We study Cr/Sc-based multilayer mirrors designed to work in the water window range using hard and soft x-ray reflectivity as well as x-ray fluorescence enhanced by standing waves. Samples differ by the elemental composition of the stack, the thickness of each layer, and the order of deposition. This paper mainly consists of two parts. In the first part, the optical performances of different Cr/Sc-based multilayers are reported, and in the second part, we extend further the characterization of the structural parameters of the multilayers, which can be extracted by comparing the experimental data with simulations. The methodology is detailed in the case of Cr/B4C/Sc sample for which a three-layer model is used. Structural parameters determined by fitting reflectivity curve are then introduced as fixed parameters to plot the x-ray standing wave curve, to compare with the experiment, and confirm the determined structure of the stack.
We use hard x-ray photoemission spectroscopy combined with x-ray standing waves to characterize a series of Pd/Y multilayers designed to work in the 7.5-11 nm wavelength range. The samples, prepared by magnetron sputtering, are deposited either with or without nitrogen introduced in the sputtering gas. The aimed period of the samples is 4 nm. The experiments consist in obtaining the core level spectra of the various elements for a series of grazing angles. The angular scan is made in the range given by the Bragg law, the multilayer period and the incident photon energy. Given the period of the multilayer and the presence of a 2.5 nm-thick B4C capping layer, the photon energy is chosen to be 10 keV in order to probe the first 5-6 periods of the stack. Thus the Bragg angle is a little less than 1°. Rotating the sample enables putting the nodes of the electric field at some particular location of the stack, thus to make the excitation depth-selective, probing one interface or another or the center of one given layer. The changes of the chemical shift in the Pd 2p and 3d, Y 2p and 3d, O 1s, N 1s, C 1s and B 1s as a function of the angle, that is to say as a function of the location in the stack will give information about the possible interfacial process taking place in the Pd/Y multilayers.
Stimulated emission is a fundamental process in nature that deserves to be investigated and understood in the EUV and X-ray regimes. Today this is definitely possible through high energy density FEL beams. In this context, we show evidence for soft x-ray stimulated emission from a MgO solid target pumped by extreme ultraviolet FEL pulses formed in the regime of travelling-wave amplified spontaneous emission in backward geometry. Our results combine two effects separately reported in previous works: emission in a privileged direction and existence of a material-dependent threshold, for the stimulated emission. We have developed a theoretical framework, based on coupled rate and transport equations taking into account the solid density plasma state of the target. Our model, accounts for both observed mechanisms that are the privileged direction for the stimulated emission of the Mg L2,3 characteristic emission and the pumping threshold.
We report on further development of the optical, structural performances and temporal stability of Al(1%wtSi)/Zr multilayers. The multilayers with variable periods (from 10 to 80) are fitted by four-layer model. Below 40 periods, the surface and interfacial roughnesses are increased with the period numbers, but not decrease the reflectivity of Al(1%wtSi)/Zr multilayers. Above 40 periods, such as 80 periods, the reflectivity is down to 34.7% due to larger roughness and worse interfacial boundary. To improve the reflectivity, we modify some parameters during deposition process. The results in the EUV reflectivity show that the reflectivity of the sample with 40 periods is increased from 41.2% to 48.2%. The temporal stability of Al(1%wtSi)/Zr samples with different annealing temperatures has been observed over 35 days.
Here is presented the spectroscopic study of the evolution of the first buried interfaces of a B4C capped Co/Mo2C multilayer mirror induced by thermal treatment up to 600°C. This kind of study is typically performed to simulate the response of multilayer optics working in extreme conditions, as for instance when irradiated by new high brilliance sources as Free Electron Lasers. In fact, the efficiency of multilayers is related to the optical contrast between the alternating high and low density layers, and then to the degree of interdiffusion and the creation or evolution of interface compounds. The analysis has been performed at the Co L23 edge with different soft x-ray spectroscopic techniques including diffuse and specular reflectivity, total electron and fluorescent yield at the BEAR beamline at Elettra (Trieste) (http://www.elettra.trieste.it/elettra-beamlines/bear.html). The presentation is focused on the spectroscopic results obtained by soft x-ray standing wave enhanced photoemission (XSW) from the Mo 3d, B 1s, C 1s, O 1s core levels by using a photon energy close to the Co L23 edge and corresponding to the first Bragg peak of the multilayer. The experimental results have been compared with simulations to obtain information both on the chemical state (e.g. oxidation state) and interface morphology in terms of profiles of distribution of elements and interdiffusion of B, oxidized B and C in the interface region. In summary, it is possible to conclude in favour of a good stability of the multilayer in the investigated temperature range, as confirmed by the good performance in terms of reflectivity. These results confirm the usefulness of XSW for this kind analysis of multilayer optics.
