Extreme ultraviolet multilayer for the FERMI@Elettra free electron laser beam transport system

In situ and real-time monitoring of structure formation during non-reactive sputter deposition of lanthanum and reactive sputter deposition of lanthanum nitride

A real-time synchrotron radiation study of the crystalline phase, texture formation and resulting surface roughness during deposition of thin La and LaN films is presented. For LaN, the theoretically predicted metastable wurtzite and zincblende structures were found, while La assumes the expected NaCl structure.

Lanthanum and lanthanum nitride thin films were deposited by magnetron sputtering onto silicon wafers covered by natural oxide. In situ and real-time synchrotron radiation experiments during deposition reveal that lanthanum crystallizes in the face-centred cubic bulk phase. Lanthanum nitride, however, does not form the expected NaCl structure but crystallizes in the theoretically predicted metastable wurtzite and zincblende phases, whereas post-growth nitridation results in zincblende LaN. During deposition of the initial 2–3 nm, amorphous or disordered films with very small crystallites form, while the surface becomes smoother. At larger thicknesses, the La and LaN crystallites are preferentially oriented with the close-packed lattice planes parallel to the substrate surface. For LaN, the onset of texture formation coincides with a sudden increase in roughness. For La, the smoothing process continues even during crystal formation, up to a thickness of about 6 nm. This different growth behaviour is probably related to the lower mobility of the nitride compared with the metal. It is likely that the characteristic void structure of nitride thin films, and the similarity between the crystal structures of wurtzite LaN and La₂O₃, evoke the different degradation behaviours of La/B and LaN/B multilayer mirrors for off-normal incidence at 6.x nm wavelength.

Nitridated Pd/B4C multilayer mirrors for soft X-ray region: internal structure and aging effects

Reactive sputtering with a mixture of argon and nitrogen (N₂ partial pressure of 4%, 8%, and 15%) as the working gas is used to develop the high reflectance Pd/B₄C multilayers for soft X-ray region application. Compared to the pure Ar fabricated sample, the interface roughness of the nitridated multilayer is slightly increased while the compressive stress is essentially relaxed from -623 MPa (pure Ar) to -85 MPa (15% N₂). A maximum reflectance of 32% is measured at the wavelength of 9.5 nm for the multilayer fabricated with 15% N₂. After storing the multilayers in an air environment for 6-17 months, a distinct aging effect is observed on the nitridated samples. The transmission electron microscopy results indicate that a large part of the top layers of the nitridated samples is deteriorated with severe interdiffusion, essential decrease in d-spacing, and compacted multilayer structure. The deterioration is less pronounced for the multilayers fabricated with a higher ratio of N₂. Energy dispersive X-ray spectroscopy reveals that the concentration of nitrogen and boron in the degraded area is much reduced compared to the intact layers. A primitive model of upward diffusion of nitrogen and boron is proposed to explain the aging effects of the nitridated structure.

Enhancement of soft X-ray reflectivity and interface stability in nitridated Pd/Y multilayer mirrors

Pd/Y multilayer mirrors operating in the soft X-ray region are characterized by a high theoretical reflectance, reaching 65% at normal incidence in the 8-12 nm wavelength range. However, a severe intermixing of neighboring Pd and Y layers results in an almost total disappearance of the interfaces inside the multilayer structures fabricated by direct current magnetron sputtering and thus a dramatic reflectivity decrease. Based on grazing incidence X-ray reflectometry and X-ray photoelectron spectroscopy, we demonstrate that the stability of the interfaces in Pd/Y multilayer structures can be essentially improved by adding a small amount of nitrogen (4-8%) to the working gas (Ar). High resolution transmission electron microscopy shows that the interlayer width is only 0.9 nm and 0.6 nm for Y(N)-on-Pd(N) and Pd(N)-on-Y(N) interfaces, respectively. A well-defined crystalline texture of YN (200) is observed on the electron diffraction pattern. As a result, the measured reflectance of the Pd(N)/Y(N) multilayer achieves 30% at λ = 9.3 nm. The peak reflectivity value is limited by the remaining interlayers and the formation of the YN compound inside the yttrium layers, resulting in an increased absorption.

