rsor ion damage is evidenced by the increasing gate current above 2 V as ion fluence was accumulated. The sudden increased current at all voltages for the 3.7-V irradiation is the characteristic signature of SEGR; this capacitor had a SEGR failure threshold of 3.7 V. 0.4 ions/cm2/year 2 with LET 40 MeV-cm2/mg), precursor ion damage is not expected to have any practical impact on reliability in space. For further information on particle-induced damage effects in thermal and nitrided oxides, the reader is directed to [209]. SEGR has been investigated in oxides thinner than 5 nm [208,209]. These oxides can behave fundamentally differently than thicker oxides. Specifically, they often exhibit a soft breakdown characteristic rather than the hard breakdown shown in Fig. 4.7. For example, 4.5-nm oxide capacitors tested in [208] showed soft breakdown leakage currents on the order of tens to hundreds of uA, compared to 1-10 mA rupture currents observed in identical capacitors with 7.5-nm oxides. Similarly, 5-nm oxide capacitors tested in [209] typically showed only a gradual increase in leakage current similar to precursor ion damage buildup, even at fields as high as 12 MV/cm. For a few capacitors clear rupture signatures were obtained, but these were rare. In cases such as these, the ability to sweep full I-V curves at the irradiation site is invaluable, as simple current monitoring is not very instructive. Interestingly, reliability studies of very thin oxides often show similar soft breakdown characteristics [210]. Another recent result of interest is the angular dependence of SEGR in thin oxides. Traditionally, normal incidence is considered to be the worst-case ion trajectory for SEGR, with the critical voltage to rupture showing a somewhat less than cos(0) dependence [194]. In early work on SEGR, Wrobel predicted that the angular dependence would disappear when the oxide thickness approached the diameter of the ion track [211]. In [209], the authors investigated the angular dependence of the critical field to ruptu