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A soft error refers to a temporary malfunction occurring with a normal semiconductor and a memory chip, etc. due to certain causes. Unlike a "hard error" that occurs due to such fatal damages as circuit destruction, there is no damage in the semiconductor itself in the case of a soft error. Thus, a normal state is regained by rebooting the computer or rewriting the data stored in the memory chip.
A soft error is caused mainly by radiation rays. When particles included in the radiation rays clash with a semiconductor, charged particles are generated within the semiconductor. The charged particles distort the data contained in the memory cells to generate a soft error.
A soft error was not recognized as a major problem until the latter half of the 1970s, when it was confirmed that alpha rays radiating from the product package caused a soft error. Moreover, since the latter half of the 1990s, attention has been paid to cosmic ray-induced neutrons as a cause of a soft error. This is because even a very small impact by cosmic ray-induced neutrons can generate a soft error as the electric charge contained in memory cells decreases in accordance with the advances in microlithography technology. At present, products featuring the functions to detect and correct errors are available in the market as a countermeasure against soft errors.