Trapped charge distributions in thin (10 nm) SiO/sub 2/ films subjected to static and dynamic stresses

Thin (10 nm) gate oxide MOS capacitors have been subjected to static and dynamic stress conditions. The evolution of the trapped charge distributions (characterized by average density and centroid) has been measured as a function of the stress time. The evolution of the average charge density for DC stresses shows that both polarities have identical trap generation rates and a constant average density of traps at breakdown. However, the final density of traps is much smaller for injection from the gate, so that the time-to-breakdown is also much shorter for this stress polarity. The evolution of the centroid shows that traps are always mainly generated near the cathodic interface. Unipolar dynamic stresses give results which are qualitatively very similar to those obtained under DC conditions and without a relevant frequency dependence. In contrast, bipolar stress experiments show significant qualitative differences, the frequency dependence being very important. In general, the trap generation and trapping rates are reduced in comparison to the DC and unipolar cases, this reduction being more important at high frequencies. In addition, the average density of trapped electrons at the breakdown is larger than that obtained in DC experiments. Both observations explain the tremendous increase in the mean-time-to-breakdown obtained under high-frequency stress conditions. The presented results are qualitatively explained in terms of microscopic degradation models.

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