Localized oxide degradation in ultrathin gate dielectric and its statistical analysis

Conventional oxide reliability studies determine oxide lifetime by measuring the time to breakdown or quasi-breakdown (QB). In ultrathin gate oxides with T_ox<14 Å, however, it is hard to observe breakdown or QB under typical stress conditions. Instead, the gate leakage current shows a continuous increase over the entire time period of electrical stress. As the magnitude of the gate current density increase eventually becomes too high to be acceptable for normal device operation, a lifetime criterion based on the increase in gate leakage current is proposed. Our paper also shows that the area-dependence of the gate leakage current density increase in 13.4 Å oxides is different from that in thicker oxide films, indicating a localized and discrete property of the leakage current. It has also been observed that the oxide lifetime based on the new lifetime criterion is shorter when the gate area is smaller, as opposed to the conventional area dependence of time-to-breakdown test. A simple model consisting of multiple degraded spots is proposed and it has been shown that localized gate leakage current can be described by Weibull´s statistics for multiple degraded spots.