Re-oxidation of NiCr thin film during XPS analysis
Résumé
NiCr films can be obtained by sputtering NiCr target on alumina substrates and annealing them at various times and temperatures. The object of this presentation is not to present the influence of process parameters on film composition but to demonstrate that XPS experimental conditions can have a significant impact on atomic composition.
It is well known that during annealing, chromium preferentially migrates to the surface with creation of an oxide layer. In order to obtain a depth profile composition, successive etchings with Ar+ ions were performed, immediately followed by an XPS analysis. After a first etching that removed carbon contamination, the surface is mainly composed of chromium and oxygen but with almost no nickel. After several etchings, Ni appears again and increases regularly whereas oxygen decreases to reach less than 3%.
During these analyses, we observed that the oxygen level is not stable and depends on whether the O1s window is registered at the beginning or at the end of analysis, i.e. ten minutes later. It seems that there is a re-oxidation of NiCr film during the XPS analysis. In order to spotlight this phenomenon, analyses were performed during several hours and showed that oxidation kinetics also changes with charge compensation.
This presentation shows that composition of an etched surface can change during XPS analyses and opens the discussion on the origin of this re-oxidation phenomenon and its growth kinetics.