02/25/2008 9:00 AM EST)

PORTLAND, Ore. — The National Aeronautics and Space Administration thinks silicon carbide is ready to replace silicon in circuitry that must withstand ultrahot temperatures--as high as 1,000 degrees F--or deliver ultrahigh power. Prototypes of the world's first commercial SiC integrated circuit, which NASA has contracted with Inprox Technology Corp. (Boston) to jointly design and fabricate, are due out by the end of 2008.

The position sensor is being designed to measure linear motion inside NASA's turbine propulsion engines, but Inprox also plans to repurpose it for automotive engine control as well as for high-power, high-temperature industrial applications.

"For NASA, the major advantage of silicon carbide circuitry is its ability to handle the high temperatures in our advanced electronic sensing and control systems, slated for the hot sections of jet engines," said NASA electrical engineer Phil Neudeck, the team leader. The silicon carbide group has been tasked to help sense the harsh environment inside aircraft engines at NASA's Glenn Research Center in Cleveland. "We need these sensors to improve the safety and fuel efficiency of jet aircraft engines, while reducing weight and pollution."

Traditional electronics must either be remotely located or liquid-cooled, Neudeck said, "which seriously hampers their ability to achieve desired safety and performance specifications." But SiC can function in an engine environment at 500 degrees C (932 degrees F) without cooling, he said. The work is funded under NASA's Aviation Safety and Fundamental Aeronautics Programs.

Silicon carbide is a rare natural material called moissanite. Its synthesized form, carborundum, is widely used in industry as an abrasive. At the leading edge of the electronics industry, highly purified SiC wafers are being used to fabricate semiconductor devices that have the potential to transform the market for ruggedized electronics by enabling ultrahigh-power, ultrahigh-temperature components.

A wide-bandgap material, silicon carbide's electron mobility is not quite as high as silicon's (900 cm2/V-s compared with 1,500 cm2/V-s). But almost all of its high-temperature and high-power electronic properties are superior to those of silicon.

SiC's advantages have been well known for more than a decade, but building integrated circuitry for durable operation at extreme temperatures well beyond limits of silicon has been a challenge. Pioneering fabricators were plagued with defects and high costs. Slowly but surely, however, over the last 10 years, many of the major engineering hurdles have been cleared.

Now, discrete devices for nonextreme environments are being fabricated by vendors like Cree Inc. (Durham, N.C.), which offers high-power silicon carbide discrete transistors and rectifiers. However, Inprox's higher-temperature design, which will be fabricated using the NASA-developed chip technology, could prove to be the first commercial IC to take advantage of SiC's extreme-temperature attributes.

Last year, NASA reported its first proof-of-concept demonstration with an experimental SiC differential amplifier that survived more than 1,700 hours at 932 degrees F--that's a hundredfold increase in operational parameters over previous silicon carbide prototypes. More recently, that same chip surpassed 5,000 hours of operation at 932 degrees F.

"Silicon carbide is much harder and more expensive to process than silicon," said Neudeck. "Our prolonged 500 degree C demonstration chip was achieved through the successful development and integration of a number of fundamental materials and processing advancements here at NASA."

One key obstacle, he said, was development of the metal-semiconductor contacts needed to carry electrical signals in and out of SiC transistors. NASA colleague Robert Okojie overcame that problem with contacts that have survived "thousands of hours of testing at 500 degrees C," Neudeck said.

In addition, he said, the team overcame other challenges "in high-temperature packaging, insulators and integration into a single process run."

NASA will use the world's first commercial silicon carbide chip to monitor linear motion inside its jet turbine engines, but Inprox sees other uses as well.

"Our device will be the first to utilize the extreme-temperature tolerances that silicon carbide enables," said Derek Weber, president and co-founder of Inprox. "Our silicon carbide device will be a standard operational device that NASA can use, but from there we can turn it into a surface-mountable device or a microelectromechanical system."

Inprox, Weber said, "feels this is a device with commercial value not only for aerospace, but also for automotive and industrial applications."

Linear position sensors ordinarily use three coils--a master coil and two slaves. A ferrite magnetic actuator moves through these coils, making linear variable differential transformers, a five-terminal device that requires complicated analog conditioning circuitry to attain high resolution.

Inprox's linear position sensor, by contrast, uses a proprietary captive-field linear-direct (CFLD) approach, an all-digital solution yielding ultrahigh resolution that nevertheless requires only a single coil, no ferrite actuator and no analog conditioning circuitry.

"The biggest reason our approach is attractive is that we cut the number of terminals from five to two, eliminate the ferrite actuator, eliminate two of three coils, reduce the mass by as much as 90 percent and require no analog signal-condition circuitry, which saves board space that is at a real premium for aerospace applications," said Weber.

Inprox's CFLD sensors provide a continuously variable square-wave output, where linear position is directly proportional to the frequency of the square wave. Instead of attaching a complicated actuator to the object whose motion is being measured, an extremely simple actuator can be built into the moving object itself to affect the flux density of a single coil, which in turn changes the frequency of the sensor's square-wave output.

The square-wave output from Inprox sensors can be set to range from as low as 50 kHz up as high as 1 MHz, providing extremely high resolution and dynamic range compared with conventional analog sensors. The only part of a captive-field linear-direct sensor that is analog is the actuator itself--everything else is digital.

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