Very thin thermal oxides are shown to exhibit a failure mode that is undetected by conventional breakdown tests. This failure mode appears in the form of excessive leakage current at low field and is induced by high-field stresses. The stress-induced oxide leakage is permanent and stable with time and thermal annealing. It becomes the dominant failure mode of thin oxides because it always precedes destructive breakdown. Experimental results and theoretical calculations show that the leakage current is not caused by positive charge generation and accumulation in the oxide. It is proposed that the oxide leakage originates from localized defect-related weak spots where the insulator has experienced significant deterioration from electrical stress. The leakage conduction mechanism appears to be thermally assisted tunneling through the locally reduced injection barrier, and the model seems to be consistent with both I-V measurements at temperatures from 77 K to 250°C and theoretical calculations. © 1988 IEEE.