Stress, Roughness and Reflectivity Properties of Sputter-Deposited B_4C Coatings for X-Ray Mirrors

Boron carbide (B_4C) coatings have high reflectivity and are widely used as mirrors for free-electron lasers in the x-ray range. However, B_4C 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 B_4C 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 B_4C 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.