Hard X-ray Microscopy Enhanced by Coherent Image Reconstruction

Abstract

Introduction 2 From plain shadowgraphy to nanoscale imaging 3 Finite-difference propagation for simulation of x-ray multilayer optics 4 Off-axis multilayer zone plate with 16 nm x 28 nm focus for high resolution x-ray beam induced current imaging 5 In-line holography with hard x-rays at sub-15 nm resolution 6 Coherent diffractive imaging with diffractive optics 7 Conclusion A Appendix Bibliography Author contributions List of publications Acknowledgements Curriculum vitae is known as the numerical aperture (NA = n λ sin θ). Over the last centuries, improve-1 A conventional definition of the resolution is the full width at half maximum (FWHM) of the point spread function: d min = 0.51 • λ/NA [BW13] 2 For the calculation of d min only the real part of the refractive index n λ is used. of soft x-rays. This restricted the variety of samples to objects which are optically thin and robust against high radiation doses [Att00]. Moreover, imperfections in the optics and related aberrations impeded x-ray microscopes from reaching the theoretical resolution limit. Over the last decades image formation in x-ray microscopes has fundamentally changed. The change was driven by the advent of synchrotrons as bright monochromatic and coherent x-ray sources, new techniques in the development and fabrication of optics, and the possibility for the quantitative analysis of the recorded data. The latter was enabled by pixelated x-ray detectors and powerful computers [SA10; CN10]. The concepts today exploit the advantages of bright radiation sources and the high penetration power of hard x-rays, overcoming challenges such as missing highly efficient point-to-point imaging optics or the lack of detectors with sub-micrometer pixel sizes. In this work, experiments were performed with two different approaches of x-ray microscopy. The first is based on optics with a large NA and thus a small focus to scan the sample, namely scanning transmission x-ray microscopy. The second approach uses the full-field illumination of the sample and the subsequent image reconstruction, namely coherent full-field imaging. Both methods can be used for imaging with absorption and phase contrast. In scanning transmission x-ray microscopy (STXM) the sample is scanned with a focused x-ray beam [Att00]. Behind the sample the transmitted beam is recorded and the detected signal is plotted as a function of the position, to generate a map of the sample structure. The signal can be either the measured transmission of the x-ray beam for absorption contrast or the deflection of the x-ray beam by the sample which results in an image of the differential phase contrast. The resolution in a scanning transmission x-ray microscope is given by the focus size of the probing x-ray beam. Newly developed diffractive x-ray optics such as multilayer zone plates (MZP) or multilayer Laue lenses (MLL) can focus an x-ray beam down to 5 nm [Dör+13], and can be utilized for hard xrays up to 100 keV [Ost+17b]. These optics fabricated by thin film multilayer deposition promise to extent the ...