Breakdown transients in ultra-thin gate oxynitrides

It has been shown that under accelerated stress at relatively low voltage thin gate oxides are subjected to the phenomenon of progressive breakdown (BD). This consists in a progressive growth of the BD spot, and as a consequence, of the gate leakage, in times that under operation conditions can be a large fraction of the time required for circuit failure. The I-V characteristics of the BD spot under progressive BD are investigated and modeled in the relevant range of voltages. The model is based on an assumption concerning the physical structure of the BD spot. This is compared to direct TEM observations. Experimental conditions in terms of voltages and geometry leading to BD runaway, i.e., the transition from progressive to very fast BD transient are shown and discussed. Moreover, the impact on the BD transient of a different material for the gate electrode is studied in the case of tungsten, used in place of the standard poly-Si.