Multilayer X-ray optics for synchrotron radiation

Multilayer X-ray optics for synchrotron radiation

Nuclear Instruments and Methods in Physics Research A 359 (199.5) 114-120 NUCLEAR INSTRUMENTS &METHoDS IN PHVSICS RESEARCH Sects ELSEVIER A Multil...

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Nuclear Instruments and Methods in Physics Research A 359 (199.5) 114-120

NUCLEAR INSTRUMENTS &METHoDS IN PHVSICS RESEARCH Sects

ELSEVIER

A

Multilayer X-ray optics for synchrotron radiation N.N. Salashchenko

*,

Yu.Ya. Platonov, S.Yu. Zuev

Institute for Physics of Microstructares of Russian Academy of Sciences, Ulyanou Str. 46, 603600 Nizhny Nougorod, Russian Federation

Abstract Layered synthetic nanostructures are considered both as normal incidence imaging and dispersive optical elements in the soft X-ray range and as filters of -y-radiation for Mijssbauer spectroscopy.

1. Introduction The current rapid advance in the studies of soft X-rays (A = 0.1-20 nm) is due to a broad variety of research and applied problems to be solved. Traditionally major results have been achieved in this wavelength region using synchrotron radiation. However, the last decade has seen a great number of works involving coherent and noncoherent pulsed plasma sources of intensive X-ray radiation. Successful development of multilayer, Bragg-Fresnel and Fresnel optics makes it possible to use the potential of synchrotron and pulsed-plasma X-ray sources in structure and biology research, applied microelectronics and materials studies. In the field of multilayer X-ray optics, synchrotron radiation (SR) has been actively used to investigate X-ray optical constants of materials and reflection characteristics of multilayer X-ray mirrors [l-5]. Now the X-ray optics started “paying back” as, in its turn, it finds increasing applications to the SR-related problems. The most important of them, pursuing combined utilization of synchrotron radiation and multilayer X-ray optics, are stated in the never-aging work by Kulipanov and Skrinsky [6]. In particular, some trivial applications of multilayer optics include monitoring of an SR beam: splitting, point/slit focusing, preliminary monochromatization, and filtering one polarization out of the beam. Active discussion is currently under way of projects to create high-efficiency X-ray projection lithography devices capable of 50-100 mn spatial resolution [7-91, projection and contact X-ray microscopes in the “water window” range (A = 2.35-4.5 nm) that would allow high spatial/temporal observation of live biological organisms [lo], and a quasimonochromatic X-ray probe on the basis of Bragg-Fresnel multilayer optics for local X-ray spectroscopy and microscopy [ 11,121.

l

Corresponding author.

High-quality multilayer X-ray mirrors are used with SR beams as generators of standing X-ray waves to study structural characteristics of superfine films of different materials and Langmuir-Blodgett film structures deposited on a mirror surface [13-151. Another application of synchrotron (undulator) radiation and multilayer X-ray mirrors for research purposes relates to resonance diffraction of y-radiation on Layered Synthetic Nanostructures (LSN) [16,17]. Resonance diffraction of y-radiation on LSN attracts considerable attention, firstly, because it facilitates further advance in the physics of coherent interaction of radiation with matter and, secondly, in connection with a promising outlook for the development of optical systems to produce y-beams with a very narrow energy spread by SR sources. A solution to the problem of filtering such radiation out of the SR spectrum is crucial for tackling X-ray holography and interferometry to obtain inverse states of Miissbauer nuclei in a crystal. A wide latitude in choosing the resonant isotope in LSN and the superfine structure of nuclear levels highlights resonant multilayer structures as entities of a unique type for studying the fundamental issues in the collective interaction of nuclear radiation with matter. Thus, research towards perfection of multilayer X-ray optics for SR covers the following fundamental and applied problems: - upgrading deposition technologies and optimization of materials for multilayer coatings with a view to spectral range broadening, upgrading the reflection capability and the spectral resolution of multilayer dispersion elements; - fabrication of multilayer structures to be used as the basis for multimirror optical systems for X-ray projection lithography; - development of deposition technology and the choice of suitable material pairs forming metastable ordered solid solutions with nanometer and subnanometer periods; study of diffusion processes in such structures; - development of normal incidence optics for projection X-ray microscopes in the “water window” spectral band;

