Stanford Research Systems RGA100, Operating Manual And Programming Reference

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Appendix A
SRS Residual Gas Analyzer
1
Appendix A
Vacuum Diagnosis with SRS RGA’’s
Introduction
Residual Gas Analyzer (RGA) is the term for a class of mass spectrometers. They are all
quadrupole mass spectrometers and typically cover mass ranges from 1 to 100 or 200
amu (atomic mass units). The RGA’’s resolution is sufficient to clearly distinguish peaks
that are 1 amu apart. They are designed for the analysis of the gases present in high and
ultra high vacuum systems. The specifications of the RGA are a perfect match for
vacuum diagnosis. Not many materials with a mass greater than 200 amu will be volatile,
and so a small mass range is suitable. The high resolution of a research grade mass
spectrometer is not necessary for the analysis of low molecular weight species. Overall,
RGA’’s are affordable instruments that can be permanently attached to a vacuum system.
Experiments and processes are performed under vacuum for two main purposes: to lower
the total pressure to a suitable level and to provide an extremely clean environment. An
ion gauge addresses the first purpose, but only an RGA can address the second. The high
sensitivity of some processes to specific impurities makes the goal of cleanness more
difficult to achieve than the required total pressure. A typical method of making a system
cleaner is to lower the total pressure. The assumption being that the partial pressure of
the impurities is directly related to the total pressure. This assumption fails in two ways.
First, it is simply not true; a total pressure measurement cannot tell the user tell the user
the level of impurities. As an example, a system operating at a total pressure of 1.1
u
10
-8
mbar might consist of 1.0
u
10
-8
mbar of water due to outgassing and 0.1
u
10
-8
mbar
of air due to a leak. An overnight bake out of the vacuum system reduces the total
pressure to 0.2
u
10
-8
mbar; 0.1
u
10
-8
mbar is water and 0.1
u
10
-8
mbar is air. Is the
vacuum system cleaner than before the bakeout? If the process or experiment is sensitive
to oxygen, the system is no cleaner than before the bakeout. Second, the assumption fails
because total pressure measurement is not very precise; measurements more precise than
10% are difficult and expensive. If a system is operating at a standard pressure of 1.0
u
10
-7
mbar and the next day the pressure is 1.1
u
10
-7
mbar would there be cause for
concern? The vacuum system could be filled with 10% oil vapor or the pressure gauge
sensitivity could have changed 10%. The RGA is designed to address exactly these types
of questions, by immediately showing the user
what is in their vacuum system.
The primary application of the RGA is to analyze the composition of a vacuum system.
The composition can be used to detect impurities, monitor gas fills, or analyze ongoing
chemistry. Another application of the RGA is as a leak detector. It serves this use very
well and in many ways is superior to a portable helium leak tester. In the remainder of
this application note we will illustrate the usefulness of an RGA. The data shown are real
results from experiments designed to illustrate certain points.