Analysis of parameters of multilayer carbon interference structures in the soft x-ray range

Article abstract of DOI:10.1063/1.118041

A new method for creating x-ray mirrors with nanometer a-C:H layers using rather simple methods of plasma deposition technology is demonstrated experimentally. The absence of concentration gradients at interfaces allows one to minimize the interface diffusion broadening and thus to obtain more perfect structures. The carbon mirrors can be considered as fundamentally new type of mirrors being one-component (carbon) structures and not two-component ones as before. Increasing the number of periods of such multilayer carbon interference structures enables one to obtain structures with high resolution and, at the same time, sufficient reflectivity in a large spectral range. Because there are no restrictions on relative layer thickness inside the period, the conditions that are necessary for quenching higher-order reflections are easily satisfied.

Full text of DOI:10.1063/1.118041

Analysis of parameters of multilayer carbon interference structures in the soft xray
range
P. E. Kondrashov, I. S. Smirnov, E. G. Novoselova, and A. M. Baranov
Citation: Applied Physics Letters 69, 305 (1996); doi: 10.1063/1.118041
View online: http://dx.doi.org/10.1063/1.118041
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/69/3?ver=pdfcov
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Analysis of parameters of multilayer carbon interference structures
in the soft x-ray range
P. E. Kondrashov,a) I. S. Smirnov, E. G. Novoselova
Institute Electronics and Mathematics, Bol. Vuzuvsky per., 3/12, Moscow 109028, Russia
A. M. Baranov
Research Institute of Vacuum Technique, Nagorny proezd 7, Moscow 113105, Russia
͑Received 19 February 1996; accepted for publication 7 May 1996͒
A new type of x-ray interference mirrors using hydrogenated carbon films of various densities is
described. The reflection coefficient and resolution of multilayer carbon structures obtained in the
soft x-ray range ͑1.54–44.7 Å͒ are investigated experimentally and theoretically. As is shown,
hydrogenated carbon films can be used to create mirrors with high resolution. © 1996 American
Institute of Physics. ͓S0003-6951͑96͒02129-8͔
The most important elements of the x-ray optics are
grazing incidence mirrors and multilayer interference struc-
tures ͑MIS͒. At present, the most commonly used structures
are MIS in which highly absorbing layers of heavy metals
͑W, Re, Ni, etc.͒ alternate with weakly absorbing layers of
light atoms ͑C, Si, etc.͒.1 However, in manufacturing x-ray
mirrors based on two-component MIS with metallic layers, a
number of problems arise which are quite difficult to resolve.
For example, it is impossible to obtain both high eflectivity
and high resolution.2 Moreover, in such structures it is diffi-
cult to create sharp interfaces because of the mutual diffusion
in the regions of large atomic concentration gradients.
These problems can be solved by using periodic
multilayer structures consisting of weakly absorbing layers
with close dielectric constants as interference mirrors in the
x-ray range. We have manifactured and studied new x-ray
mirrors consisting of hydrogenated carbon films ͑a-C:H͒ of
various densities. The choice of carbon for our purpose is
motivated by several reasons. On the one hand, carbon is one
of the elements with the lowest absorption in the range 1.54–
130 Å, being inferior in this respect only to lighter elements:
H, He, Li, Be, and B. On the other hand, the presence of
several types of bond hybridization ͑sp1, sp2, and sp3) en-
ables one to obtain various structures ͑from graphitelike to
diamondlike ones͒ with a great diversity of properties. Fur-
thermore, the variation of the hydrogen content ͑from 10% to
30% in our case͒ provides additional possibilities to control
the properties of carbon films. The presence of hydrogen can
also give rise to a decrease in x-ray absorption compared to
carbon films without hydrogen.
gets. The beam was directed to the sample via the collimator
with angular divergence 0.003° at 1.54 Å and 0.15° at 7.33 Å
and 8.33 Å. For a comparative analysis, Ni/C mirrors were
used. The measurements at 13.3 and 31.4 Å were made by
the x-ray reflectometer described elsewhere.4 The scanning
steps were 2⌬⌰ϭ0.01° at 1.54 Å and 2⌬⌰ϭ0.02° at the
other wavelengths.
The measured dependence on the incidence angle,
Rϭf͑⌰͒, of the reflection coefficients for MCIS and Ni/C is
shown in Fig. 1͑a͒ ͑curves 1 and 2, respectively͒. The calcu-
lated curves for these structures are plotted in Fig. 1͑b͒
͑curves 1 and 2͒; the calculations were made using the recur-
sion relations.5 For a MCIS with period Lϭ94 Å, mean
square substrate roughness ␴ϭ8 Å, and the number of layers
Multilayer carbon interference structures ͑MCIS͒ were
prepared by plasma deposition from the mixture
ArϩC6H 12.3 Plates of fused quartz of diameter 30 mm with
mean square surface roughness ␴ϭ8 Å were used as sub-
strates. For better adhesion, the substrates were treated by
plasma before the layer deposition. The measurements of the
structure parameters were performed in the wavelength range
͑␭͒ 1.54–44.7 Å. The measurements at 1.54 Å were made
using an x-ray reflectometer designed to study total external
reflection. Measurements at 7.13 and 8.33 Å were made us-
ing a modified fluorescence spectrometer with Si and Al tar-
FIG. 1. ͑a͒ Experimental and ͑b͒ theoretical dependencies Rϭf͑⌰͒ for
␭ϭ1.54 Å. ͑1͒ Multilayer carbon interference structure ͑Nϭ118, dϭ94 Å͒;
͑2͒ Ni/C mirror ͑Nϭ60, dϭ72 Å͒.
