Beyond EUV lithography at 6.X nm wavelength has a potential to extend EUVL beyond the 11 nm node. To implement B-based mirrors and to enable their industrial application in lithography tools, a reflectivity level of > 70% has to be reached in near future. The authors will prove that transition from conventional La/B4C to promising LaN/B4C multilayer coatings leads to enhanced optical properties. Currently a near normal-incidence reflectivity of 58.1% @ 6.65 nm is achieved by LaN/B4C multilayer mirrors. The introduction of ultrathin diffusion barriers into the multilayer design to reach the targeted reflectivity of 70% was also tested. The optimization of multilayer design and deposition process for interface-engineered La/C/B4C multilayer mirrors resulted in peak reflectivity of 56.8% at the wavelength of 6.66 nm. In addition, the thermal stability of several selected multilayers was investigated and will be discussed.
The usable power of high-power EUV light sources at 13.5 nm and also the lifetime of source and collector optics are
currently considered to be the largest challenges encountered during the transition of EUV lithography from the current beta-tool status to high-volume manufacturing. Fraunhofer IOF Jena has developed cost-effective refurbishment technologies of multilayer-based near normal incidence collector mirrors for high-power laser-produced plasma sources. Presently, the collector mirror lifetime exceeds 80 billion laser pulses which correspond to a lifetime of several months during continuous use of the source. Together with their partners Cymer is currently carrying out a focused program to improve the collector lifetime. New multilayer coatings together with new in-situ cleaning strategies during source operation are key technology development strategies to get closer to the ultimate target of about one year collector lifetime. The paper discusses different LPP collector refurbishment strategies and presents the recent status on collector refurbishment techniques.
Optical coatings are an integral part of superior optical components. Astronomical applications (ground- and space-based)
place especially high demands on these coatings, not only with regard to their optical performance but also to
their mechanical and environmental stability, their thermal properties, and their radiation resistance. This article presents
a short overview of several coating solutions developed in recent years at Fraunhofer IOF in order to meet the
challenging demands of astronomical applications. The focus is placed on high reflective coatings for different
wavelength regions including coatings for the VUV range below 100nm, coatings for the DUV wavelength range above
100nm and VIS/NIR/IR coatings. Further, amorphous silicon layers will be introduced which can be polished to very
low roughness values and therefore can act as polishing layer for the manufacture of ultraprecise optical components
from metal substrates.
The usable power and the collector optics lifetime of high-power extreme ultraviolet light sources at 13.5 nm are
considered as the major challenges in the transitioning of EUV lithography from the current pre-production phase to
high volume manufacturing. We give a detailed performance summary of the large ellipsoidal multilayer collector
mirrors used in Cymer's laser-produced plasma extreme ultraviolet light sources. In this paper we present the optical
performance - reflectance and wavelength - of the multilayer-coated ellipsoidal collectors as well as a novel approach
for the roughness characterization of large EUV mirror optics based on light scattering measurements at 442 nm. We
also describe the optical performance and characteristics during operation of the light source and the substantial increase
of collector lifetime by the implementation of new coating designs.
As EUV lithography is on its way into production stage, studies of optics contamination and cleaning under realistic
conditions become more and more important. Due to this fact an Exposure Test Stand (ETS) has been constructed at
XTREME technologies GmbH in collaboration with Fraunhofer IOF and with financial support of Intel Corporation.
This test stand is equipped with a pulsed DPP source and allows for the simultaneous exposure of several samples. In the
standard set-up four samples with an exposed area larger than 35 mm2 per sample can be exposed at a homogeneous
intensity of 0.25 mW/mm2. A recent update of the ETS allows for simultaneous exposures of two samples with
intensities up to 1.0 mW/mm2. The first application of this alternative set-up was a comparative study of carbon
contamination rates induced by EUV radiation from the pulsed source with contamination rates induced by quasicontinuous
synchrotron radiation. A modified gas-inlet system allows for the introduction of a second gas to the
exposure chamber. This possibility was applied to investigate the efficiency of EUV-induced cleaning with different gas
mixtures. In particular the enhancement of EUV-induced cleaning by addition of a second gas to the cleaning gas was
studied.
We report on optics contamination rates induced by exposure to broad-bandwidth, high-intensity EUV radiation peaked
near 8 nm in a new beamline at the NIST synchrotron. The peak intensity of 50 mW/mm2 allows extension of previous
investigations of contamination by in-band 13.5 nm radiation at intensities an order of magnitude lower. We report nonlinear
pressure and intensity scaling of the contamination rates which is consistent with the earlier lower-intensity
studies. The magnitude of the contamination rate per unit EUV dose, however, was found to be significantly lower for
the lower wavelength exposures. We also report an apparent dose-dependent correlation between the thicknesses as
measured by spectroscopic ellipsometry and XPS for the carbon deposits created using the higher doses available on the
new beamline. It is proposed that this is due to different sensitivities of the metrologies to variations in the density of the
deposited C induced by prolonged EUV irradiation.
