How do ion pumps work?

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The pumping action of sputter-ion pumps is based on sorption processes that are initiated by ionized gas particles in a Penning discharge (cold cathode discharge). By means of “paralleling many individual Penning cells” the sputter ion pump attains a sufficiently high pumping speed for the individual gases.

Operating principle of sputter-ion pumps

The ions impinge upon the cathode of the cold cathode discharge electrode system and sputter the cathode material (titanium). The titanium deposited at other locations acts as a getter film and adsorbs reactive gas particles (e.g., nitrogen, oxygen, hydrogen). The energy of the ionized gas particles is not only high enough to sputter the cathode material but also to let the impinging ions penetrate deeply into the cathode material (ion implantation). This sorption process “pumps” ions of all types, including ions of gases which do not chemically react with the sputtered titanium film, i.e. mainly noble gases.

Sputter ion pump construction

The following arrangement is used to produce the ions: stainless-steel, cylindrical anodes are closely arranged between, with their axes perpendicular to, two parallel cathodes (see Fig. 2.61). The cathodes are at negative potential (a few kilovolts) against the anode. The entire electrode system is maintained in a strong, homogeneous magnetic field of a flux density of B = 0.1 T, (T = Tesla = 10⁴ Gauss) produced by a permanent magnet attached to the outside of the pump’s casing. The gas discharge produced by the high tension contains electrons and ions. Under the influence of the magnetic field the electrons travel along long spiral tracks (see Fig. 2.61) until they impinge on the anode cylinder of the corresponding cell. The long track increases ion yield, which even at low gas densities (pressures) is sufficient to maintain a self-sustained gas discharge. A supply of electrons from a hot cathode is not required. Because of their great mass, the movement of the ions is unaffected by the magnetic field of the given order of magnitude; they flow off along the shortest path and bombard the cathode.

Fig 2.61 Operating principle of a sputter-ion pump.

← ⊕ Direction of motion of the ionized gas molecules
• → Direction of motion of the sputtered titanium
- – – - Spiral tracks of the electrons
PZ Penning cells

The discharge current i is proportional to the number density of neutral particles n0, the electron density n-, and the length l of the total discharge path: (2.25)

(2.25)

The effective cross section s for ionizing collisions depends on the type of gas. According to (2.25), the discharge current i is a function of the number particle density n0, as in a Penning gauge, and it can be used as a measure of the pressure in the range from 10-⁴ to 10^-8 mbar. At lower pressures the measurements are not reproducible due to interferences from field emission effects.

Diode-type sputter-ion pumps

In diode-type sputter-ion pumps with an electrode system configuration as shown in Fig. 2.62, the getter films are formed on the anode surfaces and between the sputtering regions of the opposite cathode. The ions are buried in the cathode surfaces. As cathode sputtering proceeds, the buried gas particles are set free again. Therefore, the pumping action for noble gases that can be pumped only by ion burial will vanish after some time and a “memory effect” will occur.

Fig 2.62 Electrode configuration in a diode sputter-ion pump.

Triode sputter-ion pumps

Unlike diode-type pumps, triode sputter-ion pumps exhibit excellent stability in their pumping speed for noble gases because sputtering and film forming surfaces are separated. Fig. 2.63 shows the electrode configuration of triode sputter-ion pumps. Their greater efficiency for pumping noble gases is explained as follows: the geometry of the system favors grazing incidence of the ions on the titanium bars of the cathode grid, whereby the sputtering rate is considerably higher than with perpendicular incidence. The sputtered titanium moves in about the same direction as the incident ions. The getter films form preferentially on the third electrode, the target plate, which is the actual wall of the pump housing. There is an increasing yield of ionized particles that are grazingly incident on the cathode grid where they are neutralized and reflected and from which they travel to the target plate at an energy still considerably higher than the thermal energy 1/ 2 · k · T of the gas particles. The energetic neutral particles can penetrate into the target surface layer, but their sputtering effect is only negligible. These buried or implanted particles are finally covered by fresh titanium layers. As the target is at positive potential, any positive ions arriving there are repelled and cannot sputter the target layers. Hence the buried noble gas atoms are not set free again. The pumping speed of triode sputter-ion pumps for noble gases does not decrease during the operation of the pump.

