Sensors and Sensor Systems

High-pressure sensors for extreme temperatures

In cooperation with the Technical University of Berlin and the University of Applied Sciences Berlin, Fraunhofer IZM has developed a high-pressure sensor that can be used at significantly higher temperatures than conventional MEMS sensors.

© Fraunhofer IZM
High-temperature sensor for extrusion facilities: SOI chips (L) and housing (R).
© MEV Verlag
Both humans and technology are often subjected to heavy loads in industrial plants. The SOI highpressure sensor from Fraunhofer IZM can withstand considerably higher temperatures than conventional sensors.

High-accuracy pressure control is a key feature in many industrial processes (e.g. plastic, ceramic, chemical, aerospace, or pharmaceutical industry). However, these processes often require measurements at elevated temperatures (> 150 °C); standard silicon pressure sensors cannot withstand at direct exposure. Measuring via coupling media restricts dynamics and precision as measured results are extrapolated subsequently. The usage of mercury or oil as a coupling medium (not allowed in the food industry and the EU) and the fact that the steel membrane for media separation is prone to rupture are additional drawbacks. For the first time, it has now been possible to develop a high-pressure sensor for use at up to 400 °C. The sensor has already proved itself in extrusion facilities for processing plastics, where it uses pressure and temperature measurement to improve quality of the extrusion process (such as product quality, throughput, protection, and safety).

A sensor with an innovative structure

The innovative pressure sensor chips fabricated from SOI substrates (Silicon-on-Insulator) featuring stand-alone, isolated piezoresistors forming a Wheatstone bridge enable the best electrical isolation. To prevent environmental effects, the SOI chip is housed in a glueless ceramic body, attached to a steel membrane that is connected to a steel cylinder. The sensor is fitted neatly in a so-called “floating” design: it floats in the casing between the electrical contacts, which avoids any need for additional filler. The SOI chip is connected to the ceramic housing by wire bonding.

Advantages when compared to conventional pressure sensors

The SOI sensor can be used at temperatures of up to 400 °C. Conventional MEMS sensors can only handle 125 °C. Furthermore, the SOI technology does not require the use of additional fluids such as oil or mercury. This means that measurement results can no longer be falsified. The sensor also represents an alternative for the future, as e.g. oil and mercury can be expected to be banned from many products. Time and material can also be saved within the injection molding process thanks to the precise measuring method used by the SOI sensor – this method makes it considerably more efficient when compared to classic sensors.

Development of high-pressure sensors is not yet complete

In the future, sensors are planned that will withstand temperatures of 600 °C and more. This requires a replacement for the silicon, as it becomes self-conducting at more than 400 °C, which would make it incompatible with pressure measurements. Silicon carbide (SiC), which retains much better electrical properties even at high temperatures, is already being analyzed as a potential replacement. The research on SiC-based sensors has already begun.

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