Sandra Stephan

Hard x-ray nanobeam characterization by coherent diffraction microscopy

We have carried out a ptychographic scanning coherent diffraction imaging experiment on a test object in order to characterize the hard x-ray nanobeam in a scanning x-ray microscope. In addition to a high resolution image of the test object, a detailed quantitative picture of the complex wave field in the nanofocus is obtained with high spatial resolution and dynamic range. Both are the result of high statistics due to the large number of diffraction patterns. The method yields a complete description of the focus, is robust against inaccuracies in sample positioning, and requires no particular shape or prior knowledge of the test object.

Hard x-ray scanning microscopy with coherent radiation: Beyond the resolution of conventional x-ray microscopes

We demonstrate x-ray scanning coherent diffraction microscopy (ptychography) with 10 nm spatial resolution, clearly exceeding the resolution limits of conventional hard x-ray microscopy. The spatial resolution in a ptychogram is shown to depend on the shape (structure factor) of a feature and can vary for different features in the object. In addition, the resolution and contrast are shown to increase with increasing coherent fluence. For an optimal ptychographic x-ray microscope, this implies a source with highest possible brilliance and an x-ray optic with a large numerical aperture to generate the optimal probe beam.

Hard X-ray nanoprobe at beamline P06 at PETRA III

We describe the hard X-ray scanning microscope planned for the new synchrotron radiation source PETRA III at DESY in Hamburg, Germany. It is based on nanofocusing refractive X-ray lenses and is designed for two-dimensional mapping and scanning tomography. It supports X-ray fluorescence and (coherent) diffraction contrast, yielding elemental and structural information from inside the sample. Spatial resolutions down to well below 50 nm are aimed for in direct space. A further increase in spatial resolution is expected by applying ptychographic scanning schemes. The optical scheme with a two-stage focusing optic is described.

Full optical characterization of coherent x-ray nanobeams by ptychographic imaging.

Scanning coherent diffraction microscopy (ptychography) is an emerging hard x-ray microscopy technique that yields spatial resolutions well below the lateral size of the probing nanobeam. Besides a high resolution image of the object, the complex wave field of the probe can be reconstructed at the position of the object. By verifying the consistency of several independent wave field measurements along the optical axis, we address the question of how well the reconstruction represents the nanobeam. With a single ptychogram the wave field can be properly determined over a large range along the optical axis, also at positions inaccessible otherwise.

Non‐destructive and quantitative imaging of a nano‐structured microchip by ptychographic hard X‐ray scanning microscopy

We used hard X‐ray scanning microscopy with ptychographic coherent diffraction contrast to image a front‐end processed passivated microchip fabricated in 80 nm technology. No sample preparation was needed to image buried interconnects and contact layers with a spatial resolution of slightly better than 40 nm. The phase shift in the sample is obtained quantitatively. With the additional knowledge of the elemental composition determined in parallel by X‐ray fluorescence mapping, quantitative information about specific nanostructures is obtained. A significant enhancement in signal‐to‐noise ratio and spatial resolution is achieved compared to conventional hard X‐ray scanning microscopy.

Hard X‐Ray Scanning Microscopy with Coherent Diffraction Contrast

A hard x‐ray scanning microscope based on nanofocusing refractive x‐ray lenses is well suited for coherent x‐ray diffraction imaging, in particular for scanning coherent diffraction microscopy also known as ptychography. Using this technique, the complex transmission function of the object can be obtained with a spatial resolution better than that given by the size of the nanofocus. In addition, the full complex wave field in the plane of the sample can be reconstructed simultaneously, allowing for a complete characterization of the illuminating nanobeam. This is illustrated by a ptychogram of a test structure recorded at 24.3 keV.

Nanofocusing refractive X-ray lenses: Fabrication and modeling

Nanofocusing refractive x-ray lenses (NFLs) form the basis of a hard x-ray scanning microscope. They are characterized by their short focal length (~ 10 mm at 15 keV to 25 keV) and large numerical aperture, allowing for the generation of hard x-ray nanobeams even at short distances from a synchrotron radiation source. These optics, made out of silicon by electron beam lithography and subsequent deep reactive ion etching, have been shown to focus hard x-rays down to 50 nm. We have modeled these optics, allowing us to characterize slight aberrations and the wave-field properties in the focus by analyzing the beam profile in the far field.

Hard x-ray nano-beam characterization by ptychographic imaging

Modern hard x-ray scanning microscopes generate x-ray beams with lateral sizes well below 100 nm. Characterizing these beams in terms of shape and size by conventional techniques is tedious, requires highly accurate test objects and stages, and yields only incomplete information. Since recently, we use a ptychographic scanning coherent diffraction imaging technique in order to characterize hard x-ray nano beams in x-ray scanning microscopes, obtaining a detailed quantitative picture of the complex wave field in the nano focus and allowing one to reconstruct the exit wave field behind the nano-focusing optic, giving detailed insight into its aberrations.

Hard X-ray scanning microscopy with fluorescence and diffraction contrast

Based on nanofocusing parabolic refractive x-ray lenses we have developed and built a hard x-ray scanning microscope that was tested and put to use at beamline ID13 of the ESRF. It can provide a monochromatic hard x-ray nanobeam with lateral extension below 100 nm (down to 50 nm) and a flux up to 109 ph/s in the energy range from 15 to 25 keV. The microscope exploits transmission, fluorescence, and diffraction contrast to obtain local elemental and nanostructural information from the sample. Tomographic scanning yields high resolution elemental maps from the inside of an object. Coherent x-ray diffraction imaging with nanofocused illumination yields images of objects with highest spatial resolution, e. g., 5 nm in a given example.