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X-ray waveguide structures of thin metal-carbon layers
S. Zheludeva, M. Kovalchuk, N. Novikova, A. Sosphenov, N. Malysheva, N.
Salashenko, A. Akhsakhalyan, Yu. Platonov
To cite this version:
S. Zheludeva, M. Kovalchuk, N. Novikova, A. Sosphenov, N. Malysheva, et al.. X-ray waveguide structures of thin metal-carbon layers. Journal de Physique III, EDP Sciences, 1994, 4 (9), pp.1581- 1587. �10.1051/jp3:1994225�. �jpa-00249208�
J. Phys. III Franc-e 4 (1994) 1581-1587 SEPTEMBER 1994, PAGE 1581
Classification Physic-s Abstiacts
61.10 07.85
X-ray waveguide structures of thin metal-carbon layers
S. I. Zheludeva ('), M. V. Kovalchuk ('), N. N. Novikova ('), A. N. Sosphenov ('),
N.E. Malysheva('), N.N. Salashenko(~), A.D. Akhsakhalyan(2) and Yu. Yu.
Platonov (2)
(') Institute of Crystallography, Russian Academy of Sciences, Leninsky pr.59, Mos-
cow 117333, Russia
(2) Institute of Applied Physics, Russian Academy of Sciences, Ulyanov Str. 46, N. Novgo- rod 603600, Russia
(Rece>i°ed J9 Not-ember /993, ac.cepied J6 Maich J994)
Abstract.- X-ray waveguide structures consisting of two ultra-thin metal layers with a carbon
layer in between them deposited on glass substrate
are investigated. The reflectivity as well as fluorescence yield from the different metal layers, are used to elucidate the role of the various
layers comprising the structure. The detailed E-field intensity distribution inside and above the
layers is presented.
The analogy between the optical interference phenomena and those in the soft and hard X- ray regions as well as the achievements of the ultra-thin film deposition technology plays an important role in the development of the X-ray optical elements such as multilayer mirrors,
Fresnel zone plates, X-ray interferometers, etc.
About 20 years ago, it has been experimentally demonstrated that thin film waveguides
which are widely used in integral optics, can also be constructed for the X-ray regions [I ]. The
X-ray waveguide (XRWG) structures attract more and more the interest nowadays as they open
a unique possibility to compress an X-ray beam in a thin film to angstrom dimensions increasing essentially the beam flux.
The opportunity of flux enhancement with several orders of magnitude for a XRWG
structure [2] on the basis of a Si mirror was demonstrated allowing X-ray wave propagation by
total external reflections within the guide. A smaller increased flux may be obtained in the simpler case of a thin film on a totally reflecting substrate without an overlayer [3, 4]. An other
interesting feature of XRWG is an amplitude modulation of the X-ray standing wave (XRSW) above its surface at total external reflection (TR) which can be applied for the structural
characterization of the organic monolayers deposited on the top [5].
The aim of the present paper is to present the complicated waveguide structure consisting of
two ultra-thin metal layers with a carbon layer in between deposited on a glass substrate. The reflectivity as well as the fluorescence from the various layers have been used to elucidate the role of the different layers.
1582 jOURNAL DE PHYSIQUE III N° 9
The layered structures C/Nilc/Rh/glass and C/Nilc/Nilglass (protected by the upper carbon
a layer and bare) have been prepared by laser beam deposition technique on melt glass (Fig. ).
The X-ray experiment was performed on with a conventional X-ray tube and a Si I I I crystal
used as a monochromator. The fluorescence was registered by a Si(Li) solid state detector.
Different lines of copper radiation (CUK
~ and CUK~) have been used to excite the fluorescence from Ni and Rh respectively. The signal from the ultra-thin metal film is rather week and the CUK~ was chosen as it is close to Ni absorption K-edge which results in sufficient intensity of the NiK~ fluorescence yield. Calculations were carried out on the basis of Fresnel-type theory for layered structures [6, 3].
c
B
~ C
v
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Fig. I. The C/Nilc/Rh/glass X-ray waveguide structure.
Let's analyse the reflectivity and NiK~ and the RhL~ fluorescence of the Nilc/Rh/glass sample (Ni-401, C-240 h, Rh-45 h) presented in figure 2 to make clear what happens inside it when the glancing angle of the incident beam increase starting from 0.
