Silicon on Sapphire Pressure Sensing Technology
What is Silicon-on-Sapphire sensor (SOS) Technology?
SOS Technology-What is it? How and why is it used?
The combination of Silicon and sapphire provides a very durable sensor. Since 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, thereby allowing the sensor to operate over a very wide temperature range without loss of performance. It can endure high over pressures and provides outstanding corrosion resistance. The sapphire’s excellent elasticity ensures high repeatability- a highly desirable characteristic for sensors.
The technical bit…………….
Silicon-on-sapphire wafers are formed by depositing silicon onto the sapphire substrate at very high temperatures.
Natural sapphire tends to contain impurities, so very pure sapphire crystal is grown in a controlled lab environment. The formed sapphire ingots are cut at a 60º angle known as the R-Plane. This plane reveals the oxygen atoms in the crystal and, because the spacing of these atoms is almost identical to those found in a silicon crystal, the silicon can be cleanly deposited onto the surface of the sapphire wafer.
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. Its ability to operate at high temperatures, chemical inertness and virtual absence of hysteresis make the strain gauges ideal for use in pressure sensors.
Utilizing Silicon-on-Sapphire sensing technology results in excellent long-term stability of <0.2%, a major advantage over non-SOS bonded silicon sensors. The silicon Wheatstone bridge formed during the manufacturing stage is free from any residual stresses that may add to hysteresis and non-repeatability errors that might decrease long-term stability. There are no bonding agents between the sensing element and sapphire substrate that may age and cause instability.
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 identified as corresponding to the R plane of sapphire.
In the mid 1960’s, researchers worked on making SOS a technology that could be manufactured. The primary application was for radiation hard circuits; however, it soon became apparent that the other benefits of SOS could lead to commercial usage.
In 1978, a further breakthrough by the California Institute of technology and Hewlett Packard led to the development of ultra thin SOS films. Until SOS was deemed ready for commercial use in 1990, a process called SPER (Solid Phase Epitaxial Re-growth) was developed. Silicon-on-Sapphire is now one of the most promising pressure sensing technologies and is gaining broader acceptance in higher volume applications.