Boron carbide (B4C) is a commonly used coating material in X-ray mirrors in Free-Electron Laser (FEL) beamlines for its good qualities such as high reflectivity, high hardness and high damage threshold. The B4C coating film fabricated by direct-current magnetron sputtering has large intrinsic compressive stress, which leads to poor adhesion on the substrate. In a traditional way, an adhesive layer of chromium is inserted between the B4C film and the substrate to improve the adhesion, which may also degrade the anti-damage resistance because of the enhanced photoabsorption inside the metal layer. For applications in FEL, the B4C film is generally fabricated under high Argon sputtering gas pressure to reduce the intrinsic stress. However, it is unclear about the impact of the Cr adhesive layer and the sputtering gas pressure on the damage resistances of B4C films. In this study, using a table-top nanosecond EUV damage instrument, single-shot damage experiments were performed on B4C films fabricated at different sputtering gas pressures with or without Cr adhesive layer. The single-shot damage thresholds are reported and the possible damage mechanisms are discussed based on the measurements using scanning electron microscope (SEM) and atomic force microscope (AFM).
With the rapid development of extreme ultraviolet (EUV) light sources, such as plasma-based light source and Free Electron Laser (FEL), it provides unprecedented powerful ultra-short EUV radiations. These extremely high intense ultra-short pulses of radiation bring great challenges to the optical components utilized for steering these light beams, especially the radiation damage issues. However, more studies on the EUV damage mechanisms on optical materials are still quite desired because of limited beamtime provided by FEL facilities. In this study, we present a table-top focused EUV optical system built at the Institute of Precision Optical Engineering (IPOE) for performing EUV damage tests on optical materials. This setup consists of a laser-plasma light source, a modified Schwarzschild objective and an EUV energy attenuator. With a large numerical aperture of 0.44 and a demagnification of 11, the Schwarzschild objective is composed of two annular spherical mirrors coated with Mo/Si multilayers. By using the Zirconium filter and Mo/Si multilayers, this setup can generate the focused radiation with an energy density of 2.27 J/cm2 at the wavelength of 13.5 nm on the image plane of the objective with ~8.3 ns pulse duration. The EUV energy can be changed using a gas attenuator by varying the gas pressure of Helium or Nitrogen inside the chamber. The performance and potentials of this setup are demonstrated by the single-shot or multi-shot damage tests on some samples, such as Au thin film, CaF2 and Mo/Si multilayer mirror. The damage thresholds were determined and the possible damage mechanisms are discussed together with available experimental results.
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