There has also been a Gettering mechanism, which has been a pumping method, since vacuum technology came into being. Historically, the first use of this method was in the early stages of making electron tubes.
The basis of the Gettering method is the absorption of some gases by some materials which results in their removal from the environment. Getter pumps fall into the general category of Capture pumps, along with cryopumps and sputter-ion pumps. The Gettering process can be divided into physical and chemical categories.
Physical Getters are often found in cryostats and Dewars. In these systems, a zeolite material such as a molecular sieve is used to physically absorb and hold water vapor. If, as is often the case, the molecular sieve is placed against a surface at liquid nitrogen temperature with good thermal contact, it will absorb and hold the common air gases as well. This is the same pumping technique used by cryosorption roughing pumps.
Usually, when referring to Gettering, chemical getters are meant. A chemical getter pump pumps the chamber through a chemical reaction in which a chemically active gas is combined with an active metal to form a low-pressure solid compound.
The active metal can be an element or alloy and called a Getter Metal (GM). The reaction of chemically active gases with the getter metal is almost one-way. They all form a low-pressure ceramic compound in which the active gas is permanently discharged from the vacuum chamber. Inert gases are not pumped at all because of their inertness and do not react with the getter metal.
Hydrogen (H2) is a different gas that does not form a chemical in reaction with the metal but soluble in the getter metal to form a solid solution.
Chemical getters can be divided into two groups: Evaporable and non-evaporable. Evaporable getters are often used in electron tubes where a mirror-like metal coating can be easily observed on the inside of the glass envelope. When the tube has been pumped down and ready for tip-off to seal it, a slug of getter metal, often barium or a barium alloy is heated to a high enough temperature to evaporate it so that it can subsequently condense on the tube’s inner surfaces. There it forms a high surface area reactive coating that will remain as an in situ pump after the tube is pinched off. Another common example is the titanium sublimation pump (TSP.)
In TSP systems, a titanium / molybdenum alloy filament is heated directly or a titanium mass is heated indirectly to the sublimation temperature of titanium (about 1450 ° C).
(Sublimation means switching from the solid state to the gas state without the material entering the liquid state.)
Titanium sublimation pumps (TSP.)
The titanium vapor then condenses on the inner surface of the chamber. This thin film of titanium is converted to an active gas and H2 pump. The pumping speed of the condensed film is directly proportional to the total film area. The pumping speed of the newly created thin film decreases due to the reaction with the gas in the chamber (pumping action) and the life time of the thin film depend on the amount of gas being pumped. The higher the amount of gas, the shorter the lifetime of the thin film is result. In the case of large amounts of gas, the titanium source requires alternating charging of the thin film to maintain pumping speed, and this can be a problem.
Schematic of TSP inside vacuum chamber
The titanium source saturates with hydrogen after cooling and releases hydrogen upon reheating. As a result, this high and low temperature cycle results in increased pressure. The solution to this potential problem is to keep the source temperature high, but lower than the sublimation temperature, at all times. The next problem is due to the deposition of the new titanium layer on the saturated titanium. As a result of this phenomenon, the film layers are affected to such a degree that they cause cracking and peeling. In addition, dissolution of H2 gas in titanium exacerbates this effect as the volume of the thin layer increases with the amount of dissolved H2. Both of these effects can cause particles in the vacuum chamber to have catastrophic results in some processes. In ultra-high vacuum (UHV) conditions where the amount of gas in the environment is low, the film life is very long and TSPs are a clean, efficient and economical source for pumping in these systems.
Non-evaporable getters (NEGs) remain as solid, as their name implies, rather than evaporation and condensation on the surface. These family of getters are usually, but not always, zirconium alloys. They can be solid in any form, but they are often seen as chunks or pellets. In some cases, they are used as a thin film on metal substrates.
Schematic of NEG pump performance
Activation is a process that’s required for NEGs. When NEG materials are exposed to air for movement or loading into the system or device, the surface of the material reacts with the gases around it and forms an inactive saturated layer on their surface. This means that the NEG will be totally enclosed in an envelope of oxides, nitrides, etc. In addition, most of the material is saturated with dissolved H2. In this case, the getter material will be essentially inert and will not play the role of the active getter acting as a pump. Therefore, the activation process is required to prepare the getter surface for proper pumping. This is done in situby heating under vacuum after being installed.
Schematic NEG activation process
During heating, the reacted primary layer is dispersed into the NEG bulk, and the H2 is driven from the solid solution into the vacuum chamber, and with a suitable pump it can be removed from the chamber.
The time and temperature required for the activation of the getter for the different alloys will vary, and this process should be performed in accordance with the procedure recommended by the manufacturer.
The vacuum level required will depend upon the actual application, but a pressure of at least 10-4 torr is usually required and pumping time must be long enough to ensure that the released H2 is pumped away.
By reaction of the reactive gases in the vacuum chamber with the NEG surface, the gases exit the chamber. This process can continue until the NEG surface is re-saturated and the gases form an inactive layer on it.
Different parts of a NEG pump
In ultra-high vacuum applications, it may take years for the pump to reach this point, but in a high vacuum process, the pump may reach this point in minutes or hours.
In systems that are frequently subjected to atmospheric pressure, it should be noted that whenever NEGs are exposed to air or any other active gas such as N2, the NEG surface must be reactivated. To solve this problem in a cyclic system, the chamber can be vented with argon instead of an active gas, or the Getter Pump can be inserted into a separate chamber to be separated from the vacuum chamber with a suitable valve.
The use of Viton O-ring in getter pumps will allow some air to flow into these systems due to the relative permeability of the gas to these O-rings. Obviously, using metallic gaskets instead of Viton is often a requirement
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