XPS study of the process of oxygen gettering by thin films of PACVD boron

Numerous collector samples have been exposed in the TdeV tokamak, either to plasma assisted chemical vapor boronization only, or to boronization plus tokamak power discharges. They have been analyzed by X-ray photoelectron spectroscopy (XPS) in order to characterize and better understand the process by which the boron-rich film getter ambient oxygen. It was found that the state of oxidation of the samples after boronization is the most reliable predictor of subsequent machine performance. The gettering capacity is high, on the order of 10 ²¹ O/m ² , and affects a surprisingly thick layer (∼100 nm), but the oxide always remains substoichiometric (with a formula BO _x, x < 1). The oxidation is clearly activated by the plasma, during both glow discharge deposition and power discharges, but in this respect the latter are much more effective than the former.