Fig 2.63 Electrode configuration in a triode sputter-ion pump.

Pumping speed of ion pumps

The pumping speed of sputter-ion pumps depends on the pressure and the type of gas. It is measured according to the methods stated in DIN 28 429 and PNEUROP 5615. The pumping speed curve S(p) has a maximum. The nominal pumping speed S_n is given by the maximum of the pumping speed curve for air whereby the corresponding pressure must be stated.

For air, nitrogen, carbon dioxide and water vapor, the pumping speed is practically the same. Compared with the pumping speed for air, the pumping speeds of sputter-ion pumps for other gases amount to approximately:

Hydrogen 150 to 200%
Methane 100%
Other light hydrocarbons 80 to 120%
Oxygen 80%
Argon 30%
Helium 28%

Sputter-ion pumps of the triode type excel in contrast to the diode-type pumps in high-noble gas stability. Argon is pumped stably even at an inlet pressure of 1 · 10^-5 mbar. The pumps can be started without difficulties at pressures higher than 1 · 10^-2 mbar and can operate continuously at an air inlet producing a constant air pressure of 5 · 10^-5 mbar. A new kind of design for the electrodes extends the service life of the cathodes by 50%.

Influence by magnetic stray fields and stray ions from the sputter-ion pump

The high-magnetic-field strength required for the pumping action leads inevitably to stray magnetic fields in the neighborhood of the magnets. As a result, processes in the vacuum chamber can be disturbed in some cases, so the sputter-ion pump concerned should be provided with a screening arrangement. The forms and kinds of such a screening arrangement can be regarded as at an optimum if the processes taking place in the vacuum chamber are disturbed by no more than the earth’s magnetic field which is present in any case.

Fig. 2.64 shows the magnetic stray field at the plane of the intake flange of a sputter-ion pump IZ 270 and also at a parallel plane 150mm above. If stray ions from the discharge region are to be prevented from reaching the vacuum chamber, a suitable screen can be set up by a metal sieve at opposite potential in the inlet opening of the sputter-ion pump (ion barrier). This, however, reduces the pumping speed of the sputter-ion pump depending on the mesh size of the selected metal sieve.

Fig 2.64 Stray magnetic field of a sputter-ion pump in two places parallel to the inlet flange (inserts) curves show lines of constant magnetic induction B in Gauss.1 Gauss = 1 ·10–4 Tesla

Non evaporable getter pumps (NEG Pumps)

The non evaporable getter pump operates with a non evaporable, compact getter material, the structure of which is porous at the atomic level so that it can take up large quantities of gas. The gas molecules adsorbed on the surface of the getter material diffuse rapidly inside the material thereby making place for further gas molecules impinging on the surface. The non-evaporable getter pump contains a heating element which is used to heat the getter material to an optimum temperature depending on the type of gas which is preferably to be pumped. At a higher temperature the getter material which has been saturated with the gas is regenerated (activated). As the getter material, mostly zirconium-aluminum alloys are used in the form of strips. The special properties of NEG pumps are:

constant pumping speed in the HV and UHV ranges
no pressure restrictions up to about 12 mbar
particularly high pumping speed for hydrogen and its isotopes
after activation the pump can often operate at room temperature and will then need no electrical energy
no interference by magnetic fields
hydrocarbon-free vacuum
free of vibrations
low weight

Combination with other pump types

NEG pumps are mostly used in combination with other UHV pumps (turbomolecular and cryopumps). Such combinations are especially useful when wanting to further reduce the ultimate pressure of UHV systems, since hydrogen contributes mostly to the ultimate pressure in an UHV system, and for which NEG pumps have a particularly high pumping speed, whereas the pumping effect for H₂ of other pumps is low. Some typical examples for applications in which NEG pumps are used are particle accelerators and similar research systems, surface analysis instruments, SEM columns and sputtering systems. NEG pumps are manufactured offering pumping speeds of several `/s to about 1000 l/s. Custom pumps are capable of attaining a pumping speed for hydrogen which is by several orders of magnitude higher.

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