Introduction to Plasma-Surface Interactions Lecture 5 Sputtering Topics Physical sputtering Sputter yields Energy distribution of sputtered atoms Chemical sputtering Yields Flux dependence of yields Global Model Comparing effect of different materials Physical sputtering This is the most common mechanism for bulk impurity atoms getting into the plasma. Sputtering occurs as a result of momentum transfer from an energetic incident ion to solid surfaces It is a well understood physical process and results agree well with calculations Calculations are normally made with the TRIM Monte Carlo code. Tabulation of data for a wide range of ions and targets & energies are available Energy threshold Because an atom leaving the surface has to overcome the surface binding energy E s there is a threshold energy E T for sputtering. This is given by Where Sputter yields for Be, C and W by D and self ions Note the increasing threshold energy with target mass. Using D + ions yield is ~ same for Be, C and W W Eckstein PMI, Garching, Report PP9/8 (1993) High energy sputtering The maximum in the yield and the decreas at high energies is due to the collision cascade occurring deeper and deeper in the solid. The surface atoms have less chance of receiving sufficient energy to be sputtered Uncertainty in yields There is an variation in yields measured experimentally ~ 2. This is not experimental error but genuine variations which depend on surface conditions which can affect the binding energy Examples are variation in the structure, surface roughness or impurity levels Effect of incident angle The sputter yield increases as the angle increase from normal ( =0) This is due to the increased probability of the incident ion being backscattered At energies