This article describes a direct numerical homogenization method for investigating the deformation mechanisms in, and the material behaviour of metal powder during and after compaction. An assembly of elasto-plastic spherical particles in frictional contact with one another is considered, and the stress response to some prescribed deformation history as well as the resulting yield surface of the aggregate is computed with the finite-element method. This is done by applying periodic boundary conditions to the assembly, in a manner that allows arbitrary stress or strain control, and by monitoring the dissipated energies during re-loading simulations with prescribed stress space directions. Closed die and isostatic compaction are considered in particular; the resulting yield surfaces are compared and the role of plastic deformation and frictional sliding on the micro-scale is investigated. Significant deviations from isotropic phenomenological models are found and the development of anisotropy is demonstrated.

A three-dimensional application of the discrete element method to free, constrained and pressure-assisted solid-state sintering of powders is presented. Special emphasis is laid on the generation of realistic random initial configuration as well as on the inertia effects. With this method, internal properties such as coordination number and pair correlation function are directly accessible and compare well with theoretical predictions and experimental data. The simulation scheme also enables one to investigate how grain rearrangements on a mesoscopic scale influence macroscopic properties such as densification rate and bulk and shear viscosity. Finally, crack formation during constrained sintering of low-density films is examined.

Simulations based on the discrete element method are employed to investigate the anisotropic sintering of alumina. The results are compared with accompanying experiments. It is shown that during sinter forging an anisotropic microstructure develops, which leads to anisotropic mechanical properties. Several ways to characterize and measure this anisotropy are presented and utilized to identify the causes for anisotropic sintering behaviour. Discrete element modelling can further be employed to determine anisotropic constitutive parameters as a function of density. Also, the influence of grain rearrangement on the development of anisotropy is investigated.

Aperiodic XUV multilayer coatings with broad spectral bandwidth and flat dispersion characteristics have been developed and fabricated as reflecting and spectrally filtering optical elements for attosecond XUV pulses. Based on a genetic computational optimization algorithm aperiodic Mo/Si multilayer structures exhibiting up to 40 eV spectral bandwidth and pulse responses down to about 100 attoseconds at 80 eV center photon energy could be simulated. An experimental multilayer design exhibiting about 15 eV spectral bandwidth at 93 eV photon energy was realized and tested in an attosecond XUV pump – IR probe photoionization experiment (attosecond streak camera) utilizing single attosecond pulses from a High Harmonic Generation source. The results display the enhanced spectral bandwidth filtered by the aperiodic mirror which holds the potential for the extraction of single XUV pulses shorter than 200 attoseconds.

Chirped Mo/Si multilayer coatings have been designed, fabricated, and characterized for use in extreme-ultraviolet attosecond experiments. By numerically simulating the reflection of the attosecond pulse from a multilayer mirror during the optimization procedure based on a genetic algorithm, we obtain optimized layer designs. We show that normal incidence chirped multilayer mirrors capable of reflecting pulses of approximately 100 attoseconds (as) duration can be designed by enhancing the reflectivity bandwidth and optimizing the phase-shift behavior. The chirped multilayer coatings have been fabricated by electron-beam evaporation in an ultrahigh vacuum in combination with ion-beam polishing of the interfaces and in situ reflectivity measurement for layer thickness control. To analyze the aperiodic layer structure by hard-x-ray reflectometry, we have developed an automatic fitting procedure that allows us to determine the individual layer thicknesses with an error of less than 0.05 nm. The fabricated chirped mirror may be used for production of 150-160 as pulses.

Extreme ultraviolet lithography (EUVL) at 13.5nm is the next generation lithography technique capable of printing sub-50nm structures. With decreasing feature sizes to be printed, the requirements for the lithography mask also become more stringent in terms of defect sizes and densities that are still acceptable and the development of lithography optics has to go along with the development of new mask defect inspection techniques that are fast and offer high resolution (preferable in the range of the minimum feature size) at the same time. We report on the development and experimental results of a new 'at wavelength' full-field imaging technique for defect inspection of multilayer mask blanks for EUV lithography. Our approach uses a photoemission electron microscope (PEEM) in a normal incidence illumination mode at 13nm to image the photoelectron emission induced by the EUV wave field on the multilayer mask blank surface. We show that by these means, buried defects in the multilayer stack can be probed down to a lateral size of 50nm, which was the smallest pre-defined structure size under study so far. The PEEMs spatial resolution has been proven to be as small as 29nm edge slope width in measurements with 250nm radiation. Therefore, the capability of this technique for actinic measurements at 13nm radiation is anticipated to be well below 50nm.

Multilayered films with nanometer sized single layers are important as mirrors for extreme ultraviolet radiation or soft x-rays. Oxide systems are promising multilayers for the so-called water-window (between 2.4 and 4.4 nm wavelength). Oxygen is transparent for radiation at this wavelength, and oxides are known to form very smooth, stable interfaces without interdiffusion. In this paper we present tungsten oxide / silicon oxide multilayers, deposited by plasma assisted chemical vapor deposition (PECVD). We deposited multilayer systems consisting of up to 80 individual layers, with single layers of less than one nanometer. The roughness of the layer interfaces is low enough to avoid island formation.

We report on the development and first experimental results of a "at wavelength" full-field imaging technique for defect inspection of multilayer mask blanks for extreme ultraviolet (EUV) lithography. According to the International Semiconductor Roadmap by Sematech, less than 5x10(-3) defects per cm(2) should be present on such multilayer mask blank to enable mass production of microelectronics using EUV lithography, thus fast high-resolution methods for mask defect inspection and localization are needed. Our approach uses a photoemission electron microscope in a normal incidence illumination mode at 13 nm to image the photoelectron emission induced by the EUV wave field on the multilayer mask blank surface. We show that by these means, buried defects in the multilayer stack can be probed down to a lateral size of 50 nm. (c) 2006 American Institute of Physics.

Tailored extreme ultraviolet (EUV) multilayer systems have been designed, fabricated and characterized for applications as optical key components for guiding or monochromatizing ultrashort sub-femtosecond EUV pulses emitted from Ti:Sa laser-driven high harmonic sources. While spectral filtering of individual harmonic requires a periodic multilayer mirror exhibiting a small reflection bandwidth <3 eV to separate adjacent harmonics, the reflection of attosecond EUV pulses without compromising the time structure of the pulse requires aperiodic multilayer systems with large reflection bandwidth and optimized phase. Experimental results of the measured EUV reflection properties for a small-bandwidth multilayer are presented as well as the optimization procedure, the fabrication process and structural characterization of an aperiodic broadband EUV multilayer mirror.