Local SiO2 formation in silicon bombarded with oxygen above the critical angle for beam-induced oxidation: new evidence from sputtering yield ratios and correlation with data obtained by other techniques

The first direct comparison of the angular dependence of the sputtering yield of silicon under bombardment with oxygen and inert gas ions of similar mass (neon) is reported. The O2+/Ne+ ratio of the yields, measured at the same energy per projectile atom (5 keV), increases smoothly from 0.33 at 0° to a constant value of 0.89 above ∼65°. This large-angle ratio agrees with the number expected from the projectile mass dependence of sputtering yield theory. The experimental data were analysed on the basis of the assumption that the measured sputtering yield is the sum of contributions from areas composed of either SiO2 or (mostly) Si. The SiO2 fractions thus derived agree quite well with results obtained recently in studies on ion-induced electron emission. Combining the present results with literature data on the angular dependence of the composition of oxygen-bombarded silicon measured by x-ray-induced photoemission spectroscopy, ion-induced electron emission and Rutherford backscattering spectrometry, the SiO2 fractions were found to increase roughly with the square of the O/Si ratio. Data presentation as a function of the atomic fraction of oxygen, on the other hand, gives rise to misleading interpretations. Implications of the results for the interpretation of secondary ion yields and ripple formation are discussed briefly.