On the properties of the gate and substrate current after soft breakdown in ultrathin oxide layers

In this work we have studied soft breakdown (SBD) in capacitors and nMOSFET's with 4.5-nm oxide thickness. It is shown that for larger area devices gate current and substrate current as a function of the gate voltage after SBD are stable and unique curves, but for smaller area devices both currents become lower and unstable. This difference can be explained by the different energy available for discharging in the SBD path. It is shown that the SBD detection strongly depends on the test structure area. In nMOSFET's for positive gate polarity, the large increase in the substrate current at the SBD moment is proposed as a sensitive SBD detector. Two level fluctuations in the gate current are investigated at different voltages and are explained by means of a model where electron capture-emission in the traps of the SBD path induces local field fluctuations causing variations in the tunneling rate across the oxide. In the substrate current directly correlated two-level fluctuations are observed.