Depth profiling and angular dependent XPS analysis of ultra thin oxide film on duplex stainless steel

Thin oxide layers were produced by exposure of polished and sputter-cleaned stainless steel samples to approximately 10−5 mbar oxygen administered into the fast entry chamber previously pumped down to 10−8 mbar. XPS as well as AES depth profiling determined principal constituents of the oxide layer as chromium and iron oxides and estimated layers to be in the nanometer range (of the order of 2–3 nm). Since thickness estimations obtained by depth profiling depend on sample stoichiometry, which may be changed by the very process of the profiling measurement, independent determination of oxide layers thicknesses by means of a non-destructive technique was also performed. A simple method was used to estimate oxide layer thickness from the peak intensity ratio vs. emission angle measurements. Full ARXPS was also attempted. For angular dependent XPS measurements, a special tilted sample holder was used to translate the instrument's available sample tilt into an emission angle range from 0 to 75°. Thus, sets of metallic/oxide peaks from Fe 2p3/2 and Cr 2p3/2 were measured for different angles. From these data, thickness estimations were derived that were roughly in agreement with thickness estimations obtained from depth profiling data. These estimations were, however, grouped around two distinct values, depending on types of metallic peaks used, which may suggest that even these ultrathin layers are formed of two distinct sublayer types.