Origin of complex impact craters on native oxide coated silicon surfaces

Abstract

Crater structures induced by impact of keV-energy ${\mathrm{Ar}}_{n}^{+}$ cluster ions on silicon surfaces are measured with atomic force microscopy. Complex crater structures consisting of a central hillock and outer rim are observed more often on targets covered with a native silicon oxide layer than on targets without the oxide layer. To explain the formation of these complex crater structures, classical molecular dynamics simulations of Ar cluster impacts on oxide coated silicon surfaces, as well as on bulk amorphous silica, amorphous Si, and crystalline Si substrates, are carried out. The diameter of the simulated hillock structures in the silicon oxide layer is in agreement with the experimental results, but the simulations cannot directly explain the height of hillocks and the outer rim structures when the oxide coated silicon substrate is free of defects. However, in simulations of $5\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$/atom ${\mathrm{Ar}}_{12}$ cluster impacts, transient displacements of the amorphous silicon or silicon oxide substrate surfaces are induced in an approximately $50\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ wide area surrounding the impact point. In silicon oxide, the transient displacements induce small topographical changes on the surface in the vicinity of the central hillock. The comparison of cluster stopping mechanisms in the various silicon oxide and silicon structures shows that the largest lateral momentum is induced in the silicon oxide layer during the impact; thus, the transient displacements on the surface are stronger than in the other substrates. This can be a reason for the higher frequency of occurrence of the complex craters on oxide coated silicon.