal environmental or exposure conditions (from atmospheric pressure up to a vacuum level of 10<sup>-6</sup> Torr) during
EUV exposure. However, it is still unknown whether residual cleaning chemicals will provide a nucleus for progressive defect
formation during exposure. This lack of understanding needs to be addressed by the industry as EUV masks are expected to undergo
more frequent cleaning cycles.
In this work, we will report on an investigation of the molecular behavior of cleaning chemicals on EUV mask surfaces during EUV
exposure. Movement (e.g., migration or aggregation) of cleaning chemical molecules near EUV exposure spots on the top surface and
beneath the mask will be examined under high vacuum (~10<sup>-8</sup> Torr). We will also investigate whether EUV exposure can trigger the
evaporation of cleaning chemical residues from the EUV mask surface, possibly contaminating the exposure environment. Better
understanding of the influences of the mask cleaning chemicals during exposure, coupled with knowledge about mask tolerance and
patterning performance affected by the cleaning chemicals, should enable the proper selection of mask cleaning processes and
chemicals to meet EUV requirements.
EUV masks include many different layers of various materials rarely used in optical masks, and each layer of material has a
particular role in enhancing the performance of EUV lithography. Therefore, it is crucial to understand how the mask quality and
patterning performance can change during mask fabrication, EUV exposure, maintenance cleaning, shipping, or storage. The fact that
a pellicle is not used to protect the mask surface in EUV lithography suggests that EUV masks may have to undergo more cleaning
cycles during their lifetime. More frequent cleaning, combined with the adoption of new materials for EUV masks, necessitates that
mask manufacturers closely examine the performance change of EUV masks during cleaning process. We have investigated EUV
mask quality and patterning performance during 30 cycles of Samsung's EUV mask SPM-based cleaning and 20 cycles of
SEMATECH ADT exposure. We have observed that the quality and patterning performance of EUV masks does not significantly
change during these processes except mask pattern CD change. To resolve this issue, we have developed an acid-free cleaning POR
and substantially improved EUV mask film loss compared to the SPM-based cleaning POR.
Extreme ultraviolet lithography (EUVL) is a leading technology to succeed optical lithography for high volume
production of 22 nm node and beyond. One of the top risks for EUVL is the readiness of defect-free masks, especially
the availability of Mo/Si mask blanks with acceptable defect level. Fast, accurate and repeatable defect inspection of
substrate and multi-layer (ML) blank is critical for process development by both blank suppliers and mask makers. In
this paper we report the results of performance improvements on a latest generation mask blank inspection tool from
Lasertec Corporation; the MAGICS M7360 at Intel Corporation's EUV Mask Pilot Line. Inspection repeatability and
sensitivity for both quartz substrates (Qz) and ML blanks are measured and compared with the previous Phase I tool
M7360. Preliminary results of high speed scan correction mirror implementation are also presented
For successful implementation of extreme ultraviolet lithography (EUVL) technology for late cycle insertion at 32 nm
half-pitch (hp) and full introduction for 22 nm hp high volume production, the mask development infrastructure must be
in place by 2010. The central element of the mask infrastructure is contamination-free reticle handling and protection.
Today, the industry has already developed and balloted an EUV pod standard for shipping, transporting, transferring,
and storing EUV masks. We have previously demonstrated that the EUV pod reticle handling method represents the best
approach in meeting EUVL high volume production requirements, based on then state-of-the-art inspection capability at
~53nm polystyrene latex (PSL) equivalent sensitivity. In this paper, we will present our latest data to show defect-free
reticle handling is achievable down to 40 nm particle sizes, using the same EUV pod carriers as in the previous study
and the recently established world's most advanced defect inspection capability of ~40 nm SiO<sub>2</sub> equivalent sensitivity.
The EUV pod is a worthy solution to meet EUVL pilot line and pre-production exposure tool development requirements.
