The fabrication of stable platinum-silicon oxide multilayers for X-ray mirrors

An investigation has been carried out to determine the conditions required for the fabrication of stable SiO₂-Pt multilayers using DC-magnetron sputtering for the Pt and RF-magnetron sputtering for the SiO₂. As a preliminary investigation, single layers of Pt on SiO₂ were analysed by X-ray reflectivity (XRR) and X-ray photoelectron spectroscopy (XPS) to develop a model of the Pt-SiO₂ interface layer. The results indicated that a distinct interface layer develops as a Pt silicate approximately 6 Å thick. SiO₂-Pt multilayers fabricated with a period d>65 Å using pure argon as the sputtering gas, display X-ray reflectivity patterns which can be accurately characterised by a repeating bilayer model. When d<65 Å the multilayer becomes unstable upon exposure to air. Additional peaks develop in the XRR pattern which increase in magnitude with time. These peaks arise from the expansion of the SiO₂ layers in the multilayer starting from the top bilayer and gradually working through the multilayer. In the as-prepared specimens the SiO₂ layers are incompletely oxidised and have a composition SiO_x (x<2) and, on exposure to air, oxygen diffuses through the multilayer surface converting the SiO_x to SiO₂. By introducing a small partial pressure of oxygen into the sputtering gas during deposition, multilayers with d<65 Å remained stable on exposure to air. Under these conditions the density of the platinum layers determined from XRR measurements was reduced by approximately 25%. XPS showed that the platinum layer contained bonded oxygen in the form of platinum oxide PtO_x (x<1). SiO₂/PtO_x multilayers have been fabricated with periods down to 13 Å, but the intensity of the first order peak drops off dramatically once the thickness of the PtO_x layer is less that 10-12 Å.