Compound optics such as lens systems can overcome the limitations concerning resolution, efficiency, or aberrations which fabrication constraints would impose on any single optical element. In this work we demonstrate unprecedented sub-5 nm point focusing of hard x-rays, based on the combination of a high gain Kirkpatrick-Baez (KB) mirror system and a high resolution W/Si multilayer zone plate (MZP) for ultra-short focal length f. The pre-focusing allows limiting the MZP radius to below 2 μm, compatible with the required 5 nm structure width and essentially unlimited aspect ratios, provided by enabling fabrication technology based on pulsed laser deposition (PLD) and focused ion beam (FIB).

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During pulsed laser deposition in ultrahigh vacuum, the deposited material consists of a large fraction of ions with kinetic energies in the range of 100 eV. In many cases, these energetic particles induce resputtering at the film surface and lead to composition deviations. For Fe–Ag the resputtering effects are quantified by monitoring the deposition rate during growth of elementary and multilayer films. It is shown that preferential resputtering and, therefore, composition deviations of alloy films can be reduced at laser fluences close to the ablation threshold or even better under Ar gas pressure of about 0.05 mbar. The experimental results are described by a model, which includes atom deposition on the film surface, implantation of energetic ions below the surface and resputtering of atoms from the top monolayer.

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