V. V. Kondratenko

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.

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).

Reflection mode imaging with nanoscale resolution using a compact extreme ultraviolet laser

We report the demonstration of reflection mode imaging of 100 nm-scale features using 46.9 nm light from a compact capillary-discharge laser. Our imaging system employs a Sc/Si multilayer coated Schwarzschild condenser and a freestanding zone plate objective. The reported results advance the development of practical and readily available surface and nanostructure imaging tools based on the use of compact sources of extreme ultraviolet light.

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).

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.

Thermally induced structural and phase transformations of Mo-Si and MoSi2-Si x-ray multilayer mirrors

The effect of elevated temperatures on the structural stability of Mo - Si and MoSi2 - Si X-ray multilayer mirrors was studied. Multilayers deposited by magnetron sputtering were annealed at temperatures ranging from 300 to 1300 K. A detailed picture of the thermally induced changes in the microstructure is obtained using several techniques including small- and large-angle X-ray scattering and transmission electron microscopy. The main causes of the degradation of Mo - Si mirrors is an interdiffusion mixing of silicon and molybdenum layers and a formation of MoSi2 in both the hexagonal and tetragonal phases. The smoothening of interfaces in MoSi2 - Si mirrors and increasing of their reflectance were observed after annealing at temperatures T < 800 K. The MoSi2 - Si mirrors undergo a catastrophic degradation at T > 1000 K caused by a crystallization of amorphous Si and a recrystallization of hexagonal MoSi2.

Determination of the optical constants of amorphous carbon in the EUV spectral region 40-450 eV

The extreme ultraviolet (EUV) optical constants δ(ω) and β(ω) of amorphous carbon were determined on the basis of transmission measurements at hω=18-450 eV, the first-principles calculation of the dielectric tensor at hω<25 eV, and the Kramers-Kronig calculation of δ(ω). Our optical constants generally agree with the CXRO data, excluding the vicinity of the K-edge. First-principles analysis shows that two thresholds of absorption (at 284 eV and 291 eV) found in the present study are caused, respectively, by the π- and σ- bonds. Their weights are controlled by an orientation of graphene sheets in a-carbon.

Evolution of structure, phase composition and x-ray reflectivity of multilayer mirrors Mo-(B+C) after annealing at 400-1100°C

Structural, phase and chemical stabilities of X-ray multilayer mirrors Mo-(B + C) with period in range 8 - 11.5 nm were studied at temperatures 250 to 1100 degree(s)C by small-angle and large-angle X-ray diffraction and electron microscopy methods. Two amorphizations at approximately equals 450 degree(s)C and approximately equals 750 degree(s)C and two crystallizations at approximately equals 650 degree(s)C and approximately equals 850 degree(s)C of Mo-based layers were observed due to formation of molybdenum carbides MoC (hex), (gamma) - MoC and Mo2C instead of metal Mo and formation of molybdenum borides MoB2 and Mo2B5 instead of molybdenum carbides, respectively. Both amorphizations of Mo-based layers were accompanied by smoothening of interfaces and by increase of multilayer X-ray reflectivity at (lambda) equals 0.154 nm. Both crystallizations of Mo-based layers promoted development of interface roughness and decrease of multilayers X-ray reflectivity. The destruction of Mo-(B + C) multilayers at approximately equals 1100 degree(s)C was caused by Mo2B5 layers recrystallization.

Thin-film optical constants and multilayer mirror reflectivity in ultrasoft x-ray range

Optical constants of Si, C, Mo, and Nb thin films as well as of fused quartz and float glass substrates have been determined experimentally in the (lambda) approximately equals 80 - 190 angstrom wavelength range. The dependence of optical constants on film thickness and film production technology is demonstrated. The factors influencing substance permittivity in the soft x-ray range are discussed. It is discovered that the main of them is the presence of impurities introduced into the film during its deposition. The chemical composition of multilayer Mo-Si x-ray mirrors is studied. It is shown that if O, N, and Ar impurities in Si films are taken into account, the available experimental data on the reflectivity and resolution of Mo-Si mirrors in the soft x-ray range can be described quantitatively.

Thermal stability of normal incidence multilayer mirrors for x-ray wavelength near carbon K-edge

Annealing effects in short-period multilayers Cr3C2/C, TiC/C, Cr3C2/(B+C) and CrB2/C were studied in wide temperature range approximately equals 200-1200 degree(s)C by X-ray scattering and cross-sectional microscopy. It was shown that thermodynamic equilibrium of layers 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.