Low field leakage current and soft breakdown in ultra-thin gate oxides after heavy ions, electrons or X-ray irradiation

The excess leakage current across ultra-thin dielectrics has been studied f or different ionizing radiation sources. Namely, X-rays, 8 MeV electrons, a nd three ion beams with different LETs values have been used on large area MOS capacitors with 4-nm thick oxides. Small oxide fields were applied duri ng irradiation, reaching 3 MV/cm at most. For ionizing radiation with relat ively low LET (<10 MeV cm(2)/mg), only Radiation Induced Leakage Current (R ILC) was observed, due to the formation of neutral defects mediating electr on tunneling via a single oxide trap. For high LET values, instead, the gat e leakage current could be described by an empirical relation proper of Sof t Breakdown (SB) phenomena detected after electrical stress. Moreover, the typical random telegraph signal noise feature of this Radiation induced Sof t Breakdown (RSB) currents was observed during and after irradiation. RSB c an be attributed to conduction through a multi-defect path across the oxide , produced by the residual damage of dense ion tracks. The oxide field appl ied during irradiation enhances the RSB intensity, but RSB can be achieved even for irradiation at zero field, being LET the main factor leading to RS B activation. The dose dependence of both RILC and QB have been investigate d, showing a quasi linear kinetics with the cumulative dose. We have also s tudied the effect of modifying the angle of incidence of the ion beam on th e intensity of the gate leakage current.