SPUTTERING BY MEV IONS

The ability of fast heavy ions to produce efficient erosion of dielectric surfaces has received only limited attention from the experimental and theoretical communities. The effect was first observed in association with studies of track formation processes in dielectrics, and the sputtering yields of a limited number of combinations of ion and substrate combinations quantified. The process is poorly understood, with significant variation of sputter yields in similar materials with the same ion. The observation that the process appears to "switch off" for dielectric layers below a critical thickness (approximately 2 nm) is also interesting and unexplained. We present results of MeV sputtering using beams from the Harwell 7 MV Tandem accelerator, using a novel technique to simultaneously sputter and assess film thickness. For thermally grown SiO2 films on silicon substrates we have observed the variation of sputter yield with ion species, charge state and incident angle. A dramatic decrease in sputter yield is observed for films less than 2 nm thick. The sputter yield is found to be roughly consistent with a (sin-theta)-1 variation in incident angle, and increases dramatically with incident charge state, indicating the process to be dominated by interactions within the near surface region. The use of isotopically labelled thin oxide layers has been used to gain an insight into the depth of origin of the sputtered species, and evidence that the process is not purely a surface phenomenon is presented.