Improvement of interface structure and polarization performance of Co/C multilayers by incorporation of nitrogen

Short-period (~3.5 nm) Co/C multilayer mirrors are fabricated by the direct current magnetron sputtering technique through the addition of a small proportion of nitrogen (4-15% partial pressure) to the working gas (Ar). The addition of nitrogen has been demonstrated to significantly suppress the interdiffusion of neighboring materials due to the nitridation of carbon layers as compared with the Co/C multilayer fabricated with the use of pure Ar. The optimal partial pressure of nitrogen was found to be 6%. At this pressure, nitrogen provides abrupt interfaces and the maximal peak value (19%) of the s-polarized radiation reflectivity at the 251-eV photon energy and 45° angle of incidence. The p-polarized radiation reflectivity proved to be less than 0.3%, demonstrating high potentialities of the nitridated Co/C multilayers as Bragg polarizers in the 4.5-6.5-nm spectral range.