Defect generation in high κ gate dielectric stacks under electrical stress: the impact of hydrogen

The role of hydrogen on the generation of defects in high κ based devices, subjected to an electrical stress, is discussed, with an emphasis on issues related to negative bias temperature instabilities (NBTI) in SiO2/HfO2 based devices. It is shown that NBTI are mainly caused by the buildup of positively charged defects in the gate stack. The defect density is found to increase with the forming gas annealing temperature of the device. The defects are robust under electron injection from the Si substrate, but they can be partly removed by annealing the devices in N2 at 200 °C. All these results suggest that protons are most probably involved in the positive charge buildup. A kinetic model is proposed, based on the dispersive transport of protons in the gate stack during the electrical stressing, followed by their trapping in the HfO2 layer, forming hydrogen-induced overcoordinated oxygen centres. Ab initio calculations further indicate that the protons are stabilized in monoclinic HfO2 by forming bonds with trivalent oxygen centres, and that these defects are not producing any energetic level in the HfO2 band gap. The kinetic model allows one to explain most of the observed experimental data, i.e. the time and voltage dependence of the positive charge buildup, the dependence of the positive charge density on the forming gas annealing temperature, as well as its robustness versus electron injection from the Si substrate.