Comparative lifetime studies of Mo/Si multilayer mirrors have been conducted at the Exposure Test Stand (ETS) using a pulsed Xe-discharge EUV source at XTREME Technologies GmbH (Göttingen, Germany). Due to the large, homogeneous exposed sample area a multi-technique study of EUV induced carbon contamination and cleaning can be conducted using standard surface science techniques. EUV-reflectometry, X-ray photoelectron spectroscopy (XPS), small-angle X-ray reflectometry (SAXR), and Out-of-band (OOB) reflectometry (200 - 1000 nm) were applied to investigate exposed samples and study EUV-induced changes of the surface composition. With this approach the
influence of EUV-dose, cleaning-gas pressure and composition, and capping-layer material of the Mo/Si multilayer samples on the degradation and cleaning mechanism can be studied.
The demand for enhanced optical resolution in order to structure and observe ever smaller details has pushed optics development in recent years. There is increasing interest in optical components for the extreme ultraviolet (EUV) spectral region mainly as a result of the production of more powerful electronic circuits with the aid of projection lithography.
Due to absorption at wavelengths below 100 nm the penetration depth of EUV radiation into matter is only a few nano-meters. Hence, reflective optics must be used for imaging and light collection such as EUV multilayer mirrors which consist of alternating thin films with different refractive indices. This basic idea can be compared to the classic, high reflective λ/4 systems: the constructive interference of all beams reflected at the film interfaces.
At Fraunhofer IOF Jena multilayer optics development cover the full range between the soft X-rays around 2 nm wave-length and the vacuum ultraviolet. However, the paper will focus on multilayer optics for EUV lithography applications at 13.5 nm. Besides the development of high-reflective multilayers with enhanced thermal and radiation stability using interface engineering and optimized capping layers collector and imaging optics for diverse applications in the EUV spectral range have been realized. The deposition of EUV collector mirrors for high-power laser produced plasma (LPP) sources is discussed.
The paper summarizes recent progress and the present knowledge in preparation and characterization of multilayer optics for the EUV spectral range with regard to maximum optical performance, minimization of structure imperfections, reduc-tion of residual stresses as well as enhanced thermal and radiation stability.
The source output power and lifetime, including the collector optics lifetime, are among the key issues for EUV lithography systems. In order to meet the requirements for the EUV collector mirror, both the reflectivity and the long-term thermal stability of its multilayer coating have been enhanced considerably during recent development efforts. Sub-aperture ellipsoidal mirrors of different substrate materials with outer diameters of about 320 mm were coated with
laterally graded high-temperature multilayers. The interface-engineered Mo/Si multilayer mirror (MLM) coatings were optimized in terms of high peak reflectivity at 13.5 nm and working temperatures above 400°C. Thin barrier layers were introduced on both interfaces to block thermally induced interdiffusion processes of molybdenum and silicon and to provide long-term optical stability of the coating at elevated temperatures. A normal-incidence reflectance of R ~ 60 %
at 13.5 nm was measured on Si wafer samples after heating up to 600°C. No degradation of the optical properties of these multilayer coatings occurred during both long-term heating tests and multiple annealing cycles. On highly polished collector substrates with improved surface roughness a reflectance for s-polarized light exceeding peak values of R = 57 % was obtained. With optimized layer gradient the degree of wavelength matching was improved, as well,
resulting in peak reflectivity values above 56 % throughout the clear aperture for a series of measurement points across the mirror. The corresponding area-weighted 2% in-band average reflectance for this collector mirror coating exceeds 52 % for unpolarized light.
The lifetime of Mo/Si multilayer-coated projection optics is one of the outstanding issues on the road of commercialization
of extreme-ultraviolet lithography (EUVL). The application of Mo/Si multilayer optics in EUVL requires both sufficient
radiation stability and also the highest possible normal-incidence reflectivity. A serious problem of conventional
high-reflective Mo/Si multilayers capped by silicon is the considerable degradation of reflective properties due to carbonization
and oxidation of the silicon surface layer under exposure by EUV radiation.
In this study, we focus on titanium dioxide (TiO2) and ruthenium dioxide (RuO2) as promising capping layer materials
for EUVL multilayer coatings. The multilayer designs as well as the deposition parameters of the Mo/Si systems with
different capping layers were optimized in terms of maximum peak reflectivity at the wavelength of 13.5 nm and longterm
stability under high-intensive irradiation. Optimized TiO2-capped Mo/Si multilayer mirrors with an initial reflectivity
of 67.0% presented a reflectivity drop of 0.6% after an irradiation dose of 760 J/mm2. The reflectivity drop was explained
by the partial oxidation of the silicon sub-layer. No reflectivity loss after similar irradiation dose was found for
RuO2-capped Mo/Si multilayer mirrors having initial peak reflectivity of 66%.