We will also discuss the technical challenges facing the industry in refining the EUV pod solution to meet 22 nm hp
EUVL production requirements and beyond.
Microfield exposure tools (METs) continue to play a dominant role in the development of extreme ultraviolet (EUV)
resists. Here we present an update on the SEMATECH Berkeley 0.3-NA MET and summarize the latest test results from
high-resolution line-space and contact-hole printing. In practice, the resolution limit of contact-hole printing is generally
dominated by contact size variation that is often speculated to originate form shot noise effects. Such observations of
photon-noise limited performance are concerning because they suggest that future increased resist sensitivity would not
be feasible. Recent printing data, however, indicates that the contact size variation problem is currently not a result of
shot noise but rather attributable to the mask in combination with the resist-dominated mask error enhancement factor
(MEEF). Also discussed is the importance of the contribution of the system-level line-edge roughness (LER) to resist
LER values currently obtained with the SEMATECH Berkeley MET. We present the expected magnitude of such effects
and compare the results to observed trends in LER performance from EUV resists over the past few years.
Extreme ultraviolet lithography (EUVL) is one of the leading candidates for next-generation lithography technology for
the 32 nm half-pitch node and beyond. The availability of EUV resists is one of the most significant challenges facing its
commercialization. A successful commercial EUV resist must simultaneously meet resolution, line width roughness
(LWR), photosensitivity, and resist outgassing specifications. Photosensitivity is of particular concern because it couples
directly to source power requirements and the source is widely viewed as the most daunting challenge facing EUV
To accelerate EUV resist development, SEMATECH has two programs that provide the resist community access to EUV
exposure capability: 1) the EUV Resist Test Center (RTC) at SEMATECH at Albany, SUNY, and 2) the SEMATECH
microexposure tool (MET) at Lawrence Berkeley National Laboratory. SEMATECH uses both facilities to benchmark
EUV resists in close cooperation with resist suppliers.
Here we summarize results from the SEMATECH EUV resist benchmarking project including process windows,
exposure latitude, and depth of focus, photospeed, LWR, and ultimate resolution. Results show that EUV resists meet
resolution and outgassing requirements for the 32nm half-pitch node. LWR and photospeed, however, remain a concern
especially for contact-hole printing. Moreover, progress towards the 22nm half-pitch node has also been demonstrated in
terms of resolvability.
Base titration methods are used to determine C-parameters for three industrial EUV photoresist platforms (EUV-
2D, MET-2D, XP5496) and twenty academic EUV photoresist platforms. X-ray reflectometry is used to measure the
density of these resists, and leads to the determination of absorbance and film quantum yields (FQY). Ultrahigh levels
of PAG show divergent mechanisms for production of photoacids beyond PAG concentrations of 0.35 moles/liter. The
FQY of sulfonium PAGs level off, whereas resists prepared with iodonium PAG show FQYs that increase beyond PAG
concentrations of 0.35 moles/liter, reaching record highs of 8-13 acids generated/EUV photons absorbed.
Extreme ultraviolet lithography (EUVL) requires nearly defect-free reflective mask blanks to be economically
viable. This has driven the development of tools and processes for depositing and characterizing ultra-low defect density
masks, the focus of our study.
The Nexus<sup>TM</sup> low defect deposition (LDD) module is an ion beam sputter deposition tool installed at the Mask
Blank Development Center (MBDC) at SEMATECH. It has achieved a reflective multilayer coating-added defect
density as low as ~0.005 def/cm<sup>2</sup> for particles 70 nm and larger on a commercial 6" square quartz substrate.<sup>1</sup> To the best of our knowledge, this is the cleanest coating deposited to date on a commercial substrate. Also, since mask substrate
defects can nucleate coating defects, ultra-clean substrates are required. Despite significant advances in mask substrate
cleaning techniques, the incoming substrates contribute more defects than the multilayer coating process to the total
number of defects on our lowest defect density mask blanks. This is because cleaning processes are ineffective against
substrate pits, which dominate the substrate defect distribution. Fortunately, defect mitigation methods have been
developed that use a silicon coat-and-etch process to planarize substrate pit and particle defects. A version of the
NexusTM LDD module designed primarily for planarizing substrate pits and particles has been installed at the MBDC.
