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PROPERTIES OF Ni/C AND NixSiy/C MULTILAYERED REFLECTION COATINGS
H. van Brug, M. van der Wiel
To cite this version:
H. van Brug, M. van der Wiel. PROPERTIES OF Ni/C AND NixSiy/C MULTILAYERED REFLECTION COATINGS. Journal de Physique Colloques, 1987, 48 (C9), pp.C9-123-C9-125.
�10.1051/jphyscol:1987920�. �jpa-00227317�
JOURNAL DE PHYSIQUE
Colloque C9, supplement au n 0 1 2 , Tome 48, dkcembre 1987
PROPERTIES OF Ni/C AND Ni,Si,/C MULTILAYERED REFLECTION COATINGS
H. van BRUG.and M.J. van der WIEL
Association Euratom/FOM-Institute for Plasma Physics, Edisonbaan 14, NL-3439 M N Nieuwegein, The Netherlands
Abstract
We present results of measurements on multilayered reflection coatings (multilayers), and discuss the possibility to use multilayers as a dispersive element in a double crystal monochromator. The multilayers discussed are produced at the FOM-Institute AMOLF in Amsterdam, and have appr. 160 layers with 4A thicknesses of Ni or NixSi , and 18A C. The use of multilayers offers a tool for the monochromatization of synchrotron rldiation in the soft x-ray region. 'The throughput spectrum of a monochromator, e&ipped with a pair of identical multilayers, will be shown. From oscillations present at the high energy side of the Ni-LZ 3 edge (at about 850 eV) in the throughput spectra, we determined bondlengths in the interfaces between 1.8 and 2.3
A.
The bondlength of 1.8A
is assigned to Ni-
C [I], 2.1A
to Ni-
Si [2] and 2.3
A
to Ni-
Ni [I]. We found strong evidence that the stoichiometq of the NixSiy layer is Ni2Si.Ni Si /C coatings deposited in a torr H2 atmosphere show a better resolution than those wi&o{t. The difference is ascribed to H2 preventing the formation of silicides and the diffusion of the Ni into the carbon, resulting in more amorphous layers.
Introduction
During the last decade there has been a largely increasing interest in research in the soft X-ray range (10 - 1000 eV). As a consequence, there is an increasing need for suitable dispersive elements, especially in the range between 200 and 900 eV, which includes the 0 and CK edges and the 3d-transition metal L2 3 edges. At present there are two classes of dispersive elements used in monochromators hth a fixed direction exit beam: gratings and crystals.
A promising contribution in the fields of research mentioned, can be offered by multilayered reflection coatings (further referred to as multilayers), which have become a practical reality for onlpone decade. We present results of throughput measurements of multilayers used in the standard double crystal monochromator configuration, and discuss some properties obtained from the EXAFS (Extended X-ray Absorption Fine S t r u c ~ e ) analysis performed on the wiggles present at the high energy side of the NiL edge.
L
D C B K J H G
Fig.1 Schematic view of the experimental setup. A: Pt mirror for high photonenergy rejection.
B: C-foil for UV rejection. C,H: Options for extra foils. D,G: Monitor grids. E,K: Diaphraps.
F: Double crystal monochromator equipped with two multilayers. J: Monitor foils. L: Proportional counter. M: Channeltron detector for electron yield S: Sample manipulator.
Experimental
Experiments were rformed with four different pairs of identical multilayers: Pair I having 80 layerpairs (4 r ~ i , 18
A
C), Pair II 90 layerpairs ((3A
Ni, 19A
C), Pair IJI 80 l a y ~ a i r s (4A
NixSi , I 8A
C), and Pair IV 81 layerpairs ( 4A
NiGi,
18A
C) deposited in a 10 torr Hz atmospKere. Pair I1 had some additional carbon layers ( 2 3 ~ ) randomly placed inArticle published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987920
C9-124 JOURNAL DE PHYSIQUE
the stack, in order to improve the energy resolution. Each pair of multilayers was mounted in the double crystal monochromator for synchrotron radiation at beamline 3.4 of the Daresbury storage ring.
The multilayers were made by deposition of the two materials on a 25x25x.5 mm Si substrate by electron beam evaporation. For details on the manufacturing of the multilayers see Ref.[3]. For a schematic view of the experimental setup see Fig.1.
As example of the results we show the throughput spectrum of Pair 11, Fig.2(a). The throughput of-the double multilayer monochro&atbr was obtained by measuring the photoelectron yield of a stainless steel plate, ~ositioned in the beam. Photoelectrons were detected by a channeltron. Since the r6flecti6n of the multilayers is a summation of the reflection of all the layers, limitted by the penetration depth of the x-rays, the absorption features of the two components of the multilayers are present in the throughput signal. In this signal we measured the EXAFS oscillation above the NiL edge (852 eV).
