Many industrial processes require exact monitoring of pressure. Fraunhofer IZM's SOI high-pressure sensor makes this exact monitoring possible at temperatures of up to 400 °C. The SOI sensor will mostly be used in extrusion systems for processing plastics. These systems must determine when a mould is completely full of plastic. The SOI sensor measures the pressure and sends a signal as soon as the plastic causes back pressure.
The sensor is encased in a barrier layer of silicon dioxide to ensure complete electrical insulation. The SOL (silicon-over-layer) covering the barrier layer has standalone piezo resistors etched into its silicon membrane. The SOI sensor works without adding liquids such as oil, which is often used in conventional sensors. The advantage here is that the sensor signals will not be distorted by the liquids expanding due to the temperature. By avoiding expensive and complicated filler technologies, the SOI sensor also prevents environmental pollution. The sensor’s precise measuring capability should also allow time and material to be saved in the injection moulding process, making it more efficient compared to conventional sensors. The key benefit of using the sensor, however, is its ability to withstand the high temperatures and rough conditions present when liquid plastics are used.
To prevent environmental effects, the SOI chip is housed in an adhesive-free ceramic body attached to a steel membrane, which, in turn, is connected to a steel cylinder. The sensor is fitted perfectly and can therefore be also described as practically “floating” in the casing between the electrical contacts, avoiding the need for other filler technologies. The SOI chip is connected electrically to the ceramic case with wire bonding. In future, high-pressure sensors are expected to be able to handle temperatures up to 600 °C and more. This will require a replacement for silicon since it becomes self-conducting at temperatures exceeding 400 °C. According to Fraunhofer IZM, a solution for this is silicon carbide, which has better electrical properties at high temperatures and is already the subject of research.