Two-trap-assisted tunneling model for post-breakdown I-V characteristics in ultrathin silicon dioxide

This paper investigated the degradation and breakdown characteristics of an ultrathin silicon dioxide film by using conductive atomic force microscopy (C-AFM) with repetitive ramped voltage stress (RVS). Two-step oxide degradation was determined from the measured current-voltage (I-V) characteristics and topographies. In the first step, bond breaking and negative-charge accumulation near the SiO₂/Si interface causes oxide thinning and an effective increase in SiO₂/Si barrier height. In this step, hard breakdown (HBD) actually does not occur until permanent damage is produced within the oxide during the second step after several times of repetitive RVS. The permanent damage produced inside the oxide film is in the form of traps, which will cause the crooked I-V curves and a larger I-V shift along the voltage axis. A two-trap-assisted tunneling (TTAT) model was proposed to explain the postbreakdown I-V behaviors. In this model, two isolated traps were generated in the oxide after breakdown. The trap location of the nearer traps determines the bending of the postbreakdown I-V curves and that of the farther traps causes the I-V oxide voltage shift along the voltage axis. The model fits the measured postbreakdown I-V curves well when the locations of both the nearer trap and the farther trap are chosen correctly.