June 11, 2004 LOS ALAMOS, NM, May 6, 2004 - Life today runs more and more on circuits. Electrons racing through increasingly tiny transistors now control our airplanes, deposit money in our checking accounts and keep our houses warm.
But these miniature devices face an invisible enemy from outer space that can strike computer chips, miniaturized controls and other complex integrated circuits, spoiling the digital work that permeates 21st century human activity.
Researchers at Los Alamos National Laboratory's Neutron Science Center, or LANSCE, are using a powerful proton accelerator to speed up the potential for failure, battering circuits in a single hour with hundreds or thousands of years of the harmful neutrons created by cosmic rays. Aviation, computer and related companies, along with the National Aeronautics and Space Administration, are putting circuits through the rigors of the LANSCE neutron testbed, located in a modest steel structure known as the ICE House, for Irradiation of Chips and Electronics.
"We can generate essentially the entire neutron spectrum produced naturally by cosmic rays, but with neutrons a million times more intense than in nature," said Steve Wender, who leads the Neutron and Nuclear Science Group at LANSCE.
When cosmic rays from deep space strike Earth's upper atmosphere, they ultimately create neutrons that shower down on the Earth's surface. These neutrons pose little health hazard because the radiation dose is relatively low. However, each neutron can interact with silicon and other elements in integrated circuits to produce charged particles, with potentially disastrous impacts on memory and chip function.
"We can't fully predict the effect of these interactions, which makes having a standardized way to test circuits extremely valuable" Wender said. "Very similar devices show radically different failure rates due to neutron interactions, and we have some evidence that the smaller transistors and lower operating voltages in newer devices produce higher failure rates."
Each individual neutron has an exceedingly small probability of damaging a single bit and thus disrupting circuit performance. However, since new electronic devices require more and more bits, manufacturers are seeing errors more frequently. Neutrons go through devices one at a time, but the increased intensity available in the ICE House greatly reduces the time between neutrons and the failure that they cause.
In the case of the latest, totally computer-controlled aircraft, these tiny cosmic gremlins could cause trouble, especially because the problem gets worse as atmospheric shielding dwindles at higher altitudes. At sea level, the shielding provided by the air is equivalent to more than ten feet of concrete shielding. The neutron flux at LANSCE, 7,000 feet above sea level is approximately three times greater than at sea level; and at 40,000 feet, the cosmic-ray neutron flux is several hundred times greater than the neutron flux seen on the earth's surface.
Circuit manufacturers understand the risk posed by cosmic ray neutrons and try to design around it, so LANSCE's ICE House increasingly is becoming an international standard for putting new circuits through their paces. In fact, the Joint Electron Device Engineering Council established JESD89, a standard for integrated circuit testing, based on the suite of experimental capabilities at the Icehouse.
Los Alamos has been putting circuits through their paces at LANSCE for more than a decade, and researchers there have published significant research on the extent of the problem.
A typical static random access memory, or SRAM, chip will generate roughly 1,200 errors per hour when subjected to the intense neutron flux at the ICE House, which can range as high as a million neutrons per square centimeter per second. Companies hold as proprietary the precise number of errors. The Laboratory supplies statistical information about the amount of neutron flux that generated the errors, so the companies leave Los Alamos with data showing the number of errors per neutron.
Once they know how many errors neutrons will generate, chipmakers can decide if error correcting, redundancy or other protective measures are needed to compensate for the neutron-induced errors. This ensures that any unreliability at the device level is compensated for at the system level.
"No one's been able to make this problem go away for key electronic applications; in fact, even the codes people use to predict failure rates are becoming less accurate," Wender said. "But if somebody eventually comes up with a possible solution, they'll have to bring the new design here to test it out."
Los Alamos currently is testing flight systems provided by NASA. The focus for this testing, through NASA aviation safety and security initiatives, is commercial aviation, with possible application to the next-generation Space Shuttle and similar systems for U.S. Air Force fighters. The Laboratory and NASA recently placed a complete aircraft control system in the LANSCE beam and linked it locally with a computer simulation for a Boeing 737.
A future experiment will examine whether pilots can compensate for control system upsets during simulated flight, by remotely linking a computer undergoing tests in the ICE House to the flight simulator located in the NASA System Airframe Failure Emulation Testing and Integration Laboratory at the Langley Research Center. Los Alamos is collaborating in NASA's development of the SAFETI Laboratory, with networked links to individual NASA labs for aircraft structures, cockpit motion and propulsion systems.
Customers who have brought electronic devices to LANSCE's ICE House represent a who's who of the global electronic and avionics industries. Repeat experiments have been run by Boeing Co., Honeywell, Sun, LSI Logic, Fujitsu, Intel and Saab, among others.
"Neutron-induced soft errors hurt how customers see circuit-based products because it's so hard to predict their impact on our systems," said Rob Baumann, Distinguished Member of the Technical Staff at Texas Instruments. "We look at dozens of potential circuit failure modes, but the rate of soft errors is significantly higher than that of all the other modes combined. As we build improved circuits with higher densities, higher speeds and lower power consumption, the soft error rate will go up."
"For my industry, the service Los Alamos provides is unique and essential because of the similarity of the LANSCE spectrum to actual terrestrial cosmic rays, as well as the ease of use: just pop it into the beam and you get your answers," Baumann said.
Richard Hess, chief fellow with Honeywell Aerospace Electronic Systems, said, "Honeywell has relied on this unique, world-class facility to help identify atmospheric neutron effects and associated effect rates for the electronic components used in our commercial aviation products.
"Linking the NASA SAFETI Laboratory facilities with the ICE House facilities has created a new, unique world-class facility for the study of neutron effects on aircraft electronic systems. ICE House is a vital resource in understanding the atmospheric neutron effects on Honeywell's aerospace electronics and enables products that are tolerant of radiation in the atmosphere," Hess added.
Demand for the testbed has grown so much that the waiting list is several months long, Wender said. Typically, companies will test circuits for up to five days, though some have used the testbed for twice that long.
Los Alamos National Laboratory is operated by the University of California for the National Nuclear Security Administration (NNSA) of the U.S. Department of Energy and works in partnership with NNSA's Sandia and Lawrence Livermore national laboratories to support NNSA in its mission.
Los Alamos develops and applies science and technology to ensure the safety and reliability of the U.S. nuclear deterrent; reduce the threat of weapons of mass destruction, proliferation and terrorism; and solve national problems in defense, energy, environment and infrastructure.
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