Stanford Research Systems RGA100, Operating Manual And Programming Reference

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Ion Detector 3-13
SRS Residual Gas Analyzer
necessary electrical contacts. A plate (CDEM Anode) mounted at the exit of the CDEM
collects the secondary electrons. The resulting electron current flows into the
electrometer through a separate feedthru of the flange.
The entire setup is self -aligning and easily serviced by the user in the field. For example,
removing the screw that fastens the clamp to the HV rod is all that is needed to replace
the CDEM (Please see the Maintenance chapter for details.)
FC detection is standard in all RGA probes . All FC-only units can be upgraded to
electron multiplier detection without the need to replace the probe. When the electron
multiplier is not turned on, multiplier systems operate exactly like the FC-only systems.
The same electrometer is used to measure the ion (or electron) currents under both
detection schemes. The ECU automatically connects the necessary electrode to the
electrometer depending on the type of detector being used.
Faraday Cup operation
The Faraday Cup (FC) detector, measures the incident ion current directly. Positive ions
enter the grounded detector, strike a metal wall, and are neutralized by electron transfer
from the metal to the ion. The electrons given up in this process establish an electrical
current that has the same intensity as the incoming ion current.
Since the nominal sensitivity of the RGA is in the order of 10
-4
Amps/Torr, the currents
measured are very small: 10
-9
to 10
-15
Amps for pressures in the order of 10
-5
to 10
-11
Torr, respectively. Minimum-detectable-partial pressures as low as 5
.
10
-11
Torr are
possible with the FC. However, in real-time applications, FC detectors are rarely used to
measure partial pressures below 10
-9
Torr. At pressures below that value, the CDEM
option becomes a much better alternative allowing for faster scan rates and improved
signal-to-noise ratios.
The best characteristics of FC detection are: simplicity, stability, large dynamic range
and lack of mass discrimination
. All ions are detected with the same efficiency
regardless of their mass .
The ECU automatically connects the FC electrode to the electrometer input upon power-
on reset, and whenever the electron multiplier is not turned on.
Electron Multiplier Operation
An Electron Multiplier upgrade (Option 01) to the standard Faraday Cup (FC) detection
setup is recommended when the SRS RGA is routinely used at pressures under 10
-9
Torr.
The detection upgrade consists of a state-of-the art Macro Multi-Channel Continuous
Dynode Electron Multiplier (CDEM), and a negative high voltage power supply (0 to -
2500V).
Macro Multi-Channel Continuous Dynode Electron Multipliers are used in the SRS
RGA models with the electron multiplier option (Option 01). They consist of a straight
four or six channel tube made out of a special resistive glass (i.e. high secondary electron
emission yield) with a cone of the same material attached to the front end. The extruded
channels run along the length of the device and are twisted at the time the part is drawn to
eliminate ion feedback. Each CDEM is placed upright next to the FC and away from the
line of sight of the ionizer (i.e. off-axis configuration). When the cone is biased
negatively (-1000 to -2500V) relative to the back end, positive ions are very efficiently