Determination of the States of Oxidation of Metals in Thin Oxide Films by X-Ray Photoelectron Spectroscopy

Abstract

Thin oxide films are widely used in microelectronics, heterogeneous catalysis, and nanotechnologies [1]. The requirements imposed on the manufacture and properties of these films are very high. One of the most important parameters responsible for the quality and physicochemical properties of oxide films is the oxidation state of metals in the films. Therefore, the qualitative and quantitative determination of the oxidation state of metals is an important analytical problem. It can be successfully solved using X-ray photoelectron spectroscopy (XPS), which provides an opportunity to perform nondestructive multielement surface analysis with an analytical sampling depth of only 3-5 nm [1,2]. It is well known that chemical shifts of the core levels of metals are observed in the XPS spectra upon surface oxidation [1,2]. These chemical shifts can be effectively used for the determination of the states of oxidation of metals in thin oxide films of variable composition.The ion-beam oxidation of metal surfaces is a promising high-tech technique for the formation of oxide films several nanometers in thickness [3][4][5][6]. Oxide films result from the irradiation of surfaces with lowenergy oxygen ions under controlled high-vacuum conditions at room temperature. An advantage of this oxidation technique is the possibility of controlling the composition and thickness of films by changing radiation dose and the energy of oxygen ions.The aim of this work was to determine the states of oxidation of metals and the concentrations of metals in different states of oxidation in thin oxide films of variable composition, which are formed on the surfaces of molybdenum, tungsten, niobium, and tantalum as a result of irradiation with low-energy oxygen ions. EXPERIMENTALAll of the experiments were performed using an ingenious procedure. The sequential and essential stages of this procedure were surface cleaning with argon ions, surface irradiation with low-energy oxygen ions, and in situ XPS analysis of the oxidized surface in a high vacuum on a Leybold LHS-10 electron spectrometer (Germany) [5].Polycrystalline foils of high-purity molybdenum, tungsten, niobium, and tantalum were used as test materials. A sample was placed in the preparation chamber of the electron spectrometer, where the surface was cleaned by argon-ion bombardment in a high vacuum (10 -5 Pa). The degree of surface cleaning was monitored by Auger spectroscopy. After cleaning, the metal surface was irradiated with oxygen ions with 1-keV energy ( E ) at room temperature. Radiation dose ( D ) was varied over the range 10 15 -10 18 cm -2 . Ultrahigh-purity gases of Ar ( ≥ 99.999 vol %) and O 2 ( ≥ 99.998 vol %) (Messer Griesheim, Germany) were used in the experiments. After irradiation, the sample was transferred to the analysis chamber of the electron spectrometer through a vacuum lock. In this analysis chamber, the states of oxidation of metals in thin oxide films were determined by XPS.The XPS spectra were measured under conditions of constant absolute energy resolution of a...

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