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Extreme ultraviolet lithography (EUVL) mask multi-layer (ML) blank surface roughness specification historically comes from blank defect inspection tool requirement. Later, new concerns on ML surface roughness induced wafer pattern line width roughness (LWR) arise. In this paper, we have studied wafer level pattern LWR as a function of EUVL mask surface roughness via High-NA Actinic Reticle Review Tool. We found that the blank surface roughness induced LWR at current blank roughness level is in the order of 0.5nm 3σ for NA=0.42 at the best focus. At defocus of ±40nm, the corresponding LWR will be 0.2nm higher. Further reducing EUVL mask blank surface roughness will increase the blank cost with limited benefit in improving the pattern LWR, provided that the intrinsic resist LWR is in the order of 1nm and above.

Extreme ultraviolet lithography (EUVL) is considered the next generation lithography to produce 32-nm feature-size or smaller. The challenge is that conventional pellicles are unavailable for protecting the EUVL masks against contaminant particles, because the EUV beam is easily absorbed by most solid materials. The masks are usually transported or stored in mask carriers. Without the protective pellicles, particles generated inside the mask carrier may deposit on the critical surface of the mask. It is therefore important to identify where the particles are generated inside the mask carrier during shipping. In this paper, two shipping carrier models of different mask holder designs were used. The mask carriers with quartz mask blanks inside were shaken manually, vibrated with a computer-controlled vibration table, or shipped via air freight. In order to simulate the EUVL mask shipping, no pellicles were used. Several online and offline particle detection techniques were employed to investigate particle generation inside the mask carrier during vibration or shipping. It was shown that particles were mostly generated at the contact points between the mask surface and the carrier element. The design of the mask-holding element in the mask carrier played an important role in reducing particle generation.

In extreme ultraviolet lithography (EUVL), conventional pellicles are unavailable for protecting the EUVL masks, since they highly absorb the EUV radiation. One of the serious challenges is therefore to prevent particulate contamination of the EUVL masks. In this paper, EUVL mask protection schemes proposed by Asbach were experimentally challenged against horizontal aerosol flow simulating particle transport from the side during mask handling, shipping, and storage at atmospheric pressure. The protection schemes include mounting the critical surface facing down, using a cover plate with particle trap, and applying electrophoresis or thermophoresis. Both electrophoresis and thermophoresis showed very good protection capabilities. Electrophoresis, however, might be counterproductive due to the unknown particle charge polarity in real situations. A particle trap, on which contaminant particles can deposit before they reach the critical surface, could then be used to collect all particles irrespective of their polarity with a sufficiently high electric field but might not work against zero-charged particles. On the other hand, thermophoresis acts on all particles and transports them in the same direction. Therefore, the upside-down mounting and thermophoresis with the cover plate and particle trap are considered the promising protection schemes for the EUVL mask carrier systems

Extreme ultraviolet lithography (EUVL) is considered as the next generation lithography for 32-nm-node or smaller in semiconductor manufacturing. One of the challenges is to protect the EUVL masks against particle contamination, due to the unavailability of conventional pellicles. In this study, the EUVL mask protection schemes of Asbach et al. (2006. Technical note: Concepts for protection of EUVL masks from particle contamination. Journal of Nanoparticle Research, 8, 705–708), who proposed to mount the mask upside-down, have a cover plate with particle trap and apply phoretic forces, were evaluated against top–down aerosol at atmospheric pressure. Experimental evaluation was performed using 150 mm wafers as witness plates, and PSL particles ranging from 125 to 700 nm. For the numerical assessment of the protection schemes against particles between 10 and 3000 nm, a statistical method using a Lagrangian particle tracking simulation tool was employed to calculate the deposition velocity. It was shown that the critical surface could effectively be protected against top–down aerosol. 2006 Elsevier Ltd. All rights reserved.

This article presents a study of particle formation during vacuum pump down and methods that can be incorporated to protect critical surfaces (like semiconductor wafers or masks) from those particles. Particle formation during pump down was reexamined with temperature measurements. Particles were intentionally produced with hard pump down for studying protection schemes. For the first step, a face-down approach for a critical surface was used to investigate the effect of protection from particle contamination. It was very effective to hold a critical surface face down for protection with the use of high gravitational settling velocity in vacuum environment. However, the face down approach did not sufficiently protect the critical surface. For the second step, a bottom protective plate was introduced below the critical surface to improve the protection efficiency. The bottom plate played a great role in protection of the critical surface with preventing particle formation near the critical surface by keeping the surrounding gas temperature high enough to avoid particle formation as well as with potential blocking of incoming particles toward the critical surface. Higher gas temperature intrinsically avoids formation of residue particles by the condensation process during pump down.

Evaluation of lithography process or stepper involves very large quantity of CD measurements and measurement time. In this paper, we report on an application of Scatterometry based metrology for evaluation of EUV photomask lithography. Measurements were made on mask level with Ellipsometric scatterometer for develop-check CD (DCCD) and final check CD (FCCD). Calculation of scatterometer profile information was performed with in-situ library-based rigorous coupled wave analysis (RCWA) method. We characterized the CD uniformity (CDU) and metal film thickness uniformity. OCD results show that high precision CD measurement EUV absorber and resist is possible with this method. A series of simulations were also performed to investigate the feasibility of Ellipsometric scatterometry for various pitches/line CD sizes, down to 11nm half-pitch at 1x magnification. The data showed that Scatterometry provides a nondestructive and faster mean of characterizing mask CD performance for various EUV process generations.

