Effects of Switched-bias Annealing on Charge Trapping in HfO2 Gate Dielectrics

Charge trapping characteristics are investigated for MOS capacitors with 6.8 nm HfO₂ layers and 1.0 nm interfacial silicon oxynitrides; the effective oxide thickness of the high-kappa gate dielectric layers is 2.1 nm. These devices were irradiated with 10-keV X-rays or subjected to constant voltage stress, and then annealed for ten minute intervals of alternating positive and negative gate bias at temperatures between 25 and 150degC. The resulting oxide-trap and interface-trap charge densities exhibit reversible buildup and annealing that depend strongly on bias and temperature. Additional defect-density growth with time was observed as a result of charge injection into the gate stack during the annealing process. This defect-density growth increases with increasing annealing time and temperature. After irradiation, the most of the reversibility in the charge trapping is due to metastable electron traps in the near-interfacial dielectric layers. After constant voltage stress, the motion, reactions, and trapping of protons at or near the Si/oxynitride interface are more important to the observed device response than are metastable electron traps. This is a result of reduced electron-hole pair creation during low-energy constant-voltage stress, as compared to high-energy X-ray irradiation. These results illustrate the importance of both electron traps and protons to the ionizing radiation response and long-term reliability of MOS devices with high-kappa gate dielectrics