1. Introduction
Burn-in test is a method used to detect infant mortality by applying higher levels of stress to accelerate the deterioration of electronic devices. However, burn-in test itself may affect the yield of devices. It results from the fact that systematic defects grow during burn-in test and some of them end up in yield-loss [1]. The breakdown mechanism of the gate oxide is in close connection with the growth of defects under the given voltage and temperature. The amount of defect growth and yield-loss depend upon the environment of burn-in test, such as stress time, temperature, voltage, and stress patterns [2].