We study a periodic Co/Mo2C multilayer prepared by magnetron sputtering. The period is 4.1 nm and the sample is
designed to work around 778 eV, i.e. close to the Co 2p3/2 threshold, at a glancing angle of 11°. In this condition, strong x-ray standing waves set up within the sample. In order to probe different depths within the stack, particularly the
interfaces, the glancing angle is moved along the first Bragg peak, while, the B 1s, C 1s, Mo 3d or O 1s photoelectron
spectra, the Co Lα x-ray spectrum as well as the drain current of the sample are measured. Boron is present in the 3.5 nm
B4C capping layer and oxygen is from surface contamination. The photoelectrons bring information from the superficial
zone, i.e. the 5 first nm, while the characteristic x-rays probe the whole stack. Clear modulations of the intensity of the
studied signals as well as core level shifts are observed when going through the Bragg peak. In order to understand what
happens in the multilayer calculations of depth distributions of the electric field and the energy loss by the radiation are
made with the IMD and OPAL codes, respectively. The combination of experimental results and theoretical simulations
will enable us to determine from which place originate the various signals and to know if some interaction exists between
the Co and Mo2C layers.
The Co-based multilayers have been shown promising optical mirrors for application in the EUV and soft x-ray ranges.
Most multilayer systems cannot attain the reflectivity and resolution requirements assumed by theory because of
interdiffusion and roughness. Therefore, it is necessary to find out the excellent material possessing optical performance
in the EUV and soft x-ray ranges and propose solution to eliminate the interface imperfections or find out new efficient
combinations. Here we propose a new system, namely the periodic Co/Mo2C multilayer. The multilayer systems are
prepared by the magnetron sputtering and characterized by x-ray reflectivity at 8048 eV (Cu Kα emission) and with
synchrotron radiation in the soft x-ray range at 778 eV. The measurements are used in order to determine the structural
parameters (thickness, roughness and density) of the layers. The simulated reflectivity at 11° grazing angle with
s-polarized is calculated to be 45% at 778 eV, if there is no interaction between the layers and no interfacial roughness,
while experimentally reflectivity is limited to 25%. The relationship between the reflectivity and annealing up to a
temperature of 600°C is also investigated. It shows that the Co/Mo2C multilayer is able to work up to 600°C. First the
reflectivity increases to 27% at 300°C. After the reflectivity slightly decreases to 25% at 500°C and then we observe a
reflectivity drop to 20% at 600°C. Relationship between the structural parameters and the reflectivity values is deduced
from the fit of the experimental curves.
We have developed and elaborated a series of Mg/Co-based periodic multilayers to build efficient mirrors for the
extreme ultraviolet (EUV) range. For s-polarized light and at 45° of grazing incidence, the reflectivity of as-deposited
Mg/Co is 42.6% at 25.1 nm. X-ray emission spectroscopy and nuclear magnetic resonance measurements do not indicate
any noticeable interdiffusion at the interfaces between layers. Scanning transmission electronic microscopy images attest
the high structural quality of the stack. X-ray reflectivity (XRR) curves in the hard x-ray and EUV domains confirm this
description and estimate a weak interfacial roughness (~ 0.5 nm). Taking advantage of the magnetic character of Co, we
have performed resonant magnetic reflectivity measurements by scanning the photon energy around the Co L absorption
edge for opposite circular polarizations. The magnetization profile of the Co layers within Co/Mg determined with an
expected depth resolution of one monolayer confirms the interface abruptness. Scanning electron microscopy images and
XRR curves give evidence of the thermal stability of Mg/Co up to 300 °C. From that value, a strong change in the
sample morphology due to the delamination of the multilayer from the substrate occurs. This should account for the drastic reflectivity drop observed above this temperature. Starting from Mg/Co, we have inserted a Zr layer at one or at
the other interface or at both interfaces to estimate the effect of the introduction of a third material within the period. We
have found that Mg/Co/Zr is more efficient (50% of reflectivity) than Mg/Zr/Co and Mg/Zr/Co/Zr (~ 40%). Through
time-of-flight secondary ion mass spectrometry depth profiling and NMR measurements, we have assigned this
difference to an intermixing process when Co layers are deposited onto Zr layers.
