Simplified model of atmospheric neutron flux variation with altitude. Data is from Dr. Eugene Normand, Boeing Radiation Effects Lab.
Atmospheric radiation is an issue for avionics designers today, with every indication of becoming a greater issue in the future. Atmospheric radiation causes single event effects (SEE) in electronics, resulting in various system failure conditions, including hazardous misleading information.
Cosmic radiation, including high energy neutrons, is constantly showering the earth. Galactic cosmic rays and solar rays hit the earth's atmosphere, interact with oxygen and nitrogen atoms, and produce particle cascades of secondary radiation. This secondary radiation leads to a high flux of energetic particles, including protons, neutrons, and pions.
Of the possible secondary particles, neutrons have been shown to be mainly responsible for causing single event upsets in memories and other devices in aircraft since the early 1990s. Neutrons interact with the silicon structures of a component and release energy that can change the state of a bit.
While the flux density varies with global position, altitude, and solar activity, all surface locations are exposed to this radiation, including ground level. Typical commercial airliners operate up to 40,000 ft, where the flux density is in the range of 300 to 450 times greater than at sea level, resulting in greater incidences of SEEs for avionics equipment compared to ground-based equipment.
Altitude variation of atmospheric neutrons derives from competition between various production and removal processes that affect how the neutrons and the initiating cosmic rays interact with the atmosphere. The result is a maximum in the flux at about 60,000 ft, called the Pfotzer maximum.
Although altitude is the largest single factor driving atmospheric neutron flux, latitude is also very important. The variation with latitude is due to the bending of primary particles' trajectory by the earth's geomagnetic field, increasing by a factor of six between equatorial latitude and the high latitude polar regions.
The energy variation of atmospheric neutrons is usually presented by plotting the differential flux (flux per unit energy interval) as a function of energy, which is often called the spectrum. Neutron spectrum measurements at altitude are more complex than the neutron flux measurements at altitude and latitude, but they have been made and are in good agreement with detailed and elaborate neutron transport calculations.
The level of atmospheric radiation is also dependent on the sun. During a typical 11-year sunspot cycle, the sun produces about 100 severe coronal mass ejections (CMEs) or solar flares and about four extreme CMEs into the solar system—only a fraction of which usually hit the earth.
Particles from these solar flares “typically” have energies much lower than particles produced by galactic cosmic rays from outside the solar system. During periods of low solar activity, the surface of the sun is comparatively stable and the particles produced “normally” contribute very little (<2%) to the resulting secondary neutron particle creation in the atmosphere.
But during times of high activity, large solar flares occur. These can produce large numbers of high energy solar energetic particles over a period of a few hours, creating SEE rates from 30 to 300 times normal. Documented occurrences have been rare over the last 60 years, and therefore this should be considered a specific risk.
During the solar maximum period, the sun's magnetic field pushes out away from the sun shielding the earth from galactic particles; the atmospheric neutron flux decreases. Conversely, during the solar minimum, the magnetic field collapses and is not effective at deflecting galactic particles, and the atmospheric neutron flux increases.
SEEs are caused by a single particle, most likely a neutron, and can take on various forms. The definition of a single event is the disturbance of an active electronic device, such as a transistor, caused by the energy deposited in a device by a single energetic particle. The effect is caused when a radiation-generated ionization charge exceeds the device's critical charge. There are various types of these events, but they all are the result of a single particle depositing sufficient energy to cause a disturbance in an electronic device.
When charged particles lose energy by ionizing the medium through which they pass, they create a path of electron-hole pairs. These electron-hole pairs collect at the source and drain of a transistor and produce a current pulse. While the majority of the neutrons passing through a microelectronic device will have no impact, if the particle deposits enough charge, a malfunction of the device results; the state of a node can change from logic 1 to logic 0 and vice versa.
Various failure modes in electronic systems can occur, such as data corruption or unplanned events. Additional types of undesirable effects may include the following:
• Damage to hardware
• Corrupted software residing in volatile memory
• Corrupted data in memory
• Microprocessor halts and interrupts
• Writing over critical data tables
The industry trend is for continued decreases in component feature size and operating voltages, while the number of gates on a given device continues to increase. As this trend continues to deep submicron gate lengths, the expected critical charge decreases and the expected sensitivity to radiation increases. Note that extrapolating the level of susceptibility and resulting behavior of future IC technologies from older devices cannot be guaranteed without measurement.
There are various types of SEEs which result in different types of failure modes. These include single and multiple event upsets, latch-up, transients, single event functional interrupts, and burnout. Hardware can be damaged, as in the case of a burnout or gate rupture, but most often the failures are nondestructive. Single event upsets are the most common type of event. Under current IC technology conditions, many devices are being fabricated with feature sizes of 90 nm, 65 nm, and below. At this technology point, many SRAMs are experiencing multiple cell upsets in which a single neutron interaction leads to two or more physically adjacent cell upsets.
Single event latch-up is another area of concern. Latch-up is caused by a charged particle creating a localized short circuit across the device. When the condition occurs, there is a loss of device functionality due to a single event induced high current state. Often the device is not permanently damaged, but power cycling is required to resume normal device operation.
The development of highly reliable and available systems requires consideration of both the occurrence of SEEs and the impact they have on system performance. An addition to the System Safety Assessment Guidelines would provide instruction and an analysis methodology for performing a system assessment of atmospheric radiation susceptibility.
The addition proposed here would provide a process for assessing each component in the design, incorporating IC effect rates, determining mitigation requirements, and finally summarizing the total SEE faults for the system. Also provided would be SEE analysis preparation steps, instructions on determining neutron cross-section data for each sensitive IC, and the calculation of the SEE effect rates. That data would then be utilized to complete the analysis with the cumulative results and impact to the system.
While many factors go into determining a hardware design, the level of susceptibility to SEEs needs to play a role in device selection and play a role in hardware and software architecture. This radiation susceptibility assessment could be a part of a component selection phase of a system design process, and it would begin with a review of available test and analysis data for each selected component. The data could be analyzed and an impact analysis on system operation performed. With this information, the need for and degree of mitigation can be determined. Finally, an on-going program to monitor the design could be used to verify that the system continues to meet requirements through the life of the product.
Because susceptibility to atmospheric radiation impacts the safety and reliability of a system, the system designer needs to address SEE as a formal part of the system development process. Having the guidelines and SEE analysis method to aid in the radiation susceptibility evaluation of ICs and system impact will result in a more complete and accurate system safety assessment.
More information on this technical paper can be found at http://papers.sae.org/2011-01-2497.
This article is based on SAE technical paper 2011-01-2497 by Mike Dion, Rockwell Collins, and Laura Dominik, Honeywell