Extreme Ultraviolet Lithography (EUVL) embedded phase shift mask (EPSM) can further extend lithography resolution limit and provide better pattern fidelity as compared to that of EUVL binary mask for 16nm node technology and beyond generations. In our previous study, we have demonstrated in wafer printing that EUVL EPSM can provide improved process window for both the dense lines and contacts and the low shadowing effect when compared to that of the EUVL binary mask. Due to limitation of current EUVL resist performances, certain advantages of EUVL EPSM, such as line width roughness (LWR) improvement, cannot be readily seen at wafer resist level. This is because that the aerial image quality improvement in LWR is over shadowed by the current large resist intrinsic and process induced LER. We believe that when EUV resist and wafer process improves in future, mask induced pattern fidelity difference will start to play an observable role in wafer printing. In this study, we focused on comparing EUV actinic aerial image performance of a EUVL EPSM and a binary mask for both lines and contacts. Without convoluting with resist effect, the mask aerial image performance comparison of two different masks can better reflect all the effects that are due to mask differences. Our analysis of the EUV actinic aerial images of a EUVL EPSM and a binary mask showed not only the process window advantages of the EPSM as demonstrated previously, but also the improved LWR performance of EUVL EPSM when compared to that of the EUVL binary mask. The matrix used to analyze the aerial images includes aerial image contrast, LWR, process windows (focus-exposure plot), etc. Our detailed analysis is performed for various line and contact features.

For Extreme Ultra-violet Lithography (EUVL) targeting at 11nm and beyond design rules, the minimum printable EUVL multilayer (ML) mask defect size can be as small as 20-25nm. As a result, the defect-free EUVL ML mask blank fabrication remains the top challenge for EUVL mask. Aspects of this challenge include high quality blank substrate material (low thermal expansion material) fabrication, substrate polishing, substrate cleaning, blank handling, ML deposition, and high sensitivity substrate and blank defect inspection. High investment cost and potential low blank yield due to stringent defect-free requirement can quickly drive up EUVL cost of ownership. It is anticipated, however, the EUVL ML blank yield can be drastically improved if we can allow a few defects on a ML blank. Utilizing such a "defective" grade mask blank to fabricate a defect-free EUVL mask requires several defect mitigation schemes during mask patterning processes. These schemes include modifying mask absorber pattern via repair tool to compensate the effect of an adjacent ML defect and using absorber pattern to cover the ML defects. In this paper, we focused on the study and demonstration of using device pattern to cover limited number of blank defects. The steps of this defect mitigation process include blank fiducial mark patterning, defect location relative to fiducial mark precision measurement, automated pattern shift solution simulation for a given ML defect map, and precision alignment of the device pattern to the blank defects during e-beam write. With these steps, we have successfully demonstrated the coverage of several targeted ML blank defects simultaneously via global device pattern shift.

Extreme UV lithography or EUVL is still the primary candidate to allow scaling below the 22 nm technological node. Three major engineering challenges need to be simultaneously solved for a smooth introduction of EUVL into high volume manufacturing: source power and reliability, mask readiness, and photoresist performance. For the EUV reticle infrastructure, most of the emphasis to date has been put on obtaining and maintaining a low number of mask defects. However, the reticle flatness requirements for EUV masks are also very stringent. Recent theoretical studies have indicated that multilayer roughness higher than 50 pm causes line edge roughness. In this paper we engineered an EUV mask having a systematic surface roughness aggravation. We exposed this mask on the IMEC ASML NXE:3100, equipped with an USHIO/XTREME discharge-produced plasma (DPP) source. Herein, we present the experimental results illustrating the impact of mask surface roughness on 27 nm half-pitch lines/spaces. No evidence of aggravated line edge roughness was found on the wafer when the mask surface roughness was lower than 500 pm.

In this paper, the performance of quadrapole illumination source under two types of high numerical aperture (NA) deep-ultra-violet (DUV) printing tools, namely, excimer laser stepper with chromatic lens design and step-and-scan systems without chromatic aberration for the application of 0.25μm lithography is studied through simulation. The results are based on both aerial image and Shipley XP89131 negative DUV resist model [1] study. Due to the unique characteristics of oblique illumination source imaging, i.e., imaging by using only zero and first diffraction order light, both stepper resolution limit and depth of focus (DOF) of dense lines are extended. As a result, the proximity effect and the chromatic aberration effect in resist printing are also different from that of conventional illumination source. The optical proximity effect increases under the oblique illumination source as compared to that of conventional illumination source, especially when the light incident angle is large. For chromatic lens aberration, unlike the conventional illumination source, aerial image degradation depends not only on the laser spectral profile and amount of the lens aberration, but also on the mask feature sizes and pattern types.

We present an improved method of phase retrieval from through-focus image series with higher precision and reduced sensitivity to noise. The previous method, developed for EUV, actinic mask measurements, was based on the Gerchberg-Saxton algorithm and made use of two aerial images recorded in different focal planes. The new technique improves the reconstruction uncertainty and increases the convergence speed by integrating information contained in multiple images from a through focus series. Simulations characterize the new technique in terms of convergence speed, accuracy and stability in presence of photon noise. We have demonstrated the phase-reconstruction method on native, mask-blank phase defects and compared the results with phase predictions made from AFM data collected after the multilayer deposition. Measurements show that a defect's top-surface height profile is not a reliable predictor of phase change in all cases. The method and the current results can be applied to improve defect modeling and to enhance our understanding of the detectability and printability of native phase defects.

Extreme ultra-violet Lithography (EUVL) alternating phase shift mask (APSM) or other optical enhancement techniques are likely needed for 16nm (half pitch) technology generation and beyond. One possible option is the combination of EUVL and APSM. The fabrication of EUVL APSM is more difficult than either the fabrication of an EUVL binary mask or a conventional optical APSM mask. In the case of EUVL APSM, the phase difference in the two regions (0 and 180-degree phase regions) is created by a phase step in the substrate prior to the multilayer (ML) coating. The step height that induces 180-degree phase mismatch in the ML is determined by [λ/(4cosθ)](2m+1), where m are integers (0, 1, 2,...). In this experiment, we targeted for a step height with m=1. The same mask design also contains the standard binary structures so that the comparison between the EUVL APSM and the EUVL binary mask can be performed under the same illumination and wafer process conditions. The EUVL APSM mask was exposed using Nikon's EUV1 scanner in Kumagaya Japan. The wafer level results showed higher dense line resolution for EUVL APSM as compared to that of EUVL binary mask. APSM also showed improved line width roughness (LWR) and depth of focus (DoF) as compared to the best EUVL binary results obtained with C-dipole off-axis illumination (OAI). The wafer CD resolution improvement obtained by APSM in this experiment is partially limited by the resist resolution and the mask phase edge spread during ML deposition. We believe that wafer CD resolution and can further be improved with imaging imbalance compensation mask design and improvements in resist resolution and the phase generation portion of the mask fabrication process. In this paper, we will discuss in detail the mask fabrication process, wafer level data analysis, and our understanding of EUVL APSM related issues.

