Towards a model for the formation of positive Si  ions T. Janssens * , C. Huyghebaert, W. Vandervorst 1 IMEC, Kapeldreef 75, B-3001 Leuven, Belgium Abstract Thedeterminationoftheoxygenconcentrationatthesurfaceisthecriticalparameterinallmodels[Surf.Sci.112(1981)168;Nucl. Instrum.Meth.Phys.Res.B14(1986)403;in:A.Benninghoven,B.Hagenhoff,H.W.Werner(Eds.),SIMS,Vol.X,Wiley,NewYork, 1997,203pp.;Phys.Rev.B50(1994)15015;in:A.Benninghoven,P.Bertrand,H.N.Migeon,H.W.Werner(Eds.),SIMS,Vol.XII, Elsevier,Amsterdam,2000,151pp.;J.Appl.Phys.90(2001)4456]thatstudythelargeenhancementoftheionisationprobabilityof positive ions by the presence of oxygen at the surface. Different analysis techniques (low energy ion scattering, Rutherford back scattering spectrometry and secondary ion mass spectrometry) are combined in order to clarify the oxygen dependence of the ionisation probability of Si  , a  Si , during O 2  bombardment. A ratio of ion signals is correlated with the surface oxygen content, determinedbyLEIS,allowingustostudytheenhancementoftheionisationprobabilityofSibyoxygenatthesurfaceforenergies between1and5keVperO 2  ion.Forawiderangeofoxidationconditionstheoxygenenhancementcanbedescribedbyapowerlaw dependenceontheoxygencontent a  Si /C O =C Si  3:24 ,wherebythepowershowsnosigni®cantenergydependenceforthestudied energy range. The steep enhancement of the ionisation probability at high oxygen content, which is energy dependent, can be correlated to a change in the surface structure: enhanced shadowing of silicon by oxygen, observed by LEIS. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Ionisation probability; Surface oxidation; Energy dependence 1. Experimental data The evolution of the oxygen concentration at the surface, C S O , and the resulting variation of Si  ion intensity, I  Si C S O , and sputter yield, Y C S O , in the transient region during oxygen bombardment is the starting point of different experiments. Since sputtered ions are ejected from the outermost surface, the technique used to determine the surface composition needs a good depth resolution. Low energy ion scat- tering (LEIS) meets this requirement. An in situ SIMS±LEIS experiment resulted in a relation between ion intensity of Si  and the oxygen surface coverage [7]. The variation of the relative oxygen coverage C S O =C S Si in the transient region during 3 keV O 2  bombardment at normal incidence is shown in Fig. 1 (right axis). The oxygen and silicon content were calibrated with a cleaved MgO (1 0 0) standard and a Si (1 0 0) 2  1 reconstructed surface, measured at elevated temperature to remove H adsorbents. Although a comparison between different standards (MgO, B 2 O 3 ) showed that the accuracy of the stan- dards can be a matter of debate (differences up to 50%), the comparison of two measurement runs shows that the method is precise (10%) [7]. It needs to be mentioned that the interpretation of the C S O =C S Si ratio (as high as 7) is in¯uenced by the degree of shadowing of silicon by oxygen. Applied Surface Science 203±204 (2003) 90±93 * Corresponding author. Tel.: 32-16-281649; fax: 32-16-281706. E-mail address: janssent@imec.be (T. Janssens). 1 Also at: KULeuven, INSYS, Kard. Mercierlaan 92, B-3100 Leuven, Belgium. 0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S0169-4332(02)00705-5