Product category:
Pressure measurement systems
News Release from: ESI Technology | Subject: TN5500
Edited by the Processingtalk Editorial
Team on 02 January 2008
Silicon-on-sapphire pressure transducer
technology
Resistive pressure measurement has taken the lead as the most popular sensing technique in the manufacture of pressure sensors: this article explains the development of silicon-on-sapphire technology
The applied pressure causes the deformation of a diaphragm This deformation is translated into a proportional electrical signal by means of a strain gauge attached to the diaphragm
This article was originally published on Processingtalk on 4 Feb 2008 at 8.00am (UK)
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Initially, the strain gauge was cemented to the diaphragm.
This three component system (strain gauge-cement-diaphragm) suffers from creep and hysteresis, a phenomenon usually caused by the cement and poor signal to noise ratio.
The demand in industry was for more stable and repeatable sensors: this opened the door for the introduction of solid-state sensing.
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Solid state silicon piezo-resistive pressure sensors offered some improvement over the bonded type of sensor with regard to sensitivity and repeatability, although it had limited ability to operate at varying temperatures without expensive temperature compensation, and could be only used on dry air.
Expensive oil filled barrier diaphragm systems are necessary to use the silicon sensor on liquids.
The techniques for the production of solid-state sensors are the same as those used for integrated circuits.
One hundred or so sensor chips are produced at one time on a 3inch round silicon wafer using diffusion techniques, these chips are then turned into pressure sensors by micro-erosion and electrochemical etching processes.
Eventually the wafer is diced into the final 4mm square sensors by a diamond saw.
This process proved to be the most cost effective in the manufacture of pressure sensors.
The poor temperature characteristics and mechanical properties of silicon with regard to exposure to liquids needed to be overcome .
The on-going industrial demand for more stable and repeatable sensors was ever present.
Hence the development of Silicon-on-sapphire sensors.
Silicon-on-sapphire technology is well established in the manufacture of specialised microcircuits for demanding high temperature operation.
This ability to operate at high temperatures, in addition to having the outstanding mechanical properties of virtually no hysteresis and chemical inertness, make it ideal for use as a pressure sensor.
Single crystal silicon is epitaxially grown onto a sapphire wafer.
This silicon layer is selectively etched to produce a four arm active resistive Wheatsone bridge.
S 0 S Technology.
In closely controlled growth conditions single crystal sapphire can be grown in production quantities.
Sapphire is a single crystal aluminium oxide, the hardest of the oxide crystals, and is chemically inert.
It has good thermal properties, excellent electrical properties, optical and dielectric characteristics, is inherently radiation hardened, and maintains its strength at elevated temperatures.
Silicon proven to be one of the most desirable pressure sensing elements, is epitaxially grown onto the sapphire wafer in a manner that the growth is an atomic extension of the sapphire itself.
The resultant layer is a pure single crystalline structure.
The doped silicon strain gauges are etched from this layer, the individual strain gauges are electrically isolated from one another by the outstanding insulating characteristics of the sapphire substrate.
This eliminates calibration shifts at high temperatures due to the variations in diode junction leakage; typically found in diffused silicon sensors, this PN diode junction for electrical isolation limits the operation of diffused silicon sensors to below 150C.
SOS Hot Melt Transducer.
Silicon-on-Sapphire pressure transducers are available in pressure ranges from 0-500mbar to 0-1500bar, temperature coefficients for zero of +/-0.001%FSO/degC are easily obtainable, nonlinearity/ hysteresis are better than +/-0.1%FSO and long term stability of better than +/-0.01%/year are quite typical.
SOS High Accuracy Pressure Transducer.
The outstanding properties of SOS pressure transducers offer a more repeatable and stable sensor compared to the diffused silicon pressure sensor, and will find a growing number of applications in industrial, process and aerospace applications.
Specification.
The homogeneous single-crystalline structure of silicon on sapphire offers a specification not generally available in prior pressure transducer technology.
The structure has a modulus of elasticity exceeding that of stainless steel.
Unlike stainless steel diaphragms, however, silicon-on-sapphire structures have virtually no hysteresis and provide excellent long-term stability and repeatability.
And because sapphire electrically isolates the silicon from the pressure medium a PN junction is not required, thus lending the transducer to high temperature operation not previously attainable.
Because of its exceptional properties SOS pressure transducers can be made in many forms.
SOS pressure transducers operate well at 450C, the sapphire is five times harder than tungsten carbide with outstanding abrasion resistance, making it ideal for use as a Hot Melt Pressure Transducer for plastic extrusions.
The sensing diaphragm is located at the flush end of the transducer, in direct contact with the hot plastic, because of its hardness it can be used directly on glass filled polycarbonate, perhaps the most abrasive material in that industry.
This construction eliminates the need for isolating fluid filled systems, particularly mercury, the most common fill in the plastics industry.
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