Ultrafast X-ray absorption spectroscopy (UXAS) offers the opportunity to investigate function-structure relationships of
complex organic molecules or biological functional subunits without the need of crystallization. Of special interest from
the viewpoint of structural biology is the region of K-edges of transition metals between 5 and 10 keV. Regardless of
successful application of time-resolved diffraction techniques to investigations of crystal dynamics using synchrotron
and laboratory based sources there are only very few examples for application of UXAS to revealing the structural
dynamics in biomolecular systems. This is mainly caused by the lack of broadband ultrafast x-ray sources as well as of
appropriate optics adapted to these sources. Due to the long-data-recording time in UXAS experiments the sample
integrity is mainly determined by the average power of the pump pulses inducing the structural changes. Using a fixed
energy of the pump pulse the latter one is determined by the repetition rate of the pump laser. In this paper we discuss the
prospects of UXAS comparing fs laser plasma sources with different repetition rates in combination with tailor-made
optics based on highly annealed pyrolytic graphite (HAPG).
X-ray microscopy in the water window has become a valuable imaging tool for a wide field of applications with a
resolution in the nanometer regime. The emergence and the development of laboratory based transmission X-ray
microscopes (LTXM) can be of great benefit to users, since LTXM provides access to a method previously limited to
synchrotron facilities only. In recent years, measuring times in the laboratory have been reduced to the point, where
tomography of aqueous cryofixated samples has become feasible.
We report on a laboratory full-field transmission X-ray microscope based on a laser induced plasma source located at the
Berlin Laboratory for innovative X-ray Technologies. A short introduction on full-field X-ray microscopy in the water
window is given.
We demonstrate that, with a thin disk laser-system (TDL), which provides an average power of ~15 W a spatial
resolution of Δx = 41 nm ± 3 nm (half-pitch) is feasible. An image of a diatom recorded at 15 W average laser power
with a magnification of 1125x captured in 5 min is presented.
Technological reasons stimulated enormous interest in the spectral range between 10 nm and 15 nm. One of the most important, apart from the potential to be applied in the microlithography, was the existence of the high-efficiency, spectrally highly selective (narrow-band) reflective multi-layer (ML) optics in this spectral range. Applying these optics to plasma based XUV (extreme ultra violett) sources the debris from the plasma is a serious problem. For transmissive multi-layer optics we have additionally the low figures of merit. For example, the best beam splitters have an efficiency of about 30% (energy in both parts of the splitted beam). This type of element is crucial for efficient single-shot interferometry being the main application using table-top soft x-ray lasers.
We applied capillary optical elements, to our knowledge for the first time, to XUV radiation at 13.9 nm. These optical elements help overcome the limits discussed above or at least remarkably reduce the existing difficulties. A capillary beam splitter and a focussing capillary were applied to an incoherent XUV radiation source. For the beam splitter we measured a throughput of about 80%. With the focussing capillary we obtained a spot size of 27 μm (FWHM) with a gain (intensity in the focal spot compared to the intensity behind a pinhole of the focal spot size) of 600. Advantages and disadvantages of these optics in the discussed spectral range are analyzed.
Thin films of highly oriented pyrolytic graphite (HOPG) give the opportunity to realize crystal optics with arbitrary geometry by mounting it on a mould of any shape. A specific feature of a HOPG is its mosaicity accompanied by mosaic focusing and high integral reflectivity. These characteristics are of interest for compact x-ray diagnostic tools and spectrometers. Another interesting feature is, due to the mosaic spread of the HOPG crystals, that it is possible also with a beam of low divergence to record a spectrum in a broad energy range even within one laser shot. That means that the HOPG spectrometer can act as a polychromator. The latter feature is important if irreversible changes in samples should be investigated or, e.g., if in time-resolved pump-probe experiments a spectrum should be recorded before sample degradation takes place due to high pump intensities. Different design considerations for a compact HOPG-spectrometer based on experimental and theoretical studies will be presented. For applications in plasma diagnostics and XAFS (x-ray absorption fine structure) the attainable energy resolution plays a central role and has been intensively investigated. The results of our investigations demonstrate that HOPG can be used as powerful optics for x-ray diagnostics as well as for x-ray absorption and emission spectroscopy.
With the development of EUV lithography there is an increasing need for high-accuracy at-wavelength metrology. In particular, there is an urgent need for metrology at optical components like mirrors or masks close to the production line. Sources for metrology have to fit different demands on EUV power and spectral shape than sources for steppers systems. We present the results of the radiometric characterization of a laser produced plasma (LPP)-source, newly developed at Max-Born-Institute Berlin for use in an EUV reflectometer. It is operated with a high-power pointing-stabilized laser beam (energy per pulse up to 700 mJ, 10 ns pulse duration, < ± 25 μrad pointing stability) at 532 nm which is focussed on a rotating Au target cylinder. The incident angle of the laser beam is set to 63°, the detecting angle 55° to the target normal. The source has been characterized regarding spectral photon flux, source size and source point stability. Two independently calibrated instruments, an imaging spectrometer and a double multilayer tool for in-band power measurements were used to obtain highly reliable quantitative values for the EUV emission of the Au-LPP source. Both instruments were calibrated by Physikalisch-Technische Bundesanstalt in its radiometry laboratory at the electron storage ring BESSY II. We obtained a source size of 30 μm by 50 μm (2s horizontal by vertical) and a stability of better than 2s=5 μm horizontally and 2s=9 μm vertically. A spectral photon flux of 1*10e14 /(s sr 0.1 nm) at 13.4 nm at a laser pulse energy of 630 mJ is obtained. The shot-to-shot stability of the source is about 5% (1s) for laser pulse energies above 200 mJ. For pulse energies between 200 mJ and 700 mJ, there is a linear relation between laser pulse energy and EUV output. The spectrum shows a flat continuos emission in the EUV spectral range, which is important for wavelength scanning reflectometry. High stability in total flux and spectral shape of the plasma emission as well as low debris was only obtained using a new target position for each shot. There is also a trade off between source size and EUV power. For a slightly defocused laser, an increase in EUV power up to a factor of two is obtained, while the source size also increases by about a factor of two. It is shown that an Au-LPP source provides spectrally flat reproducible emission with sufficient power at low debris conditions for the operation of a laboratory based EUV reflectometer.
The quality assurance for production of optical components for EUV lithography strongly requires at-wavelength metrology. Presently, at-wavelength characterizations of mirrors and masks are done using the synchrotron radiation of electron storage rings, e.g. BESSY II. For the production process of EUV optics, however, the immediate access to metrology tools is necessary and availability of laboratory devices is mandatory. Within the last years a stand alone laboratory EUV reflectometer for large samples has been developed It consists of a laser produced plasma (LLP) radiation source, a monochromator and a large goniometer systme. The manipulation system of the reflectometer can handle samples with diameters of up to 500 mm, thicknesses of up to 200 mm and weights of up to 30 kg. The wavelength can be varied from 10 nm to 16 nm. The spot size on the sample surface is about 2mm. The angle of incidence can be varied from 3° to 60°. In this paper, we describe the laboratory reflectometer in detail and discuss the achieved performance. First measurements of 4 inch mirrors are presented and discussed in comparison to the results obtained at the PTB soft x-ray radiometry beamline at BESSY II.