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

O_{2}

gas sources is shown. Results for low fluence

{(10}^{10}

{ions/cm}^{2})

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

{Ar}_{n}

(n=135,

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

O_{2}

gas clusters, but with shallower crater depth than for the higher-energy impacts. The kinetics of the Ar and

O_{2}

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.

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