Troy Walter Barbee

Layered synthetic microstructures as Bragg diffractors for X rays and extreme ultraviolet: theory and predicted performance

Recent developments in thin film technology have made possible the construction of multilayered thin film structures that act as efficient Bragg diffractors for x rays and extreme ultraviolet (EUV) radiation. These structures (which we term layered synthetic microstructures or LSMs) are analogous to multilayer interference filters for the visible spectral region and have important potential applications in many areas of x-ray/EUV instrumentation. In this paper the theory of x-ray diffraction by periodic structures is applied to LSMs, and approximate formulas for estimating their performance are presented. A more complete computation scheme based on optical multilayer theory is described, and it is shown that, by adjusting the refractive indices and thicknesses of the component layers, the diffracting properties may be tailored to specific applications. Finally, it is shown how the theory may be modified to take account of imperfections in the LSM structure and to compute the properties of nonperiodic structures.

Molybdenum-silicon multilayer mirrors for the extreme ultraviolet

Multilayer structures of molybdenum and silicon have been synthesized by sputter deposition onto flat silicon single-crystal silicon substrates and spherically ground (0.5- and 22.0-m radii) fused silica substrates and the reflectivities for 170.4-A (72.8-eV), 160.1-A (77.4-eV), and 228-A (54.4-eV) light measured at near normal incidence. Observed peak values ranged from 26.2 to 78%, the highest reflectivities occurring closest to normal incidence. Energy resolutions were ∼10 in all cases. Model calculations were performed using optical constants from the literature and experimentally determined multilayer structural parameters. In all cases the measured reflectivities were equal to or larger (by up to a factor of 2) than the calculated values, a result attributed to uncertainty in the optical constants used in the calculations. Experimental and calculated angular peak positions and energy resolutions were in good agreement. The high reflectivities of these molybdenum-silicon structures will make possible application of traditional optics approaches in the EUV and support new developments including free electron lasers.

Synthesis of amorphous niobium-nickel alloys by vapor quenching

Abstract Thin film samples (10–15 μm thick) of niobium-nickel alloys in the composition range Nb-5 to 95 at. % Ni were vapor quenched bu r.f. sputtering onto fused quartz substrates held at a temperature of 180°C. It was found that fully glassy alloys were synthesized in the composition range Nb-30 to 85 at. % Ni, which is 2.5 times larger than that reported for splat-quenched alloys. Crystallization temperatures exhibited a maximum near the deepest eutectic in this alloys system and are comparable with those of splat-quenched materials. Nearest neighbor distances are in agreement with results for splat-quenched materials while “effective particle sizes” are consistently smaller. These results are discussed relative to the mechanism by which glassy alloys are synthesized during vapor quenching and the structure of the glassy state.

Design of doubly focusing, tunable (5–30 keV), wide bandpass optics made from layered synthetic microstructures

Abstract Layered Synthetic Microstructures (LSMs) show great promise as focusing, high-throughput, hard X-ray monochromators. Experimental reflectivity vs. energy curves have been obtained on carbon-tungsten and carbon-molybdenum LSMs of up to 260 layers in thickness. Reflectivities for three flat LSMs with different bandppasses were 70% with ΔE E = 5.4%, 66% with ΔE E = 1.4%, and 19% with ΔE E = 0.6% . A new generation of variable bandwidth optics using two successive LSMs is proposed. The first element will be an LSM deposited on a substrate that can be water cooled as it intercepts direct radiation from a storage ring. It can be bent for vertical focusing. The bandpass can be adjusted by choosing interchangeable first elements from an assortment of LSMs with different bandpasses (for example, ΔE E = 0.005, 0.01, 0.02, 0.05, 0.1 ). The second LSM will consist of a multilayered structure with a 10% bandpass built onto a flexible substrate that can be bent for sagittal focusing. The result will be double focusing optics with an adjustable energy bandpass that are tunable from 5 to 30 keV.

