The mechanisms of sputtering part I.

Abstract

Sputtering processes can be classified by considering the time-scale. Thus, if an incident particle hits at t = 0, one can recognize, for 10^minus;15⩽t⩽ 10⁻¹⁴ s, prompt collisional processes involving direct (or nearly direct) particle-target interactions. The yield is often expressible as a simple integral of a differential cross section. These are followed, for 10⁻¹⁴⩽t⩽10⁻¹³ to 10⁻¹² s, by slow collisional processes due to the internal flux of target atoms in the cascade intersecting the surface. The yield for sputtered atoms is given by a relation (that of Sigmund) which is conceptually similar to that for the flux of perfect gas atoms against a surface and which can be restated very directly in these terms. An important feature, as is shown in simple derivations, is that the high-energy limit of the sputtered flux is predicted to have the form flux œ E0-2 for atoms but flux œ E0-4.5 for diatomic molecules, provided the molecules are formed by recombination. Eo is the sputtered particle energy. For 10⁻¹³ to 10⁻¹²⩽t⩽10⁻¹¹ to 10⁻¹⁰ s, one has what may be called prompt thermal processes. The yield is given ideally by ∫ ∫ p(2πmkT) ^−1/22πydydt, where p is the equilibrium vapor pressure (whether vapor is present or not); the flux is proportional to E0exp(-Eo/kT). The evidence for thermal sputtering is reviewed in particular detail in view of the controversial nature of the process. Slow thermal processes, which occur for t⩾10⁻¹¹ to 10⁻¹⁰ s, include the vaporization of metal from halides or oxides after it accumulates due to electronic sputtering of halogen or oxygen. Electronic sputtering, which may be either fast or slow (t⩽or⩾10⁻¹¹ to 10⁻¹⁰ s), comes about in events such as interatomic Auger decay or the assumed diffusion of halogen or oxygen interstitials, i.e. H centers, to the surface, with resultant vaporization or expulsion of halogen or oxygen. Finally, exfoliational sputtering, due to the rupture of gas-filled cavities, sets in at very high doses of light inert-gas atoms. There are thus six fairly distinct sputtering processes, any one of which is found to be important for particular systems or particular experiments.