Collisional versus electronic sputtering of SiO2

The SiO2 sputtering yield was determined for 170–300 keV He+ ion bombardment. The low sputtering efficiency and blistering at high ion doses make high-energy He+ ion sputtering yields difficult to determine, but by modifying a measurement method previously used for heavy ions, the sputtering yield could be quite accurately determined after sputtering only 20 Å of SiO2. The sputtering yield was found not to be proportional to the energy deposited by the ion in elastic collisions at the surface of SiO2. Comparison with SiO2 sputtering yields found in literature shows that the sputtering yield increases with increasing energy deposited in electronic excitations for similar energy deposited in elastic collisions, indicating that electronic effects probably have to be included in the description of the sputtering process. Since the electronic effects do not seem to be independent of the sputtering by elastic collisions, it is suggested that SiO2 sputtering be controlled by a mixed collisional-electronic mechanism. Sputtering yield measurements were also performed for varying angles of ion incidence and, here also, good agreement could be achieved with predictions based on a mixed sputtering mechanism.