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

Different electrical characterization (subthreshold current-voltage measurements, 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-SiO/sub 2/ interface. The energy-resolved distributions of D/sub it/ in the silicon bandgap show the emergence of two broad structures located at /spl sim/E/sub V/+0.35 eV and /spl sim/E/sub V/+0.75 eV immediately after irradiation and during the first steps of the isochronal anneal (up to /spl sim/175/spl deg/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 /spl sim//spl plusmn/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.