Fabricating defect-free extreme ultraviolet lithography (EUVL) multi-layer (ML) mask blanks presents a big challenge in EUVL technology. ML defect sources primarily come from substrate defects and ML deposition adders. Defect reduction, therefore, needs to address many development aspects, such as substrate material, substrate polishing, substrate cleaning, blank handling, and ML deposition. High investment cost and potential low blank yield can quickly drive up EUVL cost of ownership. However, allowing a few defects on the ML blank can improve the blank yield drastically. Utilizing such defect-blanks through defect mitigation schemes has been proposed. It includes directly repairing small ML phase and amplitude defects, mask absorber pattern proximity repair, and using absorber pattern to cover the ML defects. It includes directly repairing small ML phase and amplitude defects,1-2 repairing mask absorber pattern to compensate the effect of an adjacent ML defect,3 and using absorber pattern to cover the ML defects.4 In each case, the ML defects will first need to be identified and located during the ML blank defect inspection. To precisely locate the ML defects on the blank, fiducial marks on the ML blank are needed for mask alignment and defect location identification. In this paper, we will present the details of the ML defect mitigation process flow, i.e., using absorber pattern to cover the ML defects, and the corresponding experimental validation of this mitigation flow. We will also discuss the fiducial marking scheme, its application in the defect mitigation flow, the error budget of ML defect mitigation, such as defect position measurement error, fiducial mask position error, e-beam alignment errors, etc.

Extreme ultraviolet lithography (EUVL) tool development achieved a big milestone last year as two full-field Alpha Demo Tools (ADT) were shipped to customers by ASML. In the future horizon, a full field "EUV1" exposure tool from Nikon will be available by the end of 20071 and the pre-production EUV exposure tools from ASML are targeted for 20092. It is essential that high quality EUVL masks can be made and delivered to the EUVL tool users to support the technology development. In the past year, we have demonstrated mask fabrication with low stress absorber deposition and good etch process control yielding a vertical etch profile and a mask CD control of 5.7 nm for 32 nm (1x) space and 7.4 nm for 32 nm (1x) lines. Mask pattern resolution of 15 nm (1x) dense lines was achieved. Full field reflective mask die-to-die inspection at a 125nm pixel size was demonstrated after low defect multilayer blanks became available. In this paper, we will present details of the Intel EUVL Mask Pilot Line progress in EUVL mask defect reduction, pattern CD performance, program defect mask design and inspection, in-house absorber film development and its performance, and EUVL metrology tool development. We will demonstrate an overall improvement in EUV mask manufacturing readiness due to our Pilot Line activities.

It becomes increasingly important to have an integrated process for Extreme UltraViolet (EUV) mask fabrication in order to meet all the requirements for the 32 nm technology node and beyond. Intel Corporation established the EUV mask pilot line by introducing EUV-specific tool sets while capitalizing on the existing photomask technology and utilizing the standard photomask equipment and processes in 2004. Since then, significant progress has been made in many areas including absorber film deposition, mask patterning optimization, mask blank and patterned mask defect inspection, pattern defect repair, and EUV mask reflectivity metrology. In this paper we will present the EUV mask process with the integrated solution and the results of the mask patterning process, Ta-based in-house absorber film deposition, absorber dry etch optimization, EUV mask pattern defect inspection, absorber defect repair, and mask reflectivity performance. The EUV resist wafer print using the test masks that are fabricated in the EUV mask pilot line will be discussed as well.

Extreme ultraviolet lithography (EUVL) is one of the leading candidates for the next generation lithography. As the requirement on critical dimension (CD) and side wall profile control becomes ever stringent as minimum feature sizes keep shrinking following the Semiconductor Industry Association (SIA) roadmap, the patterning of the EUV mask absorber material, cost of ownership (COO) of mask, and the capability for defect free EUV masks become the crucial path in enabling the overall success of EUV lithography. The purpose of this study is to understand the etch characteristics in TaN-based EUV mask absorber etch, which will enable us to determine robust process condition in terms of CD performance and profile control. In this paper, CD bias performance in TaN-based EUV mask absorber etching is investigated within inductively coupled plasma (ICP) of fluorine-containing and chlorine-containing gas chemistries. The effects of etch parameters, such as plasma source power, bias power, and pressure, on the CD bias are evaluated through design of experiments (DOE). Some other etching characteristics like etch rate and selectivity are also correlated to the CD performance and etch profile to understand the basic etch mechanism in TaN etch. Latest etch results of the TaN-based absorber are also presented.

Recently, while performing extensive EUV irradiation endurance testing on Ru-capped multilayer mirrors in the presence of elevated partial pressures of water and hydrocarbons, NIST has observed that the amount of EUV-induced damage actually decreases with increasing levels of water vapor above {approx} 5 x 10{sup -7} Torr. It is thought that the admitted water vapor may interact with otherwise stable, condensed carbonaceous species in an UHV vacuum system to increase the background levels of simple gaseous carbon-containing molecules. Some support for this hypothesis was demonstrated by observing the mitigating effect of very small levels of simple hydrocarbons with the intentional introduction of methyl alcohol in addition to the water vapor. It was found that the damage rate decreased by at least an order of magnitude when the partial pressure of methyl alcohol was just one percent of the water partial pressure. These observations indicate that the hydrocarbon components of the vacuum environment under actual testing conditions must be characterized and controlled to 10{sup -11} Torr or better in order to quantify the damage caused by high levels of water vapor. The possible effects of exposure beam size and out-of-band radiation on mirror lifetime testing will also be discussed.

