How do you design a sunscreen for a computer chip? For that matter, why would you need to?
Lloyd Massengill, professor of electrical engineering and computer engineering, has answers, both simple and complex, to those questions. Radiation from as far away as deep space and as close as our sun poses significant dangers to both space-based and earthbound computers that control an enormous array of commercial and military equipment today.
Take, for example, the Hubble Space Telescope. Each time it passes through the Van Allen radiation belt that surrounds Earth, the telescope shuts down due to the radiation—often nine times a day. With no atmosphere in space to filter radiation from galactic objects, both Hubble and the International Space Station are vulnerable to cosmic rays crashing into them at the speed of light and impacting their systems.
On a more everyday level, radiation also threatens Earth-based equipment like missile-guidance systems, supercomputers and telecommunications systems, and even cell phones and iPods. The cost of service interruptions to such systems can run into the millions of dollars.
Massengill and his colleagues in the Vanderbilt Microelectronics Radiation Effects and Reliability (RER) research group study how radiation affects electronics and are working to reduce the damage it causes.
The mission of the RER is to help increase the reliability of systems that are exposed to ionizing radiation both on Earth and in space. Because ionizing radiation can liberate energy when it passes through semiconducting material, it can cause havoc with processing equipment.
Dedicated to Microelectronics Research
In addition to his work with the radiation group, Massengill also serves as director of engineering for Vanderbilt’s Institute for Space and Defense Electronics (ISDE). The institute is among a handful of contract engineering programs conducting microelectronics research for both military and commercial space applications.
The institute’s research has been applied to Trident II ballistic missiles, the James Webb Space Telescope that will replace Hubble, and sensitive medical devices where failures are unacceptable for any reason. ISDE clients include the Defense Department, NASA, Cisco Systems, The Boeing Co. and others.
ISDE engineers use laboratory experiments and computer modeling to simulate radiation effects on integrated circuits which form the nucleus of modern information technology. They also develop tools to analyze circuit design improvements and design radiation-hardened circuits that can resist both single-event and total-dose radiation.
Massengill’s area of expertise is the study of single-event radiation that produces soft errors in microelectronics. Soft errors are due to isolated strikes by ionized particles emitted by cosmic rays, high-speed particles discharged by the sun and objects in deep space. Although the effect of single events on a computer is localized and transient, as circuits become smaller, the effect increases exponentially.
Of additional concern is total-dose radiation, which is caused by bombardment over time of subatomic particles released from a variety of sources, including ambient or background radiation on Earth. The accumulated effect of both types of radiation impairs performance and can ultimately destroy the computer.
Smaller, Faster . . . and More Vulnerable
The problem only promises to get worse as computers become smaller and smaller. Many companies are currently building processors at the 45-nanometer level, while others like IBM and NEC are working toward 22-nanometer chips. (A one-nanometer circuit is equal in size to one billionth of a meter.)
“Smaller electronic devices are cheaper, faster and more functional,” Massengill says. “However, as these devices decrease in size, they also become more susceptible to radiation. For example, today’s desktop computers are quite sensitive to ambient radiation, which is emitted by almost everything around us.”
ISDE researchers are developing a variety of approaches to make integrated microelectronic devices more resilient to radiation.
One strategy is to incorporate back-up transistors within the integrated circuits, so that if one part fails, the component itself will still work. The engineers are also working on radiation-hardened circuits that can be produced without expensive changes to the manufacturing process. Such new techniques are being applied to a variety of applications, ranging from inertial guidance systems for the military to communications systems on satellites.
Massengill emphasizes that success and discovery stem from the entire ISDE and RER teams. Although not given to hyperbole, Massengill praises the ISDE staff engineers as “the finest I have ever known” and calls the graduate students with whom he works “the best in the country.” In further examples, he highlights the work of colleagues Professor Robert Weller, Associate Professor Robert Reed and Research Associate Professor Marcus Mendenhall, who are developing a world-class single-event analysis computer code that Massengill says is revolutionizing the integrated circuit reliability community. The ISDE director also notes that Professor Bharat Bhuva and Research Associate Professor Tim Holman are devising radiation-hardened circuit designs that are innovative and game-changing. In other important work, he cites RER faculty colleagues: Dan Fleetwood, professor and chair of the Department of Electrical Engineering and Computer Science; Ronald Schrimpf, Orrin Henry Ingram Professor of Engineering and director of ISDE; and Kenneth F. Galloway, professor and dean of the School of Engineering.
Massengill’s team made an important research discovery recently when they found that single-event particles do not affect a single isolated circuit node in technologies smaller than 100 nanometers. Instead, they actually affect multiple circuit nodes through an effect called charge sharing.
“This discovery explained why certain supposedly radiation-hardened circuits were in fact sensitive to single events,” Massengill explains. “The use of separate circuit nodes to protect the signal information was thwarted by charge sharing.
“We presented our discovery at the International Reliability Physics Symposium and published it in the IEEE Transactions on Nuclear Science. It has profoundly affected the design of soft-error-tolerant microelectronics, including the DARPA Radiation Hardened by Design Program.” DARPA, the acronym for the Defense Advanced Research Projects Agency, is the Defense Department’s central research and development office, and one of ISDE’s clients.
Called a research genius by his colleagues, Massengill brings millions of dollars in research grants to the School of Engineering each year. He recently received a $2.4 million grant from the Defense Department’s Defense Threat Reduction Agency to analyze the effects of single-event radiation in 45-nanometer integrated circuits and to develop radiation-hardened microtechnologies for spacecraft and satellites.