SOS: the game-changer technology

SOS: the game-changer technology

What is the silicon on sapphire (SOS)?

Silicon-on-sapphire (SOS) wafers are formed by depositing silicon on the sapphire substrate at very high temperatures.

Natural sapphire tends to contain impurities, so to obtain a pure crystal it has to be grown in a controlled laboratory environment.

The Sapphire ingots once formed, are cut in to wafers at a 60º angle, known as the “R Plane”. This reveals the oxygen atoms present in the crystal and since the spacing of these atoms is almost identical to that of a silicon crystal, the silicon itself can be deposited with extreme precision on the surface of the sapphire wafer.

Doped silicon strain gauges are derived from the silicon layer, and the individual
strain gauges are electrically isolated from each other thanks to the outstanding
insulating characteristics of the sapphire substrate.

What are the general advantages of using SOS pressure transducers and transmitters?

The advantages of this exclusive technology are many and make it the best choice for pressure sensors:

  • thanks to its outstanding insulating properties, sapphire protects the strain gauge from electromagnetic fields
  • it can withstand high overpressures and provides excellent resistance to corrosion
  • excellent elasticity of sapphire ensures high repeatability
  • ability to operate at elevated temperatures without loss of performance
  • chemical inertness
  • virtually no hysteresis
  • excellent long-term stability (<0.2%), even superior to that of standard silicon sensors (the silicon Wheatstone bridge formed during production is
    free from any residual stress that may increase hysteresis and non-hysterical errors such as repeatability, which reduces long-term stability)
  • there are no binding agents between the sensing element and the sapphire substrate that could age and cause instability

Furthermore, the SOS sensor is installed on a titanium diaphragm instead of the traditional stainless steel, to preserve its excellent properties. Specific titanium alloys make the sensor suitable for use with hydrogen!
With the advanced features of SOS technology and titanium construction, it is not
surprising that this is the pressure sensor chosen by customers with critical applications in Aerospace, Defence, Automotive, and Oil & Gas / Subsea. The advantages, features and performance of a such an industrialised sensor are also accessible for applications that are cost-sensitive requiring robust, reliable and
precise pressure measurement.

The advantages of the ESI differential pressure transducers

The use of SOS technology in differential applications offers a reliable, long-lasting solution with unbeatable performance and value. SOS differential pressure transmitters from ESI are bidirectional and extremely robust, allowing the monitoring of applications with high line overpressures and transients that are critical or unmanageable by traditional sensor technologies.

The robust build of the pressure transmitter does not affect its excellent precision, accuracy and stability over time.

A brief history of SOS technology

A very practical experiment in 1963 at North American Aviation (now Boeing) led to the discovery of SOS. A sapphire crystal was polished into a spherical shape and immersed in a gas containing silicon. A spherical surface will expose all of the planes that exist in a crystal system. It was found that silicon grew in certain sites on the sphere, and these were identified as corresponding to the R plane of sapphire.

In the mid 1960’s researchers worked on turning S.O.S into a manufacturable technology. The primary application was for radiation hard circuits, but it soon became apparent that the other benefits of S.O.S could lead to commercial usage.

A further breakthrough was the development of ultra-thin S.O.S films by the California Institute of technology along with Hewlett Packard in 1978. A process called SPER (Solid Phase Epitaxial Re-growth) was developed until the stage where S.O.S could be commercialised for the first time in 1990.

Silicon-on-Sapphire is now the superior pressure sensing technology and is gaining broader acceptance in higher volume applications.
ESI, founded in 1984 and joined the SUCO Group in 2009, started the production of SOS sensors in 1994, optimising processes over time and the production is still in-house.
SOS is one of the most promising pressure-sensing technologies today and is
used extensively by the SUCO Group, of which ESI is a part.

Back to top of page