Differences in sodium transport in SiO2 films caused by ion and neutral-particle bombardment

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Abstract
The transport of impurity atoms from the surface to the SiO2/Si interface of SiO2 films on silicon subjected to ion and neutral-particle bombardment has been found to depend critically on the charge state of the incident projectile. SiO2 films on silicon, intentionally contaminated with 22Na, have been bombarded by low-energy neutral N02 and Ar0, by N+2 and Ar+ ions, and by these ions with concurrent thermal electron flooding of the insulator surface. Neutral-particle bombardment in the energy range 500–2000 eV causes no measurable sodium transport to the SiO2/Si interface. Ion bombardment at the same energy and dose causes migration of much of the sodium away from the SiO2 surface to the interface. Ion bombardment in the presence of simultaneous electron flooding is an intermediate case, causing two orders of magnitude less sodium transport than ions alone, but resulting in at least two orders of magnitude more sodium transport than neutral-particle bombardment. These results suggest that neutral-atom sputtering or neutral-atom beam methods may be preferred techniques for insulator profile analysis to avoid problems associated with impurity transport, and neutral beams may be preferable to ion beams for sputter etching.