Tungsten/boron nitride multilayers for XUV optical applications

W/BN multilayers are theoretically efficient x-ray mirrors at the nitrogen and boron K-(alpha) lines (31.3 angstroms and 67.6 angstroms, respectively). Their most attractive potential application is detection of light elements by x-ray fluorescence spectrometry. The performances of W/BN mirrors depend not only on the structural quality of the multilayers but also on the stoichiometry of the boron nitride layers, especially in the water window (20 - 40 angstroms). In order to get stoichiometric BN layers with low surface roughness, the deposition of thick boron nitride films has been studied in detail. In-situ kinetic ellipsometry, x-ray photoemission, grazing x-ray reflection and scanning electron microscopy show that quasi-stoichiometric BN films with low surface roughness are obtained only with a low total deposition pressure and an additional nitrogen partial pressure. This result is related to the chemical and structural properties of the BN films. W/BN multilayers with medium period value (2d approximately equals 120 angstroms) show about 80 of the maximum reflectivity at the W M4-5 line. When the period is reduced, the performances are reduced, but good quality W/BN multilayers with very low period values (2d approximately equals 50 angstroms) and a great number of periods ( 100) have been fabricated. The best structural quality is obtained when a low nitrogen partial pressure is introduced during the deposition of the BN layers. The optical indice contrast is improved and the tungsten-boron interdiffusion is reduced.