Experimental study on origin of VTH variability under NBT stress

The origin of NBTI variability was investigated experimentally using the recovery by hydrogen annealing after NBT stressing. In the case of hydrogen-annealed devices after low voltage NBT stress, Δ_VTH and ΔI_CP values including these distributions completely coincide with those in the case of the first NBT stress, irrespective the number of both the stress and the hydrogen annealing. This result indicates that the defects generated by applying low voltage NBT stress can be completely recovered by hydrogen incorporation. On the contrary, in the case of the hydrogen-annealed devices after high voltage NBT stress, the variability of Δ_VTH under low-voltage NBT stress after hydrogen annealing becomes more marked compared to that for fresh device, while the relationship between the mean values and the variability of Δ_ICP coincides with that of fresh device. Based on the experimental results, we conclude that the variability of the quantity of hole-trapping precursors in gate oxide films is the dominant origin for the V_TH-shift distribution, rather than the variability of the quality of the gate dielectric interface.