Multilayer X-ray Research Articles

Heat resistance of electron beam deposited Mo/C X-ray multilayers

Molybdenum-Carbon multilayer structures are investigated for their thermal stability as these are of interest in the energy range of 3–18 keV, a spectrum available from most of the wigglers/undulators of synchrotron radiation sources at a relatively higher power. Mo/C multilayer structures with a period of 50 Å are prepared by electron beam evaporation in ultra high vacuum environment. X-ray reflectivity and X-ray diffraction studies at a wavelength of 1.542 Å are used for investigation. The multilayer structure is stable up to 750°C and the expansion in period is ∼2.3%. The multilayer structure is destroyed at 850°C due to the crystallization of molybdenum carbides. Possible reasons for the high thermal stability and low period expansion compared to other metal/C multilayer structures are discussed.

Stress, microstructure, and stability of Mo/Si, W/Si, and Mo/C multilayer films

The stresses in periodic Mo/Si, W/Si, and Mo/C multilayer films were determined from wafer-curvature measurements. The layer thickness of each material was varied systematically, and parametric stress contours were generated, showing contours of constant stress in the two-dimensional layer thickness parameter space. These results illustrate that the net stress in a periodic multilayer is not an intrinsic property of the film (for specific deposition conditions) but, rather, depends strongly on the individual layer thicknesses. X-ray diffraction measurements show (a) how the lattice spacing in the W and Mo crystallites varies with layer thickness, and (b) in the case of the W/Si films, how the phase composition of the polycrystalline W layers vary with W layer thickness. In the case of the W/Si and Mo/Si multilayers, irreversible stress changes were observed after the samples were stored in air at room temperature for a period of several months. Stress–temperature measurements made on the as-deposited W/Si and Mo/Si samples also reveal irreversible stress changes (both positive and negative, depending on the layer thicknesses) after thermal cycling to 300 °C; x-ray diffraction measurements were used to identify any associated changes in the W and Mo microstructure. We describe mechanisms that can explain the observed stress behavior, and also discuss the significance of these results, particularly with regard to the use of these films for high-performance multilayer x-ray optics.

Multilayer mirror soft x-ray spectrometer for fast electron temperature measurement on the compact helical system

A multilayer mirror (MLM) soft x-ray spectrometer has been installed on the Compact Helical System. The x-ray energy spectrum from ECH and neutral beam injected heated low beta plasmas was measured as a function of radiation energy. Modulations of the x-ray intensity associated with magnetohydrodynamic instability were observed. The electron temperature measurement was also investigated using the MLM x-ray spectrometer. The data suggest the possibility of a fluctuation measurement of the electron temperature Te with fast time resolution using the MLM spectrometer.

Depth-graded multilayer X-ray optics with broad angular response

A method of designing depth-graded multilayer structures with broad angular response for use as coatings in X-ray optics is presented. The design is based on the well-known Fresnel equations and recursive calculation, combined with a merit function plus random variation of the thickness of each layer. This allows the design of multilayer films for different requirements in X-ray optics. Results are presented on the layer thicknesses in depth-graded W/C multilayer films and their reflectivity as a function of the grazing incidence angle for Cu Kα radiation. The required minimum number of bilayers in depth-graded multilayer films depends on the grazing incidence angle, i.e., the saturation effect observed in the design of periodic multilayer films also emerges in the design of depth-graded multilayers. The predicted performances of multilayers designed using this method are superior to those designed using existing methods.

On the possibility of X-ray refractive radiography using multilayer mirrors with resonant absorption

An X-ray multilayer mirror, with a nonperiodic structure specially designed to produce resonant absorption at a definite angle of incidence, may be used as a dispersive element for refractive X-ray radiography instead of a standard crystal analyzer. In this scheme the direct beam can be significantly suppressed. Numerical simulation of the imaging process, based on the experimental data obtained with a tungsten–boron-carbide multilayer mirror, showed that the sensitivity of a multilayer-mirror analyzer at CuK α radiation is sufficient for the recognition of small-contrast carbon–water interfaces at fluxes of 10 4 photons/mm 2.

