On the generation of MCs+ ions in SIMS with Cs+ primary beams

The formation of MCs⁺ secondary ions in SIMS operated with Cs⁺ ion beams is discussed on the basis of a well-confirmed quadratic dependence of MCs⁺ yields on the atomic polarizabilities of elements M contained in semiconductor samples. This behavior is understood by the generation of a dipole induced in M by the positive charge of a neighboring Cs⁺ projectile and the mutual induction of a dipole in Cs⁺, both dipoles depending on the atomic polarizability of M. An evaluation of the dipole–dipole interaction energy indicates that MCs⁺ ions generated at the target are emitted according to the direct emission model (DEM). This mechanism is in contrast with an association model where constituents of MCs⁺ are assumed to combine after their independent sputter emission.