SOS for sensors

Pressure-sensor technology derived from the NASA space programme holds great promise in a range of applications. Jon Excell reports.

A Welsh electronics company has drawn on space technology to develop sensors capable of operating for long periods of time at extremely high temperatures. The firm said that the technology could be used to enhance the safety of planes, cars and trucks.

It has traditionally been difficult to manufacture sensors capable of working over a wide temperature range, and available technologies often lead to the need for expensive compensation methods, frequent replacements and downtime.

However, by exploiting Silicon-on-Sapphire (SOS) technology initially used in the development of high-temperature integrated circuits for the NASA space programme, Wrexham-based Ellison Sensors has developed a sensor capable of accurately measuring vibration, torque, force or pressure in extreme temperatures over long periods of time.

The technology is being trialled in the lift fan propulsion system for the Lockheed Joint Strike Fighter.

Manufacture begins with a sapphire wafer. Silicon, a highly desirable pressure-sensing element, is deposited on to the glass-like surface of the sapphire. Since the crystal structure of the silicon film is similar to that of the sapphire, the SOS structure appears to be one crystal with a strong molecular bond between the two materials.

Using photolithography techniques, the silicon is then etched into a four-arm active-resistive Wheatstone bridge. This is an electrical circuit for the precise comparison of resistances, and is widely used in sensor design.

The big advantage of an SOS sensor over conventional silicon-based sensors is that it’s free from any residual stresses caused during processing, which can add to hysteresis (reduced performance) and non-repeatability errors.In addition, the lack of bonding agents between the sensing element and the sapphire substrate leads to greater stability.

‘When you diffuse into a traditional silicon wafer you produce impurities when creating the track (the interconnects on the circuit). The track leaks, due to insulation breakdown, which makes the sensor unstable,’ explained managing director Albert Ellison. ‘With SOS, because sapphire is a perfect insulator, you don’t get this leakage, and the sensor is perfectly stable.’

The excellent elasticity of sapphire is another important factor. The ability to bend the wafer and bring it back to its original position – known as repeatability – is a highly desirable characteristic for sensors.

‘The homogenous single-crystalline structure offers a specification not previously available in pressure transducer technology,’ said Ellison.

‘The structure has a modulus of elasticity exceeding that of stainless steel with virtually no hysteresis and provides excellent long-term stability and repeatability.’ The result, said Ellison, is a sensor with multiple applications that is able to operate at up to 450 degrees C.

The company has by chance benefited from the growth in laser technology and the development of blue LEDs (LEDs with a sapphire lens in front). These two areas have created a massive interest in single-crystal sapphire, said Ellison.

He added that the most promising area for the sensors is in demanding aerospace applications, and claimed that many new programmes have shown a lot of interest. He said that while the sensor is suited to the aerospace and defence markets, there is no reason why it could not be adopted by the commercial mainstream.

The company recently constructed a purpose-built factory to process the SOS wafers and increased its workforce by a third to cope with demand.

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