he threshold LET for SEGR would fall below that of Fe, a significant component of the galactic cosmic ray flux. If this were to happen, the SEGR error rate in space would increase several orders of magnitude, making the usage of deep submicron devices in space problematical. This prediction has inspired recent research into scaling trends and mechanisms for SEGR in thin gate oxides [206-209]. The first of these studies investigated the impact of oxide thickness on the critical electric field to rupture in gate oxide capacitors and CMOS ICs [206]. The authors found much higher critical fields to rupture than in [204]. Their results for gate oxide capacitors with an area of ~10-3 cm2 2 are shown in Figure 4.5 as a function of particle LET and gate oxide thickness. The critical field for SEGR is higher than 5 MV/cm in all cases, and decreasing the oxide thickness increases the critical field for SEGR at any given LET. Similar trends were observed for 16 Kbit and 256 Kbit SRAMs and 12-stage delay chains, although the critical fields to rupture were somewhat lower [206]. The thinnest oxides in Figure 4.5 have critical fields approaching 8 MV/cm, suggesting that advanced technologies may in fact show improved resistance to SEGR. This improvement in critical field to rupture II-39 6.5 nm 6.0 nm 12 nm 18 nm LET (MeV-cm2/mg) Figure 4.5. Critical field to SEGR in thin oxide capacitors as a function of ion LET [206]. correlated well to improved breakdown field strengths prior to irradiation, due to reduced defect generation by hot carriers in thin oxides. It was cautioned, however, that future voltage scaling is highly uncertain and if fields do exceed 5 MV/cm, SEGR could still be a significant concern. Another issue is that not all gate oxides can be expected to behave the same. Oxides from different processes have in fact exhibited different critical fields to rupture. In [207], for example, capacitors with 4.5-nm and 7.5-nm gate oxides from a different process showed much lower critical fields to SEGR compared to the dat