Semiconductor Radiation Damage
August 3, 2012
It's no wonder that I'm often tired. All those cosmic rays shooting through my body are keeping me awake at night! The flux of cosmic ray particles arriving at the Earth's surface is considerable. There are about 10,000 of the lowest energy particles (1 GeV = 109 eV) impinging on a square meter's area every second, dropping to just 1 per square meter at a TeV (1012 eV).
Cosmic rays may have been responsible for some clicks on the room radiation detector for my Xray diffractometer, although most of these clicks would have been from the background radiation of the building materials. Brick buildings have a higher natural radiation level than wooden structures; but granite is the worst offender. Certain granites have a uranium content of about 10 - 20 parts per million (ppm).
No one on Earth is really aware of these cosmic rays, but astronauts regularly see them as flashes of light, even when their eyes are closed.[2-4] Apollo astronauts reported seeing such flashes about once every three minutes, once their eyes had become adapted to the dark. Such radiation caused no instantaneous damage, but years later, astronauts developed cataracts. The cataracts sometimes developed within five years of their space flights. There's a known correlation between radiation exposure (usually, ultraviolet or microwave radiation) and cataracts.
Cosmic rays affect not just biological material, but also electronic material. As I wrote in a previous article (Fleeting Memory, September 28, 2006), computer devices are susceptible to "soft errors," which are transient errors that happen in even perfectly manufactured integrated circuits. Cosmic rays that strike an integrated circuit can generate millions of electron-hole pairs. Since transistor logic gates and memory circuits operate from the barest trickle of electron current, these stray charges can change logic states.
In the late 1990s, when transistors were huge compared with those used today, IBM published results of an extensive multi-decade study on soft errors in computer memory circuits. At that time, they found that cosmic rays would cause a soft error each week in every gigabyte of memory.
A 2006 study at a French semiconductor testing facility at 8372 feet (2,552 meters) in the Alps, where cosmic ray flux is higher, found a soft error rate of 0.6 per million hours/per MBit for the particular devices tested. This error rate is consistent with the IBM result.[Alpine]
Of course, error correcting codes work easily to eliminate such a problem in memory if the cosmic ray affect only a single bit in a data word. Our shrinking transistors make such corrections more of a problem, since more than a single bit might be affected.
Of more concern are "hard" errors, the type that were once just problematic for communications satellites and interplanetary spacecraft. A study by scientists at Vanderbilt University and Rutgers University, just published in the Journal of Applied Physics, indicates that the damage that radiation causes in semiconductors may be an order of magnitude greater than previous estimates.[8-9]
The research team used a technique called Coherent Acoustic Phonon Spectroscopy (CAPS) to measure the radiation damage of GaAs irradiated with Ne++ ions of 400 keV energy at doses between 1011-1013 cm-2. CAPS, as shown in the figure, uses a laser pulse to generate an acoustical pulse in a material, and another laser as a probe to determine the size and location of electrical defects in the material. Such damage reduces the reflected amplitude of the probe laser in response to the acoustical pulse.
CAPS has been used for about fifteen years in other applications, and it works much the way that oil companies look for underground geological structures indicative of oil. The oil prospecting method uses things such as explosives to drive an acoustic wave into the ground. Seismic detectors probe for the return signals. In the CAPS method used to investigate structure in irradiated semiconductors, laser beams take the place of both the explosives and the seismic detectors.
The pump pulse blasts the semiconductor surface with a quick, but intense, pulse of laser light. This produces an acoustic wave that propagates through the material. The probe laser uses a property of the semiconductor that allows reflection of the light at the wavefront of the acoustic pulse. This acoustical pulse maps the electrical properties of the semiconductor material as a function of depth, and any deviation from perfection reduces the amplitude of the signal.
Said Vanderbilt's Norman Tolk, an author of the study,
"The ability to accurately measure the defects in electronic materials becomes increasingly important as the size of microelectronic devices continues to shrink... When an individual transistor contains millions of atoms, it can absorb quite a bit of damage before it fails. But when a transistor contains a few thousand atoms, a single defect can cause it to stop working."
The CAPS technique discovered that ion-implanted neon atoms caused damage over a volume of about a thousand atoms in the semiconductor. This is an order of magnitude greater than the damage inferred by prior techniques. This is an important result, since nanoscale structures, such a quantum dots, contain only about a thousand atoms.
This research was supported by the US Department of Energy, the Army Research Office, and the National Science Foundation.
- Fact Sheet on Biological Effects of Radiation U.S. Nuclear Regulatory Commission March 29, 2012.
- Flashes in astronaut's eyes, British Medical Journal, vol. 4, no. 5734 (November 28, 1970), p. 510.
- L. S. Pinsky, W. Z. Osborne, J. V. Bailey, R. E. Benson, L. F. Thompson, "Light Flashes Observed by Astronauts on Apollo 11 through Apollo 1," Science, vol. 183 no. 4128 (March 8, 1974), pp. 957-959.
- Blinding Flashes, NASA Science News, October 22, 2004.
- Anthony Cataldo, "IBM moves to protect DRAM from cosmic invaders," EE Times, October 12, 1998.
- Soft Errors from Alpha Particles, Silicon Far East Web Site.
- Junko Yoshida, "Alpine lab enters rarified air of soft-error test," EETimes, September 25, 2006.
- David Salisbury, "Radiation damage bigger problem in microelectronics than previously thought," Vanderbilt University Press Release, July, 19, 2012.
- A. Steigerwald, A. B. Hmelo, K. Varga, L. C. Feldman and N. Tolk, "Determination of optical damage cross-sections and volumes surrounding ion bombardment tracks in GaAs using coherent acoustic phonon spectroscopy," J. Appl. Phys., vol. 112, no. 1 (July 1, 2012), Document No. 013514 (7 pages).
- Radiation damage bigger problem in microelectronics than previously thought, YouTube Video by Vanderbilt University, July, 19, 2012.
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