We have used silicon nitride islands as oxidation masks for the formation of nanometer-scale silicon pillars. For the growth of silicon nitride islands on the Si(1 1 1)-7 X 7 surface, two different methods were used: (1) 100 eV N-2(+) ion exposure at room temperature followed by subsequent post annealing at 980 degrees C and (2) N-2 exposure at 800 degrees C. Scanning tunneling microscope images for the two differently prepared surfaces showed a submonolayer coverage of nanometer-size silicon nitride islands. On these surfaces, oxygen was exposed at high temperatures where silicon etching was dominant over oxide formation. It was found that those silicon nitride islands successfully worked as oxidation masks to form silicon nanopillars as high as several nanometers via a selective oxygen etching of silicon. Silicon nanopillars of uniform sizes but with nonuniform heights were obtained in the case of N-2(+) ions, while those with a distribution of nonuniform sizes but homogeneous heights were formed in the case of N-2 gas. Such different results are explained in terms of differences in the nitridation reaction between the two preparation methods.