High speed imaging of Z-pinch gas discharge in extreme ultraviolet and model-based three-dimensional reconstruction of emitting volume.

We constructed a time-resolved imaging setup with 5 ns time step and 2 ns exposure to record plasma dynamics inside the electrode system of a gas discharge extreme ultraviolet source. To analyze the three-dimensional distribution of the emitting plasma, a model-based reconstruction method is implemented. It is applied to the experimentally recorded time-series images to visualize the transient dynamics of the emitting volume in the radial and otherwise inaccessible axial direction. The focus of this paper is the description of the experimental setup and the introduction of the three-dimensional image reconstruction technique for transparent axisymmetric sources. The influence of different fuel gases on plasma dynamics is discussed as an exemplary case.

Structural properties of templated Ge quantum dot arrays: impact of growth and pre-pattern parameters.

In this study we analyze the impact of process and growth parameters on the structural properties of germanium (Ge) quantum dot (QD) arrays. The arrays were deposited by molecular-beam epitaxy on pre-patterned silicon (Si) substrates. Periodic arrays of pits with diameters between 120 and 20 nm and pitches ranging from 200 nm down to 40 nm were etched into the substrate prior to growth. The structural perfection of the two-dimensional QD arrays was evaluated based on SEM images. The impact of two processing steps on the directed self-assembly of Ge QD arrays is investigated. First, a thin Si buffer layer grown on a pre-patterned substrate reshapes the pre-pattern pits and determines the nucleation and initial shape of the QDs. Subsequently, the deposition parameters of the Ge define the overall shape and uniformity of the QDs. In particular, the growth temperature and the deposition rate are relevant and need to be optimized according to the design of the pre-pattern. Applying this knowledge, we are able to fabricate regular arrays of pyramid shaped QDs with dot densities up to 7.2 × 1010 cm-2.

Ptychographic imaging with a compact gas-discharge plasma extreme ultraviolet light source.

We report the demonstration of a scanning probe coherent diffractive imaging method (also known as ptychographic CDI) using a compact and partially coherent gas-discharge plasma source of extreme ultraviolet (EUV) radiation at a 17.3 nm wavelength. Until now, CDI has been mainly carried out with coherent, high-brightness light sources, such as third generation synchrotrons, x-ray free-electron lasers, and high harmonic generation. Here we performed ptychographic lensless imaging of an extended sample using a compact, lab-scale source. The CDI reconstructions were achieved by applying constraint relaxation to the CDI algorithm. Experimental results indicate that our method can handle the low spatial coherence and broadband nature of the EUV illumination, as well as the residual background due to visible light emitted by the gas-discharge source. The ability to conduct ptychographic imaging with lab-scale and partially coherent EUV sources is expected to significantly expand the applications of this powerful CDI method.

Analysis of buried interfaces in multilayer mirrors using grazing incidence extreme ultraviolet reflectometry near resonance edges.

Accurate measurements of optical properties of multilayer (ML) mirrors and chemical compositions of interdiffusion layers are particularly challenging to date. In this work, an innovative and nondestructive experimental characterization method for multilayers is discussed. The method is based on extreme ultraviolet (EUV) reflectivity measurements performed on a wide grazing incidence angular range at an energy near the absorption resonance edge of low-Z elements in the ML components. This experimental method combined with the underlying physical phenomenon of abrupt changes of optical constants near EUV resonance edges enables us to characterize optical and structural properties of multilayers with high sensitivity. A major advantage of the method is to perform detailed quantitative analysis of buried interfaces of multilayer structures in a nondestructive and nonimaging setup. Coatings of Si/Mo multilayers on a Si substrate with period d=16.4 nm, number of bilayers N=25, and different capping structures are investigated. Stoichiometric compositions of Si-on-Mo and Mo-on-Si interface diffusion layers are derived. Effects of surface oxidation reactions and carbon contaminations on the optical constants of capping layers and the impact of neighboring atoms' interactions on optical responses of Si and Mo layers are discussed.

Spectral sharpening algorithm for a polychromatic reflectometer in the extreme ultraviolet.

In this paper we present a practical approach to the analysis of spectra recorded from a table-top reflectometer operating in the extreme ultraviolet (UV) spectral range. Such in-lab tools, which utilize light from plasma based sources, are currently under investigation for a broad range of applications such as surface and thin-film analysis, near-edge X-ray absorption fine structure (NEXAFS) studies, or reflectivity measurements. In a polychromatic approach one is able to record surface-sensitive reflectometric spectra that are characteristic for each material or thin-film-layer stack. By monitoring the incident and emergent spectrum before and after the sample using two independent detectors, one can deduce its reflectivity. However, the analysis of the raw data, in terms of digitized spectrometric intensity values, can be difficult due to the nature of the quasi-continuous emission from such plasma sources, particularly with xenon, which is a good broad-band radiator in the 10-20 nm spectral range. The complexity of configurations involved in transitions of highly ionized xenon makes a line-by-line analysis very difficult as the real spectrum consists of thousands of unresolved transitions superimposed by distinctive lines. Additionally, sampling issues, detector geometry, and minor setup misalignments can distort the result. We propose a practical algorithm for spectral decomposition and sharpening of such data. Measurement results are presented that confirm the functionality of the algorithm.