Soft X-Ray Microscopy
Using Table-Top Pinch Plasma Sources
R. Lebert1, K. Bergmann1, A. Engel1, K. Gäbel1, O. Treichel1, G. Schriever1,
C. Gavrilescu3, W. Neff2
1RWTH Aachen, Lehrstuhl für Lasertechnik, Steinbachstraße 15, D-52074 Aachen,
Germany, E-mail: Lebert@ILT.FHG.DE
2Fraunhofer Institut für Lasertechnik, Steinbachstraße 15, D-52074 Aachen, Germany
3AL. I. Cuza University of Iasi, Copou 11, 6600-Iasi, Romania
Abstract : Table-top pinch plasma devices are intense emitters in the soft
x-ray wavelength range. This paper shows the tailoring of the emission
characteristics of a single pinch plasma device for the specific demands
dictated by imaging, contact and fluorescence microscopy. Results obtained using these schemes demonstrate that pinch plasmas can be tuned
flexibly to fulfil a variety of demands.
1 Introduction
Extreme ultraviolet (EUV) radiation (often also called "soft x-rays") offers interesting features for imaging and analytical purposes with submicron resolution:
1) High contrast between elements with similar atomic numbers.
2) Adopted penetration depth for studying of some 0.1 to 10 µm thick samples
3) Highly efficient high resolution optical components allow for the extension
of conventional imaging and x-ray analytical techniques to higher lateral
resolution.
Applications in the EUV spectral range depend critically on the availability of
laboratory-sized sources. With correct optimization, compact pinch plasma devices emit both broad- and narrowband EUV-radiation. They seem well suited as
user dedicated laboratory-scale equipment to supplement research activities
based on synchrotrons. With typical nanosecond emission duration they allow for
experiments with temporal resolution. These characteristics have prompted a
study of a pinch plasma EUV source for microscopy techniques. Of particular
importance is the optimization of their emission characteristics with respect to
the specific requirements given by each application.
2 Demands for Sources
Each application dictates a special tailoring of the source concerning the spectral
range, the bandwidth, the source size etc. The tailoring of the emission characteristics will be discussed with respect to imaging x-ray microscopy, contact xray microscopy and fluorescence microscopy:
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•
For imaging X-Ray microscopy using Fresnel- zone-plates only narrowband
radiation can be used due to chromatic aberrations of the optics. Thus high
brightness, narrowband line radiation is best suited. For imaging biological
samples with single line emission in the water window Nitrogen lines (NVII
Lyman-α and NVI He-α) and Carbon lines (C VI Lyman-α) can be used [1].
• For Soft X-ray contact microscopy (SXCM) of biological samples broadband
radiation in the water window provides natural contrast [2]. The high flux
needed to expose the photoresist is best generated using the continuous recombination emission or a line band of lithium-like ions.
• Fluorescence X-ray microscopy (FXM) requires intense broadband emission
in the vicinity of the absorption edge of the element to be detected. This
technique is demonstrated for Vanadium emission excited with Nitrogen recombination radiation.
For the discussed applications pulsed exposure is favourable, in order to capture
the image before motion or radiation damage reduces the resolution. Therefore
attempts were made to get a sufficient photon flux for single pulse imaging.
3 Experimental Set-Up
All imaging techniques were studied at a Plasma Focus device [3,4,5] with 1.8 2.5 kJ electrical storage energy and 250-350 kA peak current (2 m2 foot print,
ILT/LLT Aachen) to generate the pulsed radiation (τ < 10 ns; source diameter <
1 mm). The emission of the pinch is used in axial direction, with respect to the
plasma column. This emission is concentrated and demagnified by a condenser
mirror onto the specimen. In order to reach high transmission in the beamline, the
working gas in the discharge chamber and the beamline gas are separated by a
differential pumped aperture system. The spectral emission characteristics is influenced by device parameters which can easily be changed such as:
1) the choice of the working gas,
2) the plasma temperature (Te) which depends on the velocity of the plasma
sheath. It has been shown that plasma temperature depends on pinch current
I, radius of the anode (a) and working gas pressure (p) and is proportional to
I2/(p a2) [6],
3) the maximum electron density which is proportional to p [6],
4) the type and pressure of the beamline gas or transmission filters.
4 Results
4.1 Imaging X-Ray Microscopy of Biological Samples
The optimized source for an imaging x-ray microscope [7] uses nitrogen as working gas to generate the N VII 1s - 2p (Lyman-α) and N VI 1s2 - 1s3p (Helium-β)
around 2.5 nm; the latter is of smaller intensity and separated by ∆λ = 0.0117 nm
from the NVII Lyman-α corresponding to λ/∆λ > 200 sufficient to allow for the
use of both lines simultaneously. Figure 1 shows the spectrum influenced by the
Soft X-Ray Microscopy Using Table-Top Pinch Plasma Sources
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beamline gas (oxygen) which filters shorter wavelength radiation. The insert
shows the lines of interest with high resolution. The emitting region is about 1 mm
in diameter of which about 160 µm in diameter are used for illumination.