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- fabrication of X-ray optics components with higher radiation/temperature resistivity, applicable in schemes involving intensive synchrotron and undulating sources and in controlled nuclear fusion installations; - development of Bragg-Fresnel optics components and phase reflection diffraction gratings fabrication technology; - fabrication of multilayer filters to single out y-beams with a very narrow energy spread (E/AZ? s 1010-10’3), used in X-ray holography and interferometry schemes, from synchrotron radiation.

2. Multilayer

dispersive elements in tbe soft X-ray

range

Period,

The interest in the development of small d-spacing (d = l-10 nm) LSN arises from the possibility of their employment as focusing and imaging normal incidence optical devices as well as effective dispersive elements for monochromators in the soft X-ray range. However, there are two reasons which limit the utilization of LSN: 1) the relatively small spectral selectivity of LSN, which usually does not exceed A/A A = 100 because of period fluctuation (A/A A N d/Ad), and 2) the influence of the interface roughness on the reflectivity: R =R,

exp( -16n2cr2

sin 0,

= m A/2d,

sin20,/A2), (1)

where R, is the peak reflectivity coefficient of an ideal LSN, u is the rms roughness, 0, is the Bragg angle and m is the diffraction order. It follows that to preserve a large value of 0, and to construct normal incidence LSN for shorter wavelengths it is necessary to have smaller d-space LSN, but in this case the reflectivity of the LSN is decreased dramatically by the influence of interface roughness. Using higher diffraction orders the reflectivity does not increase as follows from Eq. (l), and it may be done to improve the spectral selectivity and increase the layer thickness [18]. The LSN working at the second diffraction order has a twice higher spectral selectivity and is thus preferable if a high spectral resolution is needed. On the contrary, if a high integral coefficient of the reflectivity is needed (microscopy and lithography, for instance) utilization of the multilayers working at the first diffraction order is better. The knowledge of the optical constants [19] allows one to systematically carry out a research of the effective pair combinations of the materials for LSN construction. The main criterion is the possibility to produce high quality LSN using the available technology. For LSN production we use the magnetron sputtering technique in the high frequency and constant current modes as well as the pulse laser deposition method [20]. The first of the methods allows a d-spacing homogeneity of 1% for LSN of 200 mm in diameter and the second one of 1.5% for LSN of 60 mm in diameter.

nm

Fig. 1. Experimental reflectivity of LSN at A = 0.154 nm.

The standard diffractometer “Dron-3M” with a Cu X-ray sealed tube is used to study the LSN by hard X-ray radiation. The X-ray tube power is 30 kV X 40 mA. The collimative slits ensure an angle divergency of 12”. From the reflectivity dependence on the incidence angle in the O-20 mode the following parameters of the LSN are determined: the layer thicknesses, the density of the layer materials and the value of the interface roughness [21]. For LSN characterization in the soft X-ray range a spectrometer-monochromator RSM-500 with a grazing incidence grating and an X-ray tube is used. The RSM-500 covers the spectral range from 0.8 nm to 50 nm with two spherical gratings and is equipped with a large size vacuum reflectometer. The reflectometer allows the characterization of the plane and curved LSN of up to 300 mm in diameter in any point of the surface. The calibration of the spectrometer and reflectometer is performed using known characteristic lines of elements and KAP crystal. The precision in the determination of the angle of incidence and the wavelength radiation is about 5’ and 0.01 nm, respectively. The radiation divergence is 12’. The experimental values of the LSN reflectivity at A = 0.154 nm are presented in Fig. 1. It is obvious that the reflectivity goes down to lower values with decreasing LSN d-spacing. A reflectivity of 0.04% and an angle selectivity O/A@= 180 were observed on W/Sb LSN with d = 0.79 nm. It follows that the period fluctuation can be estimated to be Ad = 0.005 nm: The W/Sb multilayers with d < 2 nm are the most promising for utilization in the hard X-ray range and when we need I_SN with d > 2 nm the W/Si multilayers are more preferable. The small d-spacing W/Sb LSN showed the highest reflectivity in the “water window” spectral range from 2.3 nm to 3.1 nm at normal incidence as is shown in Fig. 2. From the SC L absorption edge up to the C K edge, the SC-containing multilayers, such as W/SC, Cr/Sc, and Fe/SC, are most promising. A reflectivity of 13% at A = 4.47 nm was observed on Fe/C LSN. This multilayer III. X-RAY OPTICS

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Instr. and Meth. in Phys. Res. A 359 (1995) 114-120

. I

9

-

.

