Nanostructured Multi-Layers for Applications in X-ray Optics
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In the electromagnetic spectrum, X-rays occupy the region between gamma and ultraviolet rays, ranging from hard X-rays via soft X-rays to extreme ultraviolet radiation (EUV). Since the discovery of X-ray diffraction in 1912, it has been possible to investigate the structure of materials with atomic resolution. The success of X-ray investigations depends on the properties of the optical system; this fact has driven forward the development of various sources, optics, and detectors. During the last 30 years, new X-ray sources such as synchrotron facilities have been built to achieve sufficient intensity and collimation. For these sources, now various X-ray optics are available, e.g., crystals, grazing-incidence mirrors, zone plates, X-ray lenses, and multilayer mirrors. The use of the state-of-the-art X-ray optics can dramatically improve the outcoming X-ray beam, and it can open up totally new methods for the investigation of materials. Here, multilayer mirrors will be introduced and discussed as powerful X-ray optical elements in chemical analysis and structure determination. A multilayer consists of alternating layers of high-Z and low-Z materials with a double-layer thickness or period d in the order of nanometers (2–10 nm). In XRF, multilayer mirrors have been used as analyzer crystals to measure the elemental composition of a material, especially to detect light elements by means of wavelength-dispersive XRF. They work as a filter for a distinct emission line of the stimulated fluorescence radiation of a specimen. Because of the low atomic number of light elements, the fluorescence yield is poor. Some improvements are still possible because of advanced deposition techniques, which make more exotic materials available as thin films. Furthermore, not only the peak reflectivity but also the background suppression originating from the matrix elements plays an important role in reducing the measuring time. Their optimization leads to an improved lower limit of detection (LLD). In this section, improvements for the detection of boron, carbon, and aluminum will be presented and discussed. In XRD, laterally graded multilayers on figured substrates for beam conditioning were introduced by Göbel and Schuster. The layer thickness gradient fulfils Bragg's law for the X-ray wavelength of interest across the whole mirror length. Therefore, such a graded multilayer acts both as a monochromator and as a beam shaper for X-rays. A divergent “white” beam from an X-ray source is converted into a parallel monochromatic beam by a parabolically curved, laterally graded multilayer mirror (“Göbel-Mirror”). These multilayer mirrors combine the advantages of perfect single crystals (with large incidence angle but low acceptance angle) and total-reflection mirrors (with high reflectivity but small incidence angle). The use of high-precision prefigured optical substrates has led to a new generation of curved graded multilayer mirrors that achieve peak reflectivities higher than 70% at Bragg angles between 0.5° and 3°. Therefore, multilayer mirrors have attained an important and preferential role as optical elements for X-ray laboratory equipment.