Study of cluster ions produced from ToF-SIMS analysis of a U-6%Nb target

Abstract

Cluster ions have been previously observed during time-of-flight secondary ion mass spectrometry (ToF-SIMS) analysis of metals and metal oxides. Here, we have used ToF-SIMS to investigate cluster ions formed from the hydrocarbon-containing overlayer, the mixed U and Nb surface oxide, and underlying metal of a U-6 %Nb (U6Nb) target. In the overlayer, we observe UxOy+ oxides and U-species likely containing hydrocarbons. In the surface oxide, we observe UO2+, U2O2+, U3O5+, and U4O6+ as the most intense ions for each family of oxide ions containing x U atoms. Nb oxides for NbO1-2- were only observed in negative polarity. In contrast to the oxide, analysis of the underlying U6Nb alloy resulted in repeating units of Un+, Un(Nb)+, and Un(Nb2) + ions for n = 3–11 as the highest intensity ions for each family of ions containing n U atoms. Nbn+ clusters were not observed. Un+, Un(Nb)+, and Un(Nb2)+ clusters containing C- and O-species were observed and were likely produced from soluble C or O species or precipitates known to be present in U6Nb.Cleaning of the surface oxide with varying ion current density of 25 keV 69Ga+ to perturb local chemistry did not influence ion distributions and suggests that observed high intensity ions are stable lattice fragments ejected from the surface oxide. Cleaning of the underlying alloy with varying ion current density increased the content of Un+, Un(Nb)+, or Un(Nb2)+ clusters in comparison to the C and O-containing species. These results suggest a different mechanism of cluster ion formation than what was observed in the metal oxide. Preferential sputtering of U from the surface or low local Nb content may also contribute to the enrichment of U in observed clusters. We conclude that cluster ions may be of great value in studying local chemistry, contaminants, and impurities in oxides and alloys.