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 Angstrom to 80 Angstrom, 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).