One of the biggest challenges in extreme ultraviolet lithography (EUVL) reflective mask blank fabrication is to make a reflective multilayer (ML) blank “defect-free.” SEMI-P38 standard for EUVL mask absorbing film stacks and ML requires ML defect density in the mask quality area to be less than 0.003 defect/cm 2 at 30nm size for the 32 nm technology node. Due to the technical challenge in making such a defect-free ML EUVL mask blank, the EUVL ML blank yield is expected to be low. In order to reduce EUVL technology cost, it is desirable to reuse a qualified defect-free ML blank a few times. In this paper we will present a new EUVL ML blank design that allows one to reuse the ML blank after a patterned mask is no longer needed. A standard ML design will only allow one time use. The blank, in general, will not be reusable or reclaimable due to the top capping layer (possible a few layers underneath it) damage during the mask patterning, cleaning, and usage. The new ML blank design alters the standard capping layer structures such that the ML blanks can be reused up to a few times after the blank goes through each complete mask patterning and usage cycle. No additional ML re-deposition is needed for reusing the ML blanks. This new ML structure is particularly attractive since a blank, once it is qualified as a defect-free blank, can be reused a few times as a defect-free ML blank.

The flatness of electrostatically chucked EUVL reticles was evaluated on two Zerodur bipolar coulombic electrostatic chucks (from Invax Technologies) of different thicknesses, which represent different chuck stiffness, different hardness of the dielectric material used for chuck surface, and different surface flatness finishing. A Zygo GPI interferometer was used to measure the flatness of the chucked reticles, freestanding reticles, and chuck surfaces. The chucked reticle flatness was impacted by the flatness and shape of the front and back sides of the reticle and that of the chuck. Chucked reticle dynamics during chucking and reticle hysterisis were observed. A stable operation range for the e-chucks was found. We also observed backside-particle-induced-out of plane distortion (OPD) on the chucked reticle in the experiments when Cu particles of height 1 to 3μm were placed between the chuck and the reticle backside.

Extreme Ultraviolet Lithography (EUVL) reflective mask blank development includes low thermal expansion material fabrication, mask substrate finishing, reflective multi-layer (ML) and capping layer deposition, buffer (optional)/absorber stack deposition, EUV specific metrology, and ML defect inspection. In the past, we have obtained blanks deposited with various layer stacks from several vendors. Some of them are not commercial suppliers. As a result, the blank and patterned mask qualities are difficult to maintain and improve. In this paper we will present the evaluation results of the EUVL mask pattering processes with the complete EUVL mask blanks supplied by the commercial blank supplier. The EUVL mask blanks used in this study consist of either quartz or ULE substrates which is a type of low thermal expansion material (LTEM), 40 pairs of molybdenum/silicon (Mo/Si) ML layer, thin ruthenium (Ru) capping layer, tantalum boron nitride (TaBN) absorber, and chrome (Cr) backside coating. No buffer layer is used. Our study includes the EUVL mask blank characterization, patterned EUVL mask characterization, and the final patterned EUVL mask flatness evaluation.

The introduction of extreme ultraviolet (EUV) lithography into high volume manufacturing requires the development of a new mask technology. In support of this, Intel Corporation has established a pilot line devoted to encountering and eliminating barriers to manufacturability of EUV masks. It concentrates on EUV-specific process modules and makes use of the captive standard photomask fabrication capability of Intel Corporation. The goal of the pilot line is to accelerate EUV mask development to intersect the 32nm technology node. This requires EUV mask technology to be comparable to standard photomask technology by the beginning of the silicon wafer process development phase for that technology node. The pilot line embodies Intel's strategy to lead EUV mask development in the areas of the mask patterning process, mask fabrication tools, the starting material (blanks) and the understanding of process interdependencies. The patterning process includes all steps from blank defect inspection through final pattern inspection and repair. We have specified and ordered the EUV-specific tools and most will be installed in 2004. We have worked with International Sematech and others to provide for the next generation of EUV-specific mask tools. Our process of record is run repeatedly to ensure its robustness. This primes the supply chain and collects information needed for blank improvement.

Using ruthenium (Ru) material as an extreme ultraviolet lithography (EUVL) mask blank multi-layer (ML) capping presents many advantages over silicon (Si) capping layer. Its high resistance to oxidation has been tested in EUVL optics. 1 Ru capped ML mask blank also demonstrated very high et ch selectivity during both mask absorber etch when the blank has no buffer layer and during buffer etch when the blank contains buffer layer. 2 Due to higher EUV light absorption, Ru capping layer usually has to be much thinner than that of Si capping layer. As a result, long-term mask lifetime with thin Ru capping layer and its stability during multiple mask cleans becomes a concern. To address these concerns, we developed a process that improves Ru capped ML reflectivity for a given capping thickness. We further demonstrated the shelf lifetime stability of Ru capped ML mask blank and stability during multiple mask cleans. In this paper, we will discuss the detailed performance of Ru capped ML blanks, which includes mask blank reflectivity performance for different capping thickness, Ru capped ML mask blank shelf lifetime stability and cleaning stability performance. Mask patterning results using Ru capped ML blanks will also be presented. Keywords: EUVL ML mask blank, Ru capped ML, EUVL mask processing

One of the greatest challenges for extreme ultraviolet lithography (EUVL) technology is the development of a nearly defect-free mask blanks. For EUVL mask blanks with a few printable multi-layer (ML) defects, several defect mitigation methodologies have been proposed. It includes directly repairing small ML phase and amplitude defects,1-2 mask absorber pattern proximity repair,3 and using absorber pattern to cover the ML defects. In each case, the ML defects first need to be identified and located during the ML blank defect inspection. To precisely locate the ML defects on the blank, fiducial marks on the ML blank are needed for mask alignment and defect location identification. In this study, we have demonstrated that EUVL mask ML blank fiducial mark with 60nm trench depth, which is more than 10X the required minimum detectable height of the defect inspection tool, can be generated via local heating. No material removal occurs. Therefore, it is a very clean process. These fiducial marks work for many inspection tools such as optical inspection tool, e-beam inspection tool, and EUV Aerial Image Monitor (AIM) inspection tool. The fiducial marks generated in the experiment were realized via laser heating. The locally heated Mo/Si ML converts to molybdenum silicide that has a low volume than that of Mo/Si ML. As a result, a trench or a mark is generated. The process is expected to have large process margin as the depth of the mark is determined by completely silicide the ML. Additional heating will not be able to further deepen the trench. Therefore, the temperature and heating time control should be loose.

