Four different types of ultra-thin oxide MOS structures have been analyzed to investigate the dielectric breakdown, by using a combination of techniques: time-zero breakdown, time-dependent breakdown and low-frequency noise measurements. The experiments have shown that different devices presenting lifetime values depending on the fabrication technology are all affected, before breakdown, by sharp fluctuations of the current tunnelling through the oxide in both its components: the main and the substrate current. These new results confirm those previously obtained with a set of homogeneous devices, and show that the presence of current fluctuations is a general experimental evidence in the occurrence of dielectric breakdown. The main physical models proposed in the past to explain the origin of substrate current and breakdown are then briefly reviewed. Thereafter, a comparative analysis showing how these mechanisms could justify the experimental data coming from the noise measurements is presented. The results of this analysis seem to rule out the band-to-band impact ionization mechanism as responsible for the substrate current and breakdown. © 1995.
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