Assessing MOS gate oxide reliability on wafer level with ramped/constant voltage and current stress

In this study time to breakdown distributions are compared for MOS gate oxides which were stressed with a constant voltage (or current) stress or a pre-stressing voltage (or current) ramp followed by a constant voltage (or current) stress. Results show clearly that a pre-stress can increase time to breakdown. This increase is discussed and it is shown that it is dependent on oxide thickness, pre-stressing ramp rate and the processing conditions. The current-time (or voltage-time) characteristics of the constant stress are investigated and it is observed that charge trapping in the oxide is the reason for the time to breakdown increase. The pre-stressed oxide clearly shows a different initial charge trapping characteristic than the non prestressed oxide. The measurement results are discussed and it is demonstrated that the common understanding of oxide breakdown cannot explain the observed results. Therefore, a new parameter is proposed which is related to oxide degradation and breakdown and which has to be considered in combined ramped/constant stress measurements.