We explored the structural limits of unconventional electron-beam lithography by directly writing with an electron beam into ultrathin SiO2 films. The obtained structures were analyzed by tunneling microscopy. The Auger excitation process (Knotek-Feibelman mechanism) necessary for electron-stimulated oxygen desorption allows generation of ultrasmall structures. The subsequent processing step combines thermal desorption of the remaining monoxide and simultaneous etching promoted by thermally activated silicon atoms, which turns out to be a strongly anisotropic process close to step edges. Applying this combination of processes to a regularly stepped Si(557) sample, linewidths close to the resolution of the electron microscope of 5 nm were obtained. © 2008 American Institute of Physics.