I

l W/Sb

-

/

. W/SC

-

Wavelength.

Fig. 2. Experimental reflectivity tion range at normal incidence.

nm

of LSN in the soft X-ray radia-

was prepared by the pulse laser deposition method. The reflectivity of the magnetron sputtered normal incidence carbon containing LSN does not exceed 8% at A = 4.47 nm. We suppose that it deals with the higher electron

density modulation amplitude in the pulse laser deposited LSN. The reflectivity of the Mo/Si LSN deposited by the magnetron sputtering technique on a flat Si wafer is about 65% at A = 13.5 nm [22]. With a spherical Si substrate of 200 mm in diameter we have obtained 50% of the reflectivity and with a fuse quartz substrate of 60 mm in diameter we have obtained a value of 60%. Such a behaviour can be explained by the difference in the roughness of the substrates. Table 1 shows the experimental reflectivity of the LSN used as dispersive elements in the soft X-ray range. All of the LSN presented in the table were fabricated by the magnetron sputtering technique. Nonpolarized radiation of an X-ray tube and s-polarized synchrotron radiation [23] were used for characterization of the LSN. Similar multilayers are applied in WD EPMA spectrometers and multichannel polychromators for the soft X-ray spectroscopy of a high temperature plasma [24]. Some application of the LSN demand that their spectral selectivity should be high enough to resolve a narrow-band spectral line. For instance, using the He II (A = 30.4 nm) line for the diagnostics of ITER plasma requires a resolution power of lo3 [25]. The Multilayer Phase Grating

Table 1 Experimental reflectivity of the multilayer dispersive elements in the soft X-ray range Facility

Structure

4

a

(deg.)

R max %(deg.)

Al9

SR SR SR SR

0.30 0.41 0.89 0.99

7.1 9.5 20.7 23.7

6.0 3.4 1.7 1.9

ST

2.36

11.8

11.8

0.8

SR SR SR SR

0.30 0.41 0.89 0.99

3.7 5.1 11.0 12.3

25.8 17.6 17.2 16.7

0.08 0.11 0.19 0.22

N = 120, d = 2.09 mn, y = 0.40

ST ST ST

0.99 1.83 2.36

13.6 26.1 34.7

20.6 8.1 5.0

0.15 0.30 0.45

W/S& N = 100, d = 3.18 nm, y = 0.43

ST ST ST ST

0.99 1.33 1.83 2.36

9.0 12.2 16.9 22.1

40.2 25.3 19.1 11.5

0.16 0.24 0.28 0.43

Fe/SC, N = 35, d = 5.03 nm, y = 0.41

ST ST ST

3.14 3.50 4.47

17.9 20.6 27.3

44.5 23.3 9.0

1.00 0.81 1.09

ST

4.47

26.5

30.0

0.83

W/Sb,

N = 100, d = 1.25 nm, y = 0.56

N = 100, d = 3.0 nm, y = 0.35

W/Sb,

N = 60, d = 2.39 nm, y = 0.35

W/B&,

W/Si,

Fe/C,

N = 45, d = 5.00 nm, y = 0.38

0.11 0.22 0.25

MO/B&,

N = 30, d = 8.2 nm, y = 0.39

ST

6.76

24.5

31.2

1.07

MO/B&,

N = 25, d = 10.9 nm, y = 0.4

ST

6.76

18.8

37.5

1.20

a SR - synchrotron

radiation,

ST - X-ray tube

N.N. Salashchenko et al./Nucl.

