Fundamental study of the apparent voltage-dependence of NBTI kinetics by constant electric field stress in Si and SiGe devices

SiGe and Ge pMOSFETs, with channel passivation by Si cap and standard SiO2/HfO2 gate stack, show a very strong oxide electric field dependence of NBTI, beneficial for low voltage operation. However, standard NBTI extrapolation based on voltage accelerated stress tests is complicated for these devices due to an apparent correlation of time-dependence exponent (n) to the applied gate stress overdrive voltage (VOV). This behavior is found to be unrelated to the measurement delay, but induced by a gradual reduction of the stress oxide electric field during the NBTI stress due to the buildup of threshold voltage shift. A unique analog circuit based test setup is proposed to perform a quasi-constant oxide field BTI stress. This method eliminates the field reduction effect during NBTI stress and therefore decouples the time exponent from the applied stress VOV. This allows to calculate the time-to-failure for SiGe and Ge channel devices unambiguously, confirming the superior NBTI reliability of these pMOS technologies as compared to standard Si devices. Possible limitations of the proposed measurement method are also discussed.