In this paper we present data on improved reflectivity of interface-engineered TiO2- and RuO2-capped Mo/Si multilayer
mirrors due to the minimization of both interdiffusion processes inside the multilayer stack and absorption loss in the
oxide layer. Reflectivities of 68.5% at the wavelength of 13.4 nm were achieved for both TiO2- and RuO2-capped Mo/Si
multilayer mirrors.
The ability to predict the rate of reflectivity loss of capped multilayer mirrors (MLMs) under various conditions of
ambient vacuum composition, intensity, and previous dose is crucial to solving the mirror lifetime problem in an EUV
stepper. Previous measurements at NIST have shown that reflectivity loss of MLMs exposed under accelerated
conditions of dose and pressure can be a very complicated function of these variables. The present work continues this
effort and demonstrates that reflectivity loss does not scale linearly for accelerated exposure doses over the range of
0-350 J/mm2 either for partial pressures of MMA in the range 10-8-10-7 Torr or acetone in the range 10-7-10-6 Torr. We
suggest that this nonlinear scaling may be the result of a varying damage rate as the surface of the growing
contamination layer moves through the EUV standing wave created by exposure of any MLM to resonant radiation. To
further investigate the potential influence of these resonance effects, we report new measurements showing large
variations of the secondary electron yield as a function of thickness of carbon deposited on top of a MLM.
High-reflective multilayer coatings were designed at the wavelength of 106 nm and deposited by different deposition
technologies (magnetron sputtering, thermal and e-beam evaporations). Various capping layers were suggested to protect
Al/LiF coatings against the surface degradation. The microstructure and the surface morphology of all coatings were
studied by Small Angle X-ray Reflectometry (SAXR) and Atomic Force Microscopy (AFM) methods. The optical
properties were characterized in the Vacuum Ultraviolet (VUV) spectral range by VUV-reflectometry. Finally, the aging
stability of coatings was studied.
The application of multilayer optics in EUV lithography requires not only the highest possible normal-incidence reflectivity but also a long-term thermal and radiation stability at operating temperatures. This requirement is most important in the case of the collector mirror of the illumination system close to the EUV source where a short-time decrease in reflectivity is most likely. Mo/Si multilayer mirrors, designed for high normal reflectivity at the wavelength of 13.5 nm and deposited by dc magnetron sputtering, were directly exposed to EUV radiation without mitigation system. They presented a loss of reflectivity of more than 18% after only 8 hours of irradiation by a Xe-discharge source. Another problem of Mo/Si multilayers is the instability of reflectivity and peak wavelength under high heat load. It becomes especially critical at temperatures above 200°C, where interdiffusion between the molybdenum and the silicon layers is observed. The development of high-temperature multilayers was focused on two alternative Si-based systems: MoSi2/Si and interface engineered Mo/C/Si/C multilayer mirrors. The multilayer designs as well as the deposition parameters of all systems were optimized in terms of high peak reflectivity (≥ 60 %) at a wavelength of 13.5 nm and high thermal stability. Small thermally induced changes of the MoSi2/Si multilayer properties were found but they were independent of the annealing time at all temperatures examined. A wavelength shift of -1.7% and a reflectivity drop of 1.0% have been found after annealing at 500°C for 100 hours. The total degradation of optical properties above 650°C can be explained by a recrystallization process of MoSi2 layers.
The EUV source output power and the collector optics lifetime have been identified as critical key issues for EUV lithography. In order to meet these requirements a heated collector concept was realized for the first time. An ellipsoidal collector substrate with an outer diameter of 320 mm was coated with a laterally graded high-temperature multilayer. The interface-engineered Mo/Si multilayer coating was optimized in terms of high peak reflectivity at 13.5 nm and a working temperature of 400 °C. Barrier layers were introduced on both interfaces to block thermally induced interdiffusion processes of molybdenum and silicon to provide long-term optical stability of the multilayer at elevated temperatures. A normal-incidence reflectance of more than 40 % at 13.55 nm was measured after heating. After initial annealing at 400 °C for one hour, no degradation of the optical properties of these multilayer coatings occurred during both long-term heating tests for up to 100 hours and multiple annealing cycles. The successful realization of this high-temperature sub-aperture collector mirror represents a major step towards the implementation of the heated collector concept and illustrates the great potential of high-temperature EUV multilayer coatings.
Results of soft x-ray reflection measurements of Cr/Sc multilayer mirrors close to the Sc-L (λ = 3.11 nm) and C-K (λ = 4.44 nm) absorption edges are presented. In particular, normal-incidence reflectivity measurements performed at BESSY II facility revealed a reflectivity of R = 17.3% @ 3.11 nm and 7.0 % @ 4.44 nm. Simulation results show that the interface roughness in the best Cr/Sc structures are less than 0.4 nm and strongly depend on the crystal-line structure of the layers.
