We discuss experimental studies performed on MOS structures with ultra-thin oxynitride gate dielectrics, concerning the kinetics of the final event of breakdown (BD), during which the gate leakage increases above acceptable levels. We focus in particular to the case of low voltages, where, for the case of poly-Si gates, it has been previously shown that as the stress voltage is decreased below a threshold value, the BD transient exhibits a clear transition to a regime in which the BD growth is much slower [1-3]. This recently discovered phenomenon has been termed progressive BD, ie., a gradual growth of the BD spot and of the gate leakage, with a time scale that under operation conditions can be a large fraction of the total time to BD. In this paper, observations concerning this damage are shown and discussed. The measurement of the current, voltage, power dissipated during the BD transient through the BD spot …