Characterization of heat-treated CN films fabricated by dual-facing-target sputtering for soft X-ray multilayer mirrors

The structural characterization of heat-treated CN films fabricated by dual-facing-target sputtering for soft X-ray multilayer mirrors was performed by means of X-ray diffraction (XRD), Raman spectroscopy (RS) and X-ray photoelectron spectroscopy (XPS). The XRD analyses indicate a graphization process in the CN films during thermal annealing. The Raman analyses imply that the primary bonding in the CN films is sp2. In other words, the formation of the sp3 bonding in the CN films can be suppressed effectively by doping with N atoms, and thus the thickness expansion resulting from the changes in the density of CN films during annealing can be decreased considerably. This result is also clarified by the increased conductivity measured. The XPS results give the information of the existence of the strong covalent bonding between N and C atoms, which can slow down the tendency of the structural relaxation during annealing. These results suggest that CN films suitable for soft X-ray multilayers used at high-temperature environments can be obtained by reactive dual-facing-target sputtering. With the low-angle X-ray diffraction measurements, we do observe the enhanced thermal stability of CoN/CN multilayers.

In situ spectroscopic ellipsometry as a surface-sensitive tool to probe thin film growth

Sputtered thin film and multilayer X-ray mirrors are made routinely at the Advanced Photon Source (APS) for APS users. Precise film growth control and characterization are critical in fabricating high-quality X-ray mirrors. Film thickness calibrations were carried out using in situ and ex situ spectroscopic ellipsometry, interferometry, and X-ray scattering. To better understand the growth and optical properties of different thin film systems, we have carried out a systematic study of sputtered thin films of Au, Rh, Pt, Pd, Cu, and Cr grown on Si substrates, using in situ ellipsometry. Multiple data sets were obtained in situ for each film material with incremental thicknesses and were analyzed using both flat-film and rough-film models. We found that the initial growth of these metal films on Si is correlated with their chemical bond strength with oxygen in the native oxide of Si. Metals with higher bond strength (such as Cr) grow smoother at the early stage of growth and have a better adhesion to the Si wafer. The Cr film becomes rougher as its thickness increases. Au and Rh films grow smoothly on a thin (6-nm) Cr-covered Si wafer. But on a thick (100-nm) Cr/Si film, Rh films were initially as rough as the thick Cr film. The roughness decreases as the Rh film thickness increases. When Cr is used as a ‘glue’ layer in metal on oxide systems, it is better to use a thin (<10 nm) Cr film in order to obtain a smooth metal film. We conclude that in situ spectroscopic ellipsometry as a surface-sensitive tool can also be used to probe the growth and morphology of the thin film system.

Design and fabrication of soft X-ray multilayer for broadband reflection

A random method used for improving light throughput of a soft X-ray multilayer has been developed in the 18–20 nm spectral region, based on the traditional theory of periodic multilayer, and an 8% gain in integrated reflectance is obtained. The ensemble calculation is presented at the same time, and the multilayer is fabricated by magnetron sputtering. Finally low-angle X-ray diffraction and reflectance comparative measurement are used for testing the multilayer. The results demonstrate that layer thickness disorder yields band broadening and increased integrated reflectance in the spectral range with respect to periodic multilayer, but is accompanied by a reduction in reflectance peak. Layer thickness disorder makes it more difficult to fabricate broadband multilayer, and improving the techniques of layer thickness control is the keystone of experiments.

Thickness determination of metal thin films with spectroscopic ellipsometry for x-ray mirror and multilayer applications

Sputtered thin film and multilayer x-ray mirrors are made at the Advanced Photon Source (APS) deposition lab for the APS users. Film thickness calibrations are carried out using in situ and ex situ spectroscopic ellipsometry, interferometry, and x-ray reflectometry. Here, we present a systematic study of thickness and optical constant determination of sputtered thin films of Au, Pt, Pd, Rh, Cr, Cu, as well as W and C, using in situ ellipsometry. Multiple data sets were obtained for each film material with incremental thicknesses and were analyzed with their correlation in mind. Results are compared with those obtained from interferometry and x-ray reflectivity measurements. The applications and limitations of spectroscopic ellipsometry for metal thin films are discussed. Observations of a relaxation effect of a Rh/Si film and a difference in growth mode in the Cr/Si system compared with other metal/Si systems are presented.