N VII 1s-2p
N VII L-α
N VI 1s2- 1s3p
λ/∆λ=210
N VII He-α
2 µm
Fig. 1. Nitrogen pinch plasma is tuned
to optimized N VII Lyman-α emission
with 2J (ISB = 0.8µJ/µm2/sr) in line of
interest at 2.5 nm.
Fig. 2. Image of bacteria Leptothrix
ochracea taken with a single pulse
of N VII Lyman-α radiation from the
pinch plasma source. courtesy: M.
Diehl, Forschungseinrichtung Röntgenphysik.
In principle all ionic lines emitting into the water window can be used. K-shell
lines are advantageous over L- or M-shell lines because of their larger spectral
separation. Thus, additionally the Helium-α of N VI and the Lyman-α line of
carbon were shown to have high brilliance in the water window and are of use for
imaging microscopy.
The condenser is the only optical element to illuminate the sample. In contrast to
microscopes at storage rings a monochromator stage is obsolete since pinch plasmas can be optimized to emit freestanding line radiation. The continuum energy
in the vicinity of the lines of interest is more than a factor of 500 smaller than the
energy in the line. The source ISB reaches an average of 0.61 µJ/µm2/sr, which is
reproducible within a standard deviation of less than 20%. The total yield per
pulse in the line pair amounts to 80 mJ/sr rendering an overall conversion efficiency (plug in efficiency) of about 1 % into the lines of interest.
The x-ray microscope has been operated at the Forschungseinrichtung Röntgenphysik (Göttingen). Figure 2 shows a single pulse image of wet specimen and
sub-optical resolution as also published in [8]. The system and the optimization of
the spectral characteristics for the imaging microscope operated at Forschungseinrichtung Röntgenphysik, University Göttingen is described in [1].
4.2 Contact X-Ray Microscopy of Biological Samples
Argon is best suited for contact microscopy because it emits many lines into the
spectral range of the water window. E.g., in [9] one finds 188 such L-shell lines of
which 75 are cited with relevant "intensity" > 100 and 6 are noted as very bright.
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R. Lebert et al.
Neon-Like Ar IX to Lithium-like Ar XVI contribute to the emission, so that the
flux is much larger than for a single emission line and amounts to about 6
µJ/µm2/sr. The source is about 400 µm in diameter. Lines with wavelength shorter
than the K-edge of oxygen are filtered by 150 Pa Oxygen gas in the beamline. This
filter also absorbes longer wavelength and thus leads to the decline of the argon
spectrum for λ > 3 nm.
Using the contact microscope techniques described in [2] in a first experiment
images of samples could be obtained, although neither the condensor was optimized for this purpose nor the position of the illumination was controlled to a
great extent. The results are also discussed in [2].
Contact Microscopy
Oxygen absorption
edge
Fig. 3. Argon pinch plasma emitting a
quasi-continuum of L-shell lines in the
water window. Spectral distribution is
tuned by the use of oxygen gas in the
beamline, acting as a filter for shorter
wavelengths.
Fig. 4. Contact microscopy image of
Chlamydomonas taken with a single pulse
of Argon L-shell radiation in the water
window from the pinch plasma source.
Courtesy: A. Ford, T. Stead, Royal Holloway College, University of London.
4.3 Fluorescence Microscopy
Fluorescence microscopy needs a tailoring of broadband radiation in order to
achieve an emission maximum at wavelengths shorter than the absorption edge of
the element to be studied. In this respect the argon source for contact microscopy
could be used for fluorescence microscopy of carbon. Another way is to optimise
the emission of recombination radiation. Recombination radiation is dominating
when the pinch plasma has higher temperatures and densities which can be controlled by the device parameters as described above.
Tailored Nitrogen recombination radiation for the excitation of Vanadium was
accomplished in such a way using the source for imaging and contact x-ray microscopy. Hydrogen was used as the beamline gas instead of oxygen to avoid the
absorption of the continuous radiation. Figure 5 shows the emission spectrum
used for excitation of the x-ray fluorescence of a structured vanadium foil. The
fluorescence radiation was then imaged with a KZP 7 zone plate [10] onto a CCD
camera showing single pulse resolution of better than 100 µm.
Soft X-Ray Microscopy Using Table-Top Pinch Plasma Sources
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5 Summary
Pinch plasma x-ray sources were developed and optimized with respect to single
pulse microscopy techniques. The emission characteristics of the pinch plasma
source was successfully tailored to meet the demands of each application.
500 µm
Nitrogen
Emission
Vanadium
L-absoption
Fig. 5. Nitrogen pinch plasma can also
be tuned to emit mainly recombination
continuum of hydrogen-like ions which
is suitable to excite Vanadium fluorescence. compare to Fig. 1.
Fig. 6. Image of vanadium distribution using its L-shell fluorescence at
λ=2.5 nm excited with a single
pulse from the pinch plasma source
and imaged with a KZP 7 zone plate
onto a CCD-camera.
Acknowledgements
This work was supported by the Deutsche Forschungsgemeinschaft (DFG)
(He 979/17-1, Le-904/1-1), the Bundesministerium für Forschung und Technologie (13N6489/3). The authors wish to express their thanks to Prof. G. Schmahl
and Dr. D. Rudolph and Dr. A.D. Stead and Dr. T.W. Ford for advice and the
pictures.
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