N.V. Kovalenko

Fabrication and performance characteristics of a NiC multilayer grating for hard X-rays

Abstract We have fabricated and tested a lamella multilayer grating for the hard X-ray range with a grating period of 0.97 μm and a Ni C multilayer period of 4.0 nm. To obtain large size gratings the fabrication process included holographic lithography and ion-beam etching. The reflectivity measurements obtained using a triple-axis diffractometer have shown that the diffraction efficiency and the relative intensities of the diffracted orders strongly depend on the incident angle. A very encouraging result is the possibility to direct practically all diffracted energy to any one first order with total suppression of the other orders.

Influence of x-ray beam spatial coherence on the diffuse scattering from multilayer mirrors

The improved spatial coherence of a synchrotron radiation beam was shown experimentally to stimulate additional diffuse scattering of x rays diffracted from x-ray multilayer mirrors. Although the large-scale (tens of microns) roughness does not affect Bragg diffraction from multilayers, its presence causes phase shifts at the wave packet front. This leads to partial decay of the coherent wave packet and creates additional diffuse scattering. Additional scattering from this mechanism was observed at angles of incidence corresponding to the Bragg and Kiessig maximum angles. The properties of this scattering caused by large-scale roughness, observed due to improved x-ray beam spatial coherence, were shown experimentally to be different from those of diffuse scattering previously reported when the incoming or outgoing angle is equal to the Bragg angle. Typical breaks in the diffuse scattering intensity due to the standing-wave effect are absent, and there is obvious asymmetry of the diffuse scattering cross s...

An experimental study of the q⊥-dependence of X-ray resonant diffuse scattering from multilayers

Abstract A study of X-ray resonant diffuse scattering from the W/Si multilayer was performed to examine its dependence on the momentum transfer normally to the specular diffraction plane, q⊥. The data obtained show two evident disagreements with the present theoretical approximations. Firstly, when the incident angle was approximately equal to the Bragg angle, additional scattering concentrated in the specular diffraction plane was observed. Secondly, the q⊥-dependence of the quasi-Bragg scattering intensity obtained from these experiments is not the same, at least at the small momentum transfer, as can be obtained from the scans in the specular diffraction plane, having tendency to accumulate near this plane. The possible reasons for these phenomena are discussed.

X-ray performance of multilayer gratings: recent advances at SSRC

Abstract In this paper we present the main results at the fabrication, theoretical and experimental studies of X-ray multilayer gratings. The multilayer grating reflectivity measurements in both hard and soft X-rays were performed. The features observed in these experimental data are identified and interpreted by means of numerical simulation method of eigenvectors. The effects of diffuse X-ray scattering from Ni/C multilayer and Ni/C multilayer grating have been investigated. Novel aspects of polychromatic optics based on the application of the multilayer grating in hard X-rays are discussed.

Numerical studies of multilayer gratings using the method of eigenvectors

Abstract The method of eigenvectors has been adapted for numerical studies of lamellar multilayer gratings with a partially etched multilayer stack (phase multilayer gratings). Unlike the existing modal and differential methods, this method can be applied to thick multilayer gratings with a small grating period, and also to the case of grazing incidence of radiation. For the case of soft X-rays, the reciprocity curves have been obtained for different numbers of etched bilayers in a multilayer structure using this method. The numerical calculations based on the proposed method were used to interpret the experimental data on diffraction properties of the Ni/C multilayer grating.

