Sputtering of metals and insulators with hyperthermal singly and doubly charged rare gas ions

P. Varga and U. Diebold

Institut für Allgemeine Physik, Technische Universität Wien, A-1040 Wien, Austria

Low-Energy Ion-Surface Interactions
Ed. J.W: Rabalais (John Wiley & Sons 1994) pp. 355-386

The influence of the potential energy of singly and doubly charged rare gas ions (of He, Ne, Ar and Kr) on desorption, sputtering and secondary ion formation process has been studied. By using very low impact energy (10 - 500 eV), the dependence of particle emission from metals and insulators on the charge state of the primary ions can be demonstrated. The measurements have been performed using a CO covered Ni(111) surface and a LiF(100) singly crystal or LiF thin film, respectively.

For sputtering of the CO covered metal surface, the positive secondary ion yields (Ni+ and CO+) are slightly higher when using doubly charged rare gas ions instead of singly charged, but only at impact energies below 200 eV. No dependence of the primary ions charge state on the sputtering yields of neutrals was observed. The enhancement in the secondary ion yield is explained by sputtering and simultaneous charge exchange in a close collision between a primary ion and a target particle.

Sputtering of the LiF surface with low energetic rare gas ions has been studied as an example for an insulator surface. Extensive experiments have been performed to exclude effects caused by charging up of the surface during ion bombardment; it was found that the best method for charge neutralization was to heat the LiF samples up to 400°C.

For ejection of Li+ and F- only a small influence of the charge state of the projectile has been observed at impact energies below 100 eV whereas at higher energy no effect was seen. In contrast to this it has been shown, that the sputtering of F+ strongly increases with the potential energy of the projectile. However, the F+ yield also depends on the primary energy of the projectile and a threshold energy for F+ emission is observed. This indicates that the potential energy of the incident particle alone is not sufficient for F+ emission; a certain momentum transfer is necessary for sputtering. The observed behaviour can be explained by a combination of an Auger neutralization event followed by a collisional process. F+ is formed by Auger neutralization between the lattice F- ion and the incoming projectile provided that the potential energy of the projectile exceeds at least two times the energy of the band gap of the LiF surface. The so formed F+ ion is in a weakly bound state and will be sputtered by the incoming projectile.

Corresponding author: P. Varga (varga< encoded email address >).