Boron carbide (B4C) - due to its exceptional mechanical properties - is one of the few existing materials that can withstand the extremely high brilliance of the photon beam from free electron lasers (FELs) and is thus of considerable interest for optical applications in this field. However, as in the case of many other optics operated at modern accelerator-, plasma-, or laser-based light source facilities, B4C-coated optics are subject to ubiquitous carbon contaminations. These contaminations - that are presumably produced via cracking of CHx and CO2 molecules by photoelectrons emitted from the optical components - represent a serious issue for the operation of the pertinent high performance beamlines due to a severe reduction of photon flux and beam coherence, not necessarily restricted to the photon energy range of the carbon K-edge. Thus, a variety of B4C cleaning technologies have been developed at different laboratories with varying success [1]. Here, we present a study regarding the low-pressure RF plasma cleaning of a series of carbon-contaminated B4C test samples via an inductively coupled O2/Ar and Ar/H2 remote RF plasma produced using the IBSS GV10x plasma source following previous studies using the same RF plasma source [2, 3]. Results regarding the chemistry, morphology as well as other aspects of the B4C optical coatings and surfaces before and after the plasma cleaning process are reported.
The main advantage of Bragg reflection from a multilayer mirror as a monochromator for hard X-rays, is the higher
photon flux density because of the larger spectral bandpass compared with crystal lattice reflection. The main
disadvantage lies in the strong modulations of the reflected beam profile. This is a major issue for micro-imaging
applications, where multilayer-based monochromators are frequently employed to deliver high photon flux density. A
subject of particular interest is the origin of the beam profile modifications, namely the irregular stripe patterns, induced
by the reflection on a multilayer. For multilayer coatings in general it is known that the substrate and its surface quality
significantly influence the performance of mirrors, as the coating reproduces to a certain degree the roughness and shape
of the substrate. This proceedings article reviews recent experiments that indicate potential options for producing wave
front-preserving multilayer mirrors, as well as new details on the particular mirrors our group has extensively studied in
the past.
The field of single and multilayer based optics has seen significant improvements and new applications in recent years.
In this paper, we give an overview of the numerous types of single and multilayer optics that have been developed. The
fabrication possibilities of well-known regular periodic multilayers have been driven close to the theoretical limit,
providing high resolution or high flux optics for a wide range of photon energies. In addition to that, multilayers with a
lateral gradient have been developed to be adapted to curved substrates (for example for focusing purposes) or varying
incidence angles on long, flat substrates. Depth-graded multilayers with arbitrarily selectable layer thicknesses over the
entire layer stack have been simulated and manufactured, mainly as broadband mirrors with immense bandwidths.
Finally, new applications of high precision deposition are reference sample for XRF (having several elements in low
concentrations of few ng/mm2) and TXRF (with mass deposition in the range of 1011 atoms/cm2) pay tribute to the low
detection limits achievable in modern instruments for these techniques.
We present a systematic study in which multilayers of different composition (W/Si, Mo/Si, Pd/B4C), periodicity (from
2.5 to 5.5 nm), and numbers of layers have been characterised. Particularly, we investigated the intrinsic quality
(roughness and reflectivity) as well as the performance (flatness and coherence of the outgoing beam) as a
monochromator for synchrotron radiation hard X-ray micro-imaging. The results indicate that the material composition
is the dominating factor for the performance. This is of high importance for synchrotron-based hard X-ray imaging
which has become a widely applied tool for probing the microstructure of bulk samples. The high spatial resolution and
different contrast modalities available here strongly depend on using coherent beams from highly brilliant sources. In
order to satisfy the demand for a high flux of quasi-monochromatic photons, multilayer-coated mirrors are commonly
used as monochromators. Their properties present a good tradeoff between spectral bandwidth and photon flux density.
Since the photon flux density at the sample position is higher than with standard crystal monochromators, better spatial
resolution can be reached. This comes at the cost of reduced energy resolution and stronger non-uniformities in the
incoming beam profile. By helping scientists and engineers specify the design parameters of multilayer monochromators,
our results can contribute to a better exploitation of the advantages of multilayer monochromators over crystal-based
devices; i.e., larger spectral bandwidth and high photon flux density for X-ray imaging.
To synthesize X-ray optical multilayers showing both high resolution and high reflectivity, spacer and absorber materials with low absorption coefficients for the desired spectral range are required. Beside the well-known candidates the material system Carbon / Carbon is interesting because of its low absorption coefficient over a wide spectral range and the opportunity, to deposit C-layers with different modifications, i.e. different optical properties. Simulations of C/C multilayers with different period thicknesses d and single layer densities ρ show, that reflectivities R (Cu Kα ) > 80% and a resolution ΔΘ≈0.002° can be achieved for C/C layer stacks with d= 3 nm and N= 1000 periods. An advanced large area Pulsed Laser Deposition (LA-PLD) technology was used to deposit C/C multilayers on Si-substrates up to 4" diameter. The carbon film growth conditions for the spacer and absorber layers were optimised by the variation of selected laser parameters like pulse energy and ablation wavelength, to achieve a sufficient density contrast and smooth interfaces. C/C multilayers with period thickness d= 1.1...7.0 nm and more than 500 periods were deposited. The X-ray optical performance of the C/C multilayers was characterized by means of X-ray reflectometry. A reflectivity R > 50 % (CuKα) was measured for C/C multilayers with d= 17.2 nm and N= 106 periods. A peak resolution (Δλ/λ) ≈ 1.1 % was obtained for a C/C multilayer structure with N= 80 periods and a period thickness d= 1.1nm. Results of TEM investigations indicate a regular morphology as well as smooth interfaces in the C-C layer stacks. Low compressive stresses were determined in C/C multilayers with different period thicknesses using X-ray diffraction techniques.
