X-ray photoelectron spectroscopy studies of the surface of ion-beam reactive sputter deposited zirconia films

Abstract

Zirconia thin films have been widely used in many fields. Zirconia shows a high refractive index and low loss in the infrared region. These properties of zirconia appear to render it suitable for fabricating high-reflectance mirrors and interference filters [1]. Zirconia films have been studied by several groups [1-5]. The deposition methods applied were vacuum evaporation, magnetron sputtering, ion-beam reactive sputter deposition, ion-beam-assisted evaporation and dual ion-beam deposition techniques. X-ray photoelectron spectroscopy (XPS) with surfacesensitive core-level has widely been used to analyse the surface of the thin films since core-level shifts give rather direct information about the oxidation state, charge transfer and co-ordination. In previous studies [4, 5] samples produced by ion-beam reactive sputter deposition or by a dual ion-beam deposition technique were characterized with X-ray diffraction, electron transmission spectroscopy, ellipsometry, Rutherford backscattering, XPS etc. We reported the deposition processingdependent refractive index, atomic composition, phase structures and Zr bonding properties of the zirconia films. The composition of the zirconia labelled as ZrOx films, depending on the deposition parameters, is 0 < x ~< 2. Here the O1~ state on the natural surface of the ion beam reactive sputter deposited ZrOx films is concentrated on. In addition to the same state as in the bulk of the films, a new O1~ state was observed, which is discussed in detail below. The XPS measurements were performed on a model Np-1 XPS spectroscope for chemical analyses having a base pressure of 6.7 x 10 -8 Pa. XPS data were obtained with monochromatic MgK~ radiation (hv -1254 eV) as the X-ray source and 25 eV pass energy, a setting thus giving an overall energy resolution of 1.3 eV full width at half-maximum (FWHM) and a typical sampling depth of about 3.0 nm. XPS data were recorded in N(E) mode and corrected for inelastic backscatter so as to obtain a more accurate fit of the integral spectra. The sputtering beam was from a scanning argon-ion gun operating at 2 keV. Fig. la shows a wide-scan spectrum in the 0-9970 eV kinetic energy range of a sample on the natural surface. It shows the presence of Zr, O, C and Ar. Zirconium and oxygen are constituents of the specimen surface. The argon came from the argon-sputtering process. The carbon signal is due to