Most applications of Mo/Si multilayer optics in EUVL require a high normal incidence reflectivity. Using dc magnetron sputtering we achieved R = 68.8 % @ λ = 13.5 nm. Different interface-engineered Mo/X/Si/X multilayers with maximum reflectivity of 69.6 % at 13.5 nm were developed. These new multilayer mirrors consist of molybdenum and sili-con layers separated by different interdiffusion barriers (X = C and SiC). Microstructure and optical properties of the multilayers have been investigated by small and large angle Cu-Kα scattering, AFM and characterized by EUV reflectometry. A concept for material selection, thickness optimization of interdiffusion barriers and perspectives for their wide application in imaging EUVL optics will be discussed. Some applications of multilayer mirrors in EUVL require not only the highest possible normal incidence reflectivity but also a long-term and thermal stability at the operating temperatures. The Mo/C/Si/C interface-engineered were optimized in terms of high peak reflectivity at a wavelength near 13.5 nm (Rp ⩾ 60.0 %) and broad operating temperature range (T = 20 - 500°C). The best results were obtained with 0.8 nm thickness of carbon interlayers on both interfaces. Annealing in vacuum was carried out at elevated temperatures up to 650 °C for up to 100 hours. The combination of good optical properties and high thermal stability of interface - engineered Mo/C/Si/C multilayer mirrors underlines their potential for their use in EUVL optics.
The effect of elevated temperatures on the optical and structural stability of MoSi2/Si and Mo/C/Si/C multilayer coatings
was investigated. The multilayer mirrors were designed for normal-incidence reflectivity at a wavelength of about 13.5 nm. The multilayers were deposited by dc-magnetron sputtering and subsequently annealed at temperatures of 400 °C and 500 °C for 1, 10 and 100 hours. X-ray scattering, transmission electron microscopy, atomic force microscopy and normal-incidence reflectivity measurements were used for the characterization of the multilayer structures. We achieved maximal normal-incidence reflectivities of 41.2 % and 59.6 % for as-deposited MoSi2/S and Mo/C/Si/C multilayer mirrors. While the optical properties of Mo/C/Si/C multilayers changed monotonically during annealing time at temperatures of more than 400 °C, the MoSi2/Si multilayers showed a superior thermal stability up to 500 °C. New barrier layer materials were also suggested to enhance the thermal stability of Mo/Si multilayers. Interface-engineered Mo/Si multilayer mirrors were designed to combine both a high reflectivity of more than 60 % at 13.5 nm and a superior long-term thermal stability of up to 500 °C.
The paper proposes to review briefly steps of classical experimental progress towards resistant VUV-XUV coatings. It intends to address some of the new challenges of the VUV-XUV radiation resistant coatings, including material investigations, manufacturing, characterizations and active optical components.
Most applications of Mo/Si multilayer optics in Extreme ultraviolet lithography (EUVL) require a high normal incidence reflectivity. Using dc magnetron sputtering we achieved R = 68.8 % @ λ = 13.45 nm. High-reflective Mo/Si/C and high-temperature stable Mo/C/Si/C multilayer mirrors with reflectivity of 69.6 % and 61.0 % at 13.5 nm were developed. Microstructure and optical properties of the multilayers have been investigated by small and large angle Cu-Kα scattering and characterized by EUV reflectivity. Beside the periodic multilayer design, Mo/Si multilayer mirrors with increased as well as reduced bandwidth in their spectral and angular reflectance have been designed and deposited. A reflectivity of more than 20 % was achieved in the wavelength range from 13 nm to 15 nm. In addition, narrowband multilayer mirrors with a significantly reduced band-width (FWHM = 0.077 nm) basing on high order reflection have been designed and fabricated. Both the increase and the reduction of the reflection bandwidth are unavoidably connected with a decrease of peak reflectivity. Therefore, the application of such specially designed mirrors involves areas where a maximum peak reflectivity is not required, e.g. in EUV spectroscopy and for the metrology of EUV sources. According to the optics requirements of an EUVL tool, the accurate deposition of high reflective and laterally graded multilayers on ultraprecise polished substrates can be regarded as one of the major challenges of EUVL development today. To meet these requirements, a new dc magnetron sputtering system has been developed.
The demand to enhance the optical resolution, to structure and observe ever smaller details, has pushed the way towards the EUV and soft X-rays. Induced mainly by the production of more powerful electronic circuits with the aid of projection lithography, optics developments in recent years can be characterized by the use of electromagnetic radiation with smaller wavelength. The good prospects of the EUV and soft X-rays for next generation lithography systems (λ = 13.5 nm), microscopy in the "water window" (λ = 2.3 - 4.4 nm), astronomy (λ = 5 - 31 nm), spectroscopy, plasma diagnostics and EUV/soft X-ray laser research have led to considerable progress in the development of different mulilayer optics. Since optical systems in the EUV/soft X-ray spectral region consist of several mirror elements a maximum reflectivity of each multilayer is essential for a high throughput. This paper covers recent results of the enhanced spectral behavior of Mo/Si, Cr/Sc and Sc/Si multilayer optics.