The evolution of intrinsic stress in multilayers and its behavior with growing number of periods and/or during radiation exposure, i.e. thermal treatment, becomes important in many X-ray optical applications. In Ni/C X-ray optical multilayers fabricated by pulsed laser deposition (PLD) at room temperature with typical period thickness d approx. 4.0 nm microstructure and intrinsic stress are analyzed with growing period number up to the layer stack delamination. Microstructure of single layers and interfaces and morphology of the total layer stack were investigated by means of transmission electron microscopy (TEM), X-ray reflectometry and diffraction. A clear indication of nano-crystallites was found for the Ni-layers, whereas the carbon layers were always amorphous. The diffraction pattern of a 300 period Ni/C layer stack hint at a hexagonal structure of the Ni crystallites with the (011)-lattice plane parallel to the Si substrate surface. Thus the elastic misfit between the Ni- and C-layers seems to be minimized. To investigate the evolution of stresses in the growing up Ni/C multilayers depth dependent stresses in the Si (100) substrate were analyzed using X-ray results of the stress tensor for Si-crystal of the substrate averaged over different penetration depths (0.6 and 3.6 μm). The found dependence of the substrate stresses on the total layer thickness point at low compressive stresses in the PLD-fabricated Ni/C X-ray optics in the range below -100 MPa at least up to a total layer thickness of 1200 nm (300 periods). An increase of the compressive stresses was obtained only for stacks of more than 300 periods. A delamination of the layer stack in part was observed at 900 periods.
Performance of Ni/C, Ni/B4C, Mo/B4C and W/B4C multilayers in the energy range E > 8 keV is considered by simulation of x-ray reflectivity and resolution of 1st order Bragg reflection at three different photon energies. The results indicate, that Ni/C and Ni/B4C multilayers show highest theoretical reflectivities of R > 80% for Cu K(alpha) - radiation and also above the Mo K-edge (E equals 20.04 keV) at 30 keV. For Mo K(alpha) -radiation a reflectivity of R > 90% can be achieved by the use of Mo/B4C multilayers. For applications, where period thicknesses d < 3 nm and high reflectivities are required W/B4C multilayers can be used. Theoretical values are compared with X-ray reflectometry results, which were executed at 75 period Ni/C, Ni/B4C and Mo/B4C multilayers, fabricated by pulsed laser deposition (PLD) technology on Si substrates. Amorphous or nanocrystalline structures of single layers, smoothest interfaces and high reproducibility of single layer thickness across the entire layer stack are the results of this high precision PLD process.
Periodic multilayers are well known as Bragg reflectors for X- rays. A high reflectivity and a wide reflection width are their outstanding features. However, if multilayers shall be used as reflective coating for X-ray optics, especially for wide acceptance angles, uniform layer thicknesses cause chromatic aberrations. These aberrations can be overcome by laterally graded multilayer optics. Their Bragg angle is matched laterally to the incidence angle so that for all points on the reflector, Bragg reflection is obtained for the same wavelength. Three major types of laterally graded multilayer mirrors ('Gobel Mirrors') are applied in X-ray diffractometry: (1) parabolic, (2) elliptic and (3) planar. In this paper, we give design criteria and formulae for these mirrors. Furthermore, we discuss the requirements on the dimensions and the fabrication process. Two different processes suitable for the fabrication, sputter coating and pulsed laser deposition (PLD), are described. The X-ray optical parameters and their characterization are presented for various mirrors designed for Cu K(alpha) , Mo K(alpha) and Cr K(alpha) radiation. From Ni/C and Ni/B4C multilayers, high-photon-flux monochromators with a Cu K(beta) /K(alpha) intensity ratio of about 1:1000 have been realized. The divergence of the 'parallel' beam reflected from parabolic mirrors is about 0.02 degrees, which is one order of magnitude lower than the divergence of polycapillary optics, monocapillary optics and waveguides. Comparing the photon flux density in a high resolution diffraction setup with and without mirror optics a gain factor of 16 was achieved for parabolic Ni/B4C multilayer mirrors.
A high vacuum pulsed laser deposition system is described where an intersection of two ablation plumes from twinned simultaneously irradiated targets is used. This system allows thin film and multilayer deposition of a wide variety of materials (including low melting point metals like tin) practically without droplet contamination. The intersection region acts as a filter for droplets and high energy plasma particles. The use of twinned targets of different materials facilitates preparation of artificially mixed supersaturated thin film solid state solutions used as a media for sub-micrometer and nanometer-scale surface processing. Special design of the target holder that can carry simultaneously up to 24 targets and computer control of the deposition process make it possible to easily change targets without venting the deposition chamber and to deposit arbitrary multilayer combinations of various materials.
Pulsed laser deposition is described as a technique for the synthesis of multilayers showing X- ray optical quality. The state of the art is characterized by results that demonstrate a development of the instrument basis superior to that of conventional PLD systems. Multilayers of the Ni/C, Mo/Si- and W/C-types prove the versatility of the method and the output of layer stack characterization by HREM, SPM, XD, AES, XPS, ellipsometry and image processing ensures a high quality with regard to stack regularity, layer homogeneity and interface smoothness.
We study effects of macro- and micro-postprocessing of multilayered synthetic Ni/C film structures. The structures have nm-period 1D modulation of concentration of the major components. Initially low surface and interface roughness of such structures makes them advantageous for application as a new type of substrates in nanofabrication technologies and for information storage with nm-resolution. Metastability of microstructure and high residual stresses favor the use of the structures themselves as a media for fine-scale processing.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.