N. I. Chkhalo

Multilayer X-ray mirrors based on La/B4C and La/B9C

Multilayer X-ray mirrors that are based on La/B4C and La/B9C and intended for the reflection of X-ray radiation in the spectral region near λ = 6.7 nm are prepared and studied. Reflection coefficients at a level of 40–60% are achieved for mirrors with various periods. The difference in the interlayer roughnesses reconstructed from the data measured in the hard and soft X-ray spectral regions is explained using a structural model with an asymmetric permittivity profile in a mirror period. A proximate technique is developed to estimate the permittivity profile in a multilayer-structure period using reflectometry data. The effect of antidiffusion Sn, Cr, and Mo barriers on the reflection coefficient of multilayer La/B4C structures is studied experimentally

Multilayer Zr/Si filters for EUV lithography and for radiation source metrology

The technique for fabrication of thin film filters with high mechanical strength, capable of withstanding the prolonged heat load of 1 W/cm2, has been developed. Freestanding multilayer Zr/Si filters of size 20×150 mm2 with high transparency of 76% at wavelength λ = 13 nm were manufactured for EUV lithography tool. We have also developed and fabricated various designs of filters (freestanding or mesh supported) with lower transparency of 40-50% for experiments with intensive EUV sources. The tests of differential pressure withstandability and heat-resistance of filter samples were fulfilled. In order to model the influence on the filter of intensive radiation of the lithography source we have tested Zr/Si film samples by the Joule heating in vacuum at residual pressure of 10-8 Torr. The testing consisted in continuous heating of Zr/Si films at the electrical power per area unit from 0.5 W/cm2 to 6 W/cm2 during long period of time (up to 2 months). The influence of the long-term heat load on the transparency of samples at λ = 13 nm and within wavelength region 0.3 - 2 μm was investigated.

A source of a reference spherical wave based on a single mode optical fiber with a narrowed exit aperture

A new type of a reference spherical wave source (SWS) based on a single mode optical fiber with a narrowed down up to the submicrometer size exit aperture is proposed. It is intended for the precision point diffraction interferometers as a source of a reference wave. Systematic experimental errors which influence the measurement accuracy of the quality of the wave fronts generated by the SWSs are considered. Experimental data on wave front deformations are given. The combined root-mean-square (rms) wave deformation for a couple of the SWSs measured in a numerical aperture of NA=0.27 reaches the value of rms=0.36 nm that corresponds to rms=0.25 nm of a single SWS or about lambda2500 for the red He-Ne laser.

Free-standing spectral purity filters for extreme ultraviolet lithography

Free-standing multilayer films consisting of Si, Zr, Mo and silicides of both metals have been fabricated and studied as spectral purity filters (SPF) for extreme ultraviolet (EUV) (13.5 nm) lithography tools. Comparative tests of multilayer SPF structures of various compositions have been performed at high power loads. It was found that a Mo/ZrSi2 structure with MoSi2 capping layers is featured with capability to withstand prolonged heating in vacuum (10−7  mbar) at 900-950°C. A technique of fabrication of large aperture free-standing multilayers was developed, and a pilot sample of the above film structure of 160 mm in diameter, 50 nm thick, with transparency at 13.5 nm above 70% was fabricated as a conceptual prototype of SPFs with large dimensions.

Roughness measurement and ion-beam polishing of super-smooth optical surfaces of fused quartz and optical ceramics

The main problems and the approach used by the authors for roughness metrology of super-smooth surfaces designed for diffraction-quality X-ray mirrors are discussed. The limitations of white light interferometry and the adequacy of the method of atomic force microscopy for surface roughness measurements in a wide range of spatial frequencies are shown and the results of the studies of the effect of etching by argon and xenon ions on the surface roughness of fused quartz and optical ceramics, Zerodur, ULE and Sitall, are given. Substrates of fused quartz and ULE with the roughness, satisfying the requirements of diffraction-quality optics intended for working in the spectral range below 10 nm, are made.

