Abstract
Transition metals used in semiconductor, photolithography and fusion applications interact with low energy oxygen ions. Understanding erosion, the nature of the formed oxide and depth of oxygen transport is necessary in mitigating unexpected performance of sensors, optics or plasma facing components. Oxide formation is governed by both the ion–target combination and the incident ion energy. We study the interaction of the transition metals molybdenum, ruthenium, palladium and tungsten, with oxygen ions in the energy region of 50–500 eV. Near-threshold sputtering of metals was experimentally measured and compared to predictions by the Monte Carlo code TRIDYN. Compositional changes and oxide thicknesses following sputtering were measured using Angle resolved X-ray photoelectron spectroscopy and subsequently compared to limiting oxide formed by atomic oxygen exposures. Sputter yields in some cases (ruthenium) were found to be sensitive to ion beam impurities such as ozone (<1% of background gas) leading to chemical sputtering. Ion induced oxide thicknesses (for molybdenum and tungsten) were found to be larger than those predicted by ballistic transport where sputtering is balanced by implantation. It is hypothesized that radiation enhanced diffusion of free oxygen leads to thicker oxide films at low ion energies.
Highlights
Metal oxides are of interest to a wide range of applications, from semiconductor manufacturing, to extreme ultraviolet (XUV) lithography optics and fusion research
The sputter yields measured from M-quartz crystal microbalance (QCM) films at steady state correspond to the yield of the metal, since at steady state the mass loss due to sputtering of O atoms from the formed oxide film is compensated by implantation of new oxygen atoms in the film
Sputter yields from palladium (Fig. 1a) are well described by TRIDYN’s binary collision approximation
Summary
Metal oxides are of interest to a wide range of applications, from semiconductor manufacturing, to extreme ultraviolet (XUV) lithography optics and fusion research. Oxidation can be influenced by an increased sticking probability at low energies in which case an extrapolation from high energy data cannot be made. In this regard, studies on low kinetic energy oxygen ion induced sputtering and oxidation of metals are few and scattered. Initial reports by Hechtl and Bohdansky [13] study the sputter yields for molybdenum under O+ ion bombardment down to 100 eV. This was complemented by Hechtl et al [14] for tungsten. Given the disparities in metals, energy ranges, ion species and transport processes studied so far, a complete map of information regarding sputtering and oxidation of transition metals at low ion energies is difficult to construct for any particular metal
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