Reflectivity Of Multilayer Mirrors Research Articles

Ultrasmooth beryllium substrates for solar astronomy in extreme ultraviolet wavelengths.

The paper describes a multistage method of forming ultrasmooth substrates based on bulk beryllium. Such substrates are suggested to be used for multilayer extreme ultraviolet mirrors of spacecraft missions on solar corona investigations in the spectral range 17.1-58.4nm. The technique for chemical nickel plating of the sample surface is described. The process parameters that provide the formation of an amorphous film with a thickness of about 100 microns are presented. The results of mechanical polishing are shown. The effective roughness of 1.3nm is obtained, which is twice lower than one achievable for a nickel-free beryllium surface. The applicability of the ion beam figuring technique is demonstrated: the initial surface roughness of a nickel film after etching with Ar ions (Eion=200-800 eV) to a depth of 250nm does not deteriorate. The amorphous silicon film deposition followed by ion polishing made it possible to reduce the microroughness (atomic force microscope frame 2×2 μm) to σ2×2=0.15 nm from the initial σ2×2=0.46 nm. The reflectivity of multilayer mirrors deposited on these substrates turned out to be close to the values obtained on "witnesses" (supersmooth silicon substrates). Moreover, for the Mg/MoSi2 mirror optimized for the wavelength λ=58.4 nm the values of the reflection coefficients of structures on the beryllium substrate and on the silicon "witness" were identical (about 28%).

Attosecond photoelectron streaking with enhanced energy resolution for small-bandgap materials.

Attosecond photoelectron streaking spectroscopy allows time-resolved electron dynamics with a temporal resolution approaching the atomic unit of time. Studies have been performed in numerous systems, including atoms, molecules, and surfaces, and the quest for ever higher temporal resolution called for ever wider spectral extent of the attosecond pulses. For typical experiments relying on attosecond pulses with a duration of 200as, the time-bandwidth limitation for a Gaussian pulse implies a minimal spectral bandwidth larger than 9eV translating to a corresponding spread of the detected photoelectron kinetic energies. Here, by utilizing a specially tailored narrowband reflective XUV multilayer mirror, we explore experimentally the minimal spectral width compatible with attosecond time-resolved photoelectron spectroscopy while obtaining the highest possible spectral resolution. The validity of the concept is proven by recording attosecond electron streaking traces from the direct semiconductor gallium arsenide (GaAs), with a nominal bandgap of 1.42eV at room temperature, proving the potential of the approach for tracking charge dynamics also in these technologically highly relevant materials that previously have been inaccessible to attosecond science.

Ion polished Cr/Sc attosecond multilayer mirrors for high water window reflectivity.

Recent advances in the development of attosecond soft X-ray sources ranging into the water window spectral range, between the 1s states of carbon and oxygen (284 eV-543 eV), are also driving the development of suited broadband multilayer optics for steering and shaping attosecond pulses. The relatively low intensity of current High Harmonic Generation (HHG) soft X-ray sources calls for an efficient use of photons, thus the development of low-loss multilayer optics is of uttermost importance. Here, we report about the realization of broadband Cr/Sc attosecond multilayer mirrors with nearly atomically smooth interfaces by an optimized ion beam deposition and assisted interface polishing process. This yields to our knowledge highest multilayer mirror reflectivity at 300 eV near normal incidence. The results are verified by transmission electron microscopy (TEM) and soft/hard X-ray reflectometry.

极紫外光学器件辐照污染检测技术

This paper reviews the "in situ"surface analysis and monitoring techniques for contamination induced in Extreme Ultraviolet Lithography( EUVL). It introduces the EUV lithography,reflective multilayer mirror and the mechanism of carbon contamination induced by EUV. It points out the requirement of the "in situ"surface analysis techniques in EUV lithography. The mainly surface analysis techniques are discussed. Analyzed results show the applied potentiality of each measurement used in the EUV optical system. Finally,it points out that the Fiber-based ellipsometry has further application prospect in "in situ"surface contamination monitoring of EUV lithography.

Spectral properties of La/B - based multilayer mirrors near the boron K absorption edge

The spectral properties of La/B, La/B(4)C, and LaN/B, LaN/B(4)C multilayer mirrors have been investigated in the 6.5-6.9 nm wavelength range based on measured B and B(4)C optical constants. Experimentally it is verified to what extent measured and tabulated optical constants are applicable for simulations of the reflectivity of these short period multilayer mirrors. The measured maximum reflectance at various wavelength values around the boron-K absorption edge is compared to calculated values from model systems. The measured reflectance profiles of La/B and La/B(4)C show a maximum at a slightly larger wavelength than calculations would predict based on the measured B and B(4)C optical constants. This is explained by the influence of a formed boron-lanthanum compound on the wavelength where the multilayer shows maximum reflectance. The maximum reflectance profiles of LaN/B and LaN/B(4)C multilayers can be described accurately by using the same boron atomic scattering factors, indicating boron in the LaN/B(4)C multilayer to be in a similar chemical state as boron in the LaN/B multilayer. It also indicates that nitridation of the La layer in the multilayer prevents the formation of La-B compounds. We show that the optimal wavelength for boron based optics is about 6.65 nm and depends on the B chemical state. Finally, using the measured B optical constants we are able to calculate the spectral response of the multilayers, enabling the prediction of the optimal parameters for the above mentioned multilayers.