Fig. 3. Reflectivity of MPG for He II (30.4 incidence angle of 19”.

Instr. and Meth. in Phys. Res. A 359 (1995) 114-120

nm) line at an

(MPG) is a possible candidate to solve this problem. We have begun to develop such a grating. Fig. 3 shows the reflectivity of the plane MPG for the He II line. The grating has a light concentration for the third order. High reflectivity (3.6%) is reached because of Bragg reflection of Mo/Si LSN covering the phase structure. The reflectivity and resolution obtained are to be twice improved for alpha particle diagnostics. Moreover, the problem to produce a curved MPG with such parameters must be solved.

3. Resonant diffraction nuclear multilayer

of synchrotron

radiation

by a

A relatively new application of synchrotron radiation is the excition of low-lying nuclear levels of a multilayer target of a resonance isotope layers, and the study of nuclear fluorescence arising in this process [16,17,26-291. The nuclear target excited by synchrotron radiation serves as a source of recoilless y-radiation. If the spectral characteristics of such a source are similar to those of a radioactive source of y-radiation (a single line with a width close to the natural width re>, there is a possibility of filtering intensive directional y-radiation (E/AE N 10i3) out of the broadband synchrotron radiation. A keen interest in such beams is accounted for both by experimental investigations of X-ray holography and interferometry, and by conventional Miissbauer spectroscopy studies of physics of coherent -y-radiation interaction with matter. Another method of nuclear-resonance spectroscopy involving temporal characteristics of nuclear fluorescence excited by short synchrotron pulses has been actively developed over the past few years [30,31]. The quantum intensity beats observed in such time dependences provide comprehensive information on superfine splitting of nuclear levels, since the temporal spectrum of fluorescence is the Fourier transform of a frequency spectrum measured in conventional Mossbauer spectroscopy. Yet, in the temporal version of Mossbauer spectroscopy the y-quanta energy range must be wide enough to span all possible values of

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resonant energies for excitation of the isotope under study. Thus, for the “Fe isotope this spectral range must be larger than the maximum value of superfine splitting, on the order of 2OOre or about 1 peV. Further broadening of the energy range will lead to a higher noise signal due to nonresonance radiation. The impressive technological progress in the fabrication of synthetic multilayer structures in recent years has made it possible to solve this problem using resonance multilayer structures. What makes LSN attractive for utilization is the possibility to arrange large d-spacings in the multilayers and thus achieve small diffraction angles. As the Bragg angle is reduced to l”, the angular width of the reflection increases to several arc minutes, which is comparable with the SR beams divergence. In these conditions enhancement of the radiation channel for nuclear scattering is maximum [32,33], which makes it possible, under the conditions of collective interaction of a nuclear system with nearBragg-angle radiation, to increase the nuclear resonance width by a large factor. A small angular width of the rocking curve for natural crystals - just a few arc seconds - allows for the resonance line broadening only in a small range of angles. The authors of Refs. [34,35] su ested the g Fe/56Fe fabrication of nuclear LSN on the basis of isotopes. Calculation of the energy spectra of the y-radiation diffraction in resonance “Fe/ s6Fe LSN shows [ll] that it is possible to secure reflection in the energy range of about 0.7 FeV for a rocking curve width of - 3 arc min. Such a structure is interesting from the viewpoint of observation of a purely nuclear diffraction at LSN, since scattering at the electron shells of atoms is impossible here because there is no X-ray reflection due to the electron density distribution on such structures. However, nuclear resonance radiation that interacts only with the “Fe isotope behaves towards this structure as it would towards a linear crystal. All orders of radiation reflection on such a crystal are purely nuclear; a coherently-scattered beam contains only resonance y-quanta. Another interesting feature of LSN is a fairly wide latitude in choosing structure elements, which opens up possibilities for creating nuclear mirrors for resonance energies of a broad variety of isotopes and formation of the required superfine structure of nuclear levels by arranging the appropriate environment for the resonance isotope during the LSN fabrication process. We failed to fabricate experimental perfect LSN on the basis of Fe isotopes and attribute this failure to a strong diffusion interaction of the s7Fe/56Fe isotopes on the interfaces. The same situation was observed in manufacturing theoretically promising multilayer mirrors for cold neutrons on the basis of 62Ni/s8Ni isotopes. We are nevertheless hopeful of a better luck in our further work in this direction. The authors of Refs. [16,17,26] report fabrication and study of scattering of nuclear y-radiation in resonance LSN formed by alternating layers of 57Fe and SC. Multi-

