SPUTTER YIELDS OF INSULATORS BOMBARDED WITH HYPERTHERMAL MULTIPLY-CHARGED IONS

The total sputter yield for Au, Si, GaAs, SiO2, MgO, LiF and NaCl bomb arded with hyperthermal highly charged Arq+ ions (q = 1-9) has been me asured Only for alkali halides (LiF, NaCl) and to some extent for SiO, potential sputtering (enhancement of the sputter yield with increasin g charge state of the primary ion) has been observed. All other target s showed normal collision induced sputtering. From that result it is o bvious 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 dependen ce of the sputtering yield Alkali halides and SiO2 are materials which are known for strong electron phonon coupling where electronic excita tions in the valence band are localized by formation of self trapped e xcitons (STE) and/or self trapped holes (STH). During bombardment with highly charged ions the neutralization process in front of, at and be low 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) wh ere the decay of STH and/or STE into different colour centers leads at the end to the desorption of neutralized anions. The also created neu tral cations are either evaporated (as it is the case for the alkali h alides) or have to be removed by momentum transfer by the impinging pr ojectiles. Therefore it is very likely that in the case of SiO2 for ve ry low impact energy mainly only oxygen is enhanced sputtered, the sur face is enriched in Si and the potential sputtering effect decreases w ith increasing ion dose.