Threshold energies for sputtering

It is clear that heavy ions are present in contemporary tokamak discharges and individual species such as molybdenum from the limiter and iron from the wall have been identified spectroscopically [1]. However, the means by which they are introduced into the plasma are not clear and it is an open question whether it involves sputtering, evaporation or arcing, or possibly a combination of all these mechanisms. Recent work on the ATC Tokamak [2] shows that the evolution of materials from the wall may be associated with the presence of oxygen as an impurity in the discharge. Interpretation of this and other experiments is hindered by lack of fundamental data on sputtering, in particular the mechanism of sputtering by light ions, and the threshold energy at which sputtering by ions of any type starts. This dearth of information is due, in part, to the difficulty in making the necessary measurements, also some of the published work is unreliable because of the presence of unsuspected errors. It is the purpose of this note to discuss the available information and to show that a general pattern exists from which the threshold energy of any type of ion incident on a target of any element can be estimated. This generalization also gives an insight into the mechanisms governing sputtering. In the case of light ions the proposed mechanism of sputtering near the threshold is in good agreement with the results of a recent precision determination of the sputtering of gold by protons [3]. The results of this investigation are presented here in the hope that they will stimulate future work and discussion of the subject. Sputtering is recognized as primarily a momentum transfer process between the incident particles and the target atoms. For heavy ion sputtering where Mj, the mass of the incident ion, is comparable with Mj, the target atom mass, the experimental evidence suggests that sufficient energy must be transferred in a collision to cause an atomic displacement in the lattice before a collision cascade can be initiated and sputtering take place [4]. The energy to produce an atomic displacement is approximately 20 eV. This is much greater than the energy required to remove an atom from the surface which should be equivalent to the sublimation energy, typically 3 to 8 eV [5]. Sputtering by light ions such as helium [6] occurs at energies well below that predicted by the 20-eV displacement energy. The mechanism of sputtering by protons of energies above 30 keV has been shown [7] to be due, in part, to ions which are backscattered by the atoms of the lattice. Backscattering also occurs at lower ion energies; the proportion of light ions of 1 keV energy backscattered by a niobium surface is greater than 40% and increases as the ion energy decreases [8]. The backscattering process can result in a loss of energy for the light ions so that an initially monoenergetic beam of energy Eo is backscattered as a beam whose energy spectrum ranges from a maximum value of EQ downwards to lower values. Thus, qualitatively, it is reasonable that the backscattered ions, if their momentum were in the right direction, could interact directly with surface atoms and sputter them provided that the energy transferred in the collision were equal to or greater than the sublimation energy. The maximum energy transfer E from an ion of energy Ep to an atom at rest is obtained from the laws of conservation of energy and momentum and is