While interferometry is routinely used for the characterization and alignment of lithographic optics, the ultimate measure of performance for these optical systems is the transfer of an image or pattern into photoresist. Simple yet flexible exposure systems play an important role in this task because they allow complex system-dependent effects to be isolated from the printing results. This enables the most direct lithography evalaution of the optical system under investigation. To address tehse issues for commercial-class EUV optics, a synchrotron-based programmable illuminator exposrue station has been implemented at Lawrence Berkeley National Laboratory (the Advanced Light Source). As previously presented, this static microfield exposure system has been used to lithography characterize a 4-mirror optical system designed for the EUV Engineering Test Stand (ETS) prototype stepper. Based on the lithographic characterization, here we present a detailed performance analysis of the 0.1-NA ETS Set-2 optic. Operation of the static printing system with the Set-2 optic yielded approximately 330 exposed wafers, where each wafer contains one or more focus-exposure matrices. A wide variety of parameters were studied includign, among others, illumination conditions, resist thickness, and mask tone. Here we present a subset of this data in terms of process-window results. The resutls demonstrate a depth of focus (DOF) approximately 2μm for isolated 70-nm line features, 1 μm for nested 70-nm line features, and 0.5μm for 70-nm contacts on 270-nm pitch.

Extreme Ultraviolet Lithography (EUVL) is the leading candidate for the 45 nm node technology and beyond. A mask film structure and material selection become increasingly important for a success in the EUVL mask process development. Etch CD performance is considered in the material selection of an EUV mask absorber stack. This paper reports the study of etch CD performance and pattern loading effects in the plasma etch on two potential absorber materials of Cr and TaN. The etch process was experimentally setup on wafer samples and two different film etches were conducted in the same etch chamber. The CD data was collected on an SEM and was used for the CD performance analysis and comparison. In this study, it was found that the process etch CD bias in the TaN etch was significantly smaller than that in the Cr etch due to a fast etch rate of the TaN etch. The variation of the etch CD bias in the TaN etch was slightly higher than that in the Cr etch. The global pattern density changes had less impact to the Cr etch bias than the TaN etch bias.

In this study, the known optical alternating phase shift mask (APSM) issues, such as imaging imbalance, phase error, and EUVL APSM fabrication specific issues are explored in the case of extreme ultra-violet lithograph (EUVL). EUVL APSM fabrication starts with a phase step in the substrate followed by Mo/Si multi-layer (ML) deposition. This phase step can be created either by directly etching the step into the substrate or by depositing a thin film layer and then patterning the thin film layer. The advantage of depositing thin film layer is that the step control can be obtained via thin film deposition thickness control and the thin film high etch selectivity to the substrate. The first few orders of phase steps, which is determined by [λ/(4cosθ)](2m+1), with m=0, 1, 2, -, are viable in EUVL APSM fabrication. The smallest phase step needed to induce a 180-degree phase shift is λ/(4conθ), or 3.37nm, for λ=13.4nm. In most of cases, the thin film quality or smoothness increases as the film thickness increases. In other word, using the minimum step may not be ideal from process point of view. In the simulation, EUVL APSM with different phase step heights [all yield net 180-degreee phase difference in the adjacent regions (ignore the multiples of 360 degrees)] was evaluated. It is found that image imbalance effect in EUVL using APSM is relatively small and is negligible for the smallest step. As the step heights increase, the image imbalance effect also increases. In the case of step height of 23.58nm, which corresponding m=3, image imbalance induced space difference in the zero and 180 degree phase regions is about 1.5nm for the 25nm periodic lines. As the pitch increases, the imaging imbalance effect diminishes.

In this study, we found that there are several advantages in extreme ultra-violet lithography (EUVL) by using a dark field mask to print lines. It includes: 1) no Bossung curve tilt and focus shift effect between the dense and isolated space aerial images; 2) small proximity effect as compared to the line aerial images; 3) small flare effect as compared to the clear field mask; 4) mask fabrication requires positive mask space bias, i.e., to size mask spaces larger than the printed target critical dimension (CD) to compensate the mask shadowing effect as apposed to the negative mask line bias (i.e., to size the mask lines smaller than the printed target CD) for the clear field mask. Positive mask bias will relax mask patterning resolution limitation.

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 2 buffer layer on the square quartz mask substrate.

Development of manufacturing infrastructure is required to ensure a commercial source of mask substrates for the timely introduction of EUVL. Improvements to the low thermal expansion materials that compose the substrate have been made, but need to be scaled to production quantities. We have been evaluating three challenging substrate characteristics to determine the state of the infrastructure for the finishing of substrates. First, surface roughness is on track and little risk is associated with achieving the roughness requirement as an independent specification. Second, with new flatness-measuring equipment just coming on line, the vendors are poised for improvement toward the SEMI P37 flatness specification. Third, significant acceleration is needed in the reduction of defect levels on substrates. The lack of high-sensitivity defect metrology at the vendors' sites is limiting progress in developing substrates for EUVL.

