Tobias Liese

Sub-5 nm hard x-ray point focusing by a combined Kirkpatrick-Baez mirror and multilayer zone plate

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).

Material-dependent smoothing of periodic rippled structures by pulsed laser deposition

Any kind of processing of materials like thin film deposition, ion beam treatment or polishing often creates structures, which have lateral length scales of 20–200 nm. For many applications an efficient smoothing of such roughened surfaces is needed. To study smoothing on this length-scale, in this work periodic rippled structures with wavelengths of 55 nm and an average height of 4.5 nm were systematically covered by thin films of different material classes by pulsed laser deposition. All materials used possess a low intrinsic roughness on smooth substrates. As studied by atomic force microscopy, the surface roughness is continuously smoothed out with increasing layer thickness. In all cases, the whole process is strongly frequency dependent as expected from theoretical predictions. The dominant smoothing mechanisms have been identified as surface diffusion for small structures below a critical lateral size and sputter erosion or downhill currents for larger structures.

Two-dimensional sub-5-nm hard x-ray focusing with MZP

We present experiments carried out using a combined hard x-ray focusing set-up preserving the benefits of a large-aperture Kirckpatrick-Baez (KB) mirror system and a small focal length multilayer zone plane (MZP). The high gain KB mirrors produce a pre-focus of 400 nm × 200 nm; in their defocus, two MZP lenses of diameter of 1.6 μm and 3.7 μm have been placed, with focal lengths of 50 μm and 250 μm respectively. The lenses have been produced using pulsed laser deposition (PLD) and focused ion beam (FIB). Forward simulations including error models based on measured deviations, auto-correlation analysis and three-plane phase reconstruction support two-dimensional focus sizes of 4.3 nm × 4.7 nm (7:9 keV, W/Si)1 and 4.3 nm ×5.9 nm (13:8 keV, W/ZrO2), respectively.