In electronics and computing, a soft error is a type of error where a signal or datum is wrong. Errors may be caused by a defect, usually understood either to be a mistake in design or construction, or a broken component. A soft error is also a signal or datum which is wrong, but is not assumed to imply such a mistake or breakage. After observing a soft error, there is no implication that the system is any less reliable than before. One cause of soft errors is single event upsets from cosmic rays.
In a computer's memory system, a soft error changes an instruction in a program or a data value. Soft errors typically can be remedied by cold booting the computer. A soft error will not damage a system's hardware; the only damage is to the data that is being processed.
There are two types of soft errors, chip-level soft error and system-level soft error. Chip-level soft errors occur when particles hit the chip, e.g., when secondary particles from cosmic rays land on the silicon die. If a particle with certain properties hits a memory cell it can cause the cell to change state to a different value. The atomic reaction in this example is so tiny that it does not damage the physical structure of the chip. System-level soft errors occur when the data being processed is hit with a noise phenomenon, typically when the data is on a data bus. The computer tries to interpret the noise as a data bit, which can cause errors in addressing or processing program code. The bad data bit can even be saved in memory and cause problems at a later time.
If detected, a soft error may be corrected by rewriting correct data in place of erroneous data. Highly reliable systems use error correction to correct soft errors on the fly. However, in many systems, it may be impossible to determine the correct data, or even to discover that an error is present at all. In addition, before the correction can occur, the system may have crashed, in which case the recovery procedure must include a reboot. Soft errors involve changes to datathe electrons in a storage circuit, for examplebut not changes to the physical circuit itself, the atoms. If the data is rewritten, the circuit will work perfectly again. Soft errors can occur on transmission lines, in digital logic, analog circuits, magnetic storage, and elsewhere, but are most commonly known in semiconductor storage.
Critical charge Whether or not a circuit experiences a soft error depends on the energy of the incoming particle, the geometry of the impact, the location of the strike, and the design of the logic circuit. Logic circuits with higher capacitance and higher logic voltages are less likely to suffer an error. This combination of capacitance and voltage is described by the critical charge parameter, Qcrit, the minimum electron charge disturbance needed to change the logic level. A higher Qcrit means fewer soft errors. Unfortunately, a higher Qcrit also means a slower logic gate and a higher power dissipation. Reduction in chip feature size and supply voltage, desirable for many reasons, decreases Qcrit. Thus, the importance of soft errors increases as chip technology advances.
In a logic circuit, Qcrit is defined as the minimum amount of induced charge required at a circuit node to cause a voltage pulse to propagate from that node to the output and be of sufficient duration and magnitude to be reliably latched. Since a logic circuit contains many nodes that may be struck, and each node may be of unique capacitance and distance from output, Qcrit is typically characterized on a per-node basis.
Causes of soft errors
Alpha particles from package decay Soft errors became widely known with the introduction of dynamic RAM in the 1970s. In these early devices, ceramic chip packaging materials contained small amounts of radioactive contaminants. Very low decay rates are needed to avoid excess soft errors, and chip companies have occasionally suffered problems with contamination ever since. It is extremely hard to maintain the material purity needed. Controlling alpha particle emission rates for critical packaging materials to less than a level of 0.001 counts per hour per cm2 (cph/cm2) is required for reliable performance of most circuits. For comparison, the count rate of a typical shoe's sole is between 0.1 and 10 cph/cm2.
Radioactive contamination of chip packages were documented to cause some UltraSPARC II-based Ultra Enterprise server crashes. In this case, the crashes were traced to SRAM chips in the external CPU cache.
Package radioactive decay usually causes a soft error by alpha particle emission. The positive charged alpha particle travels through the semiconductor and disturbs the distribution of electrons there. If the disturbance is large enough, a digital signal can change from a 0 to a 1 or vice versa. In combinational logic, this effect is transient, perhaps lasting a fraction of a nanosecond, and this has led to the challenge of soft errors in combinational logic mostly going unnoticed. In sequential logic such as latches and RAM, even this transient upset can become stored for an indefinite time, to be read out later. Thus, designers are usually much more aware of the problem in storage circuits.
A 2011 Black Hat paper discusses the real-life security implications of such bit-flips in the Internet's Domain Name System. The paper found up to 3,434 incorrect requests per day due to bit-flip changes for various common domains. Many of these bit-flips would probably be attributable to hardware problems, but some could be attributed to alpha particles. These bit-flip errors may be taken advantage of by malicious actors in the form of bitsquatting.
