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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 = 10
9 eV) impinging on a square meter's area every second, dropping to just 1 per square meter at a
TeV (10
12 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.[1] Certain granites have a
uranium content of about 10 - 20
parts per million (ppm).
Environmental radiation sources.
Surprisingly, the human body, itself, emits radiation.
From NCRP Report #93, "Ionizing Radiation Exposure of the Population of the United States" (1987), via Lawrence Berkeley National Laboratory.
(Rendered using Gnumeric)
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.[4] 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]
A bank of 2102 Static Random Access Memory (SRAM) integrated circuits, manufactured by National Semiconductor, circa 1977. These had 1,024 bits of memory, each.
(Photograph by author, via Wikimedia Commons))
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 10
11-10
13 cm-2.[9] 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.[10] Such damage reduces the reflected
amplitude of the probe laser in response to the acoustical pulse.
Coherent Acoustic Phonon Spectroscopy. Still image from a YouTube video[10].[)
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.[8] 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.[8]
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.[8]
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."[8]
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.[9] This is an important result, since
nanoscale structures, such a quantum dots, contain only about a thousand atoms.[8]
This research was supported by the
US Department of Energy, the
Army Research Office, and the
National Science Foundation.[8]
References:
- 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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