Radiation-induced interface traps in hardened MOS transistors: an improved charge-pumping study

Different electrical characterization techniques (subthreshold current-voltage measurements, standard, 3-level and multi-frequency charge pumping) combined with isochronal anneals have been used to investigate the generation and the evolution of interface traps in radiation-hardened MOS transistors following exposure to 10 keV X-rays. The evolution of the interface state density ([D.sub.it]) during the anneal is found to be field-dependent and consistent with models involving a drift of positive species towards the Si-Si[O.sub.2] interface. The energy-resolved distributions of [D.sub.it] in the silicon bandgap show the emergence of two broad structures located at [approximately] [E.sub.v] + 0.35 eV and [approximately] [E.sub.v] + 0.75 eV immediately after irradiation and during the first steps of the isochronal anneal (up to [approximately] 175 [degrees] C). At higher anneal temperatures, it is shown that the recovery of [D.sub.it] is not uniform in the two halves of the silicon bandgap. In particular, the separation of the [D.sub.it] distribution related to the lower part of the bandgap in two distinct peaks (at [E.sub.v] + 0.30 eV and [E.sub.v] + 0.45 eV) agrees well with the energy distributions of [P.sub.b0] and [P.sub.b1] centers. These results are consistent with Electron Spin Resonance (ESR) studies which have shown that [P.sub.b] centers play a dominating role in the interface trap build-up and recovery mechanisms. Since ESR measurements are only accurate to [approximately] [+ or -]30% in absolute number, [P.sub.b] centers do not probably account for all the electrically active interface trap defects, as also suggested by the evident asymmetry of the [D.sub.it] distributions in the bandgap. Finally, we investigate the post-irradiation response of border traps by reducing the charge pumping frequency to low values. The implication of these results on the nature of border traps is discussed.