Stress, Roughness and Reflectivity Properties of Sputter-Deposited B4C Coatings for X-Ray Mirrors**Supported by the National Key R&D Program of China under Grant No 2016YFA0401304, the National Natural Science Foundation of China under Grant Nos 61621001, U1732268 and 11875203, and the Shanghai Municipal Science and Technology Major Project under Grant No 2017SHZDZX02.

Abstract

Boron carbide (B4C) coatings have high reflectivity and are widely used as mirrors for free-electron lasers in the x-ray range. However, B4C coatings fabricated by direct-current magnetron sputtering show a strong compressive stress of about −3 GPa. By changing the argon gas pressure and nitrogen-argon gas mixing ratio, we are able to reduce the intrinsic stress to less than −1 GPa for a 50-nm-thick B4C coating. It is found that the stress in a coating deposited at 10 mTorr is −0.69 GPa, the rms roughness of the coating surface is 0.53 nm, and the coating reflectivity is 88%, which is lower than those of coatings produced at lower working pressures. When the working gas contains 8% nitrogen and 92% argon, the B4C coating shows not only −1.19 GPa stress but also a low rms roughness of 0.16 nm, and the measured reflectivity is 93% at the wavelength of 0.154 nm.