MULTILAYER GRATING AS A SR MONOCHROMATOR FOR MOSSBAUER SPECTROSCOPY

Abstract The nuclear resonant diffraction of synchrotron radiation (SR) from a multilayer grating (MG) is theoretically investigated. The nuclear scattering of SR to one of the first orders of the MG allows one to suppress greatly the nonresonant SR scattering. The advantages of the MG as a SR monochromator for Mossbauer spectroscopy are studied. A new type of nuclear MG with the spatical lateral modulation of magnetic field is proposed. The possibility of a complete separation of Mossbauer photons from SR by this grating is shown

Smoothing of interfacial micron-scale roughness in a Ni/C X-ray multilayer mirror

Correlation between the roughness of neighboring interfaces (roughness cross correlation) in a Ni/C X-ray multilayer mirror (XMM) prepared by laser ablation was studied by measuring X-ray diffuse scattering (XDS). The XDS intensities in the vicinity of the first Bragg reflection were measured at different photon energies: slightly below (8.325 keV) and slightly above (8.350 keV) the nickel photoabsorption K edge. The effective screening of the contribution from the deep layers to the XDS cross section due to the strong damping of the wave field at a photon energy higher than the photoabsorption edge allowed information on the character of the in-depth roughness cross correlation in the sample to be obtained. In particular, the characteristic lateral correlation length of the roughness was 0.35 µm at a photon energy of 8.325 keV (the contribution to the XDS cross section of the entire XMM volume), and it increased to 0.4 µm at a photon energy of 8.350 keV (predominantly the contribution from the upper layers). These data give direct evidence for the mechanism of smoothing of the interfacial roughness in the process of Ni/C XMM growth on anomalously large (up to micron) spatial scales. It was found that only rough large-scale defects with sizes of ≥10 µm are reproduced reasonably well from layer to layer. The processes of viscous flow and (or) reevaporation of high-energy target ions during deposition, which is characteristic of the laser method of XMM preparation, may serve as a possible explanation of the observed phenomenon.

An extended anomalous fine structure of X-ray quasi-Bragg diffuse scattering from multilayers

Abstract An X-ray quasi-Bragg diffuse scattering anomalous fine structure technique was probed near the absorption Ni K-edge to study the interfacial structure of the Ni/C multilayer deposited by the laser ablation. Like other combinations of the EXAFS and diffraction techniques, this method has a spatial selectivity and was shown qualitatively to provide atomic structural information from the mixed interfacial layers. The possibilities and advantages of this technique are discussed.

Investigation of roughness cross correlation in a Ni/C multilayer mirror by X-ray diffuse scattering method

The possibility of applying X-ray diffuse scattering for studying roughness in multilayer X-ray mirrors, including the correlation of roughnesses of neighboring interfaces (roughness cross-correlation) is considered. It is shown that the reliability and informativeness of this method can be improved by rejecting the classical experimental schemes and using alternative schemes in which not only the intensity of diffuse scattering itself, but also its dependence on certain experimental parameters (conditions), vary. Such parameters can be the spatial coherence of incident radiation, the direction of the momentum transfer relative to the specular diffraction plane, or the X-ray wavelength. In the framework of this approach, the results of comparative measurements of diffuse scattering from a Ni/C multilayer X-ray mirror prepared by laser ablation are considered for two close values of photon energy: below (8.325 keV) and above (8.350 keV) the K absorption edge for nickel. It is shown that, in view of effective screening of deep layers in the hard photoabsorption mode, this method provides more reliable (as compared to the standard diffuse scattering method) information on the evolution of interfaces between the layers. It is found that the smoothing of roughness in the experimental sample occurs over large spatial scales such as the micrometer scale. Only large-scale defects with a size exceeding 10 µm are replicated well from layer to layer. Possible physical reasons for the observed effect are considered. It is shown that effective smoothing on the micrometer and submicrometer spatial scales is of fundamental importance for preparing multilayer X-ray mirrors with high reflectances.

The use of quasi-Bragg diffuse scattering for express measurement of changes in multilayer d-spacing

Abstract The possibility of the use of quasi-Bragg diffuse scattering for express measurements of changes in multilayer d-spacing was studied. The error of this method was shown to be minimal, if the incident or scattered angles are sufficiently far from the critical total external reflection or Bragg angles. As an example, the measurements of the W/Si multilayer mirror with linearly varying d-spacing are presented.