Switching events in the soft breakdown I-t characteristic of ultra-thin SiO2 layers

The application of an electrical stress to an ultra-thin ( < 5 nm) gate oxide in a MOS structure can lead to the appearance of a failure conduction mode known as soft-breakdown. The current-voltage (I-V) characteristic of this conduction regime corresponds to the superposition of highly conductive spots running between the electrodes and background tunneling conduction distributed over the whole nondamaged capacitor area. In particular, when a low constant voltage is applied to the gate, the current-time (I-t) characteristic exhibits large multilevel fluctuations in the form of random telegraph signals. We present experimental results which show that some of these fluctuations can be ascribed to on/off switching events of one or more unstable conductive spots and not to a modulation of their conductance. We have also analyzed the probabilistic distribution of the switching times for a two-level fluctuation regime.