Anna-Lena Robisch

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).

Reconstruction of wave front and object for inline holography from a set of detection planes

We illustrate the errors inherent in the conventional empty beam correction of full field X-ray propagation imaging, i.e. the division of intensities in the detection plane measured with an object in the beam by the intensity pattern measured without the object, i.e. the empty beam intensity pattern. The error of this conventional approximation is controlled by the ratio of the source size to the smallest feature in the object, as is shown by numerical simulation. In a second step, we investigate how to overcome the flawed empty beam division by simultaneous reconstruction of the probing wavefront (probe) and of the object, based on measurements in several detection planes (multi-projection approach). The algorithmic scheme is demonstrated numerically and experimentally, using the defocus wavefront of the hard X-ray nanoprobe setup at the European Synchrotron Radiation Facility (ESRF).

Multilayer Fresnel zone plates for high energy radiation resolve 21 nm features at 1.2 keV

X-ray microscopy is a successful technique with applications in several key fields. Fresnel zone plates (FZPs) have been the optical elements driving its success, especially in the soft X-ray range. However, focusing of hard X-rays via FZPs remains a challenge. It is demonstrated here, that two multilayer type FZPs, delivered from the same multilayer deposit, focus both hard and soft X-rays with high fidelity. The results prove that these lenses can achieve at least 21 nm half-pitch resolution at 1.2 keV demonstrated by direct imaging, and sub-30 nm FWHM (full-pitch) resolution at 7.9 keV, deduced from autocorrelation analysis. Reported FZPs had more than 10% diffraction efficiency near 1.5 keV.

Probe reconstruction for holographic X-ray imaging.

A comparison of different schemes for probe characterization of an X-ray nano-probe in a near-field imaging setting is presented.

Phase retrieval for object and probe using a series of defocus near-field images

Full field x-ray propagation imaging can be severely deteriorated by wave front aberrations. Here we present an extension of ptychographic phase retrieval with simultaneous probe and object reconstruction suitable for the near-field diffractive imaging setting. Update equations used to iteratively solve the phase problem from a set of near-field images in view of reconstruction both object and probe are derived. The algorithm is tested based on numerical simulations including photon shot noise. The results indicate that the approach provides an efficient way to overcome restrictive idealizations of the illumination wave in the near-field (propagation) imaging.

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.

Holographic imaging with a hard x-ray nanoprobe: ptychographic versus conventional phase retrieval

We have performed near-field x-ray imaging with simultaneous object and probe reconstruction. By an advanced ptychographic algorithm based on longitudinal and lateral translations, full-field images of nanoscale objects are reconstructed with quantitative contrast values, along with the extended wavefronts used to illuminate the objects. The imaging scheme makes idealizing assumptions on the probe obsolete, and efficiently disentangles phase shifts related to the object from the imperfections in the illumination. We validate this approach by comparison to the conventional reconstruction scheme without simultaneous probe retrieval, based on the contrast transfer function algorithm. To this end, a set of semiconductor nanowires with controlled chemical composition (InP core, insulating SiO2 layer, and indium tin oxide cover) is imaged using the quasi-point source illumination realized by the hard x-ray nanofocus (26  nm×39  nm spot size) of the ID16A Nano-Imaging beamline at the European Synchrotron Radiation Facility.

Recent advances in use of atomic layer deposition and focused ion beams for fabrication of Fresnel zone plates for hard x-rays

Developments and advances in the e-beam lithography (EBL) made it possible to reach resolutions in a single digit nanometer range in the soft x-ray microscopy using Fresnel Zone Plates (FZP). However, it is very difficult to fabricate efficient FZPs for hard x-rays via this conventional fabrication technique due to limitations in the achievable aspect ratios. Here, we demonstrate the use of alternative fabrication techniques that depend on utilization of atomic layer deposition and focused ion beam processing to deliver FZPs that are efficient for the hard X-ray range.