Ion Implantation and Synthesis of Materials - Studium

12 Surface Erosion During Implantation:Sputtering12.1 IntroductionThis chapter deals with the erosion of the sample by energetic particlebombardment. In this process, called sputtering, surface atoms are removed bycollisions between the incoming particles and the atoms in the near surface layersof a solid. Sputtering limits the maximum concentration of atoms that can beimplanted and retained in a target material. The yield of sputtered atoms – thenumber of sputtered atoms per incident ion – typically falls in the range of 0.5–20,depending on ion species, ion energy, and target material. For direct ionimplantation into a target material, the maximum concentration of implantedspecies is inversely proportional to the sputter yield. Consequently, for ion-targetcombinations with high sputter yields, the maximum concentration may be only afew atom percent.12.2 Sputtering of Single-Element TargetsSputtering is the erosion of a sample by energetic particle bombardment and ischaracterized primarily by the sputtering yield, Y, which is defined asMean Number of Emitted AtomsY = Sputtering Yield = (12.1)Incident ParticleThe sputtering yield depends on the structure and composition of the targetmaterial, the parameters of the incident ion beam, and the experimental geometry.Sputtering yield experiments have measured values of Y that span more than sevendecades; however, for the medium mass ion species and keV energies of generalinterest in ion–solid interactions, the typical values of Y are between one and ten.An extensive list of sputtering yields is given by Matsunami et al. (1984). Yieldscan also be calculated using SRIM Monte Carlo simulations.159