  Si nanopillars of less than 50 nm diameter have been irradiated in a helium
ion microscope with a focused Ne$^+$ beam. The morphological changes due to ion
beam irradiation at room temperature and elevated temperatures have been
studied with the transmission electron microscope. We found that the shape
changes of the nanopillars depend on irradiation-induced amorphization and
thermally driven dynamic annealing. While at room temperature, the nanopillars
evolve to a conical shape due to ion-induced plastic deformation and viscous
flow of amorphized Si, simultaneous dynamic annealing during the irradiation at
elevated temperatures prevents amorphization which is necessary for the viscous
flow. Above the critical temperature of ion-induced amorphization, a steady
decrease of the diameter was observed as a result of the dominating forward
sputtering process through the nanopillar sidewalls. Under these conditions the
nanopillars can be thinned down to a diameter of 10 nm in a well-controlled
manner. A deeper understanding of the pillar thinning process has been achieved
by a comparison of experimental results with 3D computer simulations based on
the binary collision approximation.
