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 Angstrom thick. SiO2-Pt multilayers, fabricated with a period d>65 Angstrom 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 Angstrom 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 SiOx (x<2) and, on exposure to air, oxygen diffuses through the multilayer surface converting the SiOx to SiO2. By introducing a small partial pressure of oxygen into the sputtering gas during deposition, multilayers with d<65 Angstrom 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 PtOx (x<1). SiO2/PtOx multilayers have been fabricated with periods down to 13 Angstrom, but the intensity of the first order peak drops off dramatically once the thickness of the PtOx layer is less that 10-12 Angstrom. (C) 2002 Elsevier Science B.V. All rights reserved.