Pd/B4C/Y multilayer coatings for extreme ultraviolet applications near 10 nm wavelength

A new extreme ultraviolet (EUV) multilayer coating has been developed comprising Pd and Y layers with thin B4C barrier layers at each interface, for normal incidence applications near 10 nm wavelength. Periodic, nonperiodic, and dual-stack coatings have been investigated and compared with similar structures comprising either Mo/Y or Pd/B4C bilayers. We find that Pd/B4C/Y multilayers provide higher reflectance than either Mo/Y or Pd/B4C, with much lower film stress than Pd/B4C. We have also investigated the performance of periodic multilayers comprising repetitions of Pd/Y, Ru/Y, or Ru/B4C/Y, as well as Pd/B4C multilayers deposited using reactive sputtering with an Ar:N2 gas mixture in order to reduce stress: these material combinations were all found to provide poor EUV performance. The temporal stability of a periodic Pd/B4C/Y multilayer stored in air was investigated over a period of 16 months, and a slight reduction in peak reflectance was observed. Periodic Pd/B4C/Y multilayers were also found to be thermally stable up to 100°C; at higher temperatures (200°C and 300°C) we observe a slight reduction in peak reflectance and a slight increase in multilayer period. High-resolution transmission electron microscopy and selected area diffraction of an as-deposited Pd/B4C/Y film indicates a fully amorphous structure, with interfaces that are both smoother and more abrupt than those observed in a comparable Pd/B4C multilayer in which the Pd layers are polycrystalline. The new Pd/B4C/Y multilayers are suitable for normal-incidence imaging and spectroscopy applications, including solar physics, plasma physics, high-brightness EUV light sources, and others.

EIS: the scattering beamline at FERMI

The Elastic and Inelastic Scattering (EIS) beamline at the free-electron laser FERMI is presented. It consists of two separate end-stations: EIS-TIMEX, dedicated to ultrafast time-resolved studies of matter under extreme and metastable conditions, and EIS-TIMER, dedicated to time-resolved spectroscopy of mesoscopic dynamics in condensed matter. The scientific objectives are discussed and the instrument layout illustrated, together with the results from first exemplifying experiments.

La/B(4)C multilayer mirrors with an additional wavelength suppression

In this paper, the authors report on La/B(4)C multilayer mirrors designed for an incidence angle of 45° with both maximum reflectivity at a wavelength of 6.7 nm and reflectivity suppression at a wavelength of 20.1 nm. These mirrors were deposited for the EIS-TIMER at the FERMI@Elettra Free Electron Laser. The multilayer structure and optical properties were characterized using grazing incidence X-ray reflectometry with Cu-K(α) radiation and EUV reflectometry in the spectral region of 6.5 - 21.0 nm. An anti-reflective coating designed at the wavelength of 20.1 nm had to be deposited on top of the high reflective La/B(4)C multilayer mirror optimized at a wavelength of 6.7 nm. Measured reflectivities of 53.4% at the wavelength of 6.72 nm and 0.15% at the wavelength of 20.1 nm were simultaneously achieved. It is shown that the reflectivity loss at the wavelength of 6.7 nm due to the utilization of antireflective coating designed at the wavelength of 20.1 nm can be minimized up to 1.0%.

Stability and normal incidence reflectivity of W/B4C multilayer mirror near the boron K absorption edge

A multilayer structure consisting of alternate layers of W and B4C has been deposited using a magnetron sputtering system. The structure of the as-deposited and vacuum-annealed W/B4C multilayer sample has been characterized using grazing incidence x-ray reflectivity, grazing incidence diffraction, and the normal incidence reflectivity has been measured using synchrotron radiation. A two-layer model consisting of tungsten and boron carbide is presented. The multilayer structure was found to be stable after 800°C annealing. Grazing incidence x-ray diffraction measurements suggested that W is polycrystalline with small grain size. No signature of tungsten carbide or tungsten boride formation could be observed during the annealing treatments. A near normal incidence soft x-ray reflectivity (SXRR) of ~8.3% was obtained at 6.8 nm wavelength. A little drop (~1%) in SXRR after 800°C annealing suggested that there were no compositional changes within the layers during the annealing treatments.