Sincrotrone Trieste S.C.p.A. IUVSTA Visual Aid Programme Capture Pumps An Introduction Module #8 by Harland G. Tompkins Adapted and revised by Fabio Mazzolini Sincrotrone Trieste S.C.p.A. Trieste, Italy

Introduction Vacuum pumps fall roughly into one of three categories: positive displacement pumps momentum exchange pumps capture pumps The distinguishing feature of capture pumps is that they do not expel the gases which they pump during operation. Some pumps never expel the gases, while some other of these type of pumps have to be periodically rejuvenated.

Capture pumps have a different pumping speed for different gases: Gas Selectivity (e.g. Ion Pump) Capture pumps can pump at high pressure for a limited time: Saturation (e.g. Ion Pump) They often have difficulty with gas burst.

Capture pumps include: sputter-ion pumps titanium sublimation pumps non evaporable getter (NEG) pumps cryosorption pumps cryogenic pumps

Sputter-ion pumps (General) If a gas is ionized and the resulting positive ions are accelerated to a negatively charged plate (and retained), atoms of the gas are effectively removed from the system, and thus a pumping action has been produced. Pumps using this mechanism, gas molecules being ionized and transported by an electric field, are referred to generically as ion-pumps. This pumping mechanism is usually combined with a gettering action. Since the gettering surface is usually generated by a sputtering process, the term sputter-ion-pump is often used.

Sputter-ion pumps (Basic pumping mechanism) The basic mechanism of an ion pump involves a cold cathode discharge (Fig.A). If a free electron wanders into the space between the plates, it will be accelerated toward the anode. If it has sufficient energy, it can ionize the gas molecule, and in the process create an additional free electron. When the electron eventually strikes the anode, it is removed from the process. There are two sources of additional free electrons: 1) the creation of an ion creates an additional electron; 2) when the ion strikes the anode, secondary electrons are released. For pressures below a 10-3 mbar most of the electrons reach the anode without creating an additional electron, and hence the process is non-sustaining.

Sputter-ion pumps (Basic pumping mechanism) If the anode is a ring (Fig. B) the electron, instead of striking the anode, passes through the hole in the ring and experiences an oscillatory motion. This provides a somewhat longer path length but the discharge is still not self-sustaining in the pressure range of interest. The addition of a strong magnetic field (Fig. C) provides a force on the electrons which is perpendicular to the direction of motion and causes the electron to move in a spiral path. The distance which the electron travels before eventually striking the anode is greatly increased.

Sputter-ion pumps The cloud of free electrons and ionized gas molecules in the region between the plates is called a "Penning discharge". (1) A free electron is needed to initiate the process, but a few free electrons are always floating around (e.g., cosmic rays). (2) Normally all that is needed is to apply the voltage in the presence of the magnetic field and the appropriate pressure. (3) When an ion strikes the cathode with the resulting energy, it is driven into the cathode material and thus retained. This burial is the pumping mechanism for noble gases and other inert molecules.

Sputter-ion pumps (Pumping mechanism) The background gas is ionized. The ions that are generated are accelerated towards the cathodes. Some atoms of the Ti cathode are emitted for sputtering and cover the anode. The background gas molecules that hit with the chemically active titanium film are chemically trapped.

the ion current is a measure of the pressure. Sputter-ion pumps The number of molecules ionized is roughly proportional to the number of molecules present: the ion current is a measure of the pressure. The relationship is: I+ = KPn where I+ is the gauge ion current K is a constant P is the pressure and 1.05 < n < 1.15

Typical Ion Pump Parameters Sputter-ion pumps (Basic Pump Construction) Typical Ion Pump Parameters Anode Voltage 2 -10 Kilovolts Magnetic Field 0.12 to 0.15 Tesla Anode Cell Diam. 1 - 3 cm Anode Length 1 - 3 cm Cathode Material Ti, Ta

Sputter-ion pumps (Noble gas pumping) The process of burial of ions and sputtering the cathode materials occurs at the same time. An ion which is accelerated to the cathode buries itself and at the same time sputters titanium from the cathode. Previously buried noble gas atoms are uncovered and returned to the gas phase (pressure burst, e.g. Argon instability) The solution is for them to not occur at the same place: 1) Differential Ion Pump 2) Triode pump

Sputter-ion pumps (Differential Ion Pump) One method of separating the sputtering area from the burial area is to make the two planar cathode plates out of different materials, so that one of them will then sputter much more easily than the other (e.g. Ta). Ti plate is the sputtering area and Ta is the burial area.

Sputter-ion pumps (Triode) In this design the cathode is a grid. The third part of the triode is an auxiliary electrode outside the cathode grid. The auxiliary electrode is held at the same potential as the anode. The sputter yield is usually higher at a grazing angle than at normal incidence. On the other hand, the burial is greater at normal incidence. This will cause most of the sputtering to occur at the cathode while most of the burial will occur in the auxiliary electrode. Because of this, the uncovering process is virtually eliminated.

