SOFT BREAKDOWN FLUCTUATION EVENTS IN ULTRATHIN SIO2 LAYERS

When an ultrathin (<5 nm) oxide is subjected to electrical stress, sev eral soft-breakdown events can occur prior to the final dielectric bre akdown. After the occurrence of such failure events, the current-volta ge (I-V) characteristic corresponds to the superposition of highly con ductive spots and background conduction through the undegraded capacit or area. Tn this conduction regime, the application of a low constant voltage gives rise to large leakage current fluctuations in the form o f random telegraph signal. Some of these fluctuations have been identi fied with ON/OFF switching events of one or more local conduction spot s, and not with a modulation of their conductance. The experimental so ft-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 Physi cs.