What is gas analysis and why is it needed?
Analysis of gas at low pressures using mass spectrometry
Analyses of gases at low pressures are useful not only when analyzing the residual gases from a vacuum pump, leak testing at a flange connection or for supply lines (compressed air, water) in a vacuum. They are also essential in the broader fields of vacuum technology applications and processes. For example, in the analysis of process gases used in applying thin layers of coatings to substrates. The equipment used for qualitative and/or quantitative analyses of gases includes specially developed mass spectrometers with extremely small dimensions which, like any other vacuum gauge, can be connected directly to the vacuum system. Their size distinguishes these measurement instruments from other mass spectrometers such as those used for the chemical analyses of gases. The latter devices are poorly suited, for example, for use as partial pressure measurement units since they are too large, require a long connector line to the vacuum chamber and cannot be baked out with the vacuum chamber itself. The investment for an analytical mass spectrometer would be unjustifiably great since, for example, the requirements as to resolution are far less stringent for partial pressure measurements. Partial pressure is understood to be that pressure exerted by a certain type of gas within a mix of gases. The total of the partial pressures for all the types of gas gives the total pressure. The distinction among the various types of gases is essentially on the basis of their molar masses. The primary purpose of analysis is therefore to register qualitatively the proportions of gas within a system as regards the molar masses and determine quantitatively the amount of the individual types of gases associated with the various atomic numbers.
Partial pressure measurement devices which are in common use comprise the measurement system proper (the sensor) and the control device required for its operation. The sensor contains the ion source, the separation system and the ion trap. The separation of ions differing in masses and charges is often effected by utilizing phenomena which cause the ions to resonate in electrical and magnetic fields.
A historical review of mass spectrometers
Following Thomson’s first attempt in 1897 to determine the ratio of charge to mass e/m for the electron, it was quite some time (into the 1950s) before a large number and variety of analysis systems came into use in vacuum technology. These included the Omegatron, the Topatron and ultimately the quadrupole mass spectrometer proposed by Paul and Steinwedel in 1958 (see Fig. 4.1). The first uses of mass spectrometry in vacuum-assisted process technology applications presumably date back to Backus’ work in the years 1943 / 44. He carried out studies at the Radiographic Laboratories at the University of California. Seeking to separate uranium isotopes, he used a 180° sector field spectrometer after Dempster (1918), which he referred to as a “vacuum analyzer”. Even today a similar term, namely the “residual gas analyzer” (RGA), is frequently used in the U.S.A. and the U.K. instead of “mass spectrometer”. Today’s applications in process monitoring are found above all in the production of semiconductor components.
Initially, the control units were quite clumsy and offered uncountable manipulation options. It was often the case that only physicists were able to handle and use them. With the introduction of PCs the requirements in regard to the control units became ever greater. At first, they were fitted with interfaces for linkage to the computer. Attempts were made later to equip a PC with an additional measurement circuit board for sensor operation. Today’s sensors are in fact transmitters equipped with an electrical power supply unit attached direct at the atmosphere side; communication with a PC from that point is via standard computer ports. Operating convenience is achieved by the software which runs on the PC.