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Choosing a Quadrupole Gas Analyzer
Application Note #9
Modern-day contamination control requirements for gas phase Residual Gas Analyzers
processes are constantly pushing the limits of performance of
quadrupole gas analyzers. The quadrupole technology is The prototypical residual gas analyzer (RGA) has an open ion
rapidly evolving and adapting to lower contamination level source (OIS) and is mounted directly on a vacuum chamber so
specifications. A good understanding of the various factors that the entire sensor is at the same pressure as the rest of the
affecting the detection capabilities of the different gas analysis vacuum system. Small physical dimensions make it possible
systems currently available is an essential tool when selecting to attach an RGA to virtually any vacuum system, including
a sensor for a specific application. As is usually the case, most both research and process setups. The maximum operating
choices involve compromises, and a good understanding of pressure is 10-4 Torr. Minimum detectable partial pressures
the basic tradeoffs associated with different detector (typically measured for N2 at 28 amu) are as low as 10-14 Torr
configurations will minimize mistakes and maximize for units equipped with an electron multiplier.
productivity.
In high vacuum applications such as research chambers,
All gas phase processing setups can benefit from the addition surface science setups, accelerators, aerospace chambers,
of a quadrupole gas analyzer. The information delivered by a scanning microscopes, outgassing chambers, etc., RGAs are
well-matched detector rapidly becomes an integral part of the effectively used to monitor the quality of the vacuum, and
process, dramatically reducing the amount of guesswork that they can easily detect even the most minute impurities in the
has traditionally been part of most vacuum troubleshooting low pressure gas environment. Trace impurities can be
procedures. As quadrupole gas analyzers become more measured down to 10-14 Torr levels, and sub-ppm detectability
affordable, they are rapidly becoming commonplace in all is possible in the absence of background interferences. During
industries that require strict control of contamination levels in system troubleshooting, RGAs are also used as very sensitive,
process gases. A smart software interface, lower detection in-situ, helium leak detectors.
limits and reduced cost of ownership are some of the features
to look for in modern instruments. In the semiconductor industry, RGAs are best used in
evaporators, sputterers, etchers or any other high vacuum
The following sections of this article describe the performance systems that are routinely pumped down to lower than
specifications of open and closed ion source quadrupole mass 10 -5 Torr. Their main application is to check the integrity of
spectrometers. The main objective of this information is to the vacuum seals and the quality of the vacuum before any
introduce the basic concepts required to choose the right wafers are committed to the process. Air leaks, virtual leaks
analyzer for any gas phase application, and also to present and many other contaminants at very low levels can easily
some of the basic operating principles that must be kept in ruin wafers and must be detected before a process is initiated.
mind to assure the optimum performance of the instrument As the semiconductor processes become more sophisticated,
selected. they also become less tolerant to contaminants. Residual gas
analysis in a process chamber increases up-time and
production yield, and reduces cost of ownership.
Open Ion Source (OIS)
RGA
RGA Cover Nipple
Mounting Flange
Quadrupole
Mass Filter
Vacuum Chamber
P< 10-4 Torr
1
2
3
4 Vacuum Port
Figure 1: Schematic diagram of OIS
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Choosing a Gas Analyzer
The Open Ion Source (OIS) platinum clad molybdenum wire. This highly inert material
exhibits decreased adsorption for many gases and provides
The standard ion source used in most commercially available reduced outgassing and ESD.
RGAs is the open ion source (OIS). This ionizer is considered
the "do it all" source for RGAs. It has been around, in its Water outgassing is a frequent interference, especially
cylindrical, axially symmetrical version, since the early important because it is a serious source of contamination in
1950's. A schematic of a generic OIS design is shown in Figure 1. many high-vacuum processes. Overnight bakeouts at greater
than 200