Results of soft x-ray reflection measurements of Cr/Sc multilayer mirrors close to the Sc absorption edge at 3.11 nm are presented. Improvements in the deposition technology and the adjustment of the multilayer period with an accuracy of better than 0.01 nm to this absorption edge enabled a step forward towards soft x-ray mirrors with an adequate reflectance that allow the realization of normal incidence optical components in the water window. In particular, reflectivity measurements performed at the PTB reflectometer at BESSY II in Berlin revealed a reflectivity of R = 14.8% at an incidence
angle of θ = 1.5° and R = 15.0% at θ = 5°. Simulation results show that the interface widths between the Cr and Sc nanolayers are less than 0.4 nm. The annealing effect in short-period Cr/Sc multilayers was studied in the temperature range from 50°C to 500°C by X-ray scattering and transmission electron microscopy. Structural and phase transformations and the corresponding changes of the optical properties are presented and discussed.
Sc/Si multilayers were designed for normal incidence reflectivity in the wavelength range from 35 to 50 nm and were deposited by dc-magnetron sputtering. X-ray scattering of CuKα radiation, transmission electron microscopy, atomic force microscopy and chemical analysis were used for the characterization of the multilayer structures. The normal incidence reflectivity was measured as a function of the wavelength, for different layer thickness ratios, number of layers and some important sputter parameters. Maximum reflectivities of 21% @ 38 nm and 54 % @ 46 nm for Sc/Si multilayer mirrors were achieved. Reflectivity up to 56 % @ 44.7 nm for a Sc/Si multilayers having enhanced interface structure
due to Cr diffusion barriers will be under discussion. The increase in reflectivity is consistent with multilayers having sharper and smoother interfaces. The evolutions of optical properties in the temperature range from 50°C to 250°C for classical Sc/Si and interface engineered Sc/Cr/Si/Cr multilayers will be compared.
Due to the unusual behavior of its optical constants the first transition element Sc with atomic configuration (3p64s23d) is a very attractive candidate for multilayer coatings optimized for the anomalous dispersion region of the 3p-3d transition around 28 eV (45 nm) and for the vicinity of the 2p absorption edge at 398 eV (3.12 nm), respectively. New normal incidence reflectivity data for Sc/Si at Sc 3p are shown with peak values up to 54% and for Cr/Sc at Sc 2p with peak values up to 17% are presented. The influence of optical performance on multilayer growth conditions and parameters are discussed in detail and the necessity of at-wavelength metrology for the final characterization is demonstrated. The results encourage e.g. applications for normal incidence optics used for high-power pulsed UV and x-ray laser systems and for x-ray microscopes operated in the water window.
Mo/Si multilayer mirrors with as well an increased as a reduced bandwidth in their spectral and angular reflectance have been designed and deposited for the wavelength about 13.5 nm. For the broadband mirrors, a non-periodic multilayer design based on the thickness optimization of each layer by a stochastic method is compared to a desing that consists of 3 different monoperiodic stacks. In addition, narrowband multilayer mirrors with a significantly reduced bandwidth based on high order reflection have been designed and fabricated. A near-normal peak reflectivity of more than 20% and 30% were achieved for the broadband mirros in the wavelength range from 13 nm to 15 nm and the incidence angles from 0° to 20°, respectively. The decrease of FWHM for Mo/Si mirrors from 0.5 nm to 0.07 nm was shown for suggested narrowband design. Both the increase and the reduction of the reflection bandwidth are unavoidably connected with a decrease of peak reflectivity. Therefore, the application of such mirrors involves areas where a maximum peak reflectivity is not required, e.g. in EUV spectroscopy, radiation filtration and for the characterization of EUV sources. Furthermore, the use of such mirrors in combination with a broadband plasma source will result in a higher integral reflectivity.
EUV mask blanks consist of two thin film systems deposited on low thermal expansion 6 inch substrates (LTEM). First there is the multilayer stack with around 100 alternating layers of elements with different optical properties which are topped by a capping layer. The absorber stack which consists of a buffer and a absorber layer is next. Here a minimum absorption of EUV light of 99 % is required. The stress in both layer systems should be as low as possible. The reduction of defects to an absolute minimum is one of the main challenges. The high-reflective Mo/Si multilayer coatings were designed for normal incidence reflectivity and successfully deposited on 6-inch LTEM substrates by ion-beam sputtering. X-ray scattering, transmission electron microscopy and atomic force microscopy were used for characterization of the multilayer interfaces and the surface morphology. The results are correlated to the measured normal incidence reflectivity using synchrotron radiation at the "Physikalisch- Technischen Bundesanstalt" (PTB) refelctometer at BESSY II, Berlin, Germany. A high resolution laser scanner was used to measure the particle distribution. First multilayer defect results are presented.
