Soft/Firm Errors Glossary

Alpha particle

A helium nucleus, which contains two protons and two neutrons. Alpha particles are emitted from larger atoms as a result of radioactive decay. An alpha particle will only travel a few centimeters in air, or about 25 microns in silicon, before interacting with the matter it is raveling through.

Antifuse

This is a two-terminal device that is a highly resistive element in its unprogrammed state and is programmed to a low impedance. Typical programmed impedances range from 25 to 500 ohms, depending on the specific antifuse material, technology, and programming. This element is generally inherently radiation-tolerant; certain versions can be made radiation-hard. The failure mode of these elements during irradiation is rupture from a heavy ion.

Asymptotic Cross Section

Asymptotic or saturation cross section (sigmasat) is the value that the cross section approaches as LET gets very large. Also called "Saturation Cross Section" or "Sigma Sat." See also "Cross Section."

Bathtub Curve

The bathtub curve refers to the failure rate of many manufactured devices when viewed over the lifetime of the device. Many devices fail within a short period of time due to manufacturing defects. This failure rate decreases with time until a relatively constant failure rate is achieved. This constant failure rate applies to the normal working life of the device, after which time the failure rate starts to increase due to parts wearing out.

CfgSER

See "Configuration Soft Error Rate."

Configuration Soft Error Rate (CfgSER)

The rate at which soft errors occur in the configuration memory of an FPGA. Because errors in the configuration memory can result in far-reaching system problems, they are referred to as firm errors.

Cosmic radiation

High energy rays from space which, in space, are primarily protons (92%) and alpha particles (6%). In space, cosmic rays come from all directions.

Cross Section

Cross section (sigma) is the device SEE response to ionizing radiation. For an experimental test for a specific LET, sigma = #errors/(ion fluence). The units for cross section are cm² per device or per bit.

Dose Rate

The rate at which ionizing radiation is applied. Dose rates greater than 10rad(Si)/sec are considered high, and dose rates less than 0.1rad(Si)/sec are considered low.

DRAM

Dynamic RAM. DRAM is typically the RAM used as the main memory in a computer system. DRAM requires fewer transistors per bit than SRAM, but each bit needs to be refreshed regularly or it will lose information. DRAM is typically slower but much less expensive than SRAM.

ECC

Error Correcting Code. An error correcting code specifies how to add extra information to data bits in a fashion that allows corrections to be made to the data if one (or possibly more) of the resulting bits is changed. The most common error correcting codes have the ability to correct a single bit error and detect double bit errors. To add SEC-DED (single error correction - double error detection) to a word of 64 bits, an 8 additional bits are usually used.

EDAC

See "Error Detection and Correction."

Electromagnetic Interference (EMI)

Noise or interference in electric circuits caused by interaction of electric and magnetic fields.

EMI

See "Electromagnetic Interference."

Error Detection and Correction (EDAC)

The use of Error Correcting Codes (ECC) in applications where data may become corrupted, for example by single event upsets due to radiation effects. See "Error Correcting Code."

ESA

European Space Agency

Firm Error

Corruption of configuration information stored in SRAM based FPGAs may take time to detect and correct. In the meantime, the function of the FPGA may have changed in an unpredictable and uncontrollable manner, causing system failure. For this reason, errors in SRAM FPGA configuration memory are referred to as Firm Errors.

Firm Error Rate (FER)

The rate at which firm errors occur in a system, caused by the corruption of SRAM FPGA configuration memory.

FIT

Failure-in-time. One FIT corresponds to one failure per billion (1E9) chip-hours.

Flash

A nonvolatile memory element that uses charge stored on a floating gate to indicate a logic 1 or a logic 0. Flash technology has recently been adopted by Actel for use as configuration storage for programmable logic, enabling a range of single chip, nonvolatile yet reprogrammable FPGAs. Flash cells are SEE tolerant.

Functional Failure

The point at which the device ceases to operate.

Fuse

This is a two-terminal device that is normally a low resistive element and is programmed or "blown" resulting in an open or high impedance. Typical materials are nichrome and polysilicon. This is element is inherently radiation-hard.

Hamming code

Hamming codes are one of the most commonly used types of error correcting codes.