a͒
Electronic mail: root@onti.miem.msk.su
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TABLE I. Parameters of the structures studied.
Mirror type
No. 1 (a-C:HI /a-C:H II
Structure parameters
)
No. 2 ͑Ni/C͒
͑1͒ Structure period ͑Å͒
͑2͒ Number of period
94
59
72
30
͑3͒ Layer thickness ͑Å͒
͑4͒ Difference in decrements
of refraction of the layers ͑⌬␦͒
47/47
1.2ϫ10Ϫ6
24/48
18ϫ10Ϫ6
Nϭ118, the best agreement between the theoretical curve
and experimental data was achieved for the difference ⌬␦
ϭ1.2ϫ10Ϫ6 in the layer decrements of refraction, which
corresponds to the layer density difference ⌬␳
ϭ0.38 g/cm3. Calculations for the Ni/C mirror were made
using the tabular values of the decrements of refraction and
absorption coefficients for nickel and graphite. The param-
eters of the mirrors of both types are listed in Table I.
Comparison of the experimental data for different struc-
tures demonstrates that for MCIS having maximal reflectiv-
ity (Rmaxϭ40.7%) half as large as for the Ni/C mirror we
obtained the resolution that is twice as large ͑the half-width
of the first Bragg reflection peak was ⌬⌰ϭ0.02°͒. We note
that the smaller value of R for MCIS is, to a large extent, due
to the large roughness of quartz substrates. For MCIS the
curve Rϭf͑⌰͒ is below the reflectivity curve for the Ni/C
structure. In fact, it is seen from Fig. 1͑a͒ that in the region of
total external reflection, R for the Ni/C mirror approaches
98%, whereas for MCIS it is only 90%. It should be noted
that the half-width of the Bragg peak of the theoretical
Rϭf͑⌰͒ curve is about half as large as that of the experimen-
tal curve. We believe that this is due to the fact that the
experimental curve is the convolution of the true curve and
the instrumental function; for this reason the resulting width
is greater than the true width. This broadening effect is ex-
pected to be especially large for MCIS, as these structures
have small peak half-widths.
FIG. 2. Experimental dependencies Rϭf͑⌰͒ for ␭ϭ8.33 Å. ͑1͒ Multilayer
carbon interference structure ͑Nϭ118, dϭ94͒; ͑2͒ Ni/C mirror ͑Nϭ60,
dϭ51 Å͒.
films with ⌬␳ϭ0.8–1 g/cm3 is quite realistic. In this case
Rmax would reach 90% at wavelength 1.54 Å and dϭ94 Å.
We note one more important advantage of MCIS when
used in x-ray spectroscopy. There are no even peaks in the
reflection spectrum of the structure studied ͓Fig. 1͑a͒, curve
1͔; this indicates that the different layer thicknesses are the
same and layer thicknesses are almost constant in any period.
One can quench the ‘‘even’’ reflection peaks for Ni/C mir-
rors as well as by the corresponding increase of the Ni layer
thickness. However, the calculations show that this decreases
the reflectivity of the structure by about 10% and broadens
the Bragg peak.
The data Iϭf͑⌰͒ for the MCIS and Ni/C mirror in the
soft x-ray range at wavelength ␭ϭ8.33 Å ͑Al anode͒ are
shown in Fig. 2. The Ni/C mirror with period Lϭ51 Å hav-
ing reflectivity of 63% at ␭ϭ1.54 Å was chosen as a refer-
ence sample.
The reflection coefficient from the MCIS at the maxi-
mum of the first Bragg peak at ␭ϭ8.33 Å was calculated
*
using the formula Rmaxϭ(Imax /I0) g, where Imax is the inten-
sity of reflected beam at the maximum, I0 is the intensity of
the incidence beam, and g is the geometry factor. The latter
was introduced to take account of the sample size. For MCIS
we have gϭ3.4 but for Ni/C—mirror gϭ1.1. In this case
Since the main parameters that affect reflection at wave-
length 1.54 Å are the difference in layer densities and sub-
strate surface roughness, we have also calculated Rmax which
can be achieved by increasing ⌬␳ and decreasing ␴. These
Rmax are listed in Table II.
R
maxϭ10%, which is half as much as the reflectivity for the
Ni/C mirror ͑Rϭ22%͒. Just as in the short wavelength range,
the half-width of the Bragg peak for MCIS was appreciably
smaller ͑⌬⌰ϭ0.12°͒ than for Ni/C mirrors ͑⌬⌰ϭ0.4°͒. The
error in the determination of R is mainly due to the small size
of experimental structures, the small angle of incidence of
x-rays, and the design of the experimental setup.
The resolution ͑⌬␭/␭͒ and the half-width of the peak
͑⌬⌰͒ are related by ⌬␭/␭ϭ⌬/tg͑⌰͒, where ⌰ is the angular
position of the Bragg peak.2 Using this relation, we evaluated
the resolutions for the MCIS and the Ni/C mirror to be 0.043
and 0.085, respectively.
Measured and calculated Rmax in the range from 7.33 to
44.7 Å are shown in Table III. The calculations were made
for the MCIS, whose parameters are listed in Table II ͑No.
1͒.
The measured reflectivity for MCIS is seen to decrease
from 12% to 0.2% as the wavelength increases from 7.33 to
44.7 Å. This is the result of the increase in the carbon ab-
sorption coefficient as ␭ approaches the carbon absorption
As is seen from Table II, increasing ⌬␳ to 0.5 g/cm3 at
fixed ␴ leads to an increase in Rmax up to 61%. Decreasing ␴
to 4 Å at fixed ⌬␳ϭ0.5 g/cm3 leads to a further increase in
R
max , namely, by 11%. A further increase in ⌬␳ to 0.6
g/cm3 enables one to reach the value of 79% for the coeffi-
cient of reflection from MCIS. It is important to note that
⌬␳ϭ0.6 g/cm3 is practically attainable in the process of car-
bon film preparation, as the carbon film density usually var-
ies from 1.2 to 3 g/cm3.6 Therefore, the goal of preparing
TABLE II. MCIS parameters used in the calculations.
⌬␳
͑g/cm3)
␴
͑Å͒
Rmax
%
No.
1
2
3
4
0.38
0.5
0.5
0.6
8
8
4
4
40.7
61
72
78
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306 Appl. Phys. Lett., Vol. 69, No. 3, 15 July 1996 Kondrashov et al.
130.239.20.174 On: Mon, 24 Nov 2014 11:19:52

