New horizons in sputter depth profiling inorganics with giant gas cluster sources: Niobium oxide thin films

X-ray photoelectron spectroscopy is used to study a wide variety of material systems as a function of depth (depth profiling). Historically, Ar+ has been the primary ion of choice, but even at low kinetic energies, Ar+ ion beams can damage materials by creating, for example, nonstoichiometric oxides. Here, we show that the depth profiles of inorganic oxides can be greatly improved using Ar giant gas cluster beams. For NbOx thin films, we demonstrate that using Ar-x(+) (x=1000-2500) gas cluster beams with kinetic energies per projectile atom from 5 to 20eV, there is significantly less preferential oxygen sputtering than 500eV Ar+ sputtering leading to improvements in the measured steady state O/Nb ratio. However, there is significant sputter-induced sample roughness. Depending on the experimental conditions, the surface roughness is up to 20x that of the initial NbOx surface. In general, higher kinetic energies per rojectile atom (E/n) lead to higher sputter yields (Y/n) and less sputter-induced roughness and consequently better quality depth profiles. We demonstrate that the best-quality depth profiles are obtained by increasing the sample temperature; the chemical damage and the crater rms roughness is reduced. The best experimental conditions for depth profiling were found to be using a 20keV Ar-2500(+) primary ion beam at a sample temperature of 44 degrees C. At this temperature, there is no, or very little, reduction of the niobium oxide layer and the crater rms roughness is close to that of the original surface.