This tool has several features, such as the ability to isolate the etch source during deposition steps, that should enable
the planarization process to be done more cleanly. We present initial results on the performance of this tool and process.
Fabrication of nearly defect-free mask blanks is one of the most significant challenges facing the
commercialization of extreme ultraviolet lithography (EUVL). Despite significant advances in our ability to clean
substrates, the incoming substrate contributes more defects than the multilayer to the total number of defects on our
lowest defect density mask blanks. This is because cleaning processes are ineffective against substrate pits, which
dominate the substrate defect distribution.
Fortunately, defect mitigation methods have been developed that use a coat-and-etch process to smooth
substrate pit and particle defects. We have designed and installed a process module specifically for smoothing substrate
pits and particles. This process module has several new features, such as the ability to isolate the etch source during the
deposition steps, and should enable cleaner planarizations than those done before.
Currently, the greatest challenge for us is to demonstrate that the smoothing process can be rendered clean
enough for manufacturing. We will present results on the particles added during planarization and the composition of
these particles, which is critical to identifying their origin and eliminating them.
As semiconductor technology nodes continue shrinking down to 45nm and below, the requirements for number of particle adders and their size during optical mask blank shipment are getting tighter and tighter. In the case of extreme ultra-violet lithography (EUVL) for 32nm and below technology nodes, the requirements for shipping the final mask product are even more stringent. It virtually requires zero particle adders or single digit particle adders (if local mask clean tool is equipped at wafer fab) at 30nm size for 32nm technology node and even smaller size for the 22nm technology node. This EUVL mask handling specific issue is due to the lack of pellicle material available at EUV wavelength, because of strong EUV light absorption by all solid materials. In the past few years, several benchmarking studies on mask handling and shipping without pellicles have been conducted by different companies. The results indicated that many improvements are needed to bring down the handling and shipping induced particle adders at the required 30nm size for the 32nm technology node.
In this study, we have evaluated particle generation at ≥60nm PSL equivalent size during mask shipment. We have demonstrated zero particle adders in shipping by using mask carriers with simple design. Our study included different commercially available carriers and non-commercially available carrier with designs to further minimize the particle generation and deposition onto the mask critical surface. The study has also shown that both the carrier design and the shipping packaging are responsible for clean mask transportation. The smallest particle size (60nm) evaluated in this study is limited by the metrology capability. Further evaluation for particle adders at size ≤60nm requires new development for higher sensitivity inspection capability.
Extreme ultraviolet lithography (EUVL) is the leading next-generation lithography (NGL) technology to succeed optical lithography at the 32-nm node and beyond. The technology uses a multilayer-based reflective optical system, and the development of suitable, defect-free mask blanks is the greatest challenge facing the commercialization of EUVL. We describe recent progress toward the development of a commercial tool and process for the production of EUVL mask blanks. Using the resources at Mask Blank Development Center at SEMATECH-North in Albany, New York, we are able to decrease the mean multilayer-coating-added defect density on 6-in. square quartz substrates by almost an order of magnitude, from ~0.5 defects/cm2 to ~0.055 defects/cm2 for particles 80 nm in size (polystyrene latex equivalent). We also obtain a "champion" mask blank with an added defect density of only ~0.005 defects/cm2. This advance is due primarily to a compositional analysis of the particles using focused ion beam and energy dispersive analysis of x-rays (EDX) followed by tool and procedural upgrades based on best engineering practices and judgment. Another important specification for masks blanks is the coating uniformity, and we have simultaneously achieved a centroid wavelength uniformity of 0.4% and a coating-added defect density of 0.06 def/cm2.
