Nanometer scale oxide structures have been fabricated on Si substrates using an atomic force microscope that has been modified to enable the application of a bias voltage to its scanning tip. Parameters affecting the width and height of an oxide line were examined systematically. Surface flatness, tip sharpness, bias potential, and scanning speed were found to play important roles in this lithographic process. By optimizing these parameters, an average line width as small as 12 nm has been achieved on specially prepared Si(111) substrates with large atomic terraces.