SOS: gamechanger 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 grow in a controlled laboratory environment.

Sapphire ingots, once formed, are cut at a 60º angle, known as “Plan R”. 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 sensors in silicon not SOS (the silicon Wheatstone bridge formed during production is free from any residual stress that may increase hysteresis and non-hysterical errors.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 makes 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 the customers with critical applications in Aerospace, Defense, Automotive, and Oil & Gas / Subsea. The advantages, features and performance of a such industrialized sensor, as in the case of the SUCO group of which ESI is a member, are also accessible for applications that are cost-sensitive requiring robust, reliable and precise sensors.

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 in to 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 commercialized 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, has started the production of SOS sensors in 1994, optimizing processes over time and the production is still in-house.

SUCO has incorporated SOS technology into its transmitters and electronic pressure switches expanding the fields of application.

SOS is one of the most promising pressure-sensing technologies today and it is used extensively by the SUCO Group, of which ESI is a part.

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