Veeco MS40 - 3.4 Description of Major Components; Mass Spectrometer

Vacuum Instrument Corp.
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3.4 Description of Major Components
VACUUM SYSTEM
Mass Spectrometer: The mass spectrometer is the component of the
vacuum system that detects the helium tracer gas present in the test object.
The unit ionizes the gas molecules, separates the helium ions from other
gas ions, and converts the helium ions into an electrical current that
represents the size of the leak. The spectrometer consists of three parts:
• the ion source assembly
• the mass spectrometer magnets, and
• the ion collector
Ion Source Assembly: To permit neutral atoms and molecules of gas
to be separated by a magnetic mass spectrometer, they must first be
ionized in the Ion Source. Inside of the ion source, there is an
electrically heated filament that provides a source of electrons. As
these electrons collide with gas molecules, they split off electrons from
the gas molecules and create positively charged gas ions. These
ions then encounter the electrostatic fields, established within the ion
source by the ion repeller, the ion chamber, the focusing plates and the
object plate. The end-result of this arrangement is to accelerate the gas
ions into a well defined, mono-energetic beam.
Mass Spectrometer Magnets: Since the gas entering the
spectrometer contains atoms and molecules of different kinds, the ion
beam will likewise contain a variety of ions including nitrogen, oxygen,
carbon dioxide, and, if a leak has been found, helium. Magnets are
mounted on the outer portion of the spectrometer, outside of the vacuum
system, and are used to create magnetic fields that are perpendicular to the
beam of gas ions. A magnetic field causes the ions to deflect off their
current trajectories, the amount of deflection depends upon the mass of the
gas ion. Lighter ions such as hydrogen are deflected to a greater extent
than heavier ions such as oxygen.
Two identical magnetic fields are used in the MS-40’s mass spectrometer.
The first magnetic field is organized so that only helium has the right
mass to pass through the field and through the narrow opening in the
baffle on the far side of the magnetic field. Most of the other ions will be
deflected in a different direction (they will be either too heavy or too
light) and will be intercepted by the baffle. A few of the non-helium ions
will be able to pass through the baffle (due to random scattering and
1109-186-00 Rev. C 3-11