Nanoscale observations of the electrical conduction of ultrathin SiO2 films with conducting atomic force microscopy

A conductive atomic force microscope (C-AFM) has been used to investigate the conduction properties of electrically stressed ultrathin (<6 nm) films of SiO₂. 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 analysed area, several features not registered during macroscopic tests are observed. In particular, before the oxide breakdown, switching between different conduction states and sudden changes of conductivity have been measured, which have been related to the pre-breakdown noise observed in conventional MOS 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 phenomenology observed with the C-AFM provides experimental evidence of the local nature of the degradation and breakdown phenomena. Therefore, the C-AFM is a powerful tool to analyse the microscopic physics of the degradation and breakdown of ultrathin SiO₂ films at the same dimensional scale at which they take place