Victor E. Asadchikov

Schwarzschild soft-x-ray microscope for imaging of nonradiating objects

A soft-x-ray optical system based on multilayer mirrors was designed and fabricated for the production of magnified images of micro-objects with a spatial resolution of ~0.2 μm at a wavelength λ ≈ 20 nm. The system consists of a laser-produced plasma source, a condenser mirror, a 20× Schwarzschild objective, a filter set, and a detector. The quality of the x-ray optics and the precision of the system adjustment enabled us to achieve, for the first time to our knowledge, ~0.2-μm resolution using the full aperture (numerical aperture 0.2) of the objective and a single shot of a frequency-doubled Nd laser (pulse energy ~0.5 J, pulse duration ~1.5 ns).

X-ray study of concave surface roughness

We propose to extend x-ray scattering method to the investigation of concave surface roughness. Our approach is based on the use of the whispering gallery effect, which consists in that an x-ray beam falling tangentially on to a concave surface slides along a surface due to successive total external reflections. During its propagation the beam gradually broadens because the scattering transfers some part of radiation to the range of larger glancing angles. Fitting of a simulated angular distribution of outgoing beam to a measure done enables the determination of statistical parameters of concave surface roughness. Possible experimental schemes are analyzed by using ray-tracing technique, radiation scattering by surface roughness being modeled by the Monte Carlo method. Results of experiments in x-ray spectral region are discussed. Carbon-coated cylindrical surfaces with the radius of curvature of 6 cm and the arc angle of 45 and 60 degrees are studied with soft x-rays. The measured angular distributions of outgoing radiation are shown to agree quantitatively with the theoretical calculations when the scattering of x-rays by the surface roughness are taken into account. The rms roughness and the correlation length of the studied surface are found to be about 1.2 nm and 0.3 micrometers , respectively. The possibility of the study of concave surface roughness with the use of hard x-rays is also discussed.

X-ray investigations of supersmooth surfaces

The possibilities of X-ray scattering methods are demonstrated for the quantitative study of supersmooth surface microgeometry. It is shown that the model of a surface which takes into account layered inhomogeneities near a surface enables one to obtain better agreement between theory and experiment in comparison with the step-like model of a surface which widely used elsewhere.

Comparative study of the roughness of optical surfaces and thin films using atomic force microscopy, x-ray scattering, and light scattering methods

The surface roughness of polished glass substrates as well as metal an dielectric coatings is studied using atomic force microscopy, hard and soft x-ray scattering at the wavelength (lambda) equals 0.154 nm and 4.47 nm, and light scattering (LS) at (lambda) equals 325 nm. It is demonstrated that all the methods, permitting the determination of PSD functions in partly overlapping, partly different ranges of spatial frequency, are in a good agreement in spite of different physical principles underlying the methods. The possible reasons for some differences in the PSD functions determined form different measurements are discussed. The main of them are a more difficult interpretation of LS data when dielectric coatings are present and a limitation on the angular range of hard x-ray scattering measurements imposed by the applicability of the perturbation theory used for experimental data processing.

X-ray study of surfaces and interfaces

The analysis of the roughness of B4C films of different thickness as well as W/B4C multilayer mirrors of different periods is performed basing on AFM and x-ray scattering (XRS) measurements. It is demonstrated that the linear model of a film growth is able to describe the whole set of experimental data including films at initial island stage of growth, if suppose the relaxation processes of a film surface to depend on the film thickness. New approach to the inverse problem of x-ray reflectometry consisting in inferring the dielectric constant profile from the reflectivity data is shortly discussed.

X-ray study of the roughness of surfaces and interfaces

The potentialities of the x-ray scattering methods (XRS) for quantitative testing of supersmooth surfaces, thin films, and multilayer structures are discussed. The results of the surface roughness study with the use of XRS technique in hard and sort x-ray spectral regions are compared with independent measurements of the roughness by atomic force microscopy (AFM). It is demonstrated that the results obtained by XRS and AFM are in a very good agreement in spite of different physical principles and underlying the methods. XRS technique is applied for the roughness study of thin films which are used in applications for x-ray and UV optics. The XRS method is demonstrated to enable quantitative evaluation of PSD functions of both the film interfaces and the correlation between the substrate and film roughnesses. X-ray investigations of the correlation of the roughnesses of short-period multilayer structures are discussed as well. The use of the whispering gallery effect is demonstrated to extend the XRS method to control of the concave surface roughness.

X-ray investigations of a near surface layer of metal samples

It is demonstrated that the simplest model of a surface, assuming step-like variation of the dielectric permeability at a vacuum-matter interface, does not permit the quantitative description of all the features of x-rays reflection and scattering observed in experiment, even though the effects of the surface roughness are taken into consideration accurately. These features are much more pronounced for metals having a large number of the conductivity electrons which are bound slightly with individual atoms. Evidently, the wave function of the electron gas of a metal cannot abrupt sharply at a surface but decreases gradually into vacuum at a distance of several angstroms. To validate or disprove the hypothesis for the presence of a near surface transition layer of the electron density in metals, a set of experiments is performed. Reflection and scattering of hard and soft x-rays, and cold neutrons (which are sensitive to nuclear density distribution and insensitive to the electron one) from metal samples are measured. The independent measurements of the surface micro-topography are performed with the use of atomic force microscopy. The results obtained demonstrate the presence of a near surface layer, which is caused by gradual change of electron density, with a thickness of several angstroms.

X-ray and AFM studies of ultrathin films for EUV and soft x-ray applications

This film roughness and its correlation with the substrate microtopography are studied using x-ray scattering at (lambda) equals 0.154 nm. The approach is applied for the investigation of superthin films of several nanometers thickness, when both interfaces are responsible for x-ray scattering, and consists in the direct determination of PSD functions from a set of x-ray scattering diagrams measured at different grazing angles of the probe beam. X-ray scattering methods are demonstrated to enable the quantitative evaluation of PSD functions of external film surfaces as well as the correlation between the substrate and film roughnesses. Results of measurements of thin films of materials widely used in the fabrication of short-period multilayer mirrors are discussed. The result of the x-ray scattering measurements are compared with independent investigations of the external film surface by atomic force microscopy.

A computationally efficient version of the algebraic method for computer tomography

We propose a new fast version of the Simultaneous Algebraic Reconstruction Technique (SART) for computer tomography. The algorithm’s iteration has been made asymptotically faster with a fast Hough transform. The algorithm’s behavior has been studied with a model experiment.

Sliced zone plate for hard x-ray sources: manufacturing and testing

Fresnel hard x-ray zone plate was manufactured by sputter- sliced technology to work on 2.29 A wavelength. Cu and Cr were used to create opaque and transparent zones. The experimental setup was prepared to investigate optical properties of obtained zone plate. The focusing effect was observed. The numerical methods and software were developed to simulate imaging properties of Fresnel zone plate. Experimental data was compared with calculations.