Soft breakdown current noise in ultra-thin gate oxides

In this work we studied the soft breakdown (SB) in ultra-thin gate oxides (<3 nm) subjected to constant current stress. SB current derives from the superposition of several random telegraph signals noises with different time constants and amplitudes. Such fluctuations derive from the conductance modulation of a damaged region inside the oxide layer, due to the electrical stress. We found that the current noise power density follows the 1/f(2) power law (with alpha between 1 and 2) over a wide range of frequency (1 Hz-100 kHz). Only at frequency smaller than 1-10 Hz a possible deviation from this low cannot be excluded. Moreover, the discrete fluctuations typical of SB are statistically independent events at least over time periods around hundreds of seconds, according to a Poisson process. This result suggest that electron trapping/detrapping in defect sites near or inside the SB conductive path can be claimed as responsible for such conductance modulation. (C) 2002 Elsevier Science Ltd. All rights reserved.