Nanoscaled Multilayer Coatings for X‐Ray Optics

Abstract

For modern X-ray analytics in the lab as well as at synchrotron sources, multilayer X-ray optics are used for X-ray beam conditioning. The optics should deflect, monochromatize, focus or collimate the X-ray beam. For X-rays the real part of the refractive index n is nearly 1, so that X-rays cannot be shaped by lenses. The refractive index n = 1-d+ib consists of the dispersion d and the absorption b. A possibility of controlled X-rays beam shaping exists by using Bragg's diffraction.According to Bragg's condition, radiation of a certain wavelength k which illuminates a periodic structure with lattice constant d is deflected under the angle h. Different orders m are deflected in different directions. The perfection of the lattice as well as the lattice materials determines the reflectivity R.Optimized periodic structures of high reflectivity with adjustable d-values can be obtained by multilayer deposition. These multilayers can consist of up to several hundred layer pairs of two materials with single layers of a few nanometers thickness. The first order (m=1) can show a very high reflectivity of up to 95 %, and the peak widths and energy bandwidths can be 1-2 orders of magnitude larger than for single crystals. The multilayers consist of a heavy reflector and a light spacer, usually with a high dispersion contrast at the interface. For a good X-ray multilayer, the absorption should be low to achieve high reflectivity and reasonably sharp Bragg reflections.Using multilayers with a lateral layer thickness profile that are deposited onto curved substrates, optical elements can created that deflect and shape X-rays. Such so-called Göbel Mirrors consist, for example, of a parabolically curved substrate, and a multilayer coating with d-values typically in the range 3-5 nm. Due to the parabolically curved shape in conjunction with the d-value gradient, the divergent beams emitted from an X-ray source are transformed into a parallel beam bundle using Bragg diffraction (see Fig. 1).

An empirical correction for the influence of low-energy contamination

“…The beam-focusing device is based on elliptically shaped silicon wafers that are coated with W/C multilayers to reflect the X-rays by constructive interference, as suggested by Schuster and Gö bel (Schuster & Gö bel, 1995;Storm et al, 2004). Two identical mirrors arranged in a 90 side-by-side geometry, as proposed by Montel (1957), enable the originally divergent beam to be focused in two dimensions (Hertlein et al, 2008). Michaelsen et al (2004) describe the design and fabrication of this type of beam-shaping equipment in detail.…”

X-ray Spectrometry

“…Finally, the potential of this technique for the direct LA-ICP-TOF-MS analysis was studied. The calibration curves of 48 Ti, 69 Ga, and 138 Ba showed good linearity, and the limits of detection (LOD) were 24 pg for 48 Ti, 3 pg for 69 Ga, and 1 pg for 138 Ba, respectively.…”

“…The fundamentals of nanoscaled multilayer coating techniques for X-ray optics were explained (48). In particular, multilayer coating on a parabolically curved substrate with the d-value gradient is useful for X-ray focusing or collimating.…”

Structured X-Ray Optics for Laboratory-Based Materials Analysis

“…The addition of a parabolic multilayer to thin-film powder XRD (22) or reflectivity (23) systems resulted in greatly improved intensity. Multilayers are also widely used in spectrometry applications such as X-ray fluorescence (XRF) and in small-angle scattering (24).…”