The success of extreme ultraviolet lithography (EUVL) mask fabrication process depends on the correct choice of EUVL mask buffer and absorber materials. There are four criterion to take into account for the right buffer and absorber materials for EUVL masks. The first is the material choice impact on the mask quality such as critical dimension (CD) control, defect control, registration control, cleaning durability. The second is the impact on the mask lithography performance such as imaging contrast, shadowing effect, focus shift effect, thermal effect. The third is the impact on the mask inspection and repair, such as repair etch rate and selectivity to the Mo/Si multiplayer (ML) and optical inspection contrast of the absorber to the buffer layer as well as that of the absorber to the ML. The last is the impact on the lithography integration, such as mask stress due to buffer and absorber versus mask chucking, mask conductivity versus mask protection scheme. In the past, most attention on buffer and absorber material consideration has been paid to the processing side (as long as the absorber materials have a relatively high EUV absorption). The mask performance difference with different buffer and absorber combinations has not been fully explored. In this paper, the impact of buffer and absorber material properties on all these aspects will be discussed with special emphasis of the material property impact to the mask lithographic performance. Detailed simulation studies of EUVL mask performance with different combinations of buffer and absorber materials will be presented. It, however, will be shown that buffer and absorber material properties such as the real and imaginary part of the index of refraction coefficient, play an important role in mask lithographic performance such as the image contrast, the shadowing effect, and the focus shift effect, especially at the small design rules, such as 30nm generations. The comparison of different buffer and absorber material properties and the corresponding mask performances will allow us to understand the tradeoffs between the choice of different buffer and absorber combinations so that the most promising buffer and absorber materials can be identified and corresponding mask fabrication process can be developed.

Static and scanned images of 100 nm dense features were successfully obtained with a developmental set of projection optics and a 500W drive laser laser-produced-plasma (LPP) source in the Engineering Test Stand (ETS). The ETS, configured with POB1, has been used to understand system performance and acquire lithographic learning which will be used in the development of EUV high volume manufacturing tools. The printed static images for dense features below 100 nm with the improved LPP source are comparable to those obtained with the low power LPP source, while the exposure time was decreased by more than 30x. Image quality comparisons between the static and scanned images with the improved LPP source are also presented. Lithographic evaluation of the ETS includes flare and contrast measurements. By using a resist clearing method, the flare and aerial image contrast of POB1 have been measured, and the results have been compared to analytical calculations and computer simulations.

The printability of an extreme ultraviolet lithography (EUVL) contact layer defect mask was studied using numerical analyses. Three-dimensional models using rigorous electromagnetic simulations were used to predict the mask printing process at the 30 nm technology node. Two types of EUVL mask contact layer defects were studied, i.e., absorber and buffer layer defects. For the absorber, corner and center defects were evaluated and compared for different contact sizes. For the buffer layer, corner defects were investigated for three different materials: ruthenium, carbon, and silicon oxide. It was found that center contact defects were worse than corner defects. For buffer layer defects, the study showed that for a given size, ruthenium defects have the worst effect, followed by carbon and silicon oxide.

While interferometry is routinely used for the characterization and alignment of lithographic optics, the ultimate performance metric for these optics is printing in photoresist. The comparison of lithographic imaging with that predicted from wavefront performance is also useful for verifying and improving the predictive power of wavefront metrology. To address these issues, static, small-field printing capabilities have been added to the EUV phase-shifting point diffraction interferometer (PS/PDI) implemented at the Advanced Light Source at Lawrence Berkeley National Laboratory. The combined system remains extremely flexible in that switching between interferometry and imaging modes can be accomplished in approximately two weeks. Relevant printing studies with the ETS projection optics require illumination partial coherence with a of approximately 0.7. However, this a value is very different from the coherent illumination naturally provided by synchrotron undulator beamline and required by the high accuracy EUV PS/PDI. Adding printing capabilities to the PS/PDI experimental system has thus necessitated the development of an alternative illumination system capable of quantitatively reducing the inherent coherence of the beamline. The implemented illuminator is an angular scanning system capable of in situ coherence control. Moreover, this illuminator design readily enables the implementation of conventional resolution-enhancing pupil fills and modeling of the unique Engineering Test Stand (ETS) pupil fill. This new static microfield exposure tool has been used to lithographically characterize the static imaging performance of the ETS Set-2 projection optics. Excellent performance has been demonstrated down to the 70-nm 1:1 line/space level with a focus latitude exceeding 1 μm and a dose latitude of approximately 8%. Moreover, dense-line printing down to a resolution of 50 nm has been demonstrated.

For optical inspection of Extreme Ultraviolet Lithography (EUVL) masks using Deep Ultraviolet (DUV) light, contrast from reflected light is used to form the image of the mask and detect the defects. The inspectability of a patterned mask depends on the optical properties, surface conditions and thickness of absorber and buffer layer. The issue in EUVL mask inspection is the relatively low image contrast in the inspection tool because both the EUV-reflective and EUV-absorbing regions reflect DUV light. The need of a buffer layer to protect the multilayer (ML) reflector during mask processing and defect repair necessitates two inspections for a patterned mask: one with the buffer layer on to find the defect for repair and one with the buffer layer removed to qualify a final mask. Since the ML appears bright at DUV inspection wavelengths, the buffer layer is also chosen to give high reflectivity. Therefore, the absorber reflectivity must be low enough to provide high image contrast and to avoid the edge interference effect. Recently, we have developed a surface treatment process to reduce the reflectivity of absorber layer and result in a DUV contrast approaching 90 percent. This greatly improves the optical inspectability of EUVL mask to a level similar to conventional transmission mask. In this paper, we describe the overall EUVL mask inspection strategy and present a comprehensive discussion on mask optimization in materials selection and modification for high inspectability. We report the reflectivity of Mo-Si multilayer, buffer layers using SiO2 and Ru, and absorber layers of Cr and TaN. We will demonstrate with DUV inspection images of the optimized EUVL masks that the image contrast and quality from reflected light are close to those of conventional photo-masks with transmitted light. This greatly enhanced EUVL mask inspectability will increase defect detectability for inspection tools and simplify image rendering in die-to-database inspection.

