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

Abstract

The role of hydrogen on the generation of defects in highκ based devices, subjected to an electrical stress, is discussed, with an emphasison issues related to negative bias temperature instabilities (NBTI) inSiO2/HfO2 based devices. It is shown that NBTI are mainly caused by the buildup of positivelycharged defects in the gate stack. The defect density is found to increase with the forminggas annealing temperature of the device. The defects are robust under electron injectionfrom the Si substrate, but they can be partly removed by annealing the devices inN2 at200 °C. All these results suggest that protons are most probably involved in the positive chargebuildup. A kinetic model is proposed, based on the dispersive transport of protons inthe gate stack during the electrical stressing, followed by their trapping in theHfO2 layer, forming hydrogen-induced overcoordinated oxygen centres. Ab initiocalculations further indicate that the protons are stabilized in monoclinicHfO2 byforming bonds with trivalent oxygen centres, and that these defects are not producing any energetic levelin 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 thepositive charge density on the forming gas annealing temperature, as well as its robustnessversus electron injection from the Si substrate.