Solar Weather
Vivian Cooper-Capps, Vanderbilt University

Cosmic rays and solar wind are just two of the invisible but powerful forces that engineers grapple with in designing onboard computers for satellites, the International Space Station, and other space-faring equipment.

When "space weather" gets rough enough to play havoc with our satellites, things don't go well on Earth (please see sidebar, next page). We are increasingly dependent on space-based infrastructure for our communications and defense needs, for everything from cell phones and pagers to weather reports and defense intelligence.

"The Night the Pagers Died" is one example. On May 19, 1998, the Galaxy IV communications satellite, which controlled communications with nearly 90 percent of North America's pagers and several major broadcast networks, unexpectedly died, the victim of "killer electrons" in space.

Interstellar cosmic rays can penetrate a five-foot-thick wall of concrete without really slowing down, so shielding isn't much help against this galactic invader. And they strike at random, hurled from deep space, leaving a cascade of showering atomic particles in their wake as they blast down to Earth. In space, with no atmosphere to hinder them, cosmic rays slam into equipment at close to the speed of light.

That's not the worst, either. For all their potential destructiveness, cosmic rays are but bit players in the drama pitting sensitive microelectronics equipment against radioactive threats in space.

Our radiant sun is the major player, the source of rapidly streaming high-energy- particle "solar wind," unpredictably erupting solar flares. The sun also emits even deadlier, gigantic bubble-like coronal mass ejections packing 10 billion tons of hot, electrically charged gas with more energy than 1 billion megatons of TNT. That's a whole lot of extra energy bombarding delicate microelectronic systems that operate equipment and make decisions on the basis of very tiny electrical charges signaling "on" or "off" states.

Houston, we have a problem.

The Vanderbilt Institute for Space and Defense Electronics (ISDE) is working out solutions. Established in 2003 by the Vanderbilt Radiation-Effects Group, ISDE is the largest academic program supporting the U.S. Department of Defense in studying radiation effects for defense applications and one of the few programs involved in microelectronics research for space applications.

"Rad Hard"

Professor of Electrical Engineering Ronald D. Schrimpf leads the group in delving into unmapped regions of the atomic-scale territory of radiation effects on electronics equipment.

As director of ISDE, Schrimpf is leading the team of engineers in finding ways to make next-generation electronics resilient, or "radiation-hard," in the face of radioactive onslaughts of any kind.

"The good news is that current technology is becoming naturally resistant to 'total dose ionization,' which is what you might think of as background radiation," he says. "As circuits and devices continue to shrink in size, so do the oxide layers within integrated circuitry that ordinarily collect positive or negative charges from radiation over time.

"The bad news is that continued miniaturization results in circuits and devices that are more vulnerable to single-event phenomena like cosmic rays or high-energy protons, and these events are increasingly likely to cause a significant failure within a system or shut it down altogether," he says.

Down to Earth

In fact, ongoing miniaturization of computer circuits is making computers more vulnerable to radiation even on Earth itself.

"Computer chip manufacturers are very concerned about the effects of radiation on microelectronic devices," says Lloyd Massengill, professor of electrical engineering and ISDE's director of engineering. "This problem will become much more significant when the 65-nanometer-chip generation comes on the market."

The next-generation technology of tiny, fast and memory-rich computer chips -expected to be available in the next year or so - will have a much greater susceptibility to memory loss, system degradation, or outright failure due to radiation.

Many of these data-corrupting changes from radiation and cosmic rays produce "soft errors" that may be corrected the next time the computer boots up or may be caught by error-detection circuitry or software. Massengill and his team are working on "radiation-hardening by design" techniques that reduce the number of these errors that occur, without requiring expensive process modifications.

"Hard errors" bring the entire system down.

Vanderbilt radiation-effects engineers determine ways to protect integrated circuits and semiconductor devices from radiation by studying radiation effects in the laboratory and by developing computer models and simulations.

They are exploring the complex dynamics of all significant radiation effects, including:

Total-dose radiation is caused by bombardment over time of particles or photons, which are emitted from a variety of sources.

The accumulated effect of the radiation degrades performance and can ultimately destroy the computer. Single-event radiation is due to isolated strikes by ionized particles, such as cosmic ions. The effect on a computer circuit is localized and transient.

"Everything emits radiation at some level," Massengill explains.

"In one famous example many years ago, it was determined that the packaging materials for the computer chips contained trace amounts of system-damaging radioactive residues. They were tracked back to virtually undetectable contaminants in the water supply used in the processing of the packaging compounds."

Hardened by Design

The ISDE researchers study the different radiation effects in the laboratory, subjecting devices and circuits to X-rays, gamma rays, and accelerated protons, neutrons and ions.

Computer models and simulations are created to develop a comprehensive understanding of radiation effects on microelectronic devices and circuits.

"We are combining all the different levels of abstraction to give a seamless understanding of radiation effects, from the atomic scale to the system level," Schrimpf says.

"As we understand and predict how devices and circuits will behave in different environments through analysis, characterization and modeling, we are better able to make design recommendations that will protect equipment from radiation."

Professor Massengill focuses on circuit design that enables integrated circuits to correct or compensate for radiation damage.

Schrimpf is studying transistors and the semiconductor materials of which they are made.

One rad-hard strategy is to incorporate back-up transistors within the integrated circuits, so that if one part fails, the component itself will still work. Since adding transistors adds to the system's complexity, slows it down, and increases manufacturing costs, the engineers are evaluating which devices within the complex circuitry are most critical to the mission and most vulnerable to radiation.

"Our computer models simulate radiation effects and help us pinpoint the most vulnerable aspects of specific devices, circuits and circuit designs affected by radiation and potentially most damaging to the entire system if they fail," Massengill says.

The ISDE team is inventing and testing device design changes, such as altering the shape of the device to keep electrical current from leaking at the edges and using different materials. The group is studying silicon germanium bipolar transistors, which are commonly used in amplifiers and high-speed applications such as a radio on a single chip. They are also studying gallium arsenide, to be used in cell phones.

Space and Defense Applications

Because space is such a complex environment, ISDE recruited Robert Reed, research associate professor of electrical engineering and a former NASA engineer, to help develop computer tools to predict reliability and survivability of electronics in space.

He's helping to map the behavior of electronics equipment subjected to solar particles, cosmic rays, and trapped magnetic fields of radiation. "The models we were using were based on '70s and '80s technology," Reed says.

He's working with Robert A. Weller, professor of electrical engineering and associate professor of physics, to develop fundamental physics models that describe and predict radiation transport at the atomic scale.

"The Hubble Space Telescope has to shut down each time it passes through the South Atlantic Anomaly (part of the Van Allen radiation belt that comes closest to Earth), which no one predicted or expected," Reed says by way of example (please see sidebar). "We found that transient radiation events would create a glitch in the infrared camera's optocoupler that shuts down portions of the system. It made it clear that we needed better models," he says.

"No one can tame the sun, but we can develop better predictive tools to deal with space weather and can design more resilient systems that are better prepared to deal with extreme conditions found in space."

 

Solar Weather

 

Vanderbilt Group to Tackle Extreme Conditions in Space

 

Advanced Integrated
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