What is a residual gas analyser?
A residual gas analyser (RGA) is a small and usually rugged quadrupole mass spectrometer, typically designed for environment analysis, process control and contamination monitoring in vacuum systems. RGAs can monitor the quality of the vacuum by detecting (and measuring) minute traces of impurities in a low-pressure gaseous environment. RGAs can also be used as sensitive in-situ leak detectors, usually using helium.
RGAs are usually mounted directly onto, and into, the vacuum chamber. RGAs perform various functions which would be difficult to obtain by other methods, including: the analysis of various gas-phase reactions; monitoring changes that occur in any gas environment; detecting vacuum leaks; and, checking mass flow controllers.
How does a residual gas analyser work?
The RGA effectively determines the chemical components of the gas within a vacuum, by ionising the various gases present to create ions before determining their mass-to-charge ratio.
A typical RGA contains several significant components: the sensor, the interface box and the controller.
In the sensor the RGA produces a beam of ions (via gas collisions with electrons emitted from a hot cathode) and filters them according to their mass-to-charge ratio, m/z. An output current is created which is a measure of the abundance of the ions with a given m/z which pass through the filter. The current is measured at the detector either by a Faraday cup or a Secondary Electron Multiplier.
The signals which are processed by the interface electronics and then the control unit, produce a trace of m/z ratio on the x-axis and ion current on the y-axis.
A feature of the RGA trace is that the data requires interpretation and spectra databases are a useful resource in order to identify the ‘fingerprint’ trace associated with gases/molecules.
The limitations of residual gas analysers
Whilst RGAs measure residual gas levels without affecting the gas composition of their vacuum environment, there are two significant limitations: firstly, outgassing of water from the chamber, outgassing of H2 from the electrodes and outgassing of most varieties of the 300-series stainless steel employed (due to temperatures greater than 1300 ͦC) of the hot-cathode; and secondly, electron stimulated desorption (ESD) which is characterised by peaks observed at 12, 16, 19 and 35 amu rather than by the electron-impact ionisation of gas species. This is frequently countered by gold-plating the ioniser, however, this reduces the adsorption of many gases; alternatively, a platinum-clad molybdenum ioniser can be employed.
Applications of residual gas analysers
RGAs are employed in vacuum where residual gas species need to be identified and where process conditions need to be monitored or controlled. For example, the concentration of water vapour in semiconductor processes, thin-film production and freeze-drying, are important, and needs to be measured and monitored. RGAs play an important part in numerous other fabricating processes, such as in coating processes, the manufacture of displays, vacuum furnaces, freeze-drying and basic R&D.
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