Soft breakdown fluctuation events in ultrathin SiO2 layers

When an ultrathin (<5 nm) oxide is subjected to electrical stress, several soft-breakdown events can occur prior to the final dielectric breakdown. After the occurrence of such failure events, the current-voltage (I-V) characteristic corresponds to the superposition of highly conductive spots and background conduction through the undegraded capacitor area. Tn this conduction regime, the application of a low constant voltage gives rise to large leakage current fluctuations in the form of random telegraph signal. Some of these fluctuations have been identified with ON/OFF switching events of one or more local conduction spots, and not with a modulation of their conductance. The experimental soft-breakdown I-V characteristics are shown to be better understood if the spot conduction is considered to be locally limited by the silicon electrodes and not by the oxide. (C) 1998 American Institute of Physics.