III. X-RAY OPTICS

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Ins&. and Meth. in Phys. Res. A 359 (1995) 114-120

layer structures were magnetron-sputtered on supersmooth (rms roughness: N 0.5 nm) glass substrates with dimensions of 60 X 30 mm. The X-ray optics characteristics of the thus-formed LSN were studied by the method of small-angle X-ray diffractometry [Zl]. The electron diffractometry study revealed a fine crystalline state of Fe and SC layers. Measurements of the Hall effect showed that the sample was free of magnetization. We studied two types of LSN. The first one was fabricated by alternative deposition of 57Fe and SC layers: L5’Fe(2.0 nm)/Sd3.3 nm)] x 20. In the second type of structure, in the same fashion, layers with different isotope contents of Fe were alternated to secure purely nuclear diffraction of y-radiation: [57Fe(2.2 nm)/Sc(l.l. nm)]/Fe(2.2 nm)/Sc$l.l nm)] x 25. The alternation period for the resonance nuclei of 57Fe in the second structure is twice that of the electron density variation. Particular attention in the fabrication of the second type of structure (four-layer ones) was paid to the problem of keeping the thicknesses of the isotope 57Fe and natural iron layers equal, since deviation from a uniform thickness might lead to unwanted periodicity (6.6 nm) in the electron density. The superfine structure of the nuclear levels, obtained by conversion-electron MSssbauer spectroscopy, was the same for both LSN (Fig. 4). The spectrum has one asymmetric peak with a width of 8.3r, (Fe = 0.097 mm/s), which can be described by either a superposition of several unsplit doublets [36], or by one asymmetric doublet. The angular and energy characteristics of the nuclear y-radiation reflection were measured on a two-crystal Mossbauer diffractometer with a Mijssbauer source of Co(Cr) [16,26], and the temporal spectra of nuclear fluorescence were studied using SR [17]. No doubt, the most interesting results on Mijssbauer diffraction were obtained for the structures [57Fe/Sc/Fe/Sc] X 25 that were specially fabricated in order to extract purely nuclear radiation from SR. The Fe (110) monochromator provided an angular collimation of u 15” for a beam from a 57Co(Cr) source. The angle dependence of the y-radiation reflection on LSN in the absence of resonance (u = 0) (Fig. 5a) contains a part of the total external reflection, which nearly disappears at an angle of incidence 0 = 15’ (the angle is counted off the sample plane). There is also a Bragg peak of the first order of diffraction, 0, = 46’ with a width - 2.1’, corresponding to an interplanar distance of 3.3 nm. Reflectivity versus angle of incidence, measured near the nuclear resonance (the source velocity was varied in the range +2.0 mm/s) (Fig. 5b) has an additional reflection with a width of N 3.2’ for an angle of incidence of 24’, which corresponds to the period of variation in the density of 57Fe nuclei, i.e. 6.6 nm. The additional reflection peak is thus a consequence of purely nuclear scattering. Calculations of the reflection coefficients, taking into account actually illuminated areas of LSN yield 10% for the purely nuclear and 8% for the structurally-resolved peaks.

5OOool -2

( -1

, 1

I 0 Velocity,

( 2

mm/s

Fig. 4. Conversion-electron MGssbauer spectrum of LSN.