Isaac Asimov received a letter congratulating him on an accidental prediction of alpha-particle RAM errors in a 1950s novel.Gold (1995): "This letter is to inform you and congratulate you on another remarkable scientific prediction of the future; namely your foreseeing of the dynamic random-access memory (DRAM) logic upset problem caused by alpha particle emission, first observed in 1977, but written about by you in Caves of Steel in 1957." [Note: Actually, 1952.] ... "These failures are caused by trace amounts of radioactive elements present in the packaging material used to encapsulate the silicon devices ... in your book, Caves of Steel, published in the 1950s, you use an alpha particle emitter to 'murder' one of the robots in the story, by destroying ('randomizing') its positronic brain. This is, of course, as good a way of describing a logic upset as any I've heard ... our millions of dollars of research, culminating in several international awards for the most important scientific contribution in the field of reliability of semiconductor devices in 1978 and 1979, was predicted in substantially accurate form twenty years [Note: twenty-five years, actually] before the events took place
Cosmic rays creating energetic neutrons and protons Once the electronics industry had determined how to control package contaminants, it became clear that other causes were also at work. James F. Ziegler led a program of work at IBM which culminated in the publication of a number of papers (Ziegler and Lanford, 1979) demonstrating that cosmic rays also could cause soft errors. Indeed, in modern devices, cosmic rays may be the predominant cause. Although the primary particle of the cosmic ray does not generally reach the Earth's surface, it creates a shower of energetic secondary particles. At the Earth's surface approximately 95% of the particles capable of causing soft errors are energetic neutrons with the remainder composed of protons and pions. IBM estimated in 1996 that one error per month per 256 MiB of RAM was expected for a desktop computer. This flux of energetic neutrons is typically referred to as "cosmic rays" in the soft error literature. Neutrons are uncharged and cannot disturb a circuit on their own, but undergo neutron capture by the nucleus of an atom in a chip. This process may result in the production of charged secondaries, such as alpha particles and oxygen nuclei, which can then cause soft errors.
Cosmic ray flux depends on altitude. For the common reference location of 40.7° N, 74° W at sea level (New York City, NY, USA), the flux is approximately 14 neutrons/cm2/hour. Burying a system in a cave reduces the rate of cosmic-ray-induced soft errors to a negligible level. In the lower levels of the atmosphere, the flux increases by a factor of about 2.2 for every 1000 m (1.3 for every 1000 ft) increase in altitude above sea level. Computers operated on top of mountains experience an order of magnitude higher rate of soft errors compared to sea level. The rate of upsets in aircraft may be more than 300 times the sea level upset rate. This is in contrast to package decay-induced soft errors, which do not change with location. As chip density increases, Intel expects the errors caused by cosmic rays to increase and become a limiting factor in design.
The average rate of cosmic-ray soft errors is inversely proportional to sunspot activity. That is, the average number of cosmic-ray soft errors decreases during the active portion of the sunspot cycle and increases during the quiet portion. This counter-intuitive result occurs for two reasons. The Sun does not generally produce cosmic ray particles with energy above 1 GeV that are capable of penetrating to the Earth's upper atmosphere and creating particle showers, so the changes in the solar flux do not directly influence the number of errors. Further, the increase in the solar flux during an active sun period does have the effect of reshaping the Earth's magnetic field providing some additional shielding against higher energy cosmic rays, resulting in a decrease in the number of particles creating showers. The effect is fairly small in any case resulting in a ±7% modulation of the energetic neutron flux in New York City. Other locations are similarly affected.
One experiment measured the soft error rate at the sea level to be 5,950 failures in time (FIT = failures per billion hours) per DRAM chip. When the same test setup was moved to an underground vault, shielded by over of rock that effectively eliminated all cosmic rays, zero soft errors were recorded. In this test, all other causes of soft errors are too small to be measured, compared to the error rate caused by cosmic rays.
Energetic neutrons produced by cosmic rays may lose most of their kinetic energy and reach thermal equilibrium with their surroundings as they are scattered by materials. The resulting neutrons are simply referred to as thermal neutrons and have an average kinetic energy of about 25 millielectron-volts at 25 °C. Thermal neutrons are also produced by environmental radiation sources, including the decay of naturally occurring radioactive elements such as uranium and thorium. The thermal neutron flux from sources other than cosmic-ray showers may still be noticeable in an underground location and an important contributor to soft errors for some circuits.
Thermal neutrons Neutrons that have lost kinetic energy until they are in thermal equilibrium with their surroundings are an important cause of soft errors for some circuits. At low energies many neutron capture reactions become much more probable and result in fission of certain materials creating charged secondaries as fission byproducts. For some circuits the capture of a thermal neutron by the nucleus of the 10B isotope of boron is particularly important. This nuclear reaction is an efficient producer of an alpha particle, 7Li nucleus and gamma ray. Either of the charged particles (alpha or 7Li) may cause a soft error if produced in very close proximity, approximately 5 μm, to a critical circuit node. The capture cross section for 11B is 6 orders of magnitude smaller and does not contribute to soft errors.