Single event upset

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A Single Event Upset ( SEU ) is a soft error that can be caused in semiconductor components when high-energy ionizing particles (e.g. heavy ions , protons ) pass through . It manifests itself, for example, as a bit flip (change in the state of a bit ) in memory modules or registers , which can lead to a malfunction of the component concerned. The classification as soft error is based on the fact that an SEU does not cause permanent damage to the affected component. An example of a hard error is the single event latch-up (SEL).

Occur

Since the magnetic field and the earth's atmosphere have a shielding effect for high-energy particles, SEUs occur only relatively rarely at sea level, although the occurrence is becoming more frequent with ever smaller structures, since smaller structures are sufficient with a smaller structure size and higher clock frequency to achieve an SEU provoke. They have a great importance in the field of aviation and aerospace . Airplanes and, above all, satellites and spacecraft are exposed to increased (particle) radiation, which is why the corresponding electronics are affected to a greater extent.

Mode of action

When passing through matter, an ionizing particle releases energy to the surrounding material, which is known as linear energy transfer (LET). In semiconductors this leads to a change in the charge distribution and can thus - to put it simply - cause a “switching” of a pn junction . The energy from which an SEU can appear in a semiconductor component is referred to as , where the index th stands for threshold (German: threshold, threshold value). The unit of measurement usually used is MeV · cm² / mg (based on Si for MOS semiconductor components). Different semiconductor components differ greatly in their susceptibility to SEUs. Components that have a level of over 100 MeV · cm² / mg are often referred to as SEU-immune , but this only means that a component has been tested up to this value and no SEU has occurred.

Countermeasures

Since an SEU in a component can lead to the failure of a complete system, various measures are taken to prevent the occurrence of SEUs or to minimize the negative effects. In English one speaks of SEU Mitigation . In addition to the use of semiconductor components which, due to the manufacturing technology used, have an increased tolerance to radiation, u. a. the following methods are used:

  • Shielding : An ionizing radiation shielding jacket is placed around the component to be protected. However, this method is only suitable for intercepting particles with relatively low energy. In the case of particles of high energy, so-called secondary radiation is generatedwhen crossing the shield. The main disadvantage, however, is the added weight of the shield.
  • Error correction : Stored data are protected against single or multi-bit errors by means of suitable procedures (e.g. RS coding ). In the case of an SEU that causes a change in the memory content, the correct data can be restored.
  • Triple Modular Redundancy (TMR for short): With this method, one module to be protected is replaced by three identical and one downstream decision level ( Voter ). The voter always passes on the output value that the majority of the three modules provide. A module can be, for example, a flip-flop of an FPGA , but also a complete processor .

See also

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