Extreme Ultraviolet lithography requires defect free multilayer-coated masks. The defects in multilayer-coated masks originate from several sources including: the incoming substrate, pre-multilayer deposition cleaning, multilayer deposition, and handling processes. A previous study showed the majority of currently detectable defects are contributed by the incoming substrate. The purpose of this study is to understand the ability of multilayer deposition to modulate the size and shape of substrate pits, and to, ultimately, enable us to determine if a defect of a particular size and shape is tolerable, and will result in a non-printable pit after coating. In order to execute a systematic study, pits with controlled sizes and shapes were required. Programmed pit arrays were generated using Focused Ion Beam (FIB). The arrays were designed to contain pits of various widths and depths. The physical size of these pits was measured using Atomic Force Microscope (AFM) and Scanning Electron Microscope (SEM) both before and after multilayer deposition. These programmed pit arrays were also used to probe the sensitivity of a state of the art Lasertec M1350 defect inspection system to defect size and shape both before and after coating. Finally, the results were compared to those from natural pits. The programmed defects generated in this study will also enable further development of defect mitigation by other planarization techniques as well as improving inspection recipes.
Extreme ultraviolet lithography (EUVL) is the leading next generation lithography (NGL) technology to succeed optical lithography at the 32 nm nodes and beyond. The technology uses a multilayer-based reflective optical system and the development of suitable, defect-free mask blanks is one of the two greatest challenges facing the commercialization of EUVL. In this paper we describe recent progress towards the development of a commercial tool and process for the production of EUVL mask blanks. Using the resources at the recently formed Mask Blank Development Center at SEMATECH-North we have been able to decrease the mean multilayer-coating-added defect density on 6” square quartz substrates by almost an order of magnitude, from ~0.5 defects/cm<sup>2</sup> to ~0.055 defects/cm<sup>2</sup> for particles ≥ 80 nm in size (PSL equivalent). We have also obtained a “champion” mask blank with an added defect density of only ~0.005 defects/cm<sup>2</sup>. This advance was due primarily to a compositional analysis of the particles using FIB/EDX followed by tool and procedural upgrades based on best engineering practices and judgment. Another important specification for masks blanks is the coating uniformity and we have simultaneously achieved a centroid wavelength uniformity of 0.4% and a coating-added defect density of 0.06 def/cm<sup>2</sup>.
Extreme ultraviolet lithography (EUVL) is the leading next generation lithography (NGL) technology to succeed optical lithography at the 32 nm nodes and beyond. The technology uses a multilayer-based reflective optical system and the development of suitable, defect-free mask blanks is one of the two greatest challenges facing the commercialization of EUVL. In this paper we describe recent progress towards the development of a commercial tool and process for the production of EUVL mask blanks. Using the resources at the recently formed Mask Blank Development Center at SEMATECH-North we have been able to decrease the mean multilayer-coating-added defect density by almost an order of magnitude, from ~0.5 defects/cm<sup>2</sup> to ~0.055 defects/cm<sup>2</sup> for particles ≥ 80 nm in size (PSL equivalent). We have also obtained a "champion" mask blank with an added defect density of only ~ 0.005 defects/cm<sup>2</sup>. This advance was due primarily to a compositional analysis of the particles followed by tool and procedural upgrades based on best engineering practices and judgment. Another important specification for masks blanks is the coating uniformity and results showing good uniformity with the low defect density coating process are also presented.
