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 pu mping) combined with isochronal anneals have been used to investigate the generation and the evolution of interface traps in radiation-harde ned MOS transistors following exposure to 10 keV X-rays. The evolution of the interface state density (D-it) during the anneal is found to b e field-dependent and consistent with models involving a drift of posi tive species towards the, Si-SiO2 interface. The energy-resolved distr ibutions of D-it in the silicon bandgap show the emergence of two broa d structures located at similar to E(v) + 0.35 eV and similar to E(v) + 0.75 eV immediately after irradiation and during the first steps of the isochronal anneal (up to similar to 175 degrees C). At higher anne al temperatures, it is shown that the recovery of D-it is not uniform in the two halves of the silicon bandgap. In particular, the separatio n of the D-it distribution related to the lower part of the bandgap in two distinct peaks (at E(v) + 0.30 eV and E(v) + 0.45 eV) agrees well with the energy distributions of P-b0 and P-b1 centers. These results are consistent with Electron Spin Resonance (ESR) studies which have shown that P-b centers play a dominating role in the interface trap bu ild-up and recovery mechanisms. Since ESR measurements are only accura te to similar to +/- 30% in absolute number, P-b centers do not probab ly account for all the electrically active interface trap defects, as also suggested by the evident asymmetry of the D-it distributions in t he bandgap. Finally, we investigate the post-irradiation response of b order traps by reducing the charge pumping frequency to low values. Th e implication of these results on the nature of border traps is discus sed.