Scanning Tunnelling Microscopy is used as a tool to produce nanometer scale modifications on hydrogen passivated silicon (100) surfaces under positive and negative sample vs, tip voltage. Experiments have been performed in air and 'in situ' in an HF solution. In air, it is found that under both polarities the surface under the tip becomes oxidized and that the oxidation is possible although no tunnelling current flows between tip and sample. Taking into account the band bending in the semiconductor, it is shown that the oxidation is a field induced process. Experiments 'in situ' demonstrate that two different mechanisms exist : oxidation at anodic polarization and direct silicon etching at cathodic polarization.