A New Failure Mode of Very Thin (<50Ã\x85) Thermal SiO2 Films

Very thin (< 50 Ã…) thermal oxides are shown to exhibit a new failure mode which is undetected by conventional tests and can lead to erroneous time-zero and time-dependent breakdown data. Breakdown in these films proceeds gradually from an increased low-field leakage current induced by electrical stress to catastrophic failure. The low-field leakage is not caused by positive charge generation and accumulation but is likely due to localized defect-related weak spots in the oxide. The leakage conduction mechanism appears to be thermally-assisted tunneling through locally-reduced injection barriers of about 0.9 eV and the model seems to be fully consistent with I-V measurements at temperatures from 77 to 523 K and theoretical calculations of the temperature dependence. Thin oxide breakdown is found to evolve by enlarging the area of the weak spots rather than propagating from one interface to the other.