Anomalous asymmetry of carbon nanopillar growth on both sides of a thin substrate irradiated with a focused electron beam

The dynamics of carbon nanopillar growth on both surfaces of amorphous carbon films 40–180 nm thick irradiated with a focused electron beam with an energy of 20 keV is studied. Prolonged irradiation causes the retardation and even complete termination of nanopillar growth on the upper side of the substrate while growth on the lower side continues. This unexpected result is explained by the dissociation of precursor molecules diffusing along the substrate by secondary electrons emitted from the conical nanopillar tip. The substrate acts as a filter allowing primary electrons to easily pass, but intercepting low energy secondary electrons. The lower efficiency of secondary electrons emitted in the lower half-space may be due to a decrease in the current density within the expanding beam of scattered electrons.