We present a new technique to produce the local oxidation of Si(100) surface with STM in air by controlling the electrical field between tip and sample. In order to extend the range of bias conditions at which the modification can be produced, we have modified previously existing techniques. When the n-type silicon sample is polarized at positive bias respect to the tip, the surface becomes quickly oxidized, being possible to produce local modifications, but degrading the surface quality. In this communication, we show how the oxidation rate can be controlled if a square waveform voltage is applied to the sample while scanning. As the feedback condition is fixed by a negative voltage applied to sample, which is chosen to be around the flat band voltage of the tip-sample system, both the tip and surface quality are preserved. Moreover, as any voltage can be applied to the sample independently of the tip to sample distance, the dependence of the modification with the electrical field between tip and surface is studied. © 1995 Elsevier Science B.V. All rights reserved.