Mask blanks for extreme ultraviolet lithography (EUVL) are fabricated by depositing Mo/Si multilayer films on 6” square super polished substrates. These mask blanks must be almost defect-free and development of a suitable multiplayer deposition tool and process is crucial for the commercialization of EUVL. We will show that using current, real-world quartz substrates and our state-of-the-art defect inspection tool, that substrate defect <i>decoration </i>is an obstacle; this means that there appear to be many non-detectable substrate defects that become detectable once a reflective coating is deposited. This makes it very challenging to conduct accurate defect root cause analysis experiments. We have overcome this obstacle: it entails characterizing an already coated substrate for defects, which provides a suitable reference from which to measure the defects in the multilayer coating that is subsequently applied. We will demonstrate that this is a viable technique and that it enables a suitable defect baseline to be obtained; this is crucial to performing accurate root cause analysis experiments for potential defect sources/mechanisms.
Mask blanks for extreme ultraviolet lithography (EUVL) are fabricated by depositing Mo/Si multilayer films on super polished substrates. These mask blanks must be nearly defect-free, and therefore particles occurring during the deposition process are a serious concern. Development of the next-generation ultra low defect deposition tool for fabricating EUVL mask blanks is crucial for the commercialization of the EUVL technology. ISMT initiated a project at the ISMT-N (Albany, NY) facility to provide an ion beam sputter deposition tool for multilayer deposition on 6” square format substrates to support the development and production of EUV mask blanks. The project has access to state-of-the-art metrology tools recently installed at the Albany facility and also has process development support from Lawrence Livermore National Laboratory (LLNL) and Veeco. The project goal is to work with suppliers, LLNL, Veeco, to baseline, perform defect and root cause analysis, and improve the current tool with an upgrade path to meet the final specification for EUV mask blanks. We will provide results on the quality of the mask blanks produced during the benchmarking phase of this tool; data will be presented for the EUV reflectivity, reflectance uniformity, centroid wavelength, and uniformity.
Using TaN as extreme ultra-violet lithography (EUVL) mask absorber has been previously explored in wafer format. Due to substrate material difference between the square mask format and Si wafer format, e.g., electrical conductivity and thermal conductivity difference, etc., the etch process does not behave the same when mask substrate switches from the Si wafer to the square quartz substrates. With low thermal conductivity on quartz material and no backside cooling, mask etch prefers low power as compared to the Si wafer etch. In the study, we found that for a given source and bias power, the cooling of the substrate plays a role in TaN etch rate and selectivity to the buffer oxide layer.
In this paper, we will present detailed study and comparison of TaN EUVL mask absorber etch characteristics for both the Si substrate and the square quartz substrate cases. The effect of source power, bias power, and backside cooling will also be discussed. The etchers used in study to etch TaN film on the wafer substrate and on the square format mask substrate have the similar configuration. With the optimized etch process, we have achieved good TaN etch profile with high etch selectivity to SiO<sub>2</sub> buffer layer on the square quartz mask substrate.
The EUV mask patterning process development depends on the choice of EUV mask absorber material, which has direct impact on the mask quality or performance such as CD control, defect control, and registration. In the past, several EUV mask absorber material candidates that include Al-Cu, Ti, TiN, Ta, TaN, and Cr have been evaluated. Our research indicated that TaN and Cr are the better candidates among the others evaluated. Cr absorber has been used for many optical lithography generations. Further extending Cr mask absorber to EUV lithography presents minimum impact to the currently mask technology infrastructure. TaN is a new film that has not been used in the currently mask technology. However, Ta based metal compound has been studied previously in x-ray mask technology. Its performance in EUV mask fabrication and printing was found compatible and comparable in many process steps and performance aspects to that of Cr absorber. In this paper, we will present our research and development work on TaN absorber EUV mask fabrication and characterization. The studies include material deposition study, etch development, cleaning compatibility evaluation, and mask printing test. The TaN absorber etch was able to achieve good etch profile and high etch selectivity to the buffer oxide layer. The cleaning benchmarking results showed that TaN absorber is compatible to the currently acid based Cr cleaning procedures and solution. No material damage or loss was found in the case of extreme harsh cleaning conditions used. The TaN thin absorber mask was successfully fabricated and printed in 10x microstepper at Sandia National Lab. Minimum feature of 70nm L/S were obtained.