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 for different ionizing radiation sources. Namely, X-rays, 8 MeV electrons, and 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 during irradiation, reaching 3 MV/cm at most. For ionizing radiation with relatively low LET (<10 MeV cm²/mg), only Radiation Induced Leakage Current (RILC) was observed, due to the formation of neutral defects mediating electron tunneling via a single oxide trap. For high LET values, instead, the gate leakage current could be described by an empirical relation proper of Soft Breakdown (SB) phenomena detected after electrical stress. Moreover, the typical random telegraph signal noise feature of this Radiation induced Soft Breakdown (RSB) currents was observed during and after irradiation. RSB can be attributed to conduction through a multi-defect path across the oxide, produced by the residual damage of dense ion tracks. The oxide field applied during irradiation enhances the RSB intensity, but RSB can be achieved even for irradiation at zero field, being LET the main factor leading to RSB activation. The dose dependence of both RILC and QB have been investigated, showing a quasi linear kinetics with the cumulative dose. We have also studied the effect of modifying the angle of incidence of the ion beam on the intensity of the gate leakage current