Active Heat-Loss Compensated Miniaturized Pirani Sensor Chip

Due to the limited measuring range of commercial Pirani sensors in lower pressure regimes, attempts are being made to extend the measuring range of the sensors. Our approach to extend the measuring range towards lower pressures is a MEMS Pirani sensor with active heat compensation on the suspensions. The sensor element has the shape of a micro hotplate and consists of a nickel heating meander embedded in two silicon nitride layers. The whole structure is built on a silicon wafer. The sensor element is suspended on all four corners to the substrate. On each of the suspensions, there is an additional heating structure which minimizes the heat flux from the hot sensor element towards the substrate. The sensor is driven on a constant temperature through a self-balancing Wheatstone bridge. A constant voltage is applied to the heating structures on the suspensions. It has been demonstrated that with these additional heaters, the sensitivity of the sensor can be increased in the high vacuum regime. This enables reliable vacuum measurements down to 10-6 mbar.