Nonlinear Behavior of Decrease in Reflectivity of Multilayer Mirrors for Extreme Ultraviolet Lithography Optics by High-Flux Extreme Ultraviolet Irradiation in Various Vacuum Environments

Nonlinear Behavior of Decrease in Reflectivity of Multilayer Mirrors for Extreme Ultraviolet Lithography Optics by High-Flux Extreme Ultraviolet Irradiation in Various Vacuum Environments

Nonlinear Behavior of Decrease in Reflectivity of Multilayer Mirrors for Extreme Ultraviolet Lithography Optics by High-Flux Extreme Ultraviolet Irradiation in Various Vacuum Environments

To estimate the optics lifetime of an extreme ultraviolet lithography (EUVL) system with a contaminated mirror surface, it is indispensable to carry out an acceleration test using high-flux EUV irradiation. In this study, Mo/Si multilayer mirrors were irradiated in various vacuum environments using a contamination evaluation system with an undulator light source, and changes in their reflectivity were evaluated. The dependence of the decrease in reflectivity per unit dose on the EUV irradiance was observed in high-flux irradiation experiments in vacuum with water vapor or n-decane gas introduction at pressures lower than about 10-5 Pa. The irradiance dependence disappeared when the pressures were increased for both water vapor and decane gas. A reflectivity distribution with a doughnut shape, namely, high at the irradiation center, low in the surrounding area, and high in the non-irradiated area, was observed in the irradiation experiments during the simultaneous introduction of water vapor and decane gas. These results can be generally explained by the mechanism of the supply-controlled limit.

Design of X-ray multilayer mirrors with maximal integral efficiency

We describe several techniques to design X-ray multilayer mirrors with enhanced bandwidth and, more especially, bandwidth with a shape that optimizes its matching with a given source angular distribution. A Gaussian angular distribution source emitting at the characteristic Mo- K α radiation ( λ=0.071 nm), a radiation regularly used in X-ray analytical applications, is used for the demonstration. Three approaches are developed and compared to design multilayer structures with maximum efficiency, where efficiency is defined here as the convolution of the source and reflectance angular profiles. The first (most general) approach, which can be applied for arbitrary source profiles, is based on the exact Euler–Lagrange equation and a general expression for the multilayer mirror reflectivity of arbitrary wideband. With this method, the depth-distribution of the multilayer bi-layer thickness was found to be smooth. Numerical refinements of the obtained analytical solution were not useful, giving insignificant efficiency increases despite significant complication of the thickness depth-distribution of the multilayer stack. The efficiency of some of these depth-graded structures exceeds that of a periodic mirror by a factor of two in our case. The main factor limiting the efficiency was absorption of the radiation by the materials. The second approach is based on designing a simpler depth-graded multilayer mirror with a linear variation of bi-layer thickness and a constant thickness ratio. Such a mirror proved to have an efficiency close to the maximum possible one. The same conclusion is also true for the simplest design approach, where several periodic substructures are stacked on top of each other, our third described approach. We found that about only four substructures are necessary. In this case the optimization procedure is based on simply three fitting parameters, independently of the number of periodic substructures. We show that the efficiency decreases quickly with increasing interface width. Therefore, multilayer structures with abrupt interfaces, e.g., Mo 2C/Si multilayers instead of the more conventional Mo/Si multilayers, are preferable in practice. The efficiency also depends strongly on the mean grazing angle incident on the mirror and the angular width of the source (for a fixed number of bi-layers). This must be taken into consideration when designing wide-band multilayer coated X-ray collimators or concentrators.

Interaction of benzene with TiO2 surfaces: Relevance to contamination of extreme ultraviolet lithography mirror capping layers

The authors focus on thermal and nonthermal (radiation-induced) surface processes that affect the reflectivity of TiO2-capped multilayer mirrors used in extreme ultraviolet (EUV) lithography. Low energy electron beams mimic excitations initiated by EUV radiation. Where appropriate, comparison is made with electron bombardment in the vapor of methyl methacrylate (C5H8O2). Benzene adsorbs and desorbs reversibly on TiO2, and the steady state coverage Θ is found to be proportional to the logarithm of the benzene pressure p. This behavior is described by the Tempkin adsorption isotherm, which has the form Θ=const+logp. This isotherm is a consequence of a linear dependence of benzene adsorption energy on Θ. In addition, measurements of cross sections σ (cm2) for electron-stimulated dissociation of benzene on clean and C-covered TiO2 in the range of 10–100eV reveal surprisingly large values (e.g., ∼3.5×10−17cm2 at 10eV primary energy). Thus, low energy secondary electrons excited by EUV lithography photons are expected to contribute substantially to carbon accumulation on clean TiO2 cap layers.

