CRITICAL aircraft control systems鈥攏ot to mention office
computers鈥攃ould 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鈥攚ith 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.
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Most PCs now in the shops are powered by microchips in which the transistors
are between 330 and 250 nanometres across
(see 鈥淐rashing the barriers鈥�, 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鈥檚 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.
鈥淲hat is currently an intermittent problem could easily become a much bigger
problem,鈥� 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. 鈥淢ost of the lead that is used today is slightly radioactive,鈥� he
says. 鈥淭here are still a few places where nonradioactive lead can be obtained,
but not many.鈥� 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. 鈥淏ecause 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,鈥� 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.