Stanford Research Systems RGA100, Operating Manual And Programming Reference

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Ion Detector 3-15
SRS Residual Gas Analyzer
mass discrimination effects, gain instabilities and finite lifetime of the device. A good
understanding of these limitations is very important to assure accurate quantitative
measurements.
The dynamic range of electron multipliers is determined by their dark current at the low
end, and by the bias current value at the high end.
The bias current is established by the external voltage drop along the resistive glass tube
and flows along the channel walls replenishing their charge as secondary electrons are
emitted.
Channel electron multipliers, operate linearly in the analog mode until the
output current is approximately 10% of the bias current.
The dark current of a multiplier is the electron current measured at its output in the
absence of an input ion current. The minimum output current that can be accurately
measured with the multiplier is equal to the dark current noise.
Example: For a typical resistance of 200 Mohms, and a bias voltage of -2000V, the bias
current is 10
P
Amps and the output current must be kept under 1
P
Amp. Since the gain at
that voltage is roughly 10
6
, the maximum input current at which the output current
behaves linearly is 10
-12
Amps (1
P
Amp / 10
6
). Typical dark currents are lower than 10
-13
Amp, and the minimum input current that can be detected is 10
-19
Amps. For a sensitivity
of 10
-4
Amp/Torr, this corresponds to an lower and upper limits of 10
-15
and 10
-8
Torr,
respectively, and 7 orders of magnitude of dynamic range.
The total gain of electron multipliers varies as a function of the mass of the incident ions.
As a rule of thumb, and for small molecules,
the gain decreases as mass increases . This
mass discrimination effect is caused by the dependence of ion-electron conversion
efficiencies on the velocities of the ions entering the detector. For example, an inverse
relationship with the square root of the mass has been reported for monoatomic ions of
the same energy. For accurate quantitative measurements, it is essential to calibrate in
advance the gain of the multiplier for the specific ionic species being detected.
An important problem when working with multipliers is that their gain changes with
time.
Gain degradation is unavoidable, and particularly serious just after the detector
has been exposed to air, or after high quantities of reactive gases have been introduced
into the vacuum system. The increased surface area provided by the extra channels in the
multi-channel devices reduces this problem; however, frequent calibration of the
multiplier gain against the FC output is recommended for reliable quantitative
measurements. This is done automatically with the RGA Windows
•
software.
Gain degradation limits the lifetime of all electron multipliers. Eventually the gain drops
to unacceptable values and the multiplier needs to be replaced. As a rule of thumb, the
multiplier should be replaced when the required gains can no longer be achieved by
increasing the bias voltage.
The lifetime of electron multipliers is ultimately dependent
upon the accumulated charge drawn from the multiplier ( Gain degradation typically
starts at accumulated output current values of a few thousand
P
Amp-hr) . However,
the lifetime also depends critically on the residual gas environment and the duration of
transient signals. Contamination by organic compounds (i.e. diffusion or mechanical
pump oil) and the interaction with highly reactive gases must be avoided at all times.
It has been found that, in many cases, channel multipliers may be successfully refreshed