By John Keller
Use of radiation-hardened and radiation-tolerant solid-state memory chips is on the rise with applications in manned and unmanned space vehicles, military electronics, and even in high-altitude aircraft avionics.
Yet scarce federal dollars for defense technology, as increasing amounts go for overseas military operations in the Middle East and for rebuilding the hurricane-wracked U.S. Gulf Coast and City of New Orleans, dictate that radiation-hardened systems be designed as economically as possible.
That financial squeeze often translates into radiation-hardened subsystems and components for space and military applications that largely use commercial-grade solid-state memory, and make the most of shielded enclosures and software error correction.
“We buy non-radiation-hardened products, and then we do radiation characterization, and integrate them into a package with our radiation-mitigation technology. The packaged product is radiation hardened,” says Larry Longden, senior director of marketing and technology for rad-hard integrated circuit supplier Maxwell Technologies Inc. in San Diego.
Maxwell supplies most of the radiation-hardened and radiation-tolerant electronically erasable programmable read-only memory (EEPROM) chips to the U.S. space industry, Longden says. Maxwell’s shielding packages, called Rad-Pak, Rad-Stak, X-ray-Pak, offer performance that depends on the space orbit in which they operate. Most of Maxwell’s memory devices offer total-dose resistance to space radiation in geosynchronous orbits of at least 100 kilorads, he says. These devices go into unmanned satellites and manned spacecraft.
“Most of the today’s space applications for memory are two types: one is just data storage for instrumentation systems collecting data, and the other is for computer memory,” Longden says. “Our radiation-hardened EEPROM is for the user or application, or sometimes the boot code. The big issue there is nobody else in the semiconductor market is working on it, and it’s really an obsolete technology-but there is not a technology to replace it yet.”
Other companies also see a trend for less radiation hardening in individual devices. “Our customers are asking for 60 to 80 MEV [million electron-volts] for single-event latchup, and about total ionizing dose of 25 to 100 kilorads. People are asking for less radiation hardness,” says Chris Avrain, field application engineer for 3D Plus USA in McKinney, Texas. “Most of the people are looking for better radiation hardness for the application, so they are using external shielding on the components. We do see demand for rad-hard memory for commercial avionics, and I believe that demand will grow.”
The trend toward introducing radiation hardness with shielding and software has induced some chip suppliers to lessen their devices’ inherent resistance to radiation in the interest of reducing manufacturing costs.
Nevertheless, some of the industry’s stalwart rad-hard suppliers are bucking the trend, and are maintaining-and even in some cases beefing-up the radiation hardness of their solid-state memories.
Megarad-level radiation resistance was common for military programs such as the MILSTAR satellites and nuclear early warning systems into the 1980s, explains Allan Hurst, microelectronics sales manager at the Honeywell Inc. Defense and Space segment facility in Plymouth, Minn. In the 1990s that changed, but now it may be coming back. “The government has gone from the very hard stuff to soft stuff, and now is back to the hard stuff,” he says.
In late 1990s, there was a shift from total-dose radiation hardness of 1 megarad down to about 300 kilorads. “The thinking then was that large commercial space programs would drive electronics, and could never meet requirements of the megarad level,” Hurst says. “The reality was that large commercial space programs never took off. Now we see a shift back to establishing megarad-level technologies.”
Honeywell Defense and Space is based in Phoenix, and its Minnesota-based wafer fab, formerly known as Honeywell Solid State, now is called Honeywell Space Systems-Plymouth.
Honeywell is introducing 150-nanometer solid-state memory chips that support radiation hardening of as much as 1 megarad total-dose resistance-largely based on U.S. government requirements, Hurst points out.
For commercial satellites, however, it is a different story. Satellite makers such as Boeing and Lockheed Martin typically require 100-kilorad total-dose radiation resistance for the solid-state memories for commercial satellites. They augment those devices with shielding and software correction.
To meet the balance of anticipated market demand, Hurst says Honeywell is making 256-kilobyte, 1-megabit, and 4-megabit rad-hard static random-access memory (SRAM) chips-and are stacking their 4-megabit devices to create 16-megabit memory stacks.
Honeywell is also working on magnetoresistive random-access-memory (MRAM) chips that provide nonvolatile memory for systems expected to receive large doses of radiation or electromagnetic pulse. “The MRAM can power off to true zero power, and data will remain in the memory,” Hurst says. “It also has nondestructive readout, so in the process of reading you don’t destroy the memory. No refresh is required, and it’s strategic-hardened to 1 megarad.”
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