Vacuum Technology Know How - Triumf

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The potential curve in a Pfeiffer Vacuum ion source is shown in Figure 4.20. The heated,
electron-emitting cathode has a potential of approximately 20 V. The Wehnelt electrode is
typically connected to the positive pole of the cathode and prevents electrons from being
scattered in the vicinity of the ion source. An anode voltage V of 80 V accelerates the
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electrons into the formation area (100 V), where they ionize penetrating neutral gas
molecules. The ions are accelerated through an orifice at a potential V of -150 V, and
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are again decelerated to V = 80 V by the focusing electrode. The shot orifice accelerates
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the ions once more before they then enter the mass filter and are decelerated by the
field axis potential V = 85 V at an energy of approximately 15 eV (difference between
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formation area and field axis).
The Pfeiffer Vacuum PrismaPlus and HiQuad mass spectrometers are characterized by their
above-described electrically biased ion source and their field axis technology.
Electrically biased ion source
In many quadrupole mass spectrometers, the cathode is grounded or even has a negative
potential. The cathode (filament) accelerates the emitted electrons to the formation area
(anode), where they ionize neutral gas particles, which are then extracted in the mass filter.
Given these field conditions, however, electrons can also strike other surfaces in the vacuum,
where they trigger electron stimulated desorption (ESD) ions. This results in undesirable
background noise and can cause considerable gas eruptions when the filament is energized if
there are highly-populated surfaces in the recipient.
Pfeiffer Vacuum ion sources have a positive potential (approximately 10 – 100 V). Electrons
emitted from them are repelled from all surfaces having a negative potential and are thus
kept away from these surfaces to avoid triggering interfering ESD ions.
Field axis technology
The ions formed in the ion source are accelerated toward the mass filter at high kinetic
energy. As a result, the ions cannot be influenced by the peripheral or interference fields, and
initially move toward the mass filter at high energy. This enables optimal shot conditions to
be achieved in the quadrupole field, even without the pre-filters that are required with other
mass spectrometers. The mass filter, itself, is appropriately biased to the field axis voltage,
which decelerates the ions to a kinetic energy of approximately 15 eV again upon entering the
filter. This energy – which the industry terms the field axis voltage – together with the mass
of the ions determines the velocity of the ions, and thus their time of flight in the mass filter.
The favorable shot conditions thus produced result in a high transmission of ions through the
mass filter over a broad mass range, thus producing the high sensitivity of the entire system.
SEM: 90 degrees off axis
An additional advantage of Pfeiffer Vacuum mass spectrometers is the arrangement of the
secondary electron multiplier (SEM), which is offset by 90° relative to the filter axis
(“SEM: 90 degrees off axis”).
If the SEM (4.1.2.3) is arranged in the axial direction behind the mass filter, all colliding particles
(neutral particles, ions, electrons, photons) will generate secondary electrons and thus
contribute to the background signal.
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