Relative Pumping Speed Sputter-ion pumps (Operation) At atmospheric pressure the mean-free-path is too short At 10-2 mbar discharge will spread, possibly into working chamber. Heating causes much gas desorption. Multiple starting efforts required. When pressure drops below 10-4 mbar: discharge quiets and is confined to cell current drops and voltage rises to its steady state value N2 Gas Relative Pumping Speed Air 100% N2 85-95% O2 60-100% H2 150-250% He 10-35% Ar, Kr, Xe 1-30% Typical pumping speed curve

Getter pumps (General) When a molecule strikes the gettering surface, it forms a chemical compound and it is removed from the gas phase. When the surface saturates, it is no longer effective and must be regenerated. Getter Films = the active surface can be obtained by sputter deposition (as in ion pumps), evaporation or sublimation. When needed, an additional film is deposited. “Bulk Getters“ = the active surface is regenerated by heating to cause the adsorbed gas to diffuse to the interior of the material and thereby reactivating the surface.

n N Molecule: - strikes the gettering surface (Chemically active) - forms a Chemical bond - is removed from gas phase n = number of molecules which strike a unit area per second = number of molecules per unit volume = average molecular velocity N

Getter pumps (General) When the surface saturates, getters must be “reactivated” (Regeneration) Getter Pumps: Flash getters  not regenerated ! Titanium Sublimation (TSP)  regenerated by additional sublimation Non Evaporable Getter (NEG)  by diffusing gas species into bulk

Flash Getter Used in sealed component, initially pumped by conventional means Chemically reactive metal, often Barium alloys Protected from atmosphere by cladding Heated to 800 - 1000°C to rupture cladding and vaporize metal Metal condenses on desired location Remaining gases are pumped to acceptable pressure One shot, Not replenished

Titanium Sublimation Pumps Often used in conjunction with ion pumps Ti deposited by sublimation from a heated source: 4 to 6 Ti filaments, heated one-at-a-time by passing 30 to 50 A of current Normally operated intermittently (regenerated by additional sublimation) Active gases (O2, N2, CO, etc.) pumped very well Stable gases (H2O, CH4, etc.) must dissociate, fragments pumped Rare gases (He, Ar, Ne, etc.) not pumped

Non-Evaporable Getter (General) Non-Evaporable Getters (NEGs) consist of a material which is very chemically active, usually zirconium or a combination of Zr and other materials (Fe, V, etc.). These materials can be: 1) Ground into powders using manufacturing methods which avoid contact with oxygen. They are then compressed into a solid form which is very porous (large surface area). 2 )Deposited as coating . When the NEG material is first exposed to air, it forms a thin protective film which prohibits further reaction with atmospheric gases. After installation, the NEG must be "activated" to allow the gas molecules access to the surface of the reactive material. (heating to temperatures of the order of 180 - 450°C for times of 1 - 2 hours). This causes the protective film to be dissolved into the bulk of the grains.

NEG (Activation: a practical example) 1) pump the vacuum system down (turbomolecular pump, vacuum valve open) 2) bake the vacuum system (24h @ 150 °C) 24 h @ 150 °C 3) hold the temperture @ 100 °C 4) “degas” ion pumps (switch them on–off 3-4 times, until the pressure decrese) 5) switch all the pressure gauges off 6) activate NEGs 1 h @ 430 °C 7) “degas” filaments 8) switch ion pumps and pressure gauges on 9) cool the vacuum system down to room temp. 10) close the valves connecting the turbos to the vacuum system NEG pump activation (efficiency curves)

Non-Evaporable Getters (Pumps) metallic strips Cartridge on a ConFlat flange “modules”

NEG Pumps (features) High pumping speed for all active gases (especially for hydrogen) High capacity Compact size Very clean Vibration-free No power required for operation at room temperature after activation Operation in the presence of high magnetic fields Light weight Reversible pumping of hydrogen and its isotopes

NEG (Mechanisms) Interaction with N2, O2, CO, CO2 forms nitrides, oxides and carbides: these compounds are stable, N, O, C will not be released. Hydrogen forms a solid solution, will be released at higher temperature: sorption of Hydrogen is reversible. It is sorbed at room temperature. Amount held in solution depends on temperature and partial pressure adjacent to NEG H2O adsorbs, dissociates, products diffuse into bulk, limited by dissociation. Hydrocarbons must also be dissociated, limited efficiency for cleaning up organic contamination Noble gases not pumped: can operate getter in noble gases to improve purity

NEG Coatings Produced by sputtering: uniform and distributed coating different alloys/compounds (composite cathodes) Getter materials: Ti, Zr, Hf, V, … Thickness: 1 micron Low activation temperature (<200 °C for Ti-Zr-V) => compatible with coating on aluminium alloy substrate

NEG Coatings (CERN) The use of a new distributed pumping concept in pipes started in 1995 at CERN for the LHC project. The idea is to pass from discrete pump … … through linear pump … … to distributed pump

Cryosorption Pump (General)