Summary Partial pressure gauges and residual gas analysis
Partial pressure gauges and residual gas analysis are important items in general vacuum technique and particularly in process technology. Instruments developed to measure the gas composition in vacuum systems are collectively referred to by the name residual gas analyzer, commonly abbreviated as RGA. Of the numerous RGA’s presented since the 1940’s, only the quadrupole mass spectrometer andautoresonant trap mass spectrometer have developed into versatile residual gas analyzers over the years. Within vacuum technology the 180° magnetic deflection mass spectrometer is still applied in helium leak detectors. In this chapter, the qualities, possibilities and limitations of these three mass spectrometers are discussed. Special attention is paid to their quality-determining parameter mass resolution, i.e. how distinct successive masses can be separately observed in the mass spectrum. Expressions for the theoretical resolution and its impact on the measured mass spectrum are derived. Application of electron multipliers greatly increases the sensitivity of an RGA. Different types and their specific qualities are adressed. Finally, paragraphs are presented about the interpretation of residual gas spectra, how to perform spectrum analysis and some illustrative examples of mass spectra in vacuum systems.
Some figures in Partial pressure gauges and residual gas analysis
Gas-tight version of an axial ion source. 1 – cathode, 2 – repeller with focusing effect on the electrons, 3 – entrance for electrons in the ionization chamber, 4 – ionization chamber (anode), 5 – metal capillary (connection to gas admission system), 6 – insulating connection between capillary and ionization chamber (source: Pfeiffer Vacuum)
Open ion source with cage-like anode, built according to the principle of the extractor gauge
Scheme of a closed ion source as a connection between a process chamber and a quadrupole mass spectrometer (QMS). 1 – process chamber, 2 – entrance aperture from process chamber to ion source, 3 – gas line to ionization chamber, 4 – closed ion source, 5 – ionization chamber, 6 – ion collimation and focusing to analyzer section, 7 – electrical leads to ion source, 8 – rod system, 9 – detector, 10 – QMS flange, 11 – to QMS pump. Typical pressure levels in the different compartments: process chamber 1 Pa, ionization chamber 10-1 Pa, analyzer section and detector 10-4 Pa.
Schematic set-up of a cross beam ion source.
1 – cathode (2x incl. spare), 2 – repeller with focussing effect on the electron beam, 3 – anode, 4 – ionization chamber, 5 – hole for the analysis of a gas/vapour beam (perpendicular to the plane of drawing), 6 – extraction aperture, 7 – focussing lens, 8 – ion beam to the analyzer section (source: Pfeiffer Vacuum)
Sketch of a quadrupole mass spectrometer. 1 – ion source, 2 – analyzer section, 3 – detector, 4 – cathode, 5 – ionization chamber, 6 – focusing system (source: Pfeiffer Vacuum)
Commercially available ARTMS, provided with an “off-axis” ion source and an electron multiplier (source: Granville Phillips)
Schematic diagram of a conventional electron multiplier (source: Pfeiffer Vacuum)
Schematic diagram of a channeltron
Example of an ‘off-axis’ position of the electron multiplier in a QMS. Channeltron at 90°. 1 – incident ions, 2 – secondary electrons on their way to channeltron, 3 – conversion electrode, 4 – Faraday cup, 5 – entrance channeltron at a negative high voltage (about-3 kV), 6 – anode, 7 – measuring signal (source: Pfeiffer Vacuum)
Typical residual gas spectrum of a turbomolecular pump showing a high hydrogen peak (M/q = M = 2)
Contents Partial pressure gauges and residual gas analysis
6.1 Introduction 452
6.2 The ion source 454
6.3 The mass analyzer section; resolution 460
6.4 180° magnetic deflection mass spectrometer 465
6.5 Quadrupole mass spectrometer 472
6.6 Autoresonant trap mass spectrometer 480
6.7 The ion collector; electron multipliers 486
6.8 Interpretation of residual gas spectra 490
6.9 Spectrum analysis 494
6.10 Spectra or vacuum systems 498 exercises
