le-ion effect, there is no question that an accumulated fluence of particles does introduce damage to the oxide. This damage (termed precursor ion damage in [209]) does not affect the SEGR threshold, but does increase capacitor leakage currents. Figure 4.7 shows measured I-V curves for a 7-nm capacitor after exposure to successive fluences of Au ions at increasing bias voltages [209]. The fluence step between each curve is 106 ions/cm2. At irradiation bias voltages below 3.7 V, increasing leakage currents beyond 2 V are observed with accumulated particle fluence. This leakage current is associated with precursor ion damage. After 3.7 V was applied during the heavy ion exposure the capacitor's I-V curve changed dramatically, with the leakage current increasing by four orders of magnitude. This pronounced change in the I-V curve is the characteristic signature of SEGR. The critical voltage to SEGR in this 7-nm oxide capacitor for 360-MeV Au ions is thus 3.7 V. Accumulated fluence damage effects such as those shown before rupture in Fig. 4.7 are an interesting topic to study from a basic mechanisms standpoint. However, the data show that precursor ion damage and SEGR in thin oxides are largely unrelated effects. Also, because of the very low flux of high-LET particles in the near-Earth environment (on the order of After prestress of Nominal Fluence 2E8 ions/cm2 w/ no bias 10000 Average lons/Step Figure 4.6. Critical voltage to rupture as a function of heavy ion fluence per step [209]. Only a weak dependence of the rupture voltage is observed over more than three orders of magnitude of particle fluence per step. For reference, it would take roughly the present age of the universe to accumulate a fluence of high-LET particles this large in the near-Earth environment. II-41 Area = 0.0025 cm 2 Irradiation Bias 3.0 V 3.2 V 3.3 V 3.4 V 3.5 V 3.6 V 3.7 V 10-10 Gate Voltage (V) Figure 4.7. Thin oxide capacitor current-voltage characteristics after successive 106 ions/cm2 2 irradiations with increasing gate bias [209]. Precu