Question: What is a soft error (SE)? How do SEs affect semiconductor devices? What are the causes of SEs?
An SE is a random, non-recurring change of state or transient in microelectronic circuits due to energetic nuclear particles interacting with the silicon. When a high-energy particle hits the silicon substrate, the kinetic energy of the particle generates electron-hole pairs as it passes through the p-n junctions. Some of the deposited charge will recombine to form a very short-duration current pulse, which causes an SE. In memory elements, these current pulses can cause bit flips (i.e., data ‘1’ becoming data ‘0’ or vice versa).
An SE may result in a single-bit upset (SBU) or in a multi-bit upset (MBU). An SBU is a type of radiation-induced upset identified when a flipped bit is physically isolated from other possible events and the physical separation from any other flipped bit is at least two memory cells. An MBU is a type of radiation-induced upset identified when two or more flipped bits are physically adjacent or have a separation of, at most, one non-failing bit.
The major causes of soft errors in semiconductor devices are thermal neutrons, alpha particles, and high-energy neutrons:
- Thermal neutrons are low-energy neutrons generated during a cosmic shower. Such neutrons start off as high-energy neutrons but lose energy due to multiple collisions as they travel towards the surface of the earth. Usually, thermal neutrons do not cause soft errors by themselves. However, 10B, an isotope of boron found in borophosphosilicate glass (BPSG) oxide films of semiconductors, interacts with thermal neutrons and decays into high-energy lithium, helium nuclei, and a gamma ray. These high-energy nuclei generate electron-hole pairs and may lead to soft errors. To prevent the effect of thermal neutrons, the semiconductor industry has shifted away from using BPSG to using phosphosilicate glass (PSG) oxide films.
- Alpha particles are emitted from packaging materials and the interaction of cosmic ray thermal neutrons with boron present in p-type regions of the semiconductor devices. The radioactive decay of 232Th and 238U present in mold compounds, packages, and other assembly materials generates alpha particles. Alpha particles cannot travel large distances so their effect is limited to the immediate vicinity of their production. Thus, external alpha-particle sources cannot usually affect packaged semiconductor devices. Manufacturers use low alpha-particle mold compounds to reduce the occurrence of soft errors caused by alpha particles.
- High-energy neutrons are generated when high-energy cosmic rays and solar particles react with the upper atmosphere. High-energy neutrons are powerful and can cause multiple upsets in a semiconductor device. The concentration of high-energy neutrons varies with altitude (peaking at about 15 km), position (worst at poles), and the 11-year solar cycle.Shielding cannot be used in the case of high-energy neutrons. They undergo very little attenuation, and 70% of them can go through one foot of concrete. Thus, device or system-level mitigation techniques are required to mitigate the effect of high-energy neutrons on semiconductor devices.
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