Understanding the working environment is crucial for the good functioning of any device as electronic circuits are prone to be influenced by external perturbations.
The cosmic environment (elevation higher than 1000 Km or 600 miles) is the most aggressive working environment for an electronic equipment from a radiation standpoint. There are many radiation sources and IC’s may encounter a very large spectrum of particle types and effects. The most important radiation sources are: solar radiations, cosmic radiations and radiation belts. There are a large number of incidents encountered during the lifetime of satellites or other man-made cosmic objects.
The atmosphere shields the Earth against radiations from the outer space. Most of the energetic particles lose their initial energy when crossing the atmosphere. The particles interact with atoms from the atmosphere, such as oxygen and nitrogen, thereby dissipating the energy of the particles down to less threatening levels. The flux and characteristics of incident particles varies according to the altitude, latitude, and solar activity. The atmospheric environment was extensively studied:
- Neutrons are the most important contributors to the failure rate. The standard flux is defined as the neutron flux at the New-York position, sea level, i.e. 14 neutrons/square cm/hour. The flux increases with the altitude and depends of the geographical position.
- the ions/protons/alpha particles are much less probable to cause problems, as these particles are rapidly filtered by the atmosphere
The Influence of the Altitude on the Neutron Flux
The variation of the neutron flux versus the altitude is summarized using the following equations:
NF is the neutron flux, NFref is the neutron flux at the reference location, A is the areal density of the location of interest, Aref is the areal density of the reference location. The areal density is given in units of g/cm2. L is the flux attenuation length for neutrons in the atmosphere, also given in units of g/cm2. For terrestrial neutrons, a good value for L is 148 g/cm2.
The areal density is computed with the following equation where a is the altitude value in feet.
The reference location is the city of New York (USA) where the altitude is 0 feet (sea-level) and the reference neutron flux is 14 neutrons/cm2hour.
The following table shows the influence of the altitude on the neutron flux for various flight altitudes:
|Altitude (feet)||Neutrons flux (n/sq.cm hour)||Neutrons flux (relative to sea level)|
The working environment of an electronic device could be a source of radiations: such as the proximity of a radiation facility, the use of nuclear materials, natural radiation background or alpha-particles generated by the natural decay of nuclear impurities in the compositions of materials used for the fabrication.
The energetic particles that are likely to cause problems to electronic devices are heavy ions, alpha particles, protons and neutrons.
Since the heavy ions are electrically charged, they directly interact with the semi-conductors through an electro-magnetic reaction. The ions deposit energy in the device, causing an unwanted flow of electrical charge. Depending on the intensity of the current, various Single Event Effects may manifest in the affected device.
However, heavy ions are very rapidly attenuated in the atmosphere due to their interaction with other atoms (fragmentation). This particular phenomenon could generate other energetic particles such as neutrons and protons. Since the secondary particles have low mass, these particles are less likely to interact with other atmospheric atoms. Thus, they are harder to stop and may reach the ground level without losing too much of their energy.
Experiments have shown that atmospheric neutrons are the primary cause for the Single Events at moderate and high altitudes. The altitude and latitude play a significant role in both the energy and the flux of the incoming neutrons. The energy of a neutron can fall into a wide range from 1 MeV up to 10 GeV.
The neutrons do not carry electrical charge and they are very small when compared to heavy ions. However, the interaction mechanism is different: The neutrons may directly hit an atom and cause a nuclear reaction (a so-called spallation reaction). Neutrons cause the majority of Single Events.
The protons have behavior and effects similar to those of the neutrons. In space, cosmic rays consist of approximately 92% protons and 6% alpha particles. As cosmic rays travel through our atmosphere, they interact with atoms in the atmosphere and produce multiple lower energy particles. At sea level, the distribution is 95% neutrons. Thus, the proton contribution is small.
Alpha particles are easily absorbed by the atmosphere and thus, a significant flux can only be produced by radioactive impurities and contaminants. Their effects are similar to those induced by heavy ions, causing direct ionization.
The peak of the cosmic ray intensity occurs at about 10-25 Km, which is also the altitude of many commercial airplane flights.
A very good source of information concerning the radiation environment for the airplane altitudes is the paper of Dr. E. Normand, “Single Event Effects in Avionics” .
A number of extracts from this document shows the importance of considering the effects of Soft Errors in microelectronic components used in avionics:
“…atmospheric neutrons have been identified as the main cause of single event effects at elevated altitudes. The neutrons in the atmosphere vary with both altitude and latitude. The altitude variation derives from the competition between the various production and removal processes that affect how the neutrons and the initiating cosmic rays interact with the atmosphere….
The major concern is in random access memories, RAMs, both static (SRAMs) and dynamic (DRAMs), because these microelectronic devices contain the largest number of bits, but other parts, such as microprocessors, are also potentially susceptible to upset. In addition, other single event effects, specifically latchup and burnout, can also be induced by atmospheric neutrons….
During 1988-89, IBM flew a series of proprietary flight experiments on three different aircraft in which upsets in a large array of 64K SRAMs were measured. A few years later, a joint IBM-Boeing study sponsored by DNA and NRL, collected the actual inflight upset data from both these IBM proprietary flights, and upsets recorded in the CC-2E flight computer on military aircraft. This study, completed in 1992, demonstrated that SEUs in avionics are real, that the measured inflight rates correlate with the atmospheric neutron flux, and that the rates can be calculated using laboratory SEU data. Once avionics SEU was shown to be an actual effect, it had to be dealt with in avionics designs. The major concern is in random access memories, RAMs, both static (SRAMs) and dynamic (DRAMs), because these microelectronic devices contain by far the largest number of bits susceptible to upset. The most common way of dealing with SEU in RAMs is by means of error detection and correction (EDAC); today a number of commercially available computer systems for upgrading military aircraft incorporate EDAC in their designs…”
In the case of the General Aviation, the flight altitudes are lower, thus the particle flux is also lower. Also, the total dose radiation effects are much less present than total dose effects in space applications. Thus, the hardening efforts should focus on the protection of the device against temporary effects such as SETs and SEUs but also permanent effects such as component destruction due to Single Events.
One of the most important contributors to the device sensitivity are the SRAM-based memory blocks. Modern COTS memories have sensibilities in the interval 100~1000 FIT at ground level and a lot more (tens or hundreds times more) at flight-level altitudes. The protection of these memories is an absolute priority.
According to radiation-testing conducted by iRoC, FLASH-based memories are less sensitive to Soft Errors than SRAM chips by at least two or three orders of magnitude. The protection of these components is a nice feature to have but not critical. However, protecting the FDM units is crucial, given the sheer size of the memory.
|||E. Normand, “Single Event Effects in Avionics,” IEEE Trans. Nucl. Sci., 43, 461, 1996.|