Transition radiation as a source of x rays

The generation of transition radiation resulting from the passage of relativistic charged particles through a periodic structure can produce intense and highly directional x rays. An optimization analysis accounting for material properties, multiple scattering, and statistical random errors in foil thicknesses is used to design an x‐ray emitter. The results demonstrate that transition radiation sources can be comparable in brightness to synchrotron emitters.

Solid Fabry-Perot etalons for X-rays

Abstract Solid Fabry-Perot etalons for X-rays have been constructed using sputter deposition techniques, each etalon consisting of two Layered Synthetic Microstructures (LSM) Bragg diffraction structures separated by a carbon spacer. The individual LS mirrors contain fifteen tungsten layers ( t w = 8.5 A) separated by carbon layers ( t c = 19.1 A. The thick carbon spacers act as resonant cavities; for the structures reported on here the spacer thicknesses, t sp , are 496.6 A and 981 A. The structures were characterized at grazing incidence in reflection using Cu Kα (λ = 1.5418 A) radiation. The measured response of the etalons agrees well with calculation. Observed reflection efficiencies for Cu K α were approximately 50 percent of that calculated. This discrepancy is believed to be the result of the interfacial roughness (∼3.25 A) between component layers and the sensitivity of the etalon response to the divergence of the incident X-ray beam.

Stability of some soft-x-ray monochromator crystals in synchrotron radiation

Abstract The stability in synchrotron radiation of β-alumina and beryl has been investigated by exposing the crystals in a white beam at CHESS operating at 5.2 GeV electron energy over a period of ∼ 100 h at an average radiant power of 28 W/cm 2 . Rocking curves at the CuK α energy were used to monitor the diffraction properties before and after exposure to synchrotron radiation. A Si(220) crystal was used as the first crystal. It was found that both materials exhibit no catastrophic material failure. β-alumina is clearly proven stable with no degradation of its rocking characteristics. There was in fact indication of reflectivity improvement after exposure. Results on beryl are not conclusive because the starting material (a mineral specimen) had a lot of inherent microstructural and crystallographic defects. Rocking curves in the soft X-ray region at 10.5 A were also determined using a (10 1 0) RAP crystal as first crystal. A 19-layer 50 A d-spacing Nb-C sputtered film was also characterized for comparison.

Bragg diffractors with graded-thickness multilayers

Abstract Possible designs, characteristics and uses of multilayer diffraction elements in which the layer thickness is non-uniform are discussed.

Observation of soft x‐ray transition radiation from medium energy electrons

Soft x‐rays (2 to 6 keV) have been observed from transition radiators using electrons at energies of 104 and 66 MeV. The observed spectra agree well with theory. Various stacks of mylar of 2.5, 3.5 and 6.35 μm thickness were used to produce the radiation. Optimum foil thicknesses and number were calculated and found experimentally to be correct.

Controlled Reactive Sputter Synthesis of Refractory Oxides , The Silicon‐Oxygen System

Reactive sputter deposition of refractory oxides, under conditions in which the reaction between the sputtered species and the oxygen is isolated to the deposition surface, has been experimentally investigated, and results have been obtained for the formation of SiO /SUB x/ (0 < x < 2). It is shown that the dependence of the deposited film stoichiometry on oxygen pressure and the incidence rate of silicon can be correlated with reported results of oxygen adsorption ont single-crystal silicon surfaces and amorphous silicon surfaces. This correlation is made in terms of oxygen exposure and, in our work, a dynamic exposure (L /SUB D/ ) is defined for steady-state synthesis conditions by placing the origin of a movin reference frame at the deposition surface. This correlation shows that the sticking coefficient of oxygen onto room temperature substrates is independent of oxygen coverage (i.e., surface stoichiometry) for coverages less than 0.8. This requires that a mobile precursor adsorption process be operative for oxygen in our experiments. The deduced pressure-independent oxygen sticking coefficient is 1.5 to 4.1 x 10/sup -3/.