Normal-incidence efficiencies of 4800-grooves/mm-ruled replica gratings with multilayer and gold coatings in the 125–325-Å wavelength region

Abstract

The normal-incidence efficiencies of two 4800-grooves/mm ruled replica gratings, one with a dual-bandpass molybdenum/silicon multilayer coating and the other with a gold coating, were measured by use of synchrotron radiation in the 125-325-A wavelength region. The peak reflectance of the multilayer coating was 22% in the first Bragg order near 235 A and 28% in the second Bragg order near 126 A. The peak efficiency of the multilayer grating was 2.6% in the first diffraction order near 225 A and 0.3% in the second diffraction order near 125 A. The efficiencies of the multilayer grating were much higher than the corresponding efficiencies of the gold grating. The characterization of the surfaces of the gratings by atomic force microscopy indicated rms microroughness values in the 5-18-microm(-1) frequency range of 12-20 A for the multilayer grating and 22-32 A for the gold grating. Both gratings had bumpy surface features larger than the nominal groove height. The rather large surface roughness and groove irregularities had a detrimental effect on the grating efficiencies.