Backgrounder

Radiation Hardening of Microelectronics Microchips -- the kind used in everything from personal computers to automobile electronics to breadmaking machines -- work fine in their intended environments. But if you try to operate one of these generic microchips in the presence of ionizing radiation, the kind that satellites encounter in outer space, for example, it will no longer function correctly. The ionizing radiation penetrates the protective packaging around the microchip and strikes the transistors in the circuit. This can change the chip's electrical parameters, adding extra electrical currents that shouldn't be there and altering the circuit's operations.

There are ways to counter the effects of ionizing radiation. Circuit engineers can make the chips themselves more resistant to the radiation that penetrates the protective packaging. They do this by changing the designs of the chips and by altering the ways that the chips are manufactured. This extra protection, called radiation hardening, has been called both a science and an art. Many microelectronics experts worldwide have long considered this expertise to be synonymous with the US Department of Energy's Sandia National Laboratories.

More difficult to design, trickier to manufacture

Radiation-resistant chips are more difficult to design and harder to manufacture than "normal" chips. Some of the -- often proprietary -- design and manufacturing tricks that speed up off-the-shelf chip operation cause extra performance vulnerabilities in the presence of ionizing radiation. Similarly, some of the shortcuts that allow chip makers to turn out more chips at lower prices introduce chemical weaknesses in the atomic structure of the transistors that are attacked by ionizing radiation. Thus, standard chip-making methods cannot be used to make chips that must survive ionizing radiation. For these reasons, special knowledge must be applied to design and manufacture chips that must survive ionizing radiation.

Sandia engineers generally use two complementary approaches in radiation- hardening microchips. First, they design radiation-hard characteristics into the chip. Then, at the processing phase, they use special techniques that mitigate radiation effects. Sandia radiation-hardening experts over the years have had to learn the device physics of the transistors and other circuit elements, and how they are affected at the atomic scale by various kinds of radiation. They've also learned, through long experience, how fabrication processes -- patterning, oxide deposition, ion implantation, and such -- affect device sensitivity to different radiation sources.

Thinner layers, lower temperatures

In general, here is how chips are made more resistant to radiation: Very thin layers of oxide are used to form the working and insulating parts of the devices and regions between the transistors. These layers are made as thin as possible without compromising reliability. Reducing these thicknesses decreases the sensitivity of transistors to ionizing radiation because it limits the amount of unwanted charge that can accumulate in those layers to change the operating characteristics of the underlying devices.

There's a catch, though, a price to be paid. Extremely thin oxide layers increase the importance of precise processing. Any imperfection in one of those layers could ruin the chip by causing an electrical short. After the gate oxide layer is grown onto the substrate, the rad-hard chip makers must ensure that all additional processing steps use temperatures lower than the temperature of the gate oxide growth steps. That's in the neighborhood of 900 to 1000 degrees C, depending on the type of chip being made, versus the 1100 degrees C that's available to commercial processes. Using higher temperatures would alter the gate oxide's atomic structure, compromising radiation resistance. The restriction against higher processing temperatures increases the difficulty of many of the later fabrication steps.

In addition to changing the way the transistors in the circuit are manufactured, other tricks change the way that the transistors are combined to form working circuits. As one example, the interconnections -- the tiny wires etched onto the chip -- are made larger than would be common for commercial chips. These wider wires can handle radiation-induced currents that are much larger than would be created during normal operation in the absence of radiation. Many years of scientific analysis and trial and error have generated many such guidelines for reducing the radiation vulnerabilities of modern electronics.

Though chip fabrication techniques can be used to increase chip radiation immunity, significant design changes are even more important in making the integrated circuits inherently radiation hard. The challenge is to design rad-hard chips that will continue to perform reliably and efficiently in spite of ionizing radiation exposure.

A unique relationship: Intel and Sandia National Labs

Intel is making the design of the Pentium, the world's most popular microprocessor, available for US space and defense systems in a unique partnership with DOE's Sandia National Laboratories. Among the national laboratories, only Sandia has both the design and the microelectronics fabrication infrastructure to attempt a project as complex as redesigning and remanufacturing a Pentium-class chip with radiation-hardened characteristics.

Intel trusts Sandia with the key design aspects of its highest-value commercial product, in part because Sandia has shown that it can protect Intel's intellectual property during previous microprocessor design transfer partnerships.

Sandia combines a detailed understanding of the art and science of manufacturing radiation-hardened chips with a working knowledge of the modern microelectronics industry obtained through numerous partnerships with leading-edge US microelectronics companies.

Intel has the highest confidence that Sandia can protect Intel's design -- the company has an investment of more than $2 billion in the Pentium design -- and develop a radiation-hardened version for manufacturing by third-party vendors. These vendors will get a manufacturing recipe that enables them to build the Pentium but not reverse-engineer the key design tricks.

Third design transfer in 18 years

The transfer of the Pentium design represents the third chip design transferred from Intel to Sandia over the last 18 years, and is part of a long-term cooperative relationship. The specifics of this transfer have been under discussion for almost two years.

Prototypes of the custom chips will be fabricated and tested in small volumes at Sandia's Albuquerque, N.M., Microelectronics Development Laboratory. For production of larger quantities, Sandia will actively seek, through an announcement in the federal Commerce Business Daily, the participation of specialty commercial suppliers that traditionally serve the rad-hard integrated circuit needs of defense or space-related markets. Suppliers who choose to participate in manufacturing the rad-hard Pentium for government use will receive masks and manufacturing instructions at no cost.

Manufacturing will occur only in the United States, and the products will be subject to strict US export controls.

Sandia and Intel have worked together on a number of mutually beneficial projects over a period of years. These include large projects like the one announced on Dec. 8 -- radiation hardening of the Pentium chip -- and massively parallel computer development, as well as about 60 smaller projects over the years. Of special interest have been 20 "rapid response" projects in which difficult emergency situations demanded immediate attention. Intel has shared improvements that have resulted from work in contamination control, environmental safety and health techniques with the other companies in the semiconductor industry without charge. Likewise, Intel is licensing the transfer of the Pentium design to Sandia for space and military applications for no fee.

see above for links and previous Spacer.Com radiation reports

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