Pirani sensors are thermal sensor elements that come into contact with the residual gas in a vacuum system and simultaneously function as resistance heaters and thermometers. They are used wherever vacuum conditions are required: in research laboratories as well as in industrial facilities, for example in coating systems, packaging machines, heat treatment, and freeze-drying. Since Pirani sensors usually can only measure a limited range of the technical vacuum, they must typically be used in combination with one or more other sensor technologies.

The Pirani sensor element developed in NEOVAK (project ref. no.: 13FH029KX0) at the OTH Regensburg was designed to measure in the low-pressure range and has a higher overall sensitivity. It is manufactured using fabrication technologies based on MEMS (micro-electro-mechanical systems), which allow for the maximization of the active sensor surface area. The sensor structure consists of a nickel heating meander embedded in silicon nitride (SiN).

With this material combination, the project team takes advantage of several key effects: Nickel has a high temperature coefficient of electrical resistance that increases sensor sensitivity, while SiN reduces heat loss due to its low thermal conductivity. Furthermore, it is mechanically and chemically stable, enabling the sensor to be used even under demanding conditions in technical systems.

In addition, the sensor structure is undercut so that it is suspended over a void, attached only at its four corners. The suspension points feature additional, independently controlled heating elements that actively counteract heat loss (based on a patent held by the project partner). Another structure, which has not been undercut, is thermally coupled to the substrate and compensates for fluctuations in ambient temperature. During operation, the sensor membrane is maintained at a constant temperature of approximately 100°C above ambient temperature. This value represents an optimal compromise: it ensures good sensor response while simultaneously limiting radiation losses, which increase disproportionately with rising temperature.  

The combined reduction of parasitic heat fluxes – through active counter-heating on the one hand and by operating the sensor at moderate temperatures on the other hand –significantly improves sensitivity in high vacuum. At the same time, this extends the measurement range toward lower pressures. Measurements show that the sensor can cover the entire range from high vacuum to atmospheric pressure. The structures designed to compensate for temperature fluctuations further improve the reproducibility of the measurements.

Text source: OTH Regensburg