CRITICAL aircraft control systems-not to mention office computers-could inexplicably fail unless the makers of the next generation of microchips take account of a growing menace: background radiation.
Electronics engineers warn that the transistors in microchips are becoming so small that background radiation could corrupt the data they hold. That means data could be lost and programs might crash-with potentially disastrous results.
Engineers from Texas Instruments and Intel in the US and STMicroelectronics in France sounded the alarm at the International Test Conference in Washington DC last month. They warn that natural emissions of alpha particles and neutrons are increasingly likely to corrupt chunks of data in the coming generations of memory and microprocessor chips.
Most PCs now in the shops are powered by microchips in which the transistors are between 330 and 250 nanometres across (see "Crashing the barriers", p 42). Radiation-induced failures are rare, and any that do occur are likely to be put down to bugs in the software. But the next generation of microprocessors, which are set to shrink below the 180 nanometres of today's best chips, will be more susceptible. Such chips are expected to become commonplace early in the next decade (see Figure). The problem will get progressively worse as manufacturers continue to design smaller, faster microprocessors, engineers say.
"What is currently an intermittent problem could easily become a much bigger problem," says Gehan Amaratunga, an electrical engineer at Cambridge University. The smaller each transistor is, the less electrical charge it needs to record each bit of information. This makes each bit more susceptible to corruption by incoming radiation, says Amaratunga.
Many materials used in electronics naturally radiate alpha particles, with lead solder the major problem, says Alan Hales, an engineer at Texas Instruments in Dallas. "Most of the lead that is used today is slightly radioactive," he says. "There are still a few places where nonradioactive lead can be obtained, but not many." Some companies are experimenting with lead-free solder, but a host of other materials used in chip making, such as silica moulds and the phosphoric acid used for etching, are also natural alpha-emitters.
Electronic systems in aircraft face the worst problems as chips shrink. At normal flying altitudes of 9 kilometres or more, cosmic radiation is a thousand times as intense as at sea level, Hales notes. A fast-moving neutron can knock electrons out of their shells. The electrons may then accumulate in the crystal lattices of chips, where they build up a charge that can corrupt data.
Chip makers cannot solve the problem simply by putting barriers around their chips. "Because most of the alpha radiation comes from the packaging materials themselves, and it takes ten feet of concrete to stop a neutron, shielding would be impractical," Hales says.
One solution being explored is to double the number of transistors used to store each bit of data. Ian Boyd, who works on electronic materials at University College London, believes this could be the way forward. Cramming replicas of each element into spare areas on a chip would provide a backup in case of a radiation hit. But this would produce an expensive chip in which only half the transistors actually do useful work.
Author: Matt Walker, New Scientist, issue 7, November 1998.
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