Correlation of charge-to-breakdown obtained from constant current stresses and ramped current stresses, and the implications for ultra-thin gate oxides

Charge-to-breakdown (Q_BD) is one of the parameters that is used as a measure of oxide quality. In this work, the influence of the measurement conditions on Q_BD is examined, as well as the relationship between Q_BD and oxide thickness. Using oxides ranging from 45 Å to 80 Å, two Q_BD measurement methods are employed: constant current stress and exponential current ramp. A variety of current densities (for the constant current stress) and delay times (for the exponential current ramp) are studied. It is shown that not only does Q_BD depend on oxide thickness, but that Q_BD depends strongly on the measurement conditions, and that depending on the test conditions, Q_BD can increase or decrease as the oxide thickness decreases. It is also shown that there is a strong agreement between the Q_BD measured with a constant current stress and the Q(BD) measured with an exponential current ramp. Finally, the equipment-related limitations of the exponential current ramp are discussed, as well as the impact these limitations have on the underestimation of the resulting Q_BD