Charge phenomena induced by low-energy ion bombardment in SiO2

In order to study the mechanisms of electrical charge creation in insulators under particle bombardment, we used different techniques: metastable atom deexcitation spectroscopy (MDS) and secondary ion mass spectroscopy (SIMS). SiO2 (quartz for MDS and a thin silica film for SIMS) was bombarded with a He* atom (MDS) and a He+ ion (SIMS). MDS involves neutralization of the incident atom, and SIMS involves a competition of two mechanisms: (i) neutralization through potential energy transfer, and (ii) a cascade of collisions through kinetic energy transfer. Under the He* metastable atom, the surface charges up positively; under the He+ ion, the initial potential on the surface is negative and slowly evolves towards a positive value, which is still under bombardment. On the basis of electron transfer models, we propose to explain the observed kinetics of charge under ions in three steps: (1) appearance of initial negative charge; (2) increase in the positive counterpart of the charge because of the neutralization of He+ with electrons trapped in the localized levels due to oxygen vacancies; (3) stabilization of the charge at a positive value because of the dominant character of the neutralization versus sputtering.