Cr/Sc multilayer mirrors were designed for grazing (30 and 60 degrees) and normal incidence reflection at about 3.16 nm (NKα line) wavelength. The multilayer coatings have been grown on Si-substrates using ion-assisted sputtering deposition. X-ray scattering of CuKα radiation and transmission electron microscopy were used for the characterization of the multilayer interface quality. The reflective properties of the Cr/Sc multilayers were measured with synchrotron radiation at different angles of incidence. Reflectivity values between R = 15% for near normal incidence (θ=5) and R = 29.6% or θ=60 were measured. Multilayer mirrors with periods less than 1.9 nm have amorphous Cr and Sc layers. The transition from amorphous to crystalline Cr and Sc layers takes place for multilayer periods of more than 3.1 nm and results in interface roughness development. The annealing effect in short-period Cr/Sc multilayers was studied in the temperature range from 50°C to 500°C by X-ray scattering and transmission electron microscopy. Structural and phase transformations and the corresponding changes of the optical properties are presented and discussed.
EUV multilayer mirrors with as well an increased as a reduced bandwidth in their spectral and angular reflectance have been designed and deposited with a commercial magnetron sputtering system. Concerning the broadband mirrors, a non-periodic multilayer design based on the thickness optimization of each layer by a stochastic method is compared to a design that consists of 3 different stacks. In addition, narrowband multilayer mirrors with a significantly reduced bandwidth based on high order reflection have been designed and fabricated. The EUV reflection of the samples was investigated with synchrotron radiation at the reflectometer of the PTB (Physikalisch-Technische Bundesanstalt) at BESSY II in Berlin. A reflectivity of more than 15% was reached in the whole wavelength range from 13 nm to 15 nm and a reflectivity of more than 30% was obtained for incidence angles from 0° to 20° with both designs. Both the increase and the reduction of the reflection bandwidth are unavoidably connected with a decrease of peak reflectivity. Therefore, the application of such mirrors involves areas where a maximum peak reflectivity is not required, e.g. in EUV spectroscopy and for the metrology for EUV sources. Furthermore, the use of such mirrors in combination with a broadband plasma source will result in a higher integral reflectivity.
EUV mask blanks consist of two thin film systems deposited on low thermal expansion 6 inch substrates (LTEM). First there is the multilayer stack with around 100 alternating layers of elements with different optical properties which are topped by a capping layer. Beside optimal optical properties it is also necessary to improve the heat stability of the layer system. The absorber stack which consists of a buffer and an absorber layer is next. Here a minimum absorption of EUV light of 99 percent is required. The stress in both layer systems should be as low as possible. The reduction of defects to an absolute minimum is one of the main challenges. The high-reflective Mo/Si multilayer coatings were designed for normal incidence reflectivity and successfully deposited on 6-inch LTEM substrates by ion-beam sputtering. X-ray scattering, transmission electron microscopy and atomic force microscopy were used for characterization of the multilayer interfaces and the surface morphology. The results are correlated to the measured normal incidence reflectivity using synchrotron radiation at the PTB reflectometer at BESSY II, Berlin, Germany.
Multilayer mirrors with a significantly increased bandwidth in spectral and angular reflectance have been designed and deposited with a commercial magnetron sputtering system. A non-periodic multilayer design based on the thickness optimization of each layer by a stochastic method is compared to a design which consists of 3 different stacks. The EUV reflection of the samples was investigated with synchrotron radiation at the reflectometer of the PTB (Physikalisch-Technische Bundesanstalt) at BESSY II in Berlin. A reflectivity of more than 15 percent was reached in the whole wavelength range from 13 nm to 15 nm and a reflectivity of more than 30 percent was obtained for incidence angles from 0 degrees to 20 degrees with both designs. The increase in bandwidth is unavoidably connected with a decrease of peak reflectivity. Therefore, the application of such mirrors involves areas where a maximum peak reflectivity is not required, e.g. in EUV spectroscopy and for the metrology for EUV sources. Furthermore, the use of such mirrors in combination with a broadband plasma source will result in a higher integral reflectivity.
Applications of multilayer mirrors for extreme ultraviolet lithography (EUVL) require not only a high normal incidence reflectivity but also a long lifetime and minimal residual stress. A serious problem of Mo-Si multilayers is the structural instability in the case of localized absorption of in- and outband radiation from the EUV source followed by the degradation of the multilayer. A number os solutions have been envisaged in the past, including the use of compound materials (MoSi2Si) as well as the use of C barrier layers. We focused our interest on two Si-based systems: Mo/Si and Mo2C/Si multilayer mirrors. The mirrors were designed for normal incidence reflectivity at about 13 nm wavelength and were deposited by dc magnetron sputtering. Maximum normal incidence reflectivities of 68.4%12.8 nm for Mo/Si multilayer mirrors and 66.8%12.8 nm for Mo2C/Si have been achieved. Investigating the thermal stability of the multilayers in the temperature range from 300 degree(s)C to 500 degree(s)C we found that the reflectivity of Mo/Si mirrors is drastically decreasing after annealing above 300 degree(s)C, whereas the Mo2C/Si multilayers show a superior stability up to 400 degree(s)C...500 degree(s)C. Another problem of EUV multilayer mirrors is the large residual compressive stress (-400 to - 500 MPa), which causes undesirable distortion of the substrate figure. The reduction of residual stress of Mo/Si and Mo2C/Si multilayers with annealing has been investigated. Using a slow thermal annealing (1 degree(s)C/min), it is possible to reduce the stress from -520 MPa to zero by heating the Mo/Si samples up to 310 degree(s)C. However, this results ina reflectivity drop of about 3...4%. On the other hand one can reduce the stress of a Mo2C/Si multilayer from -490 MPa to zero by annealing without a considerable reflectivity drop.
