Reassessing the Mechanisms of Negative-Bias Temperature Instability by Repetitive Stress/Relaxation Experiments

A major intrinsic limitation of the reaction-diffusion (R-D) model for negative-bias temperature instability (NBTI) is revealed through dynamic stress experiments. We found no evidence of self-limiting recovery, one of the key features of the transport-based R-D model, after repeating the stress and relaxation cycles alternately for many times. The amount of recovery per cycle of the parameter of interest (e.g., threshold voltage shift, change in the charge-pumping (CP) current, etc.) is shown to remain constant, independent of the number of stress/recovery cycles. Under repeated cycling of the test device between stress and recovery, it is also found that the amount of parametric shift induced by the stress cycle becomes nearly identical to that recovered during the relaxation cycle, i.e., the parametric evolution under a fixed set of stress and recovery intervals is cyclic in nature. In conjunction with the thermal activation result, this cyclic behavior of the dynamic NBTI is ascribed to an ensemble of switching hole traps having broad spectra of characteristic trapping and detrapping time constants. The same group of traps responds under a fixed set of experimental conditions, giving rise to the cyclic behavior. The interface state generation was also investigated using a CP current measurement and is found to be permanent within the range of timing examined. It is also shown that the variation in the power-law exponent of the as-measured change in the CP current with temperature could be consistently explained by considering the different thermal activation of the hole trapping and interface state components. In view of these new evidences, previous claims of consistency between the generation/recovery of the interface states and the R-D model or its dispersive counterpart must be reviewed.