As minimum semiconductor critical dimension feature sizes become smaller and smaller, many adverse effects in lithography printing due to mask topography start to stand out. These effects existed in the optical lithography in the past many generations. Usually, these effects contributed to a negligible depth of focus (DOF) consumption, and therefore, were neglected. In the case of Extreme Ultraviolet Lithography (EUVL), due to the drastic reduction in wavelength, some of these adverse effects that related to the wavelength tend to magnify in the case of EUVL as compared to the deep ultraviolet (DUV) lithography. For example, the asymmetry of Bossung curves and focus shift for semi-isolated to isolated lines is found to be much less in DUV optical lithography. However, in the case of EUVL, a maximum 40nm focus offset between the dense and the isolated lines exists. This focus shift effect in EUVL has been previously discussed by C. Krautschik, et al1 and explained via the complex phase behavior for the EUVL mask that is fundamentally different from the thin mask. In this paper, we will present a detailed and simpler explanation for the cause of the Bossung curve asymmetry and focus shift effect. Our simulation study indicated that the asymmetry and focus shift effect is simply due to a phase error at the mask pattern edge. This phase error is primarily caused by the interference of the reflected light from the reflective substrate with the diffracted light at the absorber edge or boundary and with the partially transmitted light from the absorber edge. This effect is not unique for the reflective masks; it exists in transmission masks as well. The analogy of this effect to the conventional thin transmission mask can be simply made and will also be discussed in detail.

The Engineering Test Stand (ETS) is an EUV lithography tool designed to demonstrate full-field EUV imaging and provide data required to accelerate production-tool development. Early lithographic results and progress on continuing functional upgrades are presented and discussed. In the ETS a source of 13.4 nm radiation is provided by a laser plasma source in which a Nd:YAG laser beam is focused onto a xenon- cluster target. A condenser system, comprised of multilayer-coated and grazing incidence mirrors, collects the EUV radiation and directs it onto a reflecting reticle. The resulting EUV illumination at the reticle and pupil has been measured and meets requirements for acquisition of first images. Tool setup experiments have been completed using a developmental projection system with (lambda) /14 wavefront error (WFE), while the assembly and alignment of the final projection system with (lambda) /24 WFE progresses in parallel. These experiments included identification of best focus at the central field point and characterization of imaging performance in static imaging mode. A small amount of astigmatism was observed and corrected in situ, as is routinely done in advanced optical lithographic tools. Pitch and roll corrections were made to achieve focus throughout the arc-shaped field of view. Scan parameters were identified by printing dense features with varying amounts of magnification and skew correction. Through-focus scanned imaging results, showing 100 nm isolated and dense features, will be presented. Phase 2 implementation goals for the ETS will also be discussed.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The Engineering Test Stand (ETS) is a developmental lithography tool designed to demonstrate full-field EUV imaging and provide data for commercial-tool development. In the first phase of integration, currently in progress, the ETS is configured using a developmental projection system, while fabrication of an improved projection system proceeds in parallel. The optics in the second projection system have been fabricated to tighter specifications for improved resolution and reduced flare. The projection system is a 4-mirror, 4x-reduction, ring-field design having a numeral aperture of 0.1, which supports 70 nm resolution at a k{sub 1} of 0.52. The illuminator produces 13.4 nm radiation from a laser-produced plasma, directs the radiation onto an arc-shaped field of view, and provides an effective fill factor at the pupil plane of 0.7. The ETS is designed for full-field images in step-and-scan mode using vacuum-compatible, magnetically levitated, scanning stages. This paper describes system performance observed during the first phase of integration, including static resist images of 100 nm isolated and dense features.

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.

A model has been developed to predict the cost of extreme ultraviolet lithography (EUVL) masks. The mask blank for EUVL consists of a low thermal expansion material substrate having a square photomask form factor and is coated with reflective Mo/Si multilayers. Absorber layers are deposited on the multilayer and patterned. EUVL mask patterning will use evolutionary improvements in mask patterning and repair equipment. One of the challenges in implementing EUVL is to economically fabricate multilayer-coated mask blanks with no printable defects. The model of mask cost assigns yield and time required for each of the steps in fabricating EUVL masks from purchase of a polished substrate to shipment of a patterned mask. Data from present multilayer coating processes and present mask patterning processes are used to estimate the future cost of EUVL masks. Several of the parameters that significantly influence predicted mask cost are discussed in detail. Future cost reduction of mask blanks is expected from learning on substrate fabrication, improvements in low defect multilayer coating to consistently obtain

A Cr film is one of the attractive materials that have been evaluated as an absorber in the advanced mask development for the extreme ultraviolet lithography (EUVL). The EUV absorber material needs to meet the requirement in EUV absorbance, mask process, inspection, repair and others. Two EUV masks were fabricated in a research laboratory and tested on a lOx reduction EUV exposure system. The processes of fabricating these two masks started with an 8-inch silicon wafer blank that had the Mo/Si multilayer (ML). A Cr film was selected as an absorber for both masks. A Si02 film, served as a buffer layer, was applied to one of the masks. The mask patterning was carried on a conventional I-line exposure tool following plasma dry etch for pattern transfer. The functionality of the two masks was tested in a resist image printing. This paper reports the EUV mask fabrication process and discusses the two different approaches to fabricate an EUV mask. 80 nm resist image features were resolved on a lOx reduction EUV exposure system by using these two masks.

In this paper, we will present our research work in EUVL mask absorber characterization and selection. The EUV mask patterning process development depends on the choice of EUVL mask absorber material, which has direct impact on the mask quality such as critical dimension (CD) control, and registration. EUVL mask absorber material selection consideration involves many aspects of material properties and processes. These include film absorption at EUV wavelength, film emissivity, film stress, mask CD and defect control, defect inspection contrast, absorber repair selectivity to the buffer layer, etc. The selection of the best candidate is non-trivial since no material is found to be superior in all aspects. In an effort of searching the best absorber materials and processes, we evaluated Al-Cu, Ti, TiN, Ta, TaN, and Cr absorbers. The comparison of material intrinsic properties and process properties allowed us to focus on the most promising absorbers and to further develop the corresponding processes to meet EUVL requirement.