At low angles the beam is totally reflected from the upper Ni layer (critical angle for
Nid~~, 6.0 mrad for CUK~) and penetrates inside in the form of an evanescent wave leading
to a slight increase of NiK~ fluorescence. There is no fluorescence from Rh layer as an
evanescent wave penetrating through the carbon does not touch it at low angles. At about
4.4 mrad (Fig. 2b) the evanescent wave reachs the Rh layer and is totally reflected from it (d~~~ 7.3 mrad). The specularly reflected beam propagates through the carbon layer and falls at the bottom interface of the Ni layer and is partly transmitted through it and will contribute to the reflected beam outside the sample. The main part of the beam is specularly reflected by Ni layer and pushed back into the C layer, again specularly reflected from Rh etc. (Fig. I).
As the reflections at the both interfaces are total the waves A and Aj can differ only by a
phase (2 gr/> A. (A 2 a sin d, where a the thickness of resonance cavity). In the case the
phase difference equals 2grn the beams will interfere construitively resulting in an E-field
intensity enhancement inside the waveguide [7, 8].
Besides, the interference between the coherent waves with equal amplitudes and wave
vectors directed at angles d and d to the X-axis (incident beam (A) and specularly
reflected (B) in Fig. I) results in a wave field which propagates along the X-axis and has a
« frozen » inhomogeneous distribution along the Y-axis. At d for which sin d
= n>/2 a the wavefield has a form of n (n
= 1, 2 ...) modes distributed along the Y-axis as sin (ngr ala ) with the nodes at the waveguide boundaries.
N° 9 X-RAY WAVEGUIDE STRUCTURES OF THIN METAL-CARBON LAYERS 1583
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Fig. 2. -al NiK~ and b) RhL~ fluorescence yield (upper curves) and X-ray reflection curves for a C/Nilc/Rh/glass samples, dots-experimental results, solid lines-calculations.
Thus the condition for the waves to add in phase and to result in an increased E-field
intensity is equivalent to the equation of the modes. When a mode is formed inside the
waveguide the energy of a primary beam is transferred along it, which leads to the considerable decrease of outcoming radiation (reflectivity minima on the X-ray reflection curve).
The confirmation that our layered structure operates like a waveguide and gives a resonance
enhancement of the E-field intensity inside can be obtained from the analysis of the E-field distribution which is presented in figure 3. A considerable decrease of the outcoming radiation
corresponding to the reflectivity minima I and II and hence the decrease of the XRSW amplitude generated above the Ni surface at the TR region [5] takes place just when the modes (I and II) are formed (curves 1, 2 in Fig. 3). The enhancement of the E-field intensity is about
10 in the mode I.
In fact the investigated structure is far from an ideal waveguide, because of the photoelectric absorption inside carbon leads to the attenuation of the propagating flux.
Furthermore the thickness of the Rh (and Ni) layer is not sufficient to damp the evanescent
wave totally (Fig. 3) and to form a proper specular reflected beam. This will lead to radiation
leakages through the boundaries [2] of a waveguide, to the decrease of the E-field amplitude at the reflection and thus to a rather small E-field intensity increase in the resonance conditions.
For the III minimum (Fig. 2a) d
~ d~~, and the beam specularly reflected from Rh layer will exhibit usual (not total) reflection from the upper Ni layer loosing the intensity drastically. The
mode can be also formed for the corresponding angle but the enhancement is smaller (see
Fig. 3) as A and Aj have significantly different amplitudes.
The IV and V minima are caused by the mode formation due to poor interference of
transmitted and reflected beams with different amplitudes. Thus the modulated X-ray
reflection curve is formed at low angles due to the interference phenomena between the totally
reflected beams. At higher angles it represents the interference between ordinary reflected beams (see also [2]) and hence the reflectivity is drastically decreased and spread over the d axis. It differs from the situation when one layer with the density smaller than that of the mirror substrate works as a waveguide and the main reflectivity modulations are like in the envelope for the TR from the substrate [3].
To finish with figure 2 we should emphasize that the increase of the average fluorescence yield from Rh layer is related to the increase of the evanescent wave penetration and then the
1584 JOURNAL DE PHYSIQUE III N° 9
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Fig. 3. The E-field intensity distribution inside and above C/Nilc/Rh/glass structure for I, II, III, IV reflection minima (the corresponding the X-ray reflection curve is shown in Fig. 2a).
transmitted beam amplitudes into the layer when the angle grows up from 0. At the same time the Ni layer is covered by the X-ray E-field practically over all the angular range and the yield
from it exhibits only the modulations due to the interference phenomena.
For the mode formation and its propagation along the waveguide sin d = n> /2 a should be less than unity [8]. For the hard X-rays (> ma), and a lot of modes can be excited. It is also evident that the thicker is a, the smaller is the distance between the fringes at the X-ray
reflection curve I.e. more oft recurring in the angular range is the successive mode formation.
