Effect of defects on the reflectivity of Cr/C multilayer soft X-ray mirror at 4.48 nm

Cr/C is a promising material combination for multilayer mirror in the “near water window region” (4.4-6.7 nm). In the present paper, the effect of defects on the reflectivity of Cr/C soft X-ray multilayer mirror deposited by magnetron sputtering was studied. Formation of thin interlayer due to the interdiffusion, rough interface due to the non-sharp layer and contamination of O happened during the deposition process were found by a method combined by XPS, soft X-ray reflectivity at 4.48 nm and grazing incidence hard X-ray reflectivity at 0.154 nm. The XPS results show that both interlayers (Cr-on-C and C-on-Cr) are mixture composed of C sp2, C sp3, CO, CO, CrCr and CrO bondings. No chromium carbide was found at the interlayer probably due to the blocking of oxides' formation. Through the analysis of X-ray reflectivity, we obtained the multilayer structure parameters (thickness and roughness) and optical constants of each layer at 4.48 nm. Based on those results, a further calculation was carried out. The result shows that the formation of the thin interlayer contributes little to the decrease of the reflectivity, the rough interface decreases the reflectivity most and the contaminant (O) not only decreases the reflectivity but also shifts the position of the peak.