Hard Error

A hard error is an error caused by a permanent physical defect in the memory system.

Hard Error Rate (HER)

The HER is the frequency of errors caused by permanent physical defects in the memory system. The HER is usually much lower than the soft error rate.

HER

See "Hard Error Rate"

JESD89

JEDEC specification covering the testing and measurement of radiation-induced soft errors

Latchup

A condition where the output of a circuit becomes fixed near one of the two voltage extremes and will not react to changes in the input signal. Latchup can result in high current flowing through the output circuit with possible permanent damage.

LET

See "Linear Energy Transfer."

LET Threshold

LET threshold (LET_TH) is the minimum LET to cause an effect. The JEDEC recommended definition is the first effect when the particle fluence = 10⁷ ions/cm².

Linear Energy Transfer (LET)

A measure of the energy transferred to the device per unit length as an ionizing particle travels through a material. The common unit is MeV-cm²/mg of material (Si for MOS devices).

MBU

See "Multiple Bit Upset."

MTBF

Mean time between failures

Multiple Bit Upset (MBU)

Several memory elements experience state changes due to the passage of the same particle.

NASA

National Aeronautics and Space Administration

NASDA

National Space Development Agency of Japan

Neutron

A heavy subatomic particle with no electrical charge. Neutrons are produced as a result of collisions between incoming cosmic particles and atoms of oxygen and nitrogen in the atmosphere. These neutrons travel at very high speed and will pass easily through several feet of concrete.

Neutron Flux

The frequency of occurrence of neutrons. Described as the number of neutrons passing through an area of 1 cm² per second (n/cm²-sec).

Nonvolatile

The memory elements keep their contents when power is removed from the device. The element may be one time programmable or "reprogrammable." Examples of the former include fuses and antifuses. Examples of the latter include EPROM, EEPROM and Flash storage elements. Programmable devices using Flash memory elements for configuration are both nonvolatile and reprogrammable.

One-Time Programmable

This device can be programmed only once; it's contents cannot be changed. Typically these devices are fuse or antifuses based.

Parametric Failure

The point at which the device exceeds its specified limits.

Parity

Parity memory is used to detect memory errors. Each byte of data is accompanied by a parity bit, which is determined by the number of ones in the eight data bits. Even (odd) parity ensures that the total number of one bits in the data bits and parity bit is even (odd). Parity memory is most commonly used on microcomputers with a small word size. A parity memory system that uses a 64 bit word requires the same number of bits as error correcting memory, which makes error correcting memory more appealing for 64 bit and larger word sizes.

Prompt Dose

Testing at an extremely high dose rate, to simulate the effect of a nuclear weapon detonation.

Rad

Basic unit of absorbed dose for ionized radiation. Rad = "Radiation Absorbed Dose." 1 Rad is 100 ergs of energy deposited in 1 gram of material. Because absorption is dependent on the target material, the radiation dose is denoted as Rad (x), where x is the target material. For work on radiation effects on silicon integrated circuits, scientists describe the radiation dose as Rad (Si).

RAM

Random Access Memory (see also DRAM and SRAM). Random access memory should allow equally fast access to any memory location in the system. Modern RAM systems are not quite random access, but compared to disk drives, they provide a very good approximation to random access memory. The term RAM, by itself, usually refers to the VLSI-based main memory of the computer system.

Reprogrammable

These devices can have their configuration loaded more than once. SRAM-based devices may be reloaded without restriction. Many other forms of reprogrammable elements have restrictions on the number of write cycles, although they are high enough not to be of practical concern for most applications.

Saturation Cross Section

See "Asymptotic Cross Section."

SEB

See "Single Event Burnout."

SEDR

See "Single Event Dialectric Rupture."

SEE

See "Single Event Effect."

SEFI

See "Single Event Functional Interrupt."

SEGR

See "Single Event Gate Rupture."

SEL

See "Single Event Latchup."

Sensitive Volume

Sensitive volume refers to the device volume affected by SEE-inducing radiation. The geometry of the sensitive volume is not easily known, but some information is gained from test cross section data.

SER

See "Soft Error Rate."

SET

See "Single Event Transient."

SEU

See "Single Event Upset."