Laboratory methods for investigations of multilayer mirrors in extreme ultraviolet and soft X-ray region

In the paper, we report development of two reflectometers in IPM RAS. One enables investigation of the angular and spectral characteristics of mirrors with any shape of a reflecting surface in the 0.6-20 nm spectral range. The other, designed especially to study the influence of EUV sources on the reflecting characteristics of the mirrors, allows testing of flat samples. Owing to a high aperture ratio (solid angle of 0.034 steradian), high reflection coefficient of mirrors, powerful windowless X-ray tube, and effective detector, the reflectometer provides a resolution of the reflection coefficient variation at a level of 0.1%.

Particulars of studying the roughness of substrates for multilayer X-ray optics using small-angle X-ray reflectometry, atomic-force, and interference microscopy

The abilities of standard methods for rough surface investigation are analyzed in case of supersmooth substrates for multilayered X-ray optics. The X-ray specular reflection technique is shown to be the most adequate one. X-ray diffuse scattering and AFM can be applied only in a case of PSD-function calculation and approximation over the entire spectrum range of spatial frequencies.

A stand for a projection EUV nanolithographer-multiplicator with a design resolution of 30 nm

An original stand for a nanolithographer-multiplicator with an operating wavelength of 13.5 nm and a design spatial resolution of 30 nm has been developed in Russia at the Institute for Physics of Microstructures, Russian Academy of Sciences. A detailed description of basic principles and approaches used when constructing the stand and its key elements is given. The experiments planned to be performed using the stand are discussed.

Advanced materials for multilayer mirrors for extreme ultraviolet solar astronomy

We provide an analysis of contemporary multilayer optics for extreme ultraviolet (EUV) solar astronomy in the wavelength ranges: λ=12.9-13.3  nm, λ=17-21  nm, λ=28-33  nm, and λ=58.4  nm. We found new material pairs, which will make new spaceborne experiments possible due to the high reflection efficiencies, spectral resolution, and long-term stabilities of the proposed multilayer coatings. In the spectral range λ=13  nm, Mo/Be multilayer mirrors were shown to demonstrate a better ratio of reflection efficiency and spectral resolution compared with the commonly used Mo/Si. In the spectral range λ=17-21  nm, a new multilayer structure Al/Si was proposed, which had higher spectral resolution along with comparable reflection efficiency compared with the commonly used Al/Zr multilayer structures. In the spectral range λ=30  nm, the Si/B4C/Mg/Cr multilayer structure turned out to best obey reflection efficiency and long-term stability. The B4C and Cr layers prevented mutual diffusion of the Si and Mg layers. For the spectral range λ=58  nm, a new multilayer Mo/Mg-based structure was developed; its reflection efficiency and long-term stability have been analyzed. We also investigated intrinsic stresses inherent for most of the multilayer structures and proposed possibilities for stress elimination.

Ion-beam polishing of fused silica substrates for imaging soft x-ray and extreme ultraviolet optics.

We have studied the surface treatment of polished fused silica by neutralized Ar ions with energy of 500-1500 eV and incidence angles of 0-90°. We found the following regularities: for samples that passed the standard procedure of deep polishing (initial effective roughness σ(eff)∼0.5  nm), the effective roughness decreases to the ultrasmooth level (i.e., σ(eff)∼0.25  nm in the range of spatial frequencies q∈[4.9×10(-2)-63]  μm(-1)). The effect begins to be noticeable at the material removal of 150 nm and reaches saturation at depths of removal greater than 1 μm. For supersmooth samples (σ(eff)<0.3  nm), the effective roughness keeps the initial level at material removal down to tens of micrometers. The optimal ion energy range is 800-1300 eV (maximum smoothing effect); at higher energy some surface roughness degradation is observed. All the smoothing effects are observed at the incidence angle range θ(in)=0-35°. Increasing the ion energy above 1300 eV increases the etching rate by up to 4 μm per hour (J(ion)=0.8  mA/cm2), which allows for deep aspherization of sized substrates. The technique allows for manufacturing the optical elements for extreme ultraviolet and soft x-ray wavelength ranges with a numerical aperture of up to 0.6.