Bias and frequency dependence of radiation-induced-charge trapping in MOS devices

Trapped-charge densities in 45 nm radiation-hardened MOS oxides were characterized after 0.1 Hz to 1.0 MHz AC bias irradiations using thermally stimulated-current (TSC) and capacitance-voltage (C-V) techniques. We find no significant frequency dependence for the build-up of trapped positive or negative charge in these oxides. Trapped positive charge densities during AC bias exposures are similar to densities during DC bias exposures at the same average applied field during irradiation, but trapped negative charge densities do not scale as simply with average electric field. The majority of negative trapped charge is part of a stable dipole defect, which evidently forms very quickly (<1 μs) after the creation of an electron-hole pair in a region of localized space charge. Over the entire frequency range for these devices, the trapped electrons compensate 60±5% of the trapped positive charge for 5 V/0 V exposures and 70±6% of the positive charge for 5 V/-5 V exposures. These data agree well with DC bias irradiations of these devices, in which trapped electrons compensate 59±4% of the trapped positive charge under positive bias and 76±2% at zero or negative bias, suggesting an enhancement of electron-hole dipole formation at small or negative electric fields. Mechanisms are discussed for the creation of dipoles in SiO₂, and our results are compared with previous studies of the field dependence of electron-hole recombination rates. Implications for total dose modeling are discussed