To abstract the oscillatory part of the throughput, a spline function was determined that followed the throughput spectrum, but without oscillation. This spline function was subtracted from the throughput spectrum, yielding an oscillating function that, after converting into k-vector space, could directly be Fourier trasformed to give the Radial Distribution Function.
See for example Fig.2@). In this figure only the first neighbour peak can be seen, due to the limitted resolution and energy range. This peak, at 1.6
+
0.1 A, most likely corresponds with the Ni-
C distancerl]. This value is not corrected for phaseshifts. The correction for the phaseshift, obtained by extrapolation from the tables of Teo and Lee [4], is approximately 0.2A. This value is not very accurate because the tables are not given at these low k-values.Bonddistances obtained from the EXAFS analysis of the throughput spectra of the other multilayerpairs are: 1.8
+
0.1A @air I), 2.1+
0.1A (pair 111) and 2.3+
0.1A (pair IV) (after phase shift correction). These distances can be assigned to Ni-C [I], Ni-Si [2] and Ni-Ni [I]respectively. For NiZSi the first neighbour of the absorbing Ni atom is a Si atom at a distance of 2.086A [2]. This IS a strong indication that the stoichiometry of the silicide is Ni2Si. In the case of Ni,SidC multilayers deposited in an H2 atmosphere, the hydrogen molecules prevented the diffusion of the Ni into the C , and prevented the formation of nickel silicides, resulting in an amorphous layer with a clear Ni-Ni first neighbour peak in the RDF.
It is expected that better results can be obtained using multilayers with more layerpairs and when measured in the harder x-ray region, i.e. measuring K-edge EXAFS. In these measurements second and higher neighbdur &stances can be dete&ned.-
The resolution EIAE of multilayerpair IT was determined to be approximately 155. This was determined by measuring the step width of the FeL edge, defined as-the width &tween the 10 and 90% points. This edge is known to have a very steep edge jump, so we can assume the observed width as being determined by the resolution of the monochromator. For the other multilayer pairs we found a resolution of: 140 @air I), 100 (pair IE) and 130 (pair IV).
For more details of the Ni/C- and Ni,Siy/C- multilayer EXAFS measurements at Daresbury see Ref.[Sl.
so0 900 1000
energy (eV)
Fig3(a) The throughput of a NiJC multilayer pair @air ll) in the standard double crystal configuration. Not corrected for the absorption of a 9000
A
C foil and a total of 3 pm of A1 foils in the beam.2(b) Radial Distribution function as obtained by fourier
transformation of the EXAFS oscillations in Fig.Z(a).
Conclusions
We have presented a throughput spectrum with EXAFS oscillations, and have shown the possibility to use these oscillations for the analysis of the structure of the multilayers.
The throughput-EXAFS analysis technique is used on pairs of identical Ni/C and N i S C multilayers to measure the EXAFS signal above the Ni-L2 3 edge. From the EXAFS oscill
. i'
sons above the Ni-% 3 edge, bondlengths were determined e'qual to 1.8,2.1 and 2.3A,
which could be assigned tb Ni-C, Ni-Si and Ni-Ni respectively. Evaporation in an H2 atmosphere has proven to enhance the resolution, by preventing the formation of Ni silicides and carbides.
We have shown the possibility to use multilayers as dispersive element for synchrotron radiation. The medium resolution @/AE=130) as obtained with these multilayers is sufficient
for resolving e.g. EXAFS oscillations.
Acknowledgements
The authors would l i e to thank G.P.A. Frijlink and H. Zeilemaker for the multilayer fabrication. This work is part of the research program of the Foundation for Fundamental Research on Matter and the Netherlands Foundation for Chemical Research and was made possible by financial support from the Netherlands Organization for the Advancement of Pure Research.
References
[I] Handbook of Chem. and Physics; 6 0 ~ ed. Editor Robert C. Weast (1980)
[2] Calculated from W.G. Pearson, Handbook of lattice spacings and structures of metals, Vo1.n (Pergamon, London, 1958)
[3] M.P. Bruijn, J. Verhoeven and M.J. van der Wiel, Nucl. Instr. and Meth. in Phys. Res.
219 (1984) 603
[4]
Boon- en^
Teo and P.A. Lee, J. Am. Soc.101,
(1979) 2815r51 H. van Brug, R. van der Pol, J. Verhoeven, M.J. van der Wiel, G. van der Laan and
- -
J.B. ~ o e d k g o ~ , to be published.