A detector system has been developed for the soft x-ray and extreme UV ranges. It is called DUVEX and has been
designed in order to be easy to implement and use, and cheap to operate. It consists in a YAG:Ce scintillator coupled to a
photomultiplier module working in the counting mode. The system can be operated under vacuum. We report on the
design and the performances of this detector in terms of response, noise, stability and efficiency. Soft x-ray spectra of
different elements (from B to W) obtained in the wavelength dispersive mode acquitted with DUVEX are presented.
We describe some destructive and non-destructive techniques that can be useful to examine multilayers and particularly
their interfaces. The presented non-destructive techniques allow obtaining the electron structure of the sample and then
determine the chemical states of the elements in the multilayer from the analysis of the occupied (x-ray emission and
photoemission spectroscopies) or unoccupied (x-ray absorption or electron energy loss spectroscopies) states. Among the
destructive techniques we introduce secondary ion mass spectrometry and transmission electron microscopy that bring
some information about the structural quality of the samples.
Periodic multilayers of nanometric period are widely used as optical components for the X-ray and extreme UV
(EUV) ranges, in X-ray space telescopes, X-ray microscopes, EUV photolithography or synchrotron beamlines for
example. Their optical performances depend on the quality of the interfaces between the various layers: chemical
interdiffusion or mechanical roughness shifts the application wavelength and can drastically decrease the reflectance.
Since under high thermal charge interdiffusion is known to get enhanced, the study of the thermal stability of such
structures is essential to understand how interfacial compounds develop. We have characterized X-ray and EUV siliconcontaining
multilayers (Mo/Si, Sc/Si and Mg/SiC) as a function of the annealing temperature (up to 600°C) using two
non-destructive methods. X-ray emission from the silicon atoms, describing the Si valence states, is used to determine
the chemical nature of the compounds present in the interphases while X-ray reflectivity in the hard and soft X-ray
ranges can be related to the optical properties. In the three cases, interfacial metallic (Mo, Sc, Mg) silicides are evidenced
and the thickness of the interphase increases with the annealing temperature. For Mo/Si and Sc/Si multilayers, silicides
are even present in the as-prepared multilayers. Characteristic parameters of the stacks are determined: composition of
the interphases, thickness and roughness of the layers and interphases if any. Finally, we have evidenced the maximum
temperature of application of these multilayers to minimize interdiffusion.
We present the characterization of Al/SiC periodic multilayers designed for optical applications. In some samples, a thin
layer of W or Mo is added at the SiC-on-Al interfaces. We use x-ray reflectivity (XRR) in order to determine the
parameters of the stacks, i.e. thickness and roughness of all the layers. We have performed x-ray emission spectroscopy
(XES) to identify the chemical state of the Al and Si atoms present within the structure from an analysis of the shape of
the Al Kβ and Si Kβ emission bands. Finally, time of flight secondary ion mass spectrometry (ToF-SIMS) is used to
obtain the depth profile of the different elements present within the studied stacks. A fit of the XRR curves shows that
the Al/SiC multilayer present large interfacial roughness (up to 2.8 nm), which is decreased considerably (down to 1 nm
or less) when the refractory metal layers are introduced in the periodic structure. The combination of XES and ToFSIMS
allows us to conclude that in these systems the roughness is a purely geometrical parameter and not related to
chemical interfacial reactions.
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