A conductive atomic force microscope (C-AFM) has been used to investigate the degradation and breakdown of ultrathin (<6 nm) films of SiO2 at a nanometric scale. Working on bare gate oxides, the conductive tip of the C-AFM allows the electrical characterization of nanometric areas. Due to the extremely small size of the analyzed areas, several features, which are not registered during macroscopic tests, are observed. In particular, before the oxide breakdown, switchings between different conduction states and sudden changes of conductivity have been measured, which have been related to the prebreakdown noise observed in conventional metal-oxide-semiconductor structures. Moreover, similar switchings have been also measured after the oxide breakdown, which have been related to the opening or closure of conduction channels between the electrodes. The C-AFM has also allowed the determination of the areas in which the degradation and breakdown take place. The results have shown that, although degradation takes place in areas of few hundreds of nm2, breakdown is laterally propagated to neighbor spots, affecting areas of thousands of nm2. The size of the affected area has been found to be strongly related to the hardness of the breakdown event. The phenomenology observed with the C-AFM provides experimental evidence of the local nature of the degradation and breakdown processes in ultrathin SiO2 films. Therefore, the C-AFM is a powerful tool to analyze the microscopic physics of these phenomena at the same dimensional scale at which they take place. © 2002 American Institute of Physics.