Vacuum Technology Know How - Triumf

Pfeiffer Vacuum
Page 110
Vacuum Technology
This is why the ions escaping from the filter are deflected by 90 degrees and then accelerated
to the first dynode of the SEM. Neutral particles and photons are not deflected at all by
the electrical deflection unit, and electrons are deflected to a much greater extent than ions.
This means that almost all of the ions that are allowed through the filter will strike the
amplifier, which significantly improves the signal-to-noise ratio.
Except for a few special versions, HiQuad analyzers are equipped with “90 degrees
off-axis SEMs.”
In the PrismaPlus, an axial C-SEM is offered as a current amplifier. In this case, too, the ions
exiting the mass filter are deflected slightly toward the C-SEM, and in a weakened state are
thus separated from the undesired particles.
Mass Filter
Connection Flange
Deflection Unit
Housing
Figure 4.21: Design of the detectors in a QMA 400 HiQuad analyzer with
Faraday cup and SEM
Faraday Cup
SEM 217 / 218
Mass discrimination
If ions strike the first dynode of the SEM with different pulses, differing quantities of secondary
electrons will be generated. The conversion rate at the amplifier is a function of mass.
This effect is called mass discrimination, and is less pronounced with an SEM of discrete
design than with a C-SEM. Mass discrimination can be reduced by accelerating the ions to a
high energy before they strike the conversion dynode.
Summary
Both a stable HF supply as well as a mechanically precise filter are necessary in order to
achieve maximum possible transmission over a broad mass range with a pre-selected mass
resolution. A biased ion source with suitably selected field axis technology, as well as the
“90 degrees off-axis” arrangement of the SEM considerably improve the signal-to-noise ratio.
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