In this paper, the effects of mask line edge roughness (LER) transfer in both deep ultra-violet (DUV) and extreme ultra- violet (EUV) lithography patterning is studied experimentally. In order to understand how the mask LER transfers in the wafer printing process under both DUV and EUV lithography, an optical mask with programmed LER was fabricated. The EUV mask was further fabricated by using the optical mask via optical lithography pattern transfer. The programmed LER on both optical mask, EUV mask, and printed resist images were characterized, measured, and compared. The data analysis showed that the mask LER in both DUV and EUV cases transferred to wafer with a scaling factor much less than one regardless of lithographic k 1 -factors. The LER transfer does not resemble the case of mask CD error transfer, in which scale factors greater than one for small lithographic k 1 -factor occur. The primary reason for the difference is that the amount of mask LER transferred to the wafer strongly depends on the total area of the LER, i.e. depends on both vertical and horizontal dimension of the LER. This case is very similar to that of a single mask defect transfer.

We report on the comparison of defect printability experimental results with at-wavelength defect inspection and printability modeling at extreme ultraviolet (EUV) wavelengths. Two sets of EUV masks were fabricated with nm- scale substrate defect topographies patterned using a sacrificial layer and dry-etch process, while the absorber pattern was defined using a subtractive metal process. One set of masks employed a silicon dioxide film to produce the programmed defects, whereas the other set used chromium films. Line-, proximity- and point-defects were patterned and had lateral dimensions in the range of 0.2 micrometer X 0.2 micrometer to 8.0 micrometer X 1.5 micrometer on the EUV reticle, and a topography in the range of 8 nm - 45 nm. Substrate defect topographies were measured by atomic force microscopy (AFM) before and after deposition of EUV-reflective Mo/Si multilayers. The programmed defect masks were then characterized using an actinic inspection tool. All EUVL printing experiments were performed using Sandia's 10x- reduction EUV Microstepper, which has a projection optics system with a wavefront error less than 1 nm, and a numerical aperture of 0.088. Defect dimensions and exposure conditions were entered into a defect printability model. In this investigation, we compare the simulation predictions with experimental results.

In the last two years, we have developed tow Extreme UV (EUV) mask fabrication process flows, namely the substractive metal and the damascene process flows, utilizing silicon wafer process tools. Both types of EUV mask have been tested in a 10X reduction EUV exposure system. Dense lines less than 100 nm in width have been printed using both 0.6 micrometers thick top surface imaging resists and ultra-thin DUV resist. The EUV masks used in EUV lithography development work have been routinely made by using the current wafer process tools. The two EUV mask processes that we have developed both have some advantages and disadvantages. The simpler subtractive metal process is compatible with the current reticle defect repair methodologies. On the other hand, the more complex damascene process facilitates mask cleaning and particle inspection.

Extreme UV Lithography (EUVL) is one of the leading candidates for the next generation lithography, which will decrease critical feature size to below 100 nm within 5 years. EUVL uses 10-14 nm light as envisioned by the EUV Limited Liability Company, a consortium formed by Intel and supported by Motorola and AMD to perform R and D work at three national laboratories. Much work has already taken place, with the first prototypical cameras operational at 13.4 nm using low energy laser plasma EUV light sources to investigate issues including the source, camera, electro- mechanical and system issues, photoresists, and of course the masks. EUV lithograph masks are fundamentally different than conventional photolithographic masks as they are reflective instead of transmissive. EUV light at 13.4 nm is rapidly absorbed by most materials, thus all light transmission within the EUVL system from source to silicon wafer, including EUV reflected from the mask, is performed by multilayer mirrors in vacuum.

In this paper, the Extreme Ultra Violet (EUV) mask absorber repair using focused ion beam (FIB) is investigated. It is well known that focused ion beam repair for the opaque defect removal can easily cause both damage and gallium staining or contamination to the substrate. The damage and gallium staining will induce a localized transmission loss in the case of conventional transmission optical mask. The situation become even worse in the case of reflective EUV mask since gallium is very absorbent at EUV wavelength of 13.4 nm. In addition, the incident beam travels double path through the damaged region due to the reflective nature of the mask. To minimize or even eliminate the repair induced damage, we designed a mask flow to incorporate a SiO 2 sacrificial layer beneath the metal absorber layer. This sacrificial layer will become a buffer layer for the metal defect repair. The repair, therefore, will only result in the damage on the sacrificial SiO 2 layer instead of EUV reflective multilayer (ML). After the repair, the damaged and contaminated SiO 2 layer will be etched away by either wet or dry etch. In the study, we performed both high voltage of 30 kv repair and low voltage of 10 kv repair. We found that in addition to use a sacrifice layer, low voltage focused ion beam repair is also necessary to ensure the damage and contamination free ML substrate.

We report the design and initial experimental results of an actinic inspection system for extreme ultraviolet lithography mask blank defect detection. Initial bright-field and dark- field results demonstrate sensitivity to submicron size phase defects.

In extreme ultraviolet lithography (EUVL), the technology specific requirements on the mask are a direct consequence of the utilization of radiation in the spectral region between 10 and 15 nm. At these wavelengths, all condensed materials are highly absorbing and efficient radiation transport mandates the use of all-reflective optical systems. Reflectivity is achieved with resonant, wavelength-matched multilayer (ML) coatings on all of the optical surfaces - including the mask. The EUV mask has a unique architecture - it consists of a substrate with a highly reflective ML coating (the mask blank) that is subsequently over-coated with a patterned absorber layer (the mask). Particulate contamination on the EUVL mask surface, errors in absorber definition and defects in the ML coating all have the potential to print in the lithographic process. While highly developed technologies exist for repair of the absorber layer, no viable strategy for the repair of ML coating defects has been identified. In this paper the state-of-the-art in ML deposition technology, optical inspection of EUVL mask blank defects and candidate absorber patterning approaches are reviewed.