Enhancement of the reflectivity of multilayer X-ray mirrors by ultrathin carbon film deposition

The problems related to the enhancement of the reflectivity of multilayer carbon X-ray mirrors are considered. It is shown that the main contribution to the decrease in reflectivity comes from the roughness of the top layer of the multilayer structure. A method of planarization of X-ray mirror surface that results in a reflectivity increase is suggested.

Design and performance of graded multilayers as focusing elements for x-ray optics

We have developed a design method to produce laterally graded multilayer x-ray mirrors based on sputter deposition techniques. The optimization of all relevant parameters yields an ab initio estimation of any layer gradient suitable to achieve precise x-ray focusing devices. The performance and the accuracy of this method are demonstrated. A graded W/B4C multilayer was deposited on a flat substrate that was bent to a parabola. The obtained nonlinear lateral gradient differed from the theoretical calculations by less than 1%. Focusing experiments performed at an x-ray energy of 8 keV on the ESRF optics beamline revealed an excellent performance with a focal spot size of about 7 μm.

Design and testing of a prototype multilayer analyzer X-ray fluorescence detector.

Design and testing of a prototype multilayer analyzer X-ray fluorescence detector.

Development of R.F. Sputtered Cu / Si Multilayer Coatings for X-Ray Mirror Applications

Single layers of Cu and Si and Cu/Si multilayers have been deposited on glass substrates kept at room temperature, by r.f. sputtering technique in Argon plasma. The films hare been characterised by Small-Angle X-ray reflectometry at MoKu (0.07107 A) and Cu Ku (1.54A) radiation and by Phase Modulated Ellipsometry in the spectral range of 300-800 run. It has been found by the Small Angle X-ray reflectivity measurements that the sputtering system is able to produce high quality layers with very low roughness (<A). which is very much needed for the fabrication of X-ray multilayer systems. The thickness and roughness values obtained by theoretical fitting of the Small Angle X-ray reflectivity measurements have been used in the ellipsometric analysis to obtain the optical constants of the films accurately. The multilayer system, consisting of 10 bilayers of Cu and Si, have showed Bragg peak upto the 3 rd order and about 45% reflectivity at the 1 st Bragg peak. This establishes the potentiality of the Cu/Si multilayer system in X-ray mirror applications.

Study of Pt/C X-Ray Multilayer Structure as a Function of Layer Perid Using X-Ray Scattering

Pt/C x-ray multilayer, with varying period lengths d, ranging from 4.7 nm to 3.5 nm were made by dc magnetron sputtering under identical deposition conditions. The Pt layer thickness in all the cases was maintained at a nominal value of 0.37d and a total of 20 layer periods were deposited in each case. Since the morphology in the interface plane and along the growth direction is important for imaging applications, the dependence of this on the layer period has been investigated using x-ray scattering. The grazing incidence x-ray scattering technique has been used to study both the specular and diffuse scattering behaviour of these multilayer structures. The interface roughness was determined from the specular reflectivity measured over a large momentum transfer range using both 0.154 nm and 0.834 nm wavelength x-rays. The interface roughness was found to vary from ∼ 0.32 nm to 0.40 nm with decreasing period. The diffuse component of scattering on the other hand was found to decrease with a decrease in layer period. The atomic ordering in the individual layers studied using high angle x-ray diffraction shows clearly the presence of crystallinity in the Pt layers, independent of the layer period.

