Analysis of Local Breakdown Process in Stressed Gate SiO2 Films by Conductive Atomic Force Microscopy

We have studied the processes of local degradation through current leakage leading to breakdown in gate SiO2 films by conductive atomic force microscopy (C-AFM). Electrical stress was applied to the SiO2 films in the form of metal–oxide–semiconductor capacitors. Leakage current spots caused by holes trapped at stress-induced defects appeared in current images of the stressed SiO2 films. During the C-AFM observation at the same area with a high electric field, currents at these leakage spots gradually increased and the breakdown finally occurred at these sites, whereas at background regions other than the leakage spots, leakage currents gradually decreased. In contrast, in the case of nonstressed SiO2 films, the breakdown occurs without showing any predictive signs. Degradation and breakdown mechanisms depending on the stress condition are discussed.