TABLE III. Experimental (Rmax) and theoretical (Rth) values of reflectivity
for MCIS in the soft x-ray range.
absence of concentration gradients at interfaces allows one to
minimize the interface diffusion broadening and thus to ob-
tain more perfect structures.
␭ ͑Å͒
⌰ ͑°͒
⌬⌰ ͑°͒
Rmax ͑%͒
Rth ͑%͒
The carbon mirrors studied in this letter can be consid-
ered as a fundamentally new type of mirrors being one-
component ͑carbon͒ structures and not two-component ones
as before. Increasing the number of periods of such MCIS
enables one to obtain structures with high resolution and, at
the same time, sufficient reflectivity in a large spectral range.
Since there are no restrictions on relative layer thickness in-
side the period, the conditions that are necessary for quench-
ing higher-order reflections are easily satisfied.
The authors thank Professor V. V. Kondratenko for sup-
plying the Ni/C mirror used in the comparative analysis and
Professor I. F. Mikhailov and Professor N. N. Salashenko for
their help in the rendering of measurements in the soft wave-
length range.
7.13
8.33
13.3
31.4
44.7
2.3
2.68
4.35
10.2
14
0.1
12
10
6.5
0.95
0.2
19
0.12
0.17
0.65
•••
16.5
7.6
1.1
•••
edge ͑␭ϭ44.7 Å͒. At ␭ϭ44.7 Å MCIS reflection practically
vanishes, since at the absorption edge the carbon decrement
of refraction drops abruptly. Therefore, the difference in dec-
rements of refraction of different carbon layers of the MCIS
is also decreased. In this respect the reflection from MCIS
measured with the characteristic carbon radiation radically
differs from the reflection from Me/C mirrors. In fact, for
Me/C mirrors the difference in decrements between metal
and carbon goes up near the absorption edge due to the de-
crease of the carbon decrement. Thus, the reflectivity from
Me/C mirrors increases. However, for ␭Ͼ44.7 Å the reflec-
tivity from MCIS must drastically increase, due both to the
increase of ⌬␦ and to the decrease of absorption by carbon
atoms.
1 E. Spiller, Soft X-Ray Optics ͑SPIE Optical Engineering, Washington, DC,
1994͒.
2 A. V. Vinogradov and I. V. Kozhevnikow, Trudy Fiz. Inst. Lebedev
͑FIAN͒ 196, 63 ͑1989͒.
3 P. E. Kondrashov, I. S. Smirnov, H. G. Novoselova, A. M. Baranov, and
V. V. Sleptsov, Proc. SPIE 2517, 126 ͑1995͒.
4 N. N. Salashenko, Yu. Ya. Platonov, and S. Yu. Zuev, Poverhnost: Fiz.
Chim. Mehanika N 10, 5 ͑1995͒.
Thus, we demonstrated experimentally the new method
for creating x-ray mirrors with nanometer a-C:H layers using
rather simple methods of plasma deposition technology. The
5 L. G. Parrat, Phys. Rev. 95, 359 ͑1954͒.
6 H. Tsai and B. Body, J. Vac. Sci. Technol. A 5, 3287 ͑1987͒.
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