Schwarzschild Microscopes for 18−20-nm Wavelength
Range with Submicron Resolution
I. A. Artioukov1, A. V. Vinogradov1, V. E. Levashov1, I. I. Struk1,
V. E. Asadchikov2, Yu. S. Kas’yanov3, R. V. Serov3,
V. V. Kondratenko4, A. I. Fedorenko4, S. A. Yulin4
1
3
P. N. Lebedev Physical Institute, 53 Leninsky Pr.,
Moscow 117924, Russia
2
Institute of Crystallography, 59 Leninsky Pr.,
Moscow 117333, Russia
General Physics Institute, 38 Vavilova St., Moscow 117942, Russia
4
State Polytechnic University, 21 Frunze St.,
Kharkov 310002, Ukraine
Abstract. The paper presents the results of testing and applications of soft Xray imaging reflective microscopes fabricated in X-ray Optics Group. The
objectives have been designed using Schwarzschild configuration and coated
with Mo-Si reflective multilayer structure. The test-objects have been imaged
with illumination by laser plasma X-ray source with the help of a multilayer
condenser. The accuracy of mirrors’ shape and special alignment scheme
enabled us to achieve 0.1 - 0.7 µm spatial resolution.
1 Introduction
A number of papers have been published on X-ray scanning and imaging microscopes
based on Schwarzschild objectives consisting of two mirrors covered with multilayer
reflecting coatings [1-12]. A Schwarzschild microscope can be used for investigation
of radiating (hot plasma) [2, 6, 10] or non-radiating [7-9, 12] objects that is especially
valuable for various applications in biology, microanalysis and medicine. Four years
ago the Schwarzschild objective of numerical aperture Na = 0.07 was used for the
demonstration of feasibility of 0.05 µm projection lithography [13].
The spatial resolution obtained by now with large aperture (Na > 0.2)
Schwarzschild imaging microscopes is yet far from the diffraction limit for X-rays.
The main reasons are errors in the mirror's figure and misalignment of the microscope
elements. In this paper we present the results of development of our soft X-ray
microscope designed for the investigations of non-radiating objects, which has been
started two years ago [9].
2 Schwarzschild Microscopes Fabricated in X-Ray Optics Group
2.1 10x Schwarzschild Microscope at Wavelength 18 nm
In 1993 we have constructed and tested the first Schwarzschild microscope with the
working wavelength about 18 nm. Table 1 shows the main geometrical parameters of
the Schwarzschild microscope.
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I. A. Artioukov et al.
Mo/Si reflective multilayer coatings were deposited by magnetron sputtering. The
period was measured to be d = 9.5 nm, number of bi-layers N = 20, Mo/Si spacing
ratio β = 0.3.
To test our objective we used three-mirror optical scheme including the condenser
mirror and laser produced plasma X-ray source (see Fig. 1). Laser plasma was
produced by focusing of the second harmonic of Nd-glass laser (λ = 0.53 µm, pulse
duration 1.5 ns, maximum pulse energy up to 20 J) onto a bulk rhenium target. In our
experiments the typical pulse energy was about 1 J. Two aluminum filters 4 with total
thickness 0.9 µm cut-off visible and UV radiation of laser plasma. The X-ray film
UF-4 was used as a detector.
Table 1. Main Parameters of the Schwarzschild objective 1
Magnification
Focal length
Large mirror diameter, mm
Large mirror curvature radius, mm
Small mirror diameter, mm
Small mirror curvature radius, mm
Hole diameter, mm
Mirror separation, mm
Object-image distance, mm
M
F
D1
R1
D2
R2
Dh
S
Z
10.32
23.2
60
100
12
31.66
12
68.34
288,12
Fig. 1. Soft X-ray microscope: 1 - Re target; 2 - laser plasma X-ray source; 3 - condenser
mirror; 4 - Al-filters of thickness 0.4 - 0.5 mm; 5 - te3sting object; 6 - film UF-4;
7 - Schwarzschild objective; L - focusing objective
Schwarzschild Microscopes for 18−20-nm Wavelength Range
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As test-objects we used golden mesh with 15 µm cells and golden transmission
grating with the period of 1.4 µm. The quality of the Schwarzschild objective allowed
us to demonstrate the possibility to reach submicron resolution at wavelength 18 nm.
On the other hand, the quality was not enough to produce printable image of the
1.4 µm grating and the microdensitogram scanning was used [16]. The quality of
mirror substrates was improved significantly in the next versions of the microscope
(see below).
2.2 21x Schwarzschild Microscope at Wavelength 20 nm
The main efforts were made to improve the quality of the mirrors and accuracy of the
objective's adjustment. As a result the errors of mirror's figure less than 10 nm (RMS)
and microroughness about 0.6 nm (RMS) were achieved. The new housing of the
Schwarzschild objective enables us to adjust the mirrors with accuracy down to 1
micron. Main geometrical parameters of the 21x Schwarzschild objective are shown
in Table 2.
