Oxygen and titanium sputtering yields as determined by laser fluorescence and auger electron spectroscopy for monolayer oxygen coverage of polycrystalline Ti

Abstract

The presence of an oxide layer can strongly influence the charge-state of species ejected from ion-bombarded metal surfaces, as well as the total sputtering yield. These quantities directly affect the influx of metallic impurities from the wall region into the plasmas of fusion devices. Surface coverage can also modify the distribution of sputtered atoms among electronic states and thus the apparent impurity density detected by the laser fluorescence spectroscopy (LFS) technique. The measurements reported here provide LFS data on number density and electronic state populations for the species Ti and Ti + as a function of surface oxygen coverage in a laboratory apparatus providing for direct monitoring by Auger analysis. An ultra-high vacuum chamber reached a base pressure of ⩽10 −8 Pa after 200°C bakeout, making target contamination negligible during data collection. The desorption data for O-atoms by 3 keV Ar ions exhibits four distinct rates, for coverage between 0 and 3 monolayers. Neutral titanium sputtering yield falls approximately linearly with oxygen coverage, reaching 50% of the bare metal value at about two monolayers. All excited multiplets investigated showed enhanced population with oxygen coverage. Agreement with TRIM computer modelling is achieved if oxygen is assumed to occupy some sub-surface sites.