Formation of silicon nanopillars via selective oxygen etching of Si(111) surface using silicon nitride islands in the initial stage of nitridation was investigated by scanning tunneling microscopy and low energy electron diffraction. Silicon nitride islands with diameters of 6-15 nm, which were formed by low energy nitrogen ions, were resistive to O-2 exposure at high temperatures resulting in silicon nanopillars as high as 2-3 nm. Existence of high density silicon nitride islands is considered to suppress the step flow etching of nearby silicon surfaces, resulting in a spatially nonuniform etching of silicon.