Ion Implantation and Synthesis of Materials - Studium

166 12 Surface Erosion During Implantation: SputteringY A (0), to values of N sA ( ∞)and Y sA ( ∞ ) , which occurs at times sufficient toachieve steady state.At the start of sputtering, t = 0, (12.13) takes the formbbA(0) A(0)A= r = rsbB(0) B(0)BY N NY N N(12.14)When steady state conditions have been achieved, conservation of mass requiresthat the ratio of partial yields equals the bulk concentration ratioYA( ∞ ) =Y ( ∞)BNNbAbB(12.15)For example, if there is preferential sputtering where r > 1, the sputtering yield ofA is greater than that of B, and the surface will be enriched in B. This enrichmentproduces an increase in the sputtering yield of B (more B atoms) and a decreasein the sputtering yield of A (fewer A atoms). As the process continues withmacroscopic amounts (greater than 10 nm) of material removed, the increasedconcentration of B balances out the preferential sputtering of A. Therefore, atsteady state the surface concentration ratio will differ from that of the bulk ratiowhen r ≠ 1.sAsBbAbBN ( ∞ ) 1 N =N ( ∞)r N(12.16)That is, the surface composition will be rearranged so that the total sputteringyield gives the bulk composition despite differences in yields of the individualatom species.12.4.2 Composition ChangesAn example of the change in composition of a silicide layer is shown in Fig. 12.5for PtSi that was sputtered with 20 keV argon ions and then analyzed with2 MeV 4 He ions. The Rutherford backscattering spectrum shows an enrichment ofthe Pt concentration in the surface region. The ratio of Pt/Si increased from thevalue of unity associated with that of the bulk to a value near two in the surfaceregion. The increased concentration of Pt at the surface occurs because the partial