Such interference fringes have been used since Parrat's investigation [10] for film thickness determination.
In the case of the Nilc/Nilglass XRWG structures the role of the carbon layer thicknes is demonstrated in figure 4 where the experimental results for two samples which differ only in the thickness of the carbon layer, are presented.
But there is a low limit for waveguide thickness. Although the conditions of mode formation
are satisfied for a very thin film the angle d necessary for it may be too big for
TR (d ~ d~, where d~-critical angle for the bottom layer).
In other words the waveguide should have such a minimum thickness that changing the
angle inside the TR region for the bottom layer (and hence the period of a standing wave above it) this situation may be realized when at some angle the nodes of this standing wave
(corresponding to the I mode) coincide with the XRWG boundaries (see [4]).
An interesting example of the necessity of the definite thickness for the resonance cavity is demonstrated in [I I for a rather complicated wave guide structure. A small increase at the heat treatment of the thickness of a carbon layer positioned between layered synthetic microstruc-
ture Cr/C and a thin Ni layer results in the I mode formation inside the carbon layer. This is
iminediately revealed in the formation of the I fringe on the X-ray reflection curve.
A~ ~b~~t the upper metal layer there is no doubt that it plays an extremely important role in xRwG operation. From one side it should have the proper parameters (density and thickness)
to let the evanescent wave penetrating in it, to reach the bottom mirror Surface and give Start to
N° 9 X-RAY WAVEGUIDE STRUCTURES OF THIN METAL-CARBON LAYERS 1585
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Fig. 4. The experimental NiK,, fluorescence yields (upper curves) and the X-ray reflection curves for identical Nilc/Nilglass structures with different carbon layer thicknesses (200 h(a) and 500 h(b)).
the interference phenomena. From the other side it should not only be able to reflect totally the beam falling on its internal surface but also to prevent a significant flux leakage outside the structure. There should exist some optimal thickness of such a layer (for a given density) that would result in the biggest E-field amplitude inside.
In figure 5 the experimental results are presented for C/Nilc/Rh/glass structures which differs only in the thickness of the Ni layer. It can be seen, that the thinner the layer is the more
smashed is the contrast of the reflection curve and the smaller is the E-field intensity inside.
Calculations carried out for the structures with an intermediate Ni layer thicknesses revealed that in these particular experimental conditions the optimal value of the Ni layer thickness for the E-field intensity increase is about 201 (Fig. 6).
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Fig. 5. The experimental NiK~ fluorescence yields (upper curves) and the X-ray reflection curves for identical structures with different Ni layer thicknesses (10 h(a) and 401(b)).
1586 JOURNAL DE PHYSIQUE III N° 9 dNi" 40 A
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of the Ni layer ( lo, 20 and 40 h).
In conclusion it has been experimentally demonstrated that the resonance mode formation is also possible in XRWG structures consisting of ultra-thin metal layers which play the role of
waveguide walls with a carbon layer as the resonance cavity. The opportunity to detect not
only the X-ray reflection curves but also the fluorescence yield from the different layers helped
to reveal the role of each layer in the interference phenomena. Thus the necessity of the definite thickness of the resonance cavity for the mode formation is emphasized and the significance of
the proper parameters of the upper metal layer for the E-field intensity increase is
demonstrated.
Acknowledgment.
This work was partly supported by the Russian Foundation of Fundamental Research (N° 94- 02-04372).
References
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[2] Feng Y.P., Sinha S.K., Deckman H.W., Hastings I.B. and SiddonsD.P., X-ray flux
enhancement in thin-film waveguides using resonant beam couplers. To be published.
[3] de Boer D. K. G., Glancing-incidence X-ray fluorescence of layered materials, Phys. Ret-. B 44
(1991) 498.
[4] Wang I., Bedzyk M. I. and Caffrey, Resonance-enhanced X-ray fluorescence of layered materials, Science 258 (1992) 775.
[5] Zheludeva S. I., Kovalchuk M. V., Novikova N. N.. Sosphenov A. N., Malysheva N. E.,
Salashchenko N. N., Akhsakhalyan A. D. and Platonov Yu. Yu., New modification of XRSW above the surface of layered substrate under total external reflection conditions for structure, Thin Solid Films 232 (1993) 252.
N° 9 X-RAY WAVEGUIDE STRUCTURES OF THIN METAL-CARBON LAYERS 1587
[6] Born M. and Wolf E., Principles of Optics (Pergamon, Oxford 1991).
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[10] Paratt K. G., Surface studies of solids by total reflection of X-rays, Phj's. Ret'. 95 (1954) 359.
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