SHE

See "Single Hard Error."

Sigma

See "Cross Section."

Sigma Sat

See "Asymptotic Cross Section."

Single Event Burnout (SEB)

A highly localized burnout of the drain-source in power MOSFETs. SEB is a destructive condition.

Single Event Dialectric Rupture (SEDR)

The rupturing of a dielectric layer, caused by an incoming high-energy particle, resulting in the creation of a conducting path between the conductors on either side of the dielectric.

Single Event Effect (SEE)

Generic term applied to radiation effects on a semiconductor integrated circuit, where a single bit is upset, or a single latchup occurs. Single event effects include SEBs, SEFIs, SETs, SEUs, and SHEs.

Single Event Functional Interrupt (SEFI)

A condition where the device stops operating in its normal mode, and usually requires a power reset or other special sequence to resume normal operations. It is a special case of SEU changing an internal control signal. One example would be a DRAM entering the test mode defined by JEDEC. Another example is a microcircuit with IEEE 1149.1 JTAG circuitry leaving the TEST_LOGIC_RESET state and loading an unintended instruction into the instruction register (IR). Like other SEUs, the system effects must be properly analyzed. For example, a JTAG upset can cause the device to draw high currents or turn inputs into outputs. The latter could, for example, drive a clock line to ground; thus, an independent clock signal should be used for the TCLK pin on devices without the optional TRST* pin.

Single Event Gate Rupture (SEGR)

The burnout of a gate insulator in a power MOSFET. SEGR is a destructive condition.

Single Event Latchup (SEL)

A potentially destructive condition involving parasitic circuit elements forming a silicon controlled rectifier (SCR). In traditional SEL, the device current may destroy the device if not current limited and removed "in time." A "microlatch" is a subset of SEL where the device current remains below the maximum specified for the device. A removal of power to the device is required in all non-catastrophic SEL conditions in order to recover device operations.

Single Event Transient (SET)

A current transient induced by the passage of a particle through an integrated circuit. The current can propagate to cause an output error in combinational logic.

Single Event Upset (SEU)

A change of state or transient induced by an ionizing particle such as a cosmic ray or proton in a device. This may occur in digital, analog, and optical components or may have effects in surrounding circuitry. These are "soft" bit errors in that a reset or rewriting of the device causes normal behavior thereafter. A full SEU analysis considers the system effects of an upset. For example, a single bit flip, while not damaging to the circuitry involved, may damage the subsystem or system (i.e., initiating a pyrotechnic event).

Single Hard Error (SHE)

An SEU that causes a permanent change to the operation of a device. An example is a permanent stuck bit in a memory device.

Soft Error

A soft error is an error that is not due to any permanent physical defect in the memory system. Soft errors can be fixed by either writing new data to the invalid memory area or by restarting the computer.

Soft Error Rate (SER)

The soft error rate is the frequency of data errors caused by neutrons, alpha particles, cosmic or terrestrial radiation, and other factors that do not permanently damage the memory system.

SRAM

Static RAM. SRAM is used for the cache memory and registers in computer systems. SRAM typically requires four or six transistors per bit, making it substantially more expensive than DRAM, which usually requires one transistor per bit. SRAM is able to operate at higher speeds than DRAM, and does not require refreshing.

TID

Total Ionizing Dose, see "Total Dose."

TMR

See "Triple Module Redundancy."

Total Dose

The total accumulated amount of absorbed ionizing radiation. Measured in Rads.

Transition fault

A transition fault is a fault in which a memory cell or line cannot change from one particular state to a different state.

Triple Module Redundancy (TMR)

A method of overcoming single event effects that uses three discrete instances of a circuit, with a majority vote scheme that monitors the data from each of the three circuits. The majority vote circuit itself outputs data identical to what the majority of the three circuits are outputting. This is an effective way to prevent data corruption due to single event effects, however it cannot correct situations where more than one of the three discrete circuits experiences an upset. TMR is expensive, since it uses two additional instances of the circuit being protected, in addition to the majority vote circuit.

Volatile

The memory elements lose their contents when power is removed from the device. SRAM-based devices are volatile and require another device to store their configuration program. Antifuse and Flash devices are non-volatile and retain their contents when power is removed.