Charge trapping and annealing in high-κ gate dielectrics

We examine the radiation response, annealing characteristics, and long-term reliability of capacitors with Al gates and Al₂O₃-SiO_xN_y gate dielectrics stacks which received a forming gas anneal (FGA) or an O₂ and FG anneal after high-κ deposition. By comparison to a theoretical capacitance-voltage (CV) curve, the FG annealed devices are found to have a large preirradiation interface trapped charge density of ∼7×10¹¹ cm^-2, whereas devices annealed in O₂ and FG show a large density (∼9×10¹¹ cm^-2) of negative bulk charge. The midgap voltage shift (ΔV_mg) increases monotonically with dose for both sets of devices, but the O₂ annealed devices exhibit 50% less trapping at a total dose of 2 Mrad(SiO₂). The radiation-induced voltage shifts are found to recover during long duration biased anneals as a result of tunneling and thermal annealing. For short times and large biases, the annealing response is found to be dominated by tunneling. After 1,000 s of annealing, there is a 50% reduction in ΔV_mg for devices annealed at 2.0 MV/cm and a 7.5% recovery for devices annealed at 1.0 MV/cm. For longer times, the annealing response of these devices is dominated by thermal annealing. Accelerated life testing shows these devices have a broad failure distribution with a large population of extrinsic failures. Extrapolation of the reliability data suggests these particular devices would have to be operated at an electric field less than ∼2.5 MV/cm to achieve a ten-year operational lifetime. Improved reliability is, therefore, required before insertion into a manufacturing environment.