Tungsten/boron nitride multilayers for X-UV optical applications

WjBN multilayers are theoretically efficient X-ray mirrors at the nitrogen and boron K-α lines (31.3 Å and 67.6 Å 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 Å). In order to get stoichiometric BN layers with low surface roughness, the deposition of thick boron nitride films has been studied in details. In-situ kinetic ellipsometry, X-ray pholoemission, 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 ≃ 120 Å) show about 80% of the maximum reflectivity at the W M⁴⁵ line. When the period is reduced the performances are reduced but good quality W/BN multilayers with very low period values (2d ≃ 50 Å) 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.