Applications of multilayer mirrors for extreme ultraviolet lithography (EUVL) require not only a high normal incidence reflectivity but also a long lifetime and a minimum residual stress. We focused our interests on a comparative study of two perspective Si-based multilayer systems: Mo/Si and Mo2C/Si. The mirrors were designed for normal incidence reflection at about 13 nm wavelength. The multilayer mirrors were deposited by dc magnetron sputtering. X-ray scattering, transmission electron microscopy, atomic force microscopy and mechanical stress measurements were used for the characterization of the multilayer structures. Maximum normal incidence reflectivities of 67.5% 13 nm for Mo/Si multilayer mirrors and 66.3% 12.8 nm for Mo2C/Si have been achieved. Investigating the thermal stability of the multilayers in the temperature range from 200 degree(s)C to 600 degree(s)C it was shown that the reflectivity of Mo/Si mirrors is drastically decreasing after annealing above 300 degree(s)C, whereas the Mo2C/Si multilayers show a superior thermal stability up to 600 degree(s)C. It was found that as-deposited Mo/Si and Mo2/C/Si multilayer mirrors have a similar level of compressive stress of -520+/- 20 MPa. The reduction of residual stress in Mo/Si and Mo(subscript 2C/Si multilayer systems with post-deposition thermal treatment has been investigated. Using a slow thermal annealing (2 degree(s)C/min), it is possible to reduce the stress form -520 MPa to nearly zero by heating the Mo/Si specimen up to ~310 degree(s)C. However, it results in a reflectivity drop of ~3% (absolute) at the wavelength of 13 nm. For the Mo2C/Si system, a stress reduction from -520 MPa to nearly zero is possible by a post annealing at ~290 degree(s)C without a considerable drop in the reflective properties.
The effect of elevated temperature on the structural stability of Mo/Si, Mo2C/Si and Mo/Mo2C/Si/Mo2C (dMo2C equals 0.6 nm) multilayers was investigated. The multilayers deposited by dc magnetron sputtering are annealed at temperatures ranging from 200 degree(s)C to 700 degree(s)C. The multilayer mirrors were designed for normal incidence reflectivity at about 13 nm wavelength. X-ray scattering, transmission electron microscopy and atomic force microscopy were used for characterization of the multilayer structures. The results are correlated to the measured normal incidence reflectivity using synchrotron radiation. We achieved maximal normal incidence reflectivities of 61.8% at 13.0 nm wavelength for Mo2C/Si and 59.9% at 13.3 nm for Mo/Si multilayers having Mo2C diffusion barriers. While the reflectivity of Mo/Si multilayers decreased considerably after annealing above 300 degree(s)C the Mo2C/Si multilayers showed a superior thermal stability up to 600 degree(s)C.
Many applications of multilayers in the EUV spectral region require not only high normal incidence reflectivity but also high thermal stability. We instigated the thermal stability of Mo/Si multilayers in comparison with the new material combination Mo2C/Si in the temperature range from 200 degrees Celsius to 700 degrees Celsius. Additionally, we deposited and studied Mo/Si multilayers having Mo2C diffusion barriers with 0.6 nm single layer thickness. The multilayer mirrors were designed for normal incidence reflectivity at about 13 nm wavelength and were deposited by dc magnetron sputtering. X-ray scattering, transmission electron microscopy and atomic force microscopy were used for characterization of the multilayer structures. The results are correlated to the measured normal incidence reflectivity using synchrotron radiation. We achieved maximal normal incidence reflectivities of 61.8% 13.0 nm wavelength for Mo2C/Si and 59.9% 13.3 nm for Mo/Si multilayers having Mo2C diffusion barriers. While the reflectivity of Mo/Si multilayers decreased considerably after annealing above 300 degrees Celsius the Mo2C/Si multilayers showed a superior thermal stability up to 600 degrees Celsius.
