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Theory of Operation
Sputter ion pumps operate by ionizing
gas within a magnetically confined cold cathode discharge. The events that
combine to enable pumping of gases under vacuum are:
- Entrapment of electrons in orbit by a magnetic field.
- Ionization of gas by collision with electrons.
- Sputtering of titanium by ion bombardment.
- Titanium gettering of active gases.
- Pumping of heavy noble gases by ion burial.
- Diffusion of hydrogen and helium into titanium.
- Dissociation of complex molecules into simple ones for pumping ease,
e.g., CH4 breaks down into C and 2H2. Hydrogen
is pumped separately. Carbon is no longer part of the residual gas and
resides in solid form.
Burial is the basic means of pumping heavy noble gases. Argon ions neutralized
via glancing collisions with a sputter cathode impact the pump wall and
are coated with sputtered titanium. Triode pumps are specially designed
to maximize the kind of collisions that produce energetic neutrals.
Argon is permanently pumped on the wall behind the cathode in these
pumps. The wall area receives titanium for inert gas burial but, because
of a retarding electrical field between the cathode and the wall, it is
not subjected to ion bombardment and thus gases are not resputtered.
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![[Drawing]](/Legacy/graphics/iondraw.gif) |
- Permanent magnets
- Pump envelope
- Titanium cathodes
- Anode cell array
- Positive high voltage lead
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| The main elements of a standard diode
pump are a vacuum tight envelope, external magnets, and an element consisting
of multiple anode cells and two cathodes. The application of a positive
high voltage to the anode creates a plasma discharge. Ions are formed from
the gas molecules present in the system. These ions are accelerated toward
one of the cathodes. When they strike, they can be buried or reflected to
be buried elsewhere. In addition, titanium is sputtered from the cathode
to be deposited elsewhere in the pump where it acts as a getter for active
gases. An ion pump, then, does not remove gas from the vacuum system. It
binds gases down chemically and physically so they can no longer contribute
to the pressure in the system. Diode pumps cover a very wide pressure range.
Recommended starting pressure is 5 x 10-3 Torr or lower. Operation
for extended periods of time at high pressure, other than starting, is not
recommended because if shortens the pump life.
The ultimate pressure after bakeout is generally in the region of 2
x 10-11 Torr. Pumping speeds fall at these pressures because
wall effects diminish discharge intensity. Between this extreme and 10-5
Torr, discharge intensity is proportional to pressure and thus the pump
can be used as a gauge.
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- Permanent magnets
- Pump envelope
- Titanium strip cathodes
- Anode cell array
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![[Drawing]](/Legacy/graphics/triode.gif) |
| The figure above is a sketch of a
triode sputter ion pump. Item 1 refers to the external magnets which intensify
the discharge. Item 2 is the stainless steel, internally welded pump envelope.
Item 3 refers to the two sputter cathodes consisting of multiple strips
of titanium held at a negative high voltage. Item 4 is an array of stainless
steel anode cells which are at ground potential. As in the diode, a plasma
discharge is created within the anode cells upon the application of high
voltage to the cathode grid. The ions impinge upon the sputter cathode and
dislodge titanium atoms as in the diode. At this point there is a significant
difference. Because the cathode grids are open, considerable titanium reaches
the pump walls where it cannot be further disturbed by ion bombardment.
This has at least two favorable results. Undisturbed deposits mean less
regurgitation of previously pumped gases. And deposits at the pump wall
mean that titanium compounds are kept cooler in the starting mode.
A further benefit of the open cathode
grid structure is a high production rate for energetic neutral atoms.
These energetic neutrals are produced by glancing collisions at the cathode
and are readily buried at the pump wall. This burial without reemission
accounts for the triode’s high speed for noble gases.
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Ion Pump Guide
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