Silicon carbide ready for prime time

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.