Sensor technology

Piezo-resistive silicon sensors

The composition of a finished silicon sensor is relatively complex. No strain gauges are used here as has previously been described. Instead, the pressure-sensitive element consists of a silicon chip that is just a few millimeters in size.

The crystal structure of this chip has the same function as a metal strain gauge, but this is not obvious from looking at it. It is manufactured using doping, i.e. through the targeted introduction of foreign atoms. The semiconductor strain gauges produced in this way are up to 100 times more sensitive to deformation in comparison to metallic strain gauges. The change in resistance here is not caused by the purely geometric deformation, but insteadby the resulting change in the crystal lattice structure, and therefore the electron mobility or, respectively, the conductivity. The individual sensor chips are part of a wafer. These silicon discs are approximately 150 mm in size and contain several hundred chips.

Figure 1: Membrane with silicon chips

The individual sensor chip is fixed to a glass socket by anodic bonding. For relative pressure versions, the glass socket has a drilled hole all the way through. The ambient pressure is fed through this drilled hole to the back of the chip. The glass base is fixed to the TO header. The header guides the ambient pressure through the glass socket via a ventilation tube. Along with the ventilation tube and the filling tube, the connection pins are also located on the back. These are connected to the chip via anodic bonding wires.

Fig. 2 Bonded silicon chip on TO header

Fig. 3: Finished measuring cell

The sensor element is very sensitive to external influences and therefore does not come into direct contact with the medium to be measured. The header is welded into a stainless steel case which has a thin metal membrane welded onto its front. Bys electing different membrane materials (stainless steel, Hastelloy C, titanium, etc.), the measuring cells can meet the requirements of different media.

The inside is filled with a synthetic oil via the filling tube in the header and then sealed pressure-tight. The process pressure has an indirect effect on the measuring cell: via the membrane to the filling oil, and therefore to the sensor chip (media separation). In addition to the described version with media separation, a version without separating membrane and filling oil is available. This variant can be used for measuring inert media. In this case, the sensor chip is in direct contact with the medium and therefore offers better measuring accuracy because interfering influences, such as the filling oil and membrane, are eliminated.

Due to the measuring cell design, the variance is greatest with this sensor technology. The material for the membrane and housing is selected independently of the measuring system and the shape or size can also be adapted to
the specific application. The measuring cell can be welded directly to the process connection of the finished pressure transmitter (Fig. 3), which eliminates the need for an additional seal and ultimately only requires one type of material to be compatible with the medium.

Fig. 4: High temperature version

The filling oil transfers the pressure from the separation membrane to the sensor chip. The filling oil used most frequently is silicone oil, which has good features with respect to temperature-related expansion. Filling oils that are non-hazardous for human consumption are usually required in processes in the food industry. Apart from the oils specifically approved by the American Food and Drug Administration (FDA), medical white oils can also be used. Fluorocarbons are suitable for measuring oxygen. For chlorine measurements, the escape of silicone oils can result in an explosive reaction. Fluorocarbons are also suitable as a filling medium here.

JUMO. Pressure transmitter of the dTRANS p31 series