Table 2. Main parameters of the Schwarzschild objective 2
Magnification
Focal length
Large mirror diameter, mm
Large mirror curvature radius, mm
Small mirror diameter, mm
Small mirror curvature radius, mm
Hole diameter, mm
Mirror separation, mm
Object-image distance, mm
M
F
D1
R1
D2
R1
Dh
S
Z
21.26
26.9
50
100
10.6
35
10.6
65
627.59
Fig. 2. Images of golden transmission gratings produced by scanning electron microscope (a,c)
and soft X-ray microscope (b,d); the period of grating is 1.4 µm (a,b) and 0.2 µm (c,d)
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I. A. Artioukov et al.
A multilayer Mo/Si coating was deposited by the method of dc-magnetron
sputtering simultaneously on both mirrors of the Schwarzschild objective and on the
condenser. The reflectivity of the mirrors was enhanced by selecting molybdenum as
the topmost layer for all the optical components The molybdenum was itself
protected by a thin (1.5 - 2.0 nm) silicon film, which prevented reflectivity
degradation with time [18]. The period of the deposited coating was deduced form
the reflectivity curve measured for grazing incidence of Cu-Kα radiation. The coating
period was d = 9.89 nm, the fraction of the Mo layer in the period was β= 0.34, and
the number of periods was 20.
Preliminary alignment of the microscope was carried out in the visible range. Fine
alignment in the X-ray range involved displacement of a film cassette along the
optical axis of the system after each exposure when the test object was kept fixed.
The size of each image was 3 x 3 mm2; it was defined by the size of an aluminum
filter (4 in Fig. 1) placed immediately in front of the film. The high brightness of the
laser-plasma source ensured that images with a normal density were recorded on the
photographic film in one laser shot of 0.5 J energy with about 1 ns exposure.
The spatial resolution of the objective in the soft X-rays was tested were two gold
transmission gratings: one with a period of 1.4 µm and a gap 0.5 µm wide and the
other with a period of 0.2 µm and a gap of less than 0.1 µm.
Figure 2 shows photographs of our test objects. Fig. 2d reveals clearly the defects
in fabrication of the grating with the 0.2 µm period. The irregularity of the grating
was confirmed independently by electron microscopy (Fig. 2c). Therefore, X-ray
microscopy may be used to monitor the quality of samples at the submicron level.
The thickness of the object can be considerably greater than in electron microscopy,
because the pathlength of X-ray photons in matter is greater than the depth of
penetration of electrons.
In contrast to our previous experiments, the high quality of the mirrors and the
better alignment made it possible to utilize the full aperture of the objective (i.e. the
numerical aperture of ∼ 0.2) and to attain a spatial resolution of 0.2 µm (Fig. 2d),
which was possibly limited by the grain size of our UF-4 photographic film.
2.3 Compact Schwarzschild Microscope
The modern version of our submicron resolution Schwarzschild objective can be
applied with any other X-ray source. We conducted imaging experiments with a
compact X-ray plasma source which demonstrate the feasibility of a table-top soft Xray microscope for investigation of non-radiating objects. The parameters of used
YAG:Nd3 laser are: the pulse duration was: about 3 ns, pulse energy - 0.3 J,
frequency up to 10 Hz. The size of beam spot on a target was 60 µm. The geometrical
parameters of the objective and microscope were the same as earlier (see Section
3.2).
The Fig. 3 presents the soft X-ray image of 1.4 µm grating produced by compact
soft X-ray microscope with 100 laser shots. The image again demonstrates the limits
of resolution of the detector (UF-4 film grains) but not of the soft X-ray optics used.
Schwarzschild Microscopes for 18−20-nm Wavelength Range
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Fig. 3. Soft X-ray image of 1.4 µm
period grating produced by table-top
microscope on the basis of repetitionrate laser
3 Conclusions
The table-top soft X-ray microscope with Schwarzschild objective for investigation
of non-radiating objects has been designed, fabricated and tested. The spatial
resolution achieved is 0.1 µm. In our experiments the working wavelengths 18-20 nm
correspond to multilayer coatings and aluminum filters used. The same scheme can
be extended to shorter wavelengths down to 10 nm by changing reflective coatings
and filters.
The images were obtained both with repetition-rate YAG:Nd laser and one-pulse
Md-glass laser. In the latter case the image was taken in one 1 J laser shot with 1 ns
exposure.
Acknowledgements
The work was supported by Russian Foundation of Basic Research (Project No. 9502-04908) and Department of Science and Technology of the Russian Federation.
The authors are indebted to Academician K.A. Valiev for fruitful discussion.
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