Nanometer‐scale structures as small as 1 nm were fabricated on graphite surfaces using a scanning tunneling microscope in the presence of low‐pressure (10−4Torr) trimethylaluminum. The studies were performed under controlled conditions of gas purity and gas pressure, allowing systematic measurements. We studied the voltage threshold and other features of the fabrication process as a function of the tip‐surface biasing voltage and the tunneling current. The studies lead us to believe that the structures were formed by bombardment of the graphite surface by ions produced by electron‐assisted field ionization localized in the region of the tunneling gap.