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 data—the electrons in a storage circuit, for example—but 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.
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