Ion Implantation and Synthesis of Materials - Studium

174 12 Surface Erosion During Implantation: Sputtering2.52.01.51N Si1.0N Pt80.520.00 2 4 6 10 12 14YFig. 12.10. Steady-state surface composition predictions of high-dose implantation of Si (1)and Pt (2) into PtSi. Note that the implantation of Si into the PtSi sample will result in adepletion of Si if the total sputtering yield, S, is higher than three. This is because ofpreferential sputtering of Si. The composition is determined by a competition betweenimplantation and sputtering, as seen in (12.26) (from Liau and Mayer 1980)can protect the material from being sputtered, and therefore can strongly affect theparameters Y and r, which in turn determine the state of the implanted materials.The surface conditions are influenced by several factors, such as residual gas inthe vacuum, the target material, and the current density of the incident ion beam.For example, it is well known that ion implantation in a bad vacuum can cause theformation of a carbon layer on the sample surface. Formation of thin oxide layersare often encountered in the sputtering of easily oxidized materials. A goodvacuum and high ion-beam current (high sputtering rate) are often desirable tominimize surface oxidation. Both carbon and oxide layers can greatly reduce thesputtering yield of the material, which can significantly increase the maximumimplanted concentration. The surface oxide layer can also affect the preferentialsputtering parameter, r, because of the segregation effects of oxides.It might appear desirable to have surface oxide and carbon layers intentionallyto enhance the implant concentration. However, because of atomic mixing, thesurface oxygen and carbon can become mixed into the implanted layer afterprolonged implantation. These impurities can cause significant side effects.Sputtering can also give rise to surface roughness, which can affect the high-doseimplantation. Surface roughness has been found to be related to crystallographic