Ni/C and Rh/C multilayers for soft x-ray optics: influence of the deposition conditions on the nanostructure

Carbon alternated with metallic elements such as Ni, Co, Cr, Rh, Fe, W, or Ti offers the best potentialities in terms of soft X-ray reflection in the range 44 - 100 A. Deposition of this type of device requires a very accurate control of nanometric film deposition with a high regularity and sharp interfaces. We have made a precise investigation of the influence of the deposition conditions on the nanostructure of these systems using ultrahigh vacuum rf-sputtering technique. Conventional plasma conditions such as pressure and power levels have been investigated and also more unusual conditions such as target resistivity or sample temperature. It appears that the optimization process always results in a reduction of the amount of energy applied to the surface during growth. Medium argon pressure, low power levels, low target autobias and deposition at low temperature give the best structural quality and the best performance at the carbon K-alpha line. Metallic layer crystallization and the parasitic etching process are limited especially for the most sensitive Ni/C and Rh/C systems. The second part of the study is devoted to the comparison of Ni, Rh, Co, Fe, Cr, Ti, and W alternated with carbon. Thermal stability of the different systems is evaluated and related to the nanostructural behavior. When carbide formation is thermodynamically favorable, parasitics effects due to the crystallization are limited and the thermal stability is improved.