Quantitative X-ray microanalysis of beryllium using a multilayer diffracting device

H. Kleykamp, J. Anal. At. Spectrom., 1999, 14, 377 DOI: 10.1039/A806747C

Comparative analysis of diamond-like carbon and metal–carbon X-ray mirrors

Abstract The characteristics of a RAP crystal (RbHC8H4O4), metal–carbon multilayer X-ray mirror and diamond-like carbon (DLC) X-ray mirror are compared. The advantages of DLC mirrors in resolution power are described. Temperature and radiation on the reflectivity and bandwidth of Bragg peak for diamond-like mirrors are studied. It is shown that the parameters of DLC mirrors after annealing in vacuum are stable up to 570 °C.

A New Versatile Instrument for Characterization of Thin Films and Multilayers Using Spectroscopic Ellipsometry and Grazing X-Ray Reflectance

A new versatile instrument which integrates different experimental techniques has been developed recently by SOPRA. The base is the SOPRA GESP5 instrument (Gonio-Ellipso-Spectro-Photometer) which offers the possibilities of a spectroscopic ellipsometer, a spectroscopic photometer and a scatterometer. We have added a grazing X-Ray reflectometer option which allows measurements on the same spot at the same time and combined analysis of the different data. The source is a compact ceramic Cu fine focus X-ray tube. The X-ray beam is defined by Soller slits. Optionally, a graded multilayer X-ray mirror is used to produce a monochromatic parallel beam on the sample surface. After reflection, the beam is monochromatized using a curved graphite crystal and focused on the detector (scintillator coupled with photomultiplier tube). Performances of the system are discussed hereafter with some results obtained on various kinds of samples.

Modulation structure stability of heat-treated Co/C and CoN/CN soft X-ray multilayers

Modulation structure stability of Co/C and CoN/CN soft X-ray multilayers has been investigated by X-ray diffraction and Raman spectroscopy. The graphitization of the amorphous carbon layers in Co/C multilayers causes a period expansion of 12% at annealing temperatures below 400°C. An enormous period expansion (∼40%) induced by the crystallization and agglomeration of Co layers has been observed at 500°C. While the period expansion of CoN/CN multilayers is only 4% at 400°C. The interface pattern of the CoN/CN multilayers still exists even if they were annealed at 700°C. The relatively good thermal stability of CoN/CN multilayers can be attributed to the suppression of the formation of thesp 3 bonding and, at annealing temperatures higher than the phase transition temperature of 420°C (from hcp to fcc), the coexistence of hcp- and fcc-Co structures through doping nitrogen.

X-ray optical multilayers: microstructure limits on reflectivity at ultra-short periods

Multilayer X-ray mirrors of W and B 4C with period from 50 Å down to 5 Å, of interest as linear polarizers for soft X-rays, have been studied by X-ray diffraction and high-resolution transmission electron microscopy (HRTEM). Over this region soft X-ray reflectivity falls from 80% to 4% of the theoretical value. Hard X-ray reflectivity also falls dramatically, but the multilayers still show reflectivity for periods as small as d=4.7 Å. The interface width (representing both roughness and interdiffusion) decreases from 7.5 Å to 2.5 Å, but exceeds the thickness of the individual layers for the smallest periods. HRTEM shows disruption of layer continuity at d=10 Å and a loss of visible composition modulation at d=8 Å, while the reflectivity begins to fall sharply near d=15 Å. This is consistent with incomplete or discontinuous W layers below about d=15 Å, and compares reasonably with other experimental and theoretical data on growth of very thin metal films.

Characterization of a multilayer soft X-ray reflector fabricated by pulsed laser deposition

A Mo/Si multilayer (ML) has been fabricated as a reflector in the soft X-ray spectral region by pulsed laser deposition (PLD), using the second harmonic of Nd/YAG pulsed laser (5 ns, 532 nm light). The ML structure was characterized by transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS) and photoelectron spectroscopy for chemical analysis (ESCA). The near-normal incidence reflectivity in the spectral range of 14–17 nm was measured using a soft X-ray reflectometer based on a laser-produced plasma. The structural parameters were evaluated by fitting to both the SAXS profile and the soft X-ray reflectance measurement with asymmetric interface profile, roughness and composition taken into account.