Atomic force microscopy ~AFM! and high-resolution transmission electron microscope ~HRTEM! cross section imaging of individual gas cluster ion impact craters on Si~100! and Si~111! substrate surfaces is examined. The comparison between 3 and 24 kV cluster impacts from Ar and O2 gas sources is shown. Results for low fluence (1010 ions/cm2) 24 kV Ar individual cluster impacts onto a Si~100! and Si~111! substrate surfaces are compared with hybrid molecular dynamics ~HMD! simulations. A HMD method is used for modeling impacts of Arn (n5135, 225! clusters, with energies of 24–50 eV/atom, on Si~100! and Si~111! surfaces. On a Si~100!, craters are nearly triangular in cross section, with the facets directed along the close-packed ~111! planes. The Si~100! craters exhibit four-fold symmetry as imaged by cross-sectional HRTEM, and AFM top view, in agreement with modeling. In contrast, the shape of craters on a Si~111! shows a complicated six-pointed shape in the modeling, while AFM indicates three-fold symmetry of the impact. The lower energy 3 kV individual cluster impacts reveal the same crater shape in HRTEM cross section for both Ar and O2 gas clusters, but with shallower crater depth than for the higher-energy impacts. The kinetics of the Ar and O2 crater impacts may explain the successful use of higher-energy cluster impacts for etching material of higher initial surface roughness followed by the lower energy impacts as an effective finishing step to achieve smoother surfaces
