Damage study on B4C films using a table-top nanosecond EUV damage instrument

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).