Effects of oxygen dosing on Ca cluster yields and energy distributions

Abstract

Sputtering from an oxygen-dosed calcium surface under 4 keV Ar + ion bombardment has been studied. Both the secondary neutral and positive secondary ion fluxes have been analyzed using time-of-flight mass spectrometry to characterize the abundance yields of atoms and molecules. The neutrals were probed using laser post-ionization by an ArF excimer laser. A large number of molecules of the form Ca m O n ( m=1–5 and n=0–5) were observed in both the neutral and positive ion channels. Measurements to characterize the sputter yield and abundance distributions of these species were performed at a series of oxygen doses ranging from 0.03 to 300 L. Energy distributions for the secondary neutral species have also been measured as a function of oxygen coverage. The results for neutral atomic calcium agree well with previous measurements. A simple model to describe mono-atomic oxide formation was used to relate oxygen dose to oxygen surface concentration. It was shown that sputter-cleaning the calcium surface could reduce the oxygen surface atom fraction concentration to about 0.01 and that at this level the sputter yield and abundance distribution for Ca clusters was essentially the same as for an oxygen-free calcium surface. The model was extended to larger molecules and found to describe their formation in the CaO system quite well. The model offers some insights into the unexpected result that substantial numbers of Ca 2 dimers form during the sputtering process.