SOS process provides robust pressure transducers

A novel high-pressure sensor is capable of accurately measuring vibration, torque, force or pressure in extreme temperatures and adverse conditions over a long duration.

By exploiting the silicon-on-sapphire (SOS) technology previously used in the development of ICs for the NASA space programme, Ellison Sensors, the pressure transducer division of ESI Technology based in Wrexham, has developed a high-pressure sensor capable of accurately measuring vibration, torque, force or pressure in extreme temperatures and adverse conditions over a long duration.

"In the past it was very difficult to manufacture sensors which could be used over a wide temperature range.

This resulted in expensive compensation methods, frequent replacements and a lot of downtime for maintenance work", said Managing Director, Albert Ellison.

To enhance the stability of the sensors ESI explored using previously neglected SOS technology -well recognised for its good insulation properties and ability to operate under high temperatures.

"Although SOS technology is quite mature in the UK, with wafers manufactured in the Midlands for the past 30 years, this technology has not been successfully adapted for use in sensor technology".

"By growing single crystal silicon epitaxially onto a sapphire wafer and then selectively etching it to produce a four arm active resistive Wheatsone bridge, we have created a product which is highly stable and eliminates the need for costly maintenance or compensation".

To create the pure single crystalline structure, silicon - which is a highly desirable pressure sensing element - is grown onto the sapphire wafer so that the growth becomes an atomic extension of the sapphire itself.

The sapphire - one of the hardest known minerals to man - is chemically inert, has excellent thermal and electrical properties and acts as a perfect insulator for the isolated silicon strain gauges.

According to Ellison, the advantages are endless: "The homogenous single-crystalline structure of SOS offers a specification not previously available in pressure transducer technology.

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: the creation of a very stable sensor with multiple applications and the ability to operate at high temperatures previously unattainable".

"SOS pressure transducers operate at temperatures of up to 450C with optimal efficiency achieved at 350C.

The sapphire is five times harder than tungsten carbide and is highly abrasion resistant, making it ideal for use as a hot metal pressure transducer for plastic extrusion".

Current applications include testing of jet engines, military vehicles and aircraft, and oil and gas exploration.

"As the demand for new technology increases, so too does the demand for higher stability, better memory and no drift of electric signals.

By replacing the SOI technology with SOS we're able to create a sensor which is super stable, reliable and reduces drift by up to 95%".

"While niche markets like aerospace and defence, which form the core of our business currently, will no longer be subject to inaccurate measurements or expensive maintenance and instability compensation, we believe this technology needs to become consumer friendly by adapting it for the commercial mainstream", Ellison added.

This was endorsed by Dr Chris Young, Chief Executive of the Welsh Electronics Forum (WEF) who said: "ESI is at the forefront of SOS technology in the UK and USA and this development proves that small Welsh companies can develop leading-edge products and applications, which in turn have a vital part to play in moving the entire industry higher up the value chain".

"While adapting military applications for the mainstream consumer markets might seem daunting at first, it still is far easier than developing the technology from scratch in the first place".

"Just as the microchip transformed the global economy over the past few decades, so too do we believe this type of technology spearheaded by a small manufacturer will lead to the development of new products and applications which meet growing consumer demand".

Ellison added: "Over the past five years our sensors have undergone rigorous testing both by ourselves and our consumers and we feel confident that we are now in a position to supply the sensors - suitable for 99% of industrial applications - to other market players".

ESI recently constructed a purpose built factory in Wrexham to process the SOS wafers and increased its workforce by a third to cope with increasing demand.

"The manufacture of the sensors which proved costly at first, has been made more cost-effective by wider application and with our competitive pricing we expect to increase our turnover significantly in the future", said Ellison.

Back to top