The combination of Silicon and sapphire provides a very durable sensor. Because the silicon is grown onto the surface of the sapphire it is incredibly stable and exhibits virtually no hysteresis. With its outstanding insulation properties, the sapphire protects the strain gauge from electromagnetic pulse radiation, and allows the sensor to operate over a very wide temperature range without loss of performance.

Why is Silicon on Sapphire Sensor Technology so good?

It can endure high over pressures and provides superb corrosion resistance. The excellent elasticity of the sapphire ensures high repeatability which is a highly desirable characteristic for sensors.

Technical Information

Silicon-on-sapphire wafers are formed by depositing silicon onto the sapphire substrate at a high temperature. Natural sapphire tends to contain certain impurities so a very pure crystal is grown in a controlled lab environment. Download the SOS brochure.

The sapphire ingots are then cut at 60º. This is known as the R-plane. This plane reveals the oxygen atoms in the crystal, and because they are very similar to the spacing of atoms in a silicon crystal, the silicon can be deposited cleanly onto the surface of the sapphire wafer. This is known as epitaxial growth.

Doped silicon strain gauges are etched from the layer of silicon and individual strain gauges are electrically isolated from each other by the outstanding insulating characteristics of the sapphire substrate.

History of Silicon on Sapphire Sensor Technology

SOS was discovered in 1963 at North American Aviation (now Boeing) during an experiment. As sapphire crystal was being polished in to sphere, and researchers discovered that when placed in a chamber of gas containing silicon, the silicon could be seen to grow on the surface of the sapphire sphere.

In the mid 1960’s researchers worked on making SOS a manufacturable technology. The primary application was for radiation hard circuits, but it soon became apparent that the speed and low power benefits of SOS could lead to commercial usage.

The main drawback of SOS in its early stages was the high number of defects in the film. The defects caused leakage current and reduced carrier mobility. It had been known for some time that if the silicon film is grown thicker, the quality of the film improves and mobility increases. SOS was successfully used with thicker films, but in order to achieve really high performance, ultra thin film is required.

The breakthrough that enabled ultra thin SOS films was developed by the California Institute of technology and researchers at Hewlett Packard in 1978. A process called SPER (Solid Phase Epitaxial Regrowth) was developed until the stage where SOS could be commercialised for the first time in 1990.  SOS is now one of the most promising pressure sensing technologies and is gaining broader acceptance in high volume commercial applications.

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