Structure and Phase Composition of W-Si Multilayer X-Ray Mirrors

Abstract

X-ray diffractometry in a hard region (l~0.154 nm) was used to study the phase structure, composition and construction of W/Si multilayer X-ray mirrors (MXMs) with thicknesses of t_W<10 nm for tungsten layers obtained by direct-current magnetron sputtering. Two series of samples were fabricated with different tungsten deposition rates, which differ approximately by a factor of 4: ~0.60 nm/s and ~0.15 nm/s. It is shown that tungsten layers have a polycrystalline (BCC) structure at thicknesses t_W>2.7 nm, and at t_W<1.9 nm they are amorphous. Using the sin²Y-method, it was found that in thin crystalline layers of tungsten (t_W<10 nm), more than 3 at.% Si can be contained. Tensile stresses in the layers of crystalline tungsten do not exceed 1.1 GPa. The construction of the radial distribution functions of atoms made it possible to establish that amorphous layers of tungsten have an arrangement of atoms close to b-W. In all samples, formation of silicide interlayers is observed at the interfaces, as a result of which the actual thickness of the tungsten layers is less than the nominal one. Amorphous silicide layers, necessarily formed at the stage of MXM manufacturing, contain tungsten disilicide. Depending on the deposition rate, disilicide can have an arrangement of atoms close to either the tetragonal phase, t-WSi₂ (~0.6 nm/s), or to the hexagonal phase, h-WSi2 (~0.15 nm/s). An improved model for the construction of amorphous W/Si MXMs is presented. Mechanisms for the formation of silicide layers are proposed, according to which the bottom silicide interlayers (W-on-Si) are formed mainly by ballistic mixing of tungsten and silicon atoms, and the top ones due to diffusion inermixing. The interdiffusion coefficients were estimated, which made it possible to establish that the deposited surface of the layers can be heated at least 250° above the substrate temperature. The ways of reducing the interface interaction are suggested.