Ternary systems and the effect of parity on the multilayer mirror reflectance

The possibility of increasing the maximum reflectance of multilayer x-ray mirrors based on ternary systems is demonstrated. The effect is due to separate profiling of the real and imaginary components of the dielectric permittivity, which is impossible in binary systems.

High-accuracy measurements of the normal specular reflectance

The French Laser Megajoule (LMJ) is designed and constructed by the French Commissariat à l'Energie Atomique (CEA). Its amplifying section needs highly reflective multilayer mirrors for the flash lamps. To monitor and improve the coating process, the reflectors have to be characterized to high accuracy. The described spectrophotometer is designed to measure normal specular reflectance with high repeatability by using a small spot size of 100 mum. Results are compared with ellipsometric measurements. The instrument can also perform spatial characterization to detect coating nonuniformity.

Characterization of the measurement uncertainty of a laboratory EUV reflectometer for large optics

Quality assurance for the production of optical components for extreme ultraviolet lithography (EUVL) strongly requires at-wavelength metrology. Initially, the at-wavelength characterization of mirrors and masks was developed using synchrotron radiation of electron storage rings, e.g. BESSY II. For the production process of EUV optics, however, the immediate access to metrology tools is necessary and the availability of laboratory devices is mandatory. Recently, an EUV reflectometer (EUVR) for large samples has been put into operation. It consists of a laser-produced plasma (LPP) radiation source, a monochromator and a large goniometer system. It can handle samples with diameters of up to 500 mm, thicknesses of up to 200 mm and weights of up to 30 kg. The wavelength can be varied from 10 nm to 16 nm. The spot size on the sample surface is about 2 mm in both directions. The angle of incidence can be varied from 3° to grazing incidence. In this paper, we describe the measurement uncertainties achieved in the EUVR with reference to measurements by the Physikalisch-Technische Bundesanstalt at its soft x-ray radiometry beamline at BESSY II. Using appropriate reference points for the wavelength calibration of the monochromator, the absolute wavelength uncertainty is below 4 pm in the spectral range from 11 nm to 15 nm. The measured peak reflectance of multilayer mirrors shows a constant offset of about 1% absolute and is reproduced within 0.2%.

New Extreme Ultraviolet Irradiation and Multilayer Evaluation System for Extreme Ultraviolet Lithography Mirror Contamination in the NewSUBARU

A new contamination evaluation system that can irradiate high-flux extreme ultraviolet (EUV) and measure, in situ, the reflectivity of multilayer mirrors for EUV lithography (EUVL) projection optics was constructed to develop a contamination inhibition mechanism at the NewSUBARU synchrotron radiation (SR) facility. The vacuum chambers of the systems are all metal sealed. All automatic stages in the system are driven by motors set outside the chambers. The optimum pressure of the chamber was 2 ×10-7 Pa, two orders of magnitude higher than that in the system reported last year. The partial pressure of the hydrocarbon components was also two orders of magnitude smaller than that in the previously reported system. In the first experiment using the system, the lifetime of Si-capped Mo/Si multilayer mirrors was evaluated as a function of water vapor pressure. The system can also be used to measure and map X-ray absorption near-edge structure (XANES) spectra in the irradiated area, which is very important for the in situ evaluation of the contamination mechanism.

Si adhesion interlayer effects in hydrogen passivated Si/W soft X-ray multilayer mirrors

The use of hydrogen passivation of the silicon layers in Si/W soft X-ray reflective multilayer mirrors is investigated. Standard passivation, corresponding to Si:H/W structures, led to reduced growth properties of the W layers. The additional use of atomically thin Si adhesion layers, corresponding to Si:H/Si/W, led to improved growth and increased soft X-ray reflectivity. The effects taking place at the interfaces are analysed by bright field planar TEM and in situ X-ray reflectivity, and are described in terms of interface and surface energies, with quantitatively analysis of intermixing, materials density, and geometrical optical effects.

