Craters on silicon surfaces created by gas cluster ion impacts

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 ${\mathrm{O}}_2$ gas sources is shown. Results for low fluence ${\text{(10}}^{10}$ ${\mathrm{ions/cm}}^2)$ 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 ${\mathrm{Ar}}_n$ $\text{(n=135,}$ 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 ${\mathrm{O}}_2$ gas clusters, but with shallower crater depth than for the higher-energy impacts. The kinetics of the Ar and ${\mathrm{O}}_2$ 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.