The intensity of the “Co y-radiation was too low to ensure accurate estimates of the extent to which electron scattering is suppressed in the purely nuclear peak. The electron component in the purely nuclear scattering is likely to relate to both total external reflection and the difference between the thicknesses of the 57Fe and natural Fe layers. To resolve this question we measured the angular dependence of the reflection of 14.4 keV X-ray radiation extracted from the bremsstrahlung spectrum of an X-ray sealed tube (Fig. 5~). The angular divergence of this radiation was - 1’. Total external reflection intensity rapidly decreases with increasing angle of incidence, and at 0 = 20’ the reflection coefficient is no more than 10m3. As the angle of incidence is reduced further, diffise scattering becomes a major contributor in the measured intensity. For an angle 0 P 24’ at the level N 10K3 there is a slight peak that can be attributed to a difference in the thicknesses of the 57Fe and Fe layers. Thus, at the angle corresponding to a purely nuclear reflection the multilayer structure in question yields N 10’ as the ratio of coefficients of the nuclear and electron scattering. Energy spectra of the purely nuclear diffraction of y-radiation for several angular positions near the Bragg peak (+ 50”) are given in Fig. 6. Each spectrum has two lines corresponding to source velocities of +0.5 mm/s, but their contribution in the overall intensity depends on the angle of incidence in different ways. The peak on the left is predominant at smaller angles of incidence, while that on the right is predominant at larger angles. This behaviour supports the suggestion that a superfine structure of the nuclear levels of LSN is approximately a single doublet. The fact that the peaks do not appear simultaneously reflects the difference in the Bragg angles for the left

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Instr. and Meth. in Phys. Res. A 359 (1995) 114-120

and right resonances, that relates to the difference between the effective dielectric constants of 57Fe layers in the peaks of a split nuclear doublet. The width of the Mijssbauer reflection spectrum exactly at the Bragg angle (middle spectrum in Fig. 6) reaches 4Or, (AE - 0.2 peV), which is 5 times the width of the nuclear resonance in an isolated nucleus, measured by conversion-electron Mossbauer spectroscopy (Fig. 4). This broadening is the evidence that collective effects play an important role in forming a reflected wave, in particular, the effect of enhancement of the radiation channel of nuclear scattering. The distance between the energy spectrum peaks is larger than the splitting of the quadrupole doublet. In the gap between these peaks there is a minimum which is not found in the spectrum of conversion electrons. The spectrum has this shape because radiation scattering by two resonances occurs in a destructive fashion in the gap between the resonances, and in a constructive fashion in the resonance wings, which was mentioned in Ref. [34]. This study shows that resonance LSN hold promise for filtering monochromatic beams of resonance radiation out

0.2-50" Od -

0.2

0.2

r

-25”

r 8

+25”

0.2r

Velocity,

mmls

Fig. 6. Massbauer spectra of the diffraction of the y-radiation in the purely nuclear reflection

10

20

30 Angle

40

50

60

of incidence

Fig. 5. Angular distributions of the intensity of the scattered y-radiation (a) away from the resonance and (b) at the resonance. (c) Reflection coefficient of 14.4 keV X-ray radiation.

for various angular positions.

of the synchrotron radiation spectrum. Most interesting results have been obtained in the study of structure [57Fe/Sc/Fe/Sc] X 25 in which a purely nuclear reflection of y-radiation was realized in the energy band of 0.2 p,eV. The average reflection coefficient is - 10% for a level of electron scattering of - 0.1%. The energy spectra of diffraction, measured for various angles of incidence, provide a complete and detailed picture of the coherent interaction of y-radiation with the nuclear structure of the III. X-RAY OPTICS

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sample. A large angular width of reflection allows one to increase the brightness and to appreciably simplify the measurement procedure compared with ordinary crystals. This circumstance, together with a wide latitude in choosing the resonant isotope of LSN and a superfine structure of nuclear levels, makes resonance multilayer structures a unique group of entities for the study of fundamental problems in the collective interaction of nuclear radiation with matter. On the other hand, the sensitivity of Miissbauer radiation to the magnetic order of LSN means that nuclear diffraction can be used as a method for studying the properties of layered magnetic nanostructures. A major problem in the fabrication of resonance LSN is to drastically reduce the effect of electron scattering on purely nuclear reflection. To this end a LSN surface needs to be antireflection coated. Now we can make such structures for “Fe, lEITa, and 169Tm isotopes.

Acknowledgement The research described in this article was made possible in part by Grants No. RSROOO and No. RS8000 from the International Science Foundation.

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