We have implemented a variety of stigmatic high-throughput high-resolution spectroscopic configurations in the XUV using focusing multilayer mirrors (MMs), transmission gratings (TGs), and conventional plane reflection gratings. A Type-I 1-m-long spectrograph, which comprises a couple of identical MMs with reflection peaks centered at 180 angstrom and a 1800 line/mm blazed grating operating in the second outside spectral order, has a residual astigmatism of 16 microns, a plate scale of 0.35 Angstrom/mm, a bandwidth of approximately 15 angstrom (FWHM), and a resolution of 24,000 (demonstrated). The solid angle of acceptance is a square which measures 0.03 rad multiplied by 0.015 rad. A density-dependent Stark shift of the 2p43s levels of Mg IV was observed in a laser plasma. A Type-II highly versatile spectrometer, which comprises one focusing MM and a TG used to disperse a converging beam, offers a high throughput and a moderate dispersion, with a plate scale typically in the range 5 - 50 angstrom/mm. With respect to these applications, a number of Mo-Si MMs were synthesized on fused silica substrates (r equals 2000 mm, D equals 60 mm). In combination with a point-like laser-plasma broadband radiation source, the Type-II configuration is by itself inherently suited for spectroscopic characterization of imaging MMs. Our capacity to evaluate the spectral response of MMs has improved dramatically after invoking a 5-cm2-aperture TG with a density of about 1000 lines/mm initially intended for x-ray astronomy. Stigmatic line spectra in a range of 165 - 185 angstrom were obtained in the Type-II configuration, and a resolution of 500 was demonstrated.
Thermal stability of structure, period and x-ray reflectivity of multilayer mirrors for the whole range of soft x rays with wavelengths 1 - 30 nm were studied in wide temperature range 350 - 1400 K by x-ray scattering and cross-sectional electron microscopy methods. Irradiation by He+ particles with energy 30 keV and doses 1 multiplied by 1019 -4 multiplied by 1020 ion/m2 and by 10 MeV-electrons with dose up to 10-4 Gray was carried out for evaluation of radiation stability of Mo/Si and MoSi2Si multilayer mirrors. It was shown that thermodynamic equilibrium of layer materials at their interfaces and stabilization of layer structure by impurities and heat treatment are an effective approach to multilayer x-ray optics with enhanced thermal and radiation stability.
Space test of x-ray multilayer mirrors on the surface of Russian orbital station 'Mir' revealed that after a 5-month space exposure, multilayers consisting of chemically interacting materials (Mo - Si and WSi2 - Si) preserved their periods. Preannealing at 570 K for one hour was sufficient to stabilize reflectivity at 1.54 angstroms of mirror WSi2 - Si with period 33.6 angstroms. Preannealing at 770 K for one hour was not enough to stabilize reflectivity at 1.54 angstroms of MoSi2 - Si mirror with period 77.7 angstroms (after space exposure its reflectivity was increased by 18% as a result of interface smoothing influenced by space effect agents).
The effect of elevated temperatures on the structural stability of Mo - Si and MoSi2 - Si X-ray multilayer mirrors was studied. Multilayers deposited by magnetron sputtering were annealed at temperatures ranging from 300 to 1300 K. A detailed picture of the thermally induced changes in the microstructure is obtained using several techniques including small- and large-angle X-ray scattering and transmission electron microscopy. The main causes of the degradation of Mo - Si mirrors is an interdiffusion mixing of silicon and molybdenum layers and a formation of MoSi2 in both the hexagonal and tetragonal phases. The smoothening of interfaces in MoSi2 - Si mirrors and increasing of their reflectance were observed after annealing at temperatures T < 800 K. The MoSi2 - Si mirrors undergo a catastrophic degradation at T > 1000 K caused by a crystallization of amorphous Si and a recrystallization of hexagonal MoSi2.
A novel diffraction spectroscopic instrument comprising two focusing multilayer mirrors (MMs) at near-normal incidence and a conventional blazed plane grating at grazing incidence has been implemented. A nearly perfect stigmatism and a theoretical resolving power above 6 X 104 are due to the separation of the focusing and dispersing functions. For higher throughput, MMs with nearly identical resonance reflection curves around (lambda) 0 approximately equals 135 angstroms have been synthesized employing a magnetron ion sputtering source. The instrument performance has been assessed using a laser-plasma XUV radiation source. The spectral resolution in excess of 4 X 103 and the applicability to space-resolved spectroscopy and plasma diagnosis have been demonstrated.
The high throughput soft X-ray optical system was designed, constructed and tested for obtaining of magnified images of nonradiating objects at wavelength (lambda) equals 175...200 angstroms. The optical system consists of normal incidence multilayer mirrors including condenser and Schwarzschild objective with 10X magnification and it was applied together with laser produced plasma as an X-ray source. The magnified images with 0.7 micrometers resolution were produced in one shot exposure at 0.8...1 J of incident laser energy. These results show the potential for development of table-top soft X-ray microscopes with 1 nanosecond exposure and submicron spatial resolution.
Problems of short period multilayer mirrors fabrication are discussed. Results of synthesis of multilayer structures with nanometer period are presented. The shortest period observed is 13 angstroms for W - Si and W - B4C sputtered multilayers. Measurements of near normal incidence reflectivity at (lambda) equals 31 - 32 angstroms are described for W - Sc multilayers with period about 16 angstroms. Measured reflectivity achieves 3.3% and is in good agreement with theoretical model.
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