Sputter yields of insulators bombarded with hyperthermal multiply charged ions

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Abstract
The total sputter yield for Au, Si, GaAs, SiO2, MgO, LiF and NaCl bombarded with hyperthermal highly charged Arq+ ions (q = 1-9) has been measured. Only for alkali halides (LiF, NaCl) and to some extent for SiO2 potential sputtering (enhancement of the sputter yield with increasing charge state of the primary ion) has been observed. All other targets showed normal collision induced sputtering. From that result it is obvious that the mechanisms for sputtering can not be explained by the Coulomb explosion model, because in this model insulators like MgO and semiconductors like Si and GaAs should also show charge state dependence of the sputtering yield. Alkali halides and SiO2 are materials which are known for strong electron phonon coupling where electronic excitations in the valence band are localized by formation of self trapped excitons (STE) and/or self trapped holes (STH). During bombardment with highly charged ions the neutralization process in front of, at and below the surface causes the formation of STE and/or STH. Therefore the potential sputtering can be explained as a defect mediated sputtering process which is well known in electron stimulated desorption (ESD) where the decay of STH and/or STE into different colour centers leads at the end to the desorption of neutralized anions. The also created neutral cations are either evaporated (as it is the case for the alkali halides) or have to be removed by momentum transfer by the impinging projectiles. Therefore it is very likely that in the case of SiO2 for very low impact energy mainly only oxygen is enhanced sputtered, the surface is enriched in Si and the potential sputtering effect decreases with increasing ion dose.