Abstract

X-ray multilayer structures offer high normal incidence reflectance in the extreme ultraviolet and soft x-ray range. The choice of materials for use in these structures depends largely on their optical properties, which change rapidly with wavelength. The bi-layer thickness or period of multilayers is directly proportional to λ, and is measured in nanometers. Molybdenum/silicon multilayers are efficient reflectors for λ > 12.4 nm, with reflectance > 50%. At shorter λ, absorption from the Si L_II,III levels makes Si less ideal for use in multilayers. C and B have low absorption in this range, and are hence incorporated into multilayer structures. As wavelengths decrease below 12.4 nm, the ratio of the reflectance measured from actual multilayers compared to that calculated for ideal multilayers decreases. Presumably this decrease results from the presence of structural imperfections, which have a larger effect on reflectance as the wavelength, and multilayer period, become smaller. The measured soft x-ray reflectance of sputtered multilayers composed of Ru and C, B and B₄C are presented and compared to theoretical calculations. Microstructural characterizations including transmission electron microscopy and non-specular x-ray scattering are also presented, and used to discuss the nature of structural imperfections which may be important in degrading multilayer reflectance.

© 1991 Optical Society of America