Stanford Research Systems RGA100, Operating Manual And Programming Reference

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Sensitivity Tuning Procedure 7-11
SRS Residual Gas Analyzer
Sensitivity Tuning Procedure
All quantitative calculations performed with the SRS RGA rely on the assumption that
there is a linear relation between the partial pressure and the corresponding RGA signals
of the gases. Each gas ionizes differently, and its ions make it through the mass filter with
different efficiencies. As a result the proportionality constant relating the ion current of a
gas to its partial pressure is very dependent on the specific gas.
The partial pressure sensitivity of the RGA to a gas g,
S
g
, is defined as the ratio of the
change
(H-H
0
)
in principal mass peak height to the corresponding change
(P-P
0
)
in
total pressure due to a change in partial pressure of the particular gas species.
H
0
and
P
0
are background values.
S
g
= (H-H
0
) / (P-P
0
)
The units of
S
g
are of ion current per unit pressure (amp/Torr, for example).
The sensitivity of the RGA changes with time due to aging of the head, and is a strong
function of the operating conditions of the instrument. Careful quantitative analysis
requires that the sensitivity factor,
S
g
, be determined for every gas which may be a
component gas in the system being analyzed. The sensitivity factors must be obtained
under the same operating conditions that will be used during general partial pressure
analysis since they depend on many instrumental parameters, including: ionization
energy, emission current, mass filter setting, type of detector, etc.
Important: In order to separate the gain of the electron multiplier from the intrinsic
sensitivity of the RGA head, the sensitivity factors of the SRS RGA are defined for
Faraday Cup detection. A separate Electron Multiplier Gain Factor, is used to correct the
ion signals when the electron multiplier is used. See the Electron Multiplier Tuning
section for details.
The partial pressure sensitivity of the RGA to a particular gas is determined following
the steps listed below.
1. Refresh the calibration of the electrometer using the Calibrate Detector command
(Head Menu) of the RGA Windows program or the CL command of the RGA
command set.
2. Start with the vacuum system at base pressure (preferably under 10
-7
Torr).
3. Select Faraday cup detection and choose the desired operational parameters for the
instrument (See note at the end).
4. Perform a few analog scans on the chamber to assure that the contributions from all
other gases to your measurements can be neglected. Check the quality of the analog
spectra, and peak tune the instrument if needed.
5. Introduce the pure gas into the vacuum system, at a known or calculable pressure
(typically around 10
-6
Torr).