Soft X-ray imaging of thick carbon-based materials using the normal incidence multilayer optics.

The high transparency of carbon-containing materials in the spectral region of "carbon window" (lambda approximately 4.5-5nm) introduces new opportunities for various soft X-ray microscopy applications. The development of efficient multilayer coated X-ray optics operating at the wavelengths of about 4.5nm has stimulated a series of our imaging experiments to study thick biological and synthetic objects. Our experimental set-up consisted of a laser plasma X-ray source generated with the 2nd harmonics of Nd-glass laser, scandium-based thin-film filters, Co/C multilayer mirror and X-ray film UF-4. All soft X-ray images were produced with a single nanosecond exposure and demonstrated appropriate absorption contrast and detector-limited spatial resolution. A special attention was paid to the 3D imaging of thick low-density foam materials to be used in design of laser fusion targets.

Nanoimaging with a compact extreme-ultraviolet laser.

Images with a spatial resolution of 120-150 nm were obtained with 46.9 nm light from a compact capillary-discharge laser by use of the combination of a Sc-Si multilayer-coated Schwarzschild condenser and a free-standing imaging zone plate. The results are relevant to the development of compact extreme-ultraviolet laser-based imaging tools for nanoscience and nanotechnology.

Damage to extreme-ultraviolet Sc/Si multilayer mirrors exposed to intense 46.9-nm laser pulses.

The damage threshold and damage mechanism of extreme-ultraviolet Sc/Si multilayer mirror coatings are investigated with focused nanosecond pulses at 46.9-nm radiation from a compact capillary-discharge laser. Damage threshold fluences of approximately 0.08 J/cm2 are measured for coatings deposited on both borosilicate glass and Si substrates. The use of scanning and transmission electron microscopy and small-angle x-ray diffraction techniques reveals the thermal nature of the damage mechanism. The results are relevant to the use of newly developed high-flux extreme-ultraviolet sources in applications.

Efficient method for the determination of extreme-ultraviolet optical constants in reactive materials: application to scandium and titanium.

The chemical reaction of a sample with atmospheric gases causes a significant error in the determinantion of the complex refractive index n = 1 - delta + ibeta in the extreme-ultraviolet region. The protection of samples removes this effect but hampers the interpretation of measurements. To overcome this difficulty, we derive the exact dependences on film thickness of the reflectivity and transmissivity of a protected film. These dependences greatly simplify the determination of delta and beta when the spectra of several films with different thickness and identical protection are measured. They also allow the verification of the delta(omega) obtained from the Kramers-Kronig relation and even make the Kramers-Kronig method unnecessary in many cases. As a practical application, the optical constants of Sc and Ti are determined at h omega = 18-70 eV and 18-99 eV, respectively. The essential feature of our experimental technique is deposition of a film sample directly on a silicon photodiode that allows easy operation with both thin (approximately 10-nm) and thick (approximately 100-nm) films. The comparison of calculated reflectivities of Si-Sc multilayers with the measured values shows the high accuracy of the determined delta(omega) and beta(omega).

Skylab 3600 groove/mm replica grating with a scandium-silicon multilayer coating and high normal-incidence efficiency at 38-nm wavelength.

A Sc-Si multilayer coating was applied to a replica of the 3600 groove/mm grating, developed for the SO82A spectroheliograph that flew on the Skylab mission, for the purpose of enhancing the normal-incidence efficiency in the extreme-ultraviolet region. The efficiency, measured at an angle of incidence of 6 degrees with synchrotron radiation, had a maximum value of 7.2% at a wavelength of 38 nm and was a factor of 3 higher than the efficiency of the gold-coated Skylab grating. The measured efficiency of the Sc-Si grating was in good agreement with the efficiency calculated by use of the modified integral method.

Focusing of a tabletop soft-x-ray laser beam and laser ablation.

We focused the beam of a high-repetition-rate capillary-discharge tabletop laser operating at a wavelength of 46.9 nm, using a spherical Si/Sc multilayer mirror. The energy densities significantly exceeded the thresholds for the ablation of metals. Single-shot laser ablation patterns were used in combination with ray-tracing computations to characterize the focused beam. The radiation intensity within the 2-mum -diameter central region of the focal spot was estimated to be approximately 10(11)W/cm(2), with a corresponding energy density of ~100 J/cm(2).

High-reflectivity multilayer mirrors for a vacuum-ultraviolet interval of 35-50nm.

Sc-Si multilayers are suggested as high-reflectivity coatings for a VUV interval of 35-50 nm. Fabricated mirrors show normal-incidence reflectivity of 30-54%, which is high enough for effective manipulation of the beams of compact-discharge, laser-driven x-ray lasers. The values obtained are not, however, limits for Sc-Si coatings. Theoretical estimations as well as electron microscopy studies of Sc-Si interfaces indicate a large potential for a further increase in their reflectivity.

Evolution of structure, phase composition, and x-ray reflectivity of multilayer mirrors mo-(B + C) after annealing at 250-1100°c.

Structural, phase, and chemical stabilities of x-ray multilayer mirrors Mo-(B + C) with periods in the range 8-11.5 nm were studied at temperatures of 250-1100°C by small-angle and large-angle x-ray diffraction and electron microscopy methods. Two amorphizations at ~450 and ~750°C and two crystallizations at ~650 and ~850°C of Mo-based layers were observed, which were due to the formation of the molybdenum carbides MoC (hex), γ-MoC, and Mo2C instead of the metal Mo, and to the formation of the molybdenum borides MoB2 and Mo2B5 instead of molybdenum carbides, respectively. Both amorphizations of Mo-based layers were accompanied by smoothing of interfaces and by an increase of the multilayer x-ray reflectivity at λ = 0.154 nm. Both crystallizations of Mo-based layers promoted the development of interface roughness and a decrease of multilayer x-ray reflectivity. The destruction of Mo-(B + C) multilayers at ~ 1100°C was caused by the recrystallization of Mo2B5 layers.

Thermal Stability of Normal Incidence Multilayer Mirrors for the X-Ray Wavelength near Carbon K-Edge.

Annealing effects in the short-period multilayers Cr3C2/C, TiC/C, Cr3C2/(B + C), and CrB2/C were studied in a wide temperature range ~200-1200°C by x-ray scattering and cross-sectional electron microscopy. It was shown that the thermodynamic equilibrium of the layer materials at their interfaces and stabilization of layer structure by impurities and heat treatment are effective approaches to short-period multilayers with enhanced thermal stability of their structure and optical properties.

Thermal stability of soft x-ray Mo-Si and MoSi(2)-Si multilayer mirrors.

The thermal stability of Mo-Si multilayers prepared by magnetron sputtering is studied. It is found that degradation of x-ray reflectivity of Mo-Si multilayers under heat loading is connected with the roughening of Mo-Si interfaces and the formation of compounds Mo(x)Si(y),. To avoid these degradation mechanisms we fabricated and tested MoSi(2)-Si multilayers under heat loading. The MoSi(2)-Si multilayer appeared to be much more stable both in period and x-ray reflectivity because of thermodynamic equilibrium of the components MoSi(2) and Si at the interface. The working temperature of MoSi(2)-Si multilayers reaches 1000 K.