Oxidation of metal thin films by atomic oxygen: A low energy ion scattering study

Abstract

In this study, we combine Low Energy Ion Scattering (LEIS) static and sputter depth profiles for characterization of the oxidation kinetics on Zr, Mo, Ru and Ta films of various thicknesses, following exposure to atomic oxygen at room temperature (∼20 • C). A method for non-destructive determination of the oxide growth rate via LEIS static depth profiling (static DP) is presented in detail. This method shows high sensitivity to the oxide thickness formed, and the results are in agreement with those obtained by X-ray reflectometry (XRR) and sputter depth profiling (sputter DP). Sequential exposures of oxygen isotopes in combination with LEIS sputter DP are applied to elucidate the growth mechanism of the oxide films. The results indicate that the oxidation kinetics at the applied experimental conditions is directly influenced by the metal work function, characterizing a Cabrera-Mott growth type. The maximum thickness of the formed oxide and oxide growth rate are in the order: Zr ≈ Ta > Mo > Ru. The combining of analysis by LEIS static DP and isotope tracing sputter DP is decisive in the characterization of oxidation kinetics in the room temperature regime.

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“…This is corroborated by the well documented fact that the diffusion of oxygen into a Cu lattice is limited. 55,56 Although XPS analysis shows that the surface is oxidized, this does not imply that the surface is oxidized during operation. During the transport of the metal from the reactor to the XPS machine, the sample is exposed to air for a long time and will be…”

“…This will lead to a buildup of an oxygen atom coverage on the surface. Since in-diffusion of O-atoms is limited, 49,55,56 the adsorption of O-atoms will lead to recombinative desorption, resulting in the ejection of O 2 molecules. The reaction can be both a Langmuir-Hinshelwood type or an Eley-Rideal reaction.…”

“…There is no oxidation of the bulk after the formation of a thick surface oxide. 55,56 There are no studies in the literature to our knowledge in which metal oxides are directly exposed to atomic oxygen.…”

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“…Considering that such oxygen diffusion, not observed on the TiO 2 @SiO 2 , was enabled by loading AuPd nanoparticles on the silica shell, we hypothesize that AuPd enables oxygen to permeate the silica shell by dissociating the O 2 molecule. AuPd nanoparticles supported on metal oxides have previously been reported to facilitate O 2 molecule dissociation. , In addition, it is known that the transport of atomic oxygen species through oxide thin film at room temperature is dominated by a field-induced drift, which is generated by the chemisorption of reactive oxygen species. , Although no direct evidence was obtained for such seemingly counterintuitive oxygen dissociation process, it is the most plausible rationale for the series of results presented above (supplementary text in Supporting Information).…”

“…40,41 In addition, it is known that the transport of atomic oxygen species through oxide thin film at room temperature is dominated by a fieldinduced drift, which is generated by the chemisorption of reactive oxygen species. 42,43 Although no direct evidence was obtained for such seemingly counterintuitive oxygen dissociation process, it is the most plausible rationale for the series of results presented above (supplementary text in Supporting Information). We further confirmed the effect of the SiO 2 shell and AuPd by measuring the radical generation with different catalysts (Figure 4b,c).…”