X-ray Nanobeam Characteristics

As smaller and smaller nanobeams are produced, the more difficult becomes their characterization. While beams of several micrometers in size can be directly imaged using high resolution x-ray cameras, beams in the submicrometer range are typically characterized by scanning a sharp edge through the beam and recording the transmission, x-ray fluorescence or scattering signal from it. This technique yields the beam profile in scanning direction, integrating over the perpendicular direction. For highest spatial resolutions, phase shifting edges and analyzers based on thin film technology are scanned through the beam.

Recently, scanning coherent diffraction microscopy also known as ptychography was introduced in the field of x-ray microscopy. In this technique, an object is scanned through a confined coherent x-ray beam(Fig. 1). At each position of the scan, a far-field diffraction pattern is recorded. From these data, the complex transmission function and the complex wave field at the position of the object can be consistently reconstructed, provided there is a sufficient overlap between the illuminated areas of neighboring scan points. The spatial resolution of this technique goes beyond that of classical scanning microscopy, where the beam size limits the spatial resolution. As an example, the reconstructed image of a resolution test pattern is shown in Fig. 2a (a detailed description of the experiment is given in). It was recorded with a scanning microscope based on nanofocusing refractive x-ray lenses at an x-ray energy of 15.25 keV. For comparison, the scanning fluorescence image(tantalum L radiation) is shown in Fig. 2b, demonstrating the improvement in spatial resolution obtained by this technique. In ptychography, the spatial resolution is limited by the largest momentum at which diffracted photons are recorded by the detector in the far field. Thus increasing the dose on the sample increases the spatial resolution.

Besides the object transmission function, the complex wave field at the sample position can be reconstructed. In this way, a detailed image of the x-ray beam is obtained. Figure 3a and b shows the re-constructed nanobeam obtained in the example given above. In Fig. 3b the beam shows weak side maxima that can be accurately reconstructed. They are responsible for the fact that the object can be reconstructed outside of the scanning region delineated by the white line in Fig. 2a. As the complex wave field in the plane of the object is available, the full caustic of the beam can be reconstructed by propagation (Fig. 3c). Therefore, the sample does not need to be perfectly in focus. Indeed, the exact focal position can be determined by propagation.

Fig. 1 Scanning coherent diffraction microscopy also known as ptychography: the sample is scanned through a focused coherent beam. At each position of the scan, a for-field diffraction pattern is recorded. From these data, the object’s complex transmission function can be reconstructed

Fig. 2(a)Transmission(phase)of a test structure reconstructed by ptychography. The area delineated by the white line was scanned with the central maximum of the focus. (b)Ta L fluorescence radiation recorded in parallel with the ptychogram

Fig. 3 (a)Reconstructed complex wave field in the nanofocus. (b) Amplitude of the wave field is scaled to highlight low intensity side maxima.(c)3-D view of the focused beam determined by forward and backward propagation of the complex wave field in the plane of the sample given in (a). Amplitude and phase of the wave field are encoded by brightness and hue, respectively (compare color wheel)

The intensity distribution is given by the absolute square of the amplitudes. In the given example, a lateral beam size of 78×86nm2 was determined. The method does not rely on a special test object, but works best for strongly scatter-ing objects with high structural diversity on the scale of the lateral beam size. This technique is a quick and reliable method to fully characterize diffraction limited nanobeams, and has been applied to diffraction-limited beams from different optics. In the meantime it is routinely used to characterize the focus in hard x-ray microscopes at the European Synchrotron Radiation Facility in Grenoble, France, and at the synchrotron radiation source PETRA III at DESY in Hamburg, Germany.