Electrons on a straight path

Vacuum gauges or partial pressure gauges are used for process control in many areas of industry, especially in the semi-conductor industry. Ionization vacuum gauges are used for on-site calibration and for monitoring in the high and ultra-high vacuum ranges. The principle of these devices consists in ionizing gas molecules by means of fast electrons and then measuring the resulting ion current. For traceability, conventional ionization vacuum gauges must be individually calibrated not only for nitrogen, but also for the gas used in each specific case, since sensitivity to a particular type of gas depends on the individual device used.

To simplify the calibration process, a novel ionization vacuum gauge was developed within the scope of the EMPIR project titled “Ion Gauge”. In contrast to conventional vacuum meters with undefined electron trajectories, electrons are, in this case, led on a straight path through the ionization space from the cathode into a Faraday detector. In addition, the ion collector is used as an electrostatic lens to focus the electrons on the exit orifice of the anode. The electrons are led through a deflector onto the Faraday detector in such a way that the X-rays emitted when the electrons hit the detector cannot reach the ion collector, where they would distort the measurement result.

Due to the ionization vacuum gauge’s construction principle, both the length of the electron trajectory and the area from which ions can get to the collector are well defined and do not depend on the type of device used. Thus, sensitivity to a given type of gas can be determined either by means of a known ionization probability or by a merely type-specific – but not instrument-specific – calibration for all vacuum meters of the given type, which saves a considerable amount of effort and money.

In addition, previously obtained experimental results have shown that the new device type has a reproducibility and a transport instability of better than 1 %, which had so far been unprecedented in the case of ionization vacuum gauges. This device can therefore be used both as a reference measuring instrument for calibration laboratories and as a transfer standard for comparisons. For its broad-scale application, it is planned that this novel type of ionization vacuum gauge will be used as a standard by ISO.