Stress-induced leakage current and defect generation in nFETs with HfO2/TiN gate stacks during positive-bias temperature stress

The stress-induced leakage current (SILC) in nFETs with SiO₂/HfO₂/TiN dual-dielectric gate stacks with metal electrodes is studied during positive-bias temperature stress at high temperatures and at high gate stress voltage. It is shown that strong defect creation in the HfO₂ causes a linear increase of the SILC with stress time. The SILC generation is found to be thermally activated with an activation energy, E_a ~ 1 eV. In addition, the SILC formation exhibits a strong correlation with the threshold voltage (V_t) instability DeltaI_g/I_g ~ dV_t ³. Both degradation phenomena show a strong hysteretic behavior with gate bias; the SILC and V_t-degradation are observed to be substantially reduced by applying a negative gate bias after stress. All these observations may be rationalized in terms of charge trapping in shallow HfO₂ defects -such as oxygen vacancy - and by the generation of new shallow defects during stress. The defect generation process has a low activation energy, likely because of thin-film effects. Therefore, the SILC and the V_t instability are large under accelerated TDDB test conditions. It is also shown that the observed low activation energy in combination with the reversibility of the SILC has important implications for dielectric breakdown detection in dual-dielectric gate stacks.