Scanning tunneling microscopy (STM) was used to study the surface structure of ultrathin single-crystal Si3N4 film prepared by thermal nitridation, on an -oriented Si substrate. Hi.-h-resolution STM images indicate that both 8 x 8 and 8/3 x 8/3 orderings exist on the surface of the single-crystal Si3N4 film. We also found that ultrathin Si3N4 films (< 5 nm) formed by thermal nitridation or low-pressure chemical vapor deposition on doped Si(111) and Si(001) substrates are excellent mask materials for nanolithography. Local oxidation of crystalline or amorphous silicon nitride films can be reliably performed with a conductive-probe atomic force microscope (AFM) with an extremely fast initial oxidation rate (five to six orders of magnitude higher than the thermal oxidation rate at 1000 degreesC) at a sample bias of +10 V. The nanopatterned silicon nitride masks can be used for selective-area high-aspect-ratio etching and epitaxial growth with large selectivities. Using an AFM-patterned SiO2/Si3N4 bilayer mask, selectively grown Si dots and lines of high crystalline perfection were successfully obtained.