Pre-breakdown noise in electrically stressed thin SiO2 layers of MOS devices observed with C-AFM

A conductive atomic force microscope (C-AFM) has been used to analyse the degradation stage (before breakdown, BD) of ultrathin (<6 nm) films of SiO₂ at a nanometer 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 analysed areas, several features, which can be masked by the current that flows through the overall test structure during standard electrical tests, are observed. In particular, switching between different conduction states and sudden changes of conductivity have been measured during ramped voltage tests, which have been related to the trapping and detrapping of single electronic charges in the defects generated during the electrical stress. This phenomenon, which has been observed during constant voltage stresses in the form of random telegraph signals, has been associated to the pre-breakdown noise measured in poly-gated structures. The C-AFM has also allowed to directly measure the I–V characteristics of the fluctuating spot.