X-ray Characterization

Each multilayer coating is examined after synthesis using X-ray reflectometry. The 1st Bragg order is usually used for reflection. High reflectivity is always desired. Furthermore, the peak width is of interest. Small peaks lead to high resolution, while broader peaks may be used to maximize the integral intensity. The XRR measurements allow a deduction of the interface roughness and the density of the layers. The aim of X-ray optical film deposition is to obtain interfaces which are as sharp and smooth as possible, and have an optimal density difference.

The Bragg peak angles can be used to calculate the layer thickness period, the so-called d-spacing. For optical elements requiring a lateral d-spacing gradient (as necessary in virtually all elements used today in XRD) we are able to plot the measured d-spacings as a function of the position across the mirror length. This plot shown in the figure below is included in our shipping documents for each of our optics. An extremely high d-spacing accuracy of several tens of picometers is required for an optimal performance.

Laterally Graded Multilayer Optics: The film thickness variation along the substrate and the ideal and tolerable thickness gradients are shown. The thicknes was measured by X-ray reflectometry.

The film thickness and the gradient must be within 0.1 nm on the length of 6 cm. This is comparable to 1 mm on a distance of 600 km.

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