Radiation Induced Errors in Flash Memory Systems

The radiation environment that low earth-orbit satellites are exposed to is much harsher than that on earth due to the earth being shielded by the magnetosphere. Therefore flash memory systems will be highly affected by the radiation and can be damaged and even become defective if not protected. A completely damaged memory system can render a satellite, such as SUNSAT. Therefore measures must be taken to protect the memory system from radiation induced errors and increase its reliability in space environment operations.

Radiation Effects

When energetic particles pass through matter, they lose energy through a variety of interactions and scattering mechanisms. This energy transfer from radiation to the material gives rise to the two main effects of radiation: ionization and atomic displacement. These effects cause degradation of performance in materials which might or might not be permanent.

1.    Atomic Displacement

A collision between an energetic particle and silicon can result in the formation of a Frenkel defect pair, a displaced atom ( Interstitial: 'I') and the vacancy left by it (Vacancy : 'V'). This is called atomic displacement. The vacancy has a tendency to recombine with impurities, while the interstitial atom, which is extremely mobile in the lattice, has a strong tendency to displace an impurity.

2.   Ionization

Energy loss from high-energy radiation results in the formation of electron-hole pairs. This a result of the valence band electrons in a solid being excited to the conduction band. If an electric field is applied then the electrons are highly mobile. Therefore any solid conducts at a higher level than is normal for that solid. "The positive charged holes are also mobile, and their production and trapping cause major degradation in bipolar devices.

3.    Transient Effects

Local ionization effects on extremely dense electronic devices can lead to a strong transient electrical response. The electron-hole pair concentration generated by ionization can reach well above the doping densities of most semiconductor elements. This can cause the junctions to become 'swamped' and results in current flowing in directions which are normally blocked as well as higher voltages than were being used. These transient effects lead to a special case called 'single-event effects'. This is a special case of ionization effect which is common in space electronics, especially in memory components. The two main classes of single-event effects are single-event upsets (soft errors) and single-event latch up (hard errors).