Reduced Preferential Sputtering of TiO 2 using Massive Argon Clusters

Abstract

In this study, we demonstrate low damage etching of titania for depth profile analysis using large Ar cluster ions. Sample damage caused by impinging ion beams is discussed and a comparison is made between monatomic Ar + ions and Ar n + cluster ions with respect to surface damage. Monatomic ions of beam energy 5 kV cause considerable changes to the structure of amorphous titania resulting in the preferential removal of oxygen and damage to the lattice structure. Titanium is reduced from the +4 oxidation state to +3 and +2 oxidation state during this preferential removal. In contrast, the use of clusters greatly diminishes this damage and also reduces the incorporation of the impinging ion into the sample surface. Here we discuss the use of Ar n + clusters as a new methodology for reducing the damaging effects caused by ion bombardment. 1. Overview Depth profiles have been employed extensively to accurately gain information regarding elemental concentration and chemical composition of complex heterogeneous materials. The process involves repetitive cycles of analysis-in this case employing XPS as the analytical technique-followed by ion bombardment. Ion bombardment removes the top layers each time, exposing bulk material. The difference in the chemical composition of the surface relative to the sub-surface or bulk is often significant to the mechanical or electrical performance of the material. Until the recent development of Ar n + gas cluster ion sources such depth profile studies have been limited to Ar + as the primary ion for sputter erosion however we show that by using cluster ions, where the energy per atom can be as low as 5-40 eV, it is possible to significantly reduce bulk damage and preferential sputtering of these complex surfaces. Here we will discuss the use of clusters for depth profiling a titania thin-film. Titania is used in a wide variety of applications from solar cells to heterogeneous catalyst supports. Its many uses can be attributed to its formidable electron transport properties. Unfortunately analysis using depth profile methods has until now been limited due to its readiness to reduce under ion bombardment. Here we will introduce a new methodology which limits this damaging process. 2. Experimental All measurements were taken using the Kratos Axis Nova XPS instrument equipped with a Kratos Gas Cluster Ion Source (GCIS) for sample sputtering using Ar n + cluster ions or monatomic Ar +. Clusters are created in the GCIS through the supersonic expansion of high pressure Ar gas through a de Laval nozzle into a medium vacuum region [1]. Nascent clusters are transmitted through a differentially pumped region into an electron impact ionisation source. Following ionisation, Ar n + cluster ions are extracted through the length of a Wien filter to eliminate all small ions and limit the transmitted cluster size distribution spread around a chosen median value. The Ar n + cluster ions are then deflected through 2 °