Effects of interface traps and border traps on MOS postirradiation annealing response

Threshold-voltage and charge-pumping measurements are combined to estimate densities of radiation induced bulk-oxide, interface, and border traps in transistors with soft 45-nm oxides. Immediately after irradiation, nearly all effects usually attributed to interface traps are actually due to border traps in these devices. During positive-bias anneal at 80°C, the interface-trap density grows by more than a factor of 10, and the border-trap density changes by less than 30%. The increase in interface-trap density is matched by a decrease in bulk-oxide-trap charge. This raises the possibility that slowly transporting or trapped protons in the oxide may be responsible for this effect. An alternate explanation is offered by H-cracking models. Latent “interface-trap” growth in harder 27.7-nm oxides is associated with (true) interface traps, not border traps. Switched-bias annealing of the soft 45-nm oxides reveals fast and slow border traps with different annealing responses. Trivalent Si defects associated with O vacancies in SiO₂, the E_γ´ center and the O_3-xSi_xSi· family, are excellent candidates for slow and fast border traps, respectively. For O_3-xSi_xSi, x=0 is the E_s´ defect; x=3 is the D center; and x=1 or 2 have been proposed as candidates for the “P_b1” defect on (100) Si. A hydrogen-related complex (e.g. OH^-) may also be a border trap. The practical significance of these results is discussed for (1) bias-temperature instabilities in thin oxides, (2) effects of burn-in on MOS radiation response, and (3) enhanced bipolar gain degradation at low dose rates