Recovery of NBTI degradation in HfSiON/metal gate transistors

The instability of threshold voltage in PMOS HfSiON devices with TiN gate electrode under negative bias and temperature stress (NBTI) is investigated. The amount of threshold voltage shift with negative bias stress is found to be more extreme than the traditional NBTI in SiO₂. It is found that the transconductance and subthreshold swing of the PMOS transistors are relatively unaffected by the stress, merely a lateral shift in threshold voltage is observed. A low activation energy (0.042 eV) is found for the temperature dependence of the threshold instability. Based on these observations, we assert that charge trapping is responsible for the threshold voltage instability rather than the traditional hydrogen reaction-diffusion model of SiO₂. Using our previous results on ab initio calculations of the defects created by introduction of nitrogen into hafnium dioxide, a model is introduced that suggests that traps deep in the silicate bandgap are contributing to electron trapping and de-trapping of the dielectric. This assertion is further reinforced by the observation of charge trapping of identical NMOS devices but at similar dielectric field strengths.