Fundamentals of Vacuum Technology (1248463), страница 21
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2.63 shows the electrode configurationof triode sputter-ion pumps. Their greater efficiency for pumping noblegases is explained as follows: the geometry of the system favors grazingincidence of the ions on the titanium bars of the cathode grid, whereby thesputtering rate is considerably higher than with perpendicular incidence.The sputtered titanium moves in about the same direction as the incidentatoms•Ö TitaniumGas moleculesüä IonsElectronsA Anode cylinder(same as in the diodepump)B Magnetic fieldF Target plate(pump housing)as the third electrodeK Cathode gridions. The getter films form preferentially on the third electrode, the targetplate, which is the actual wall of the pump housing. There is an increasingyield of ionized particles that are grazingly incident on the cathode gridwhere they are neutralized and reflected and from which they travel to thetarget plate at an energy still considerably higher than the thermal energy1/2 · k · T of the gas particles.
The energetic neutral particles can penetrateinto the target surface layer, but their sputtering effect is only negligible.These buried or implanted particles are finally covered by fresh titaniumlayers. As the target is at positive potential, any positive ions arriving thereare repelled and cannot sputter the target layers. Hence the buried noblegas atoms are not set free again. The pumping speed of triode sputterion pumps for noble gases does not decrease during the operation ofthe pump.The pumping speed of sputter-ion pumps depends on the pressure and thetype of gas. It is measured according to the methods stated in DIN 28 429and PNEUROP 5615.
The pumping speed curve S(p) has a maximum. Thenominal pumping speed Sn is given by the maximum of the pumping speedcurve for air whereby the corresponding pressure must be stated.For air, nitrogen, carbon dioxide and water vapor, the pumping speed ispractically the same. Compared with the pumping speed for air, thepumping speeds of sputter-ion pumps for other gases amount toapproximately:HydrogenMethaneOther lighthydrocarbonsOxygenArgonHelium150 to 200 %100 %80 to 120 %80 %30 %28 %Sputter-ion pumps of the triode type excel in contrast to the diode-typepumps in high-noble gas stability.
Argon is pumped stably even at an inletpressure of 1 · 10-5 mbar. The pumps can be started without difficulties atpressures higher than 1 · 10-2 mbar and can operate continuously at an airinlet producing a constant air pressure of 5 · 10-5 mbar. A new kind ofdesign for the electrodes extends the service life of the cathodes by 50 %.Influence on processes in the vacuum chamber by magnetic strayfields and stray ions from the sputter-ion pump.The high-magnetic-field strength required for the pumping action leadsinevitably to stray magnetic fields in the neighborhood of the magnets.
As aresult, processes in the vacuum chamber can be disturbed in some cases,so the sputter-ion pump concerned should be provided with a screeningarrangement. The forms and kinds of such a screening arrangement can beregarded as at an optimum if the processes taking place in the vacuumchamber are disturbed by no more than the earthÕs magnetic field which ispresent in any case.Fig. 2.64 shows the magnetic stray field at the plane of the intake flange ofa sputter-ion pump IZ 270 and also at a parallel plane 150 mm above. Ifstray ions from the discharge region are to be prevented from reaching thevacuum chamber, a suitable screen can be set up by a metal sieve atopposite potential in the inlet opening of the sputter-ion pump (ion barrier).This, however, reduces the pumping speed of the sputter-ion pumpdepending on the mesh size of the selected metal sieve.Fig.
2.63 Electrode configuration in a triode sputter-ion pump52HomeVacuum generationsince hydrogen contributes mostly to the ultimate pressure in an UHVsystem, and for which NEG pumps have a particularly high pumping speed,whereas the pumping effect for H2 of other pumps is low. Some typicalexamples for applications in which NEG pumps are used are particleaccelerators and similar research systems, surface analysis instruments,SEM columns and sputtering systems. NEG pumps are manufacturedoffering pumping speeds of several `/s to about 1000 l/s. Custom pumpsare capable of attaining a pumping speed for hydrogen which is by severalorders of magnitude higher.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 ·10Ð4 Tesla2.1.8.4Non evaporable getter pumps (NEGPumps)The non evaporable getter pump operates with a non evaporable, compactgetter material, the structure of which is porous at the atomic level so that itcan take up large quantities of gas.
The gas molecules adsorbed on thesurface of the getter material diffuse rapidly inside the material therebymaking place for further gas molecules impinging on the surface. The nonevaporable getter pump contains a heating element which is used to heatthe getter material to an optimum temperature depending on the type of gaswhich is preferably to be pumped. At a higher temperature the gettermaterial which has been saturated with the gas is regenerated (activated).As the getter material, mostly zirconium-aluminum alloys are used in theform 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 willthen need no electrical energy• no interference by magnetic fields• hydrocarbon-free vacuum• free of vibrations• low weightNEG pumps are mostly used in combination with other UHV pumps(turbomolecular and cryopumps).
Such combinations are especially usefulwhen wanting to further reduce the ultimate pressure of UHV systems,53HomeVacuum generation2.1.9 CryopumpsAs you may have observed water condenses on cold water mains orwindows and ice forms on the evaporator unit in your refrigerator. Thiseffect of condensation of gases and vapors on cold surfaces, water vapor inparticular, as it is known in every day life, occurs not only at atmosphericpressure but also in vacuum.This effect has been utilized for a long time in condensers (see 2.1.5)mainly in connection with chemical processes; previously the baffle ondiffusion pumps used to be cooled with refrigerating machines. Also in asealed space (vacuum chamber) the formation of condensate on a coldsurface means that a large number of gas molecules are removed from thevolume: they remain located on the cold surface and do not take part anylonger in the hectic gas atmosphere within the vacuum chamber.
We thensay that the particles have been pumped and talk of cryopumps when theÒpumping effectÓ is attained by means of cold surfaces.Cryo engineering differs from refrigeration engineering in that thetemperatures involved in cryo engineering are in the range below 120 K(< -153 ¡C).
Here we are dealing with two questions:••••Gifford-McMahon processStirling processBrayton processClaude processThe Gifford-McMahon process is mostly used today and this process is thatwhich has been developed furthest. It offers the possibility of separating thelocations for the large compressor unit and the expansion unit in which therefrigeration process takes place. Thus a compact and low vibration coldsource can be designed. The cryopumps series-manufactured byLEYBOLD operate with two-stage cold heads according to the GiffordMcMahon process which is discussed in detail in the following.The entire scope of a refrigerator cryopump is shown in Fig. 2.65 andconsists of the compressor unit (1) which is linked via flexible pressure lines(2) Ð and thus vibration-free Ð to the cryopump (3). The cryopump itselfconsists of the pump casing and the cold head within.
Helium is used asthe refrigerant which circulates in a closed cycle with the aid of thecompressor.a) What cooling principle is used in cryo engineering or in cryopumps andhow is the thermal load of the cold surface lead away or reduced?b) What are the operating principles of the cryopumps?2.1.9.1 Types of cryopumpDepending on the cooling principle a difference is made between• Bath cryostats• Continuous flow cryopumps• Refrigerator cryopumpsIn the case of bath cryostats Ð in the most simple case a cold trap filledwith LN2 (liquid nitrogen) Ð the pumping surface is cooled by direct contactwith a liquefied gas. On a surface cooled with LN2 (T Å 77 K) H2O and CO2are able to condense.
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