Multilayer mirrors for µ‐XPS using a Schwarzschild objective

AbstractThe development of highly reflective multilayer mirrors for use in the wavelength region around 6 nm has made it possible to use a multilayer‐coated Schwarzschild objective for micro‐x‐ray photoemission spectroscopy. For this purpose, CoxCr1−x/C and W/C multilayers were fabricated by magnetron sputtering and the soft x‐ray reflectivities were measured. The peak reflectivity of the Cr/C multilayer was found to be the highest with ∼18.9% at a wavelength of 6.42 nm and an incident angle of 88°. It is believed that a Cr/C multilayer will provide sufficient performance for use on a Schwarzschild mirror for x‐ray photoemission spectroscopy using 6 nm x‐rays. Copyright © 2005 John Wiley & Sons, Ltd.

Creation of Multilayer Neutron Mirror with Very Sharp Interfaces and IBS Coating System with Large Deposition Area

The reflectivity of multilayer neutron mirrors depends on the interface roughness. In order to smoothen the interfaces, we have investigated multilayer deposition techniques using ion beam sputtering (IBS) method. NiC/Ti multilayers were deposited as a function of carbon mixing ratio, and were characterized. It was confirmed that the nickel grain size was decreased by deposition of Ni3C compound and the reflectivity of supermirror with 2.4Qc was improved. Furthermore, ion polishing technique was developed in combination with IBS. The dependencies of ion polishing time, ionacceleration energy and incident angle were studied. It was observed that the reflectivities and the interface roughness are obviously improved using ion-polishing. We have constructed an IBS coating system with large deposition area for the mussproduction of high performance supermirrors and curved mirrors.

EUVリソグラフィ技術の位置付けと開発動向

EUV lithography is the most promising candidate for the replication of patterns for the 45-nm technology node and below. There are many issues in EUV lithography currently under intensive investigation. They concern (1) a high-power light source, (2) aspherical optics, (3) the exposure system, (4) multilayer coatings, (5) defect-free masks and (6) resist processes. This paper reports on recent research in all these areas except the high power light source. The required output power of the light source is discussed based on a detailed analysis of throughput. However, there are many parameters related to throughput. In this regard, it should be noted that not only a light source with a higher output power but also more sensitive resists and more reflective multilayer mirrors are indispensable if EUV lithography is to become a practical tool.

Multilayer resonances sharpened by grating waveguide resonance

A multilayer planar structure comprising a highly reflective multilayer dielectric mirror and a corrugated waveguide is proposed for use as a narrow passband optical filter. The proposed filter has a much narrower linewidth than a usual Fabry–Perot cavity with two multilayer dielectric mirrors. It is shown that the narrowing of the linewidth is due to the strong spectral dependence of the phase of the wave reflected from the waveguide grating mirror. The shape of the pass band can be made symmetrical by a proper choice of the grating groove profile.

Modeling of a low-loss spot-size converter for hollow waveguides with sub-wavelength air core

We propose the integration of a tapered spot-size converter for coupling between a narrow air-core hollow waveguide and a single-mode fiber (SMF). The linear taper waveguide consisting of SiO2 is integrated with a hollow waveguide composed of highly reflective multilayer mirrors. Light is guided with total internal reflections at a SiO2 taper surface and the multiple reflections in the multilayer mirror. The core thickness of a taper SiO2 waveguide is linearly changed from 15µm to 0.6µm. The modeling result shows a coupling loss of less than 1dB between a SMF and a hollow waveguide with an air-core thickness of 1µm. A taper waveguide-induced loss is less than 0.1dB for a two-dimensional linear taper waveguide with a length of less than 1mm. Using the proposed structure, we can expect the dramatic reduction in coupling losses by 10dB. The device structure can be formed by fully planar fabrication processes based on lithography, etching and coating. The proposed spot-size converter enables us to achieve a large tunability of over 10% in propagation constant in hollow waveguides with a variable air-core.

14.5% near-normal incidence reflectance of Cr/Sc x-ray multilayer mirrors for the water window.

Cr/Sc multilayer mirrors, synthesized by ion-assisted magnetron sputter deposition, are proved to have a high near-normal reflectivity of R = 14.5% at a grazing angle of 87.5 degrees measured at the wavelength lambda = 3.11 nm, which is an improvement of more than 31% compared with previously published results. Elastic recoil detection analyses show that the mirrors contained as much as 15 at. % of N and traces of C and O. Soft x-ray reflectivity simulations reveal interface widths of sigma = 0.34 nm and an exceptionally small layer thickness drift of approximately 1.6 x 10(-5) nm/multilayer period throughout the stack. Simulations show that a reflectivity of R = 25.6% is attainable if impurities and layer thickness drift can be eliminated. The abrupt interfaces are achieved with ion assistance with a low ion energy of 24 eV and high ion-to-metal flux ratios of 7.1 and 23.1 during Cr and Sc sputter deposition, respectively. In addition, a near-normal incidence reflectivity of 5.5% for the C VI emission line (lambda = 3.374 nm) from a laser plasma source was verified.