Preparation and characterization of X-ray mirrors with three single layers of a-C, B4C and Ni onto two 820 mm long Si substrate

Advanced research light sources, such as free-electron lasers, require ultra-precise and long x-ray mirrors that provide high reflectivity, high flux and a wide wavelength range. An X-ray mirror is a combination of a substrate and a coating. The demand for large mirrors has increased during the last few years, since surface finishing technology is able to process longer substrate lengths on the rms-level of a few nanometers. A state-of-the-art X-ray mirror could be coated with more than one single layer to allow a selection of thin-film materials suitable for the large wavelength range of a free-electron laser. Presented here is an X-ray mirror fabrication method to achieve low variation in thickness of less than 1 nm (peak-to-valley) over the whole mirror length of about 1 m. Low figure errors and low roughness are essential for a wave front preserving transport of photons and a high reflectance of a mirror surface. At FLASH II, the new extension of the Free-electron LASer in Hamburg (FLASH) at DESY, Germany, the wavelength range will be 4-80 nm. It is further expected that the photon beam will possess average single pulse energy of 1-500 μJ, pulse duration of 10-300 fs (FWHM), and peak power of 1-5 GW. At the Helmholtz-Zentrum Geesthacht, an in-house designed magnetronsputtering facility enabled us to deposit single layers and multilayers on up to 1.5 m long substrates. Earlier results confirmed the excellent uniformity of X-ray optical coating properties in the tangential and sagittal direction of the mirrors. Moreover, the deposition facility provided the simultaneous fabrication of two mirrors to achieve identical properties. Thin films of amorphous carbon (a-C), boron carbide (B4C) and nickel (Ni) are deposited by means of magnetron sputtering. The thin-film properties were investigated and analyzed by means of X-ray reflectometry (XRR), atomic force (AFM), and interference microscopy. The experimental results were analysed using simulations for the determination of layer thickness, density and roughness.