The Space Environment

The inner and outer van Allen belts

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Ionizing radiation and semiconductor damage

Silicon dioxide develops a positive charge when irradiated. An ionizing particle passes through, and generates hole-electron pairs. The electrons are highly mobile, and diffuse or drift out, while the holes get trapped, and leave a positive charge. Hafnium oxide develops a negative charge, trapping electrons. A stack of both shows promise as a rad-hard gate oxide, withstanding 10Mrad from a Cobalt 60 source with minimal shifts. I wonder if that is tuned for Co60? Perhaps a wider spectrum of radiation energies, as would be found in the Van Allen belt, would preferentially charge either the !HfO or the SiO2, leaving a residual imbalance? In any case, it does demonstrate how modern gate oxides may be much more rad hard than older technologies.

Tantalum shielding

Tantalum capacitors over the tops and bottoms of chips can act as both shielding and bypass. Tantalum has a density of 16.7 gm/cm3 at 300K, so a 30 micron thick layer would add 0.05 grams/cm2 of shielding. According to SMAD figure 8-18, that might reduce the 1-15keV dose by a factor of 3-5, and perhaps a factor of 2 for electrons (figure 8-19). [ More accurate information needed] . To accommodate the extra thickness in the launch stack, the processors and memory and other chips might sit in interdigitated positions on every other server-sat.

Question - does a small tantalum cap short out when an ionizing particle passes through it? That would actually be good, because it could remove power from the segment of a chip that is being hit, possibly removing drift fields from the gate oxide and helping immobilize charges.

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Ionizing radiation, charge upsets and latchup

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Drag

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References

http://www.isde.vanderbilt.edu/content/muri_2008/dixit_muri2008.pdf Sriram Dixit et. al. at Vanderbilt University. Recent work on HfO/SiO2 stacked gates and radiation resistance.

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