Stanford Research Systems RGA100, Operating Manual And Programming Reference

Page: 255 / 362

Appendix A
SRS Residual Gas Analyzer
7
Single Mass Measurement
For vacuum systems that only need to be clean, the mass spectrum is the most useful
measurement. During experiments and processes the partial pressure of certain species is
of more interest. The RGA software provides three modes that are used to measure
selected peaks. The selection of which mass is associated with which species is usually
straightforward,
i.e . the mass of the molecule is chosen. When two species have
overlapping peak patterns, the user chooses the strongest peak that does not interfere.
For example N
2
and CO both have a mass of 28. In a system with a large CO
interference, N
2
could be measured at mass 14. Because the peak at 14 is smaller than the
major peak, a scaling factor is required. The set of peaks of interest are entered into the
RGA software by the user. During measurements, the RGA measures only each peak.
Because the whole spectrum is not measured, data is acquired much faster. The two
examples that follow show an interesting pressure vs. time experiment and a method of
increasing the dynamic range of measurements.
Pressure vs. time
Figure 4 shows the use of the P vs. t Mode to monitor an air lock sequence. The process
is to open an air lock, place the sample in it, and move the sample into the main vacuum
chamber. The air lock is pumped from atmospheric to rough vacuum using the same
mechanical pump as the main vacuum chamber. This requires isolating the foreline of
the turbo pump during the time the sample is in the air lock. To keep oxygen out of the
main vacuum chamber, the air lock is flushed with dry nitrogen then pumped to rough
vacuum. The flush is repeated two additional times before the sample is finally
transferred. If this procedure is successful, the main vacuum chamber should not be
disturbed.