Titanium sublimation pump
Encyclopedia
A Titanium sublimation pump (TSP) may be used as a component of ultra high vacuum
systems.
filament through which a high current (typically around 40 Amps) is passed periodically. This current causes the filament to reach the sublimation temperature of titanium, and hence the surrounding chamber walls become coated with a thin film of clean titanium. Since clean titanium is very reactive, components of the residual gas in the chamber which collide with the chamber wall are likely to react and to form a stable, solid product. Thus the gas pressure in the chamber is reduced.
After some time, the titanium film will no longer be clean and hence the effectiveness of the pump is reduced. Therefore, after a certain time, the titanium filament should be heated again, and a new film of titanium re-deposited on the chamber wall. Since the time taken for the titanium film to react depends on a number of factors (such as the composition of the residual gas, the temperature of the chamber and the total pressure), the period between successive sublimations requires some consideration. Typically, the operator does not know all of these factors, so the sublimation period is estimated according to the total pressure and by observing the effectiveness of the outcome. Some TSP controllers use a signal from the pressure gauge to estimate the appropriate period.
Since the TSP filament has a finite lifetime, TSPs commonly have multiple filaments to allow the operator to switch to a new one without needing to open the chamber. Replacing used filaments can then be combined with other maintenance jobs.
The effectiveness of the TSP depends on a number of factors. Amongst the most critical are; the area of the titanium film, the temperature of the chamber walls and the composition of the residual gas. The area is typically maximised when considering where to mount to TSP. The reactivity of the new titanium film is increased at lower temperatures, so it is desirable to cool the relevant part of the chamber, typically using liquid nitrogen. However, due to the cost of the nitrogen and the need to ensure a continuous supply, TSPs are commonly operated at room temperature. Finally the residual gas composition is important - typically the pump works well with the more reactive components (such as CO
and O2
), but is very ineffective at pumping inert components such as the noble gasses. Therefore TSP must be used in conjunction with other pumps.
Other pumps which use exactly the same working principle, but using something other than titanium as a source are also relatively common. This family of pumps are usually called 'getter
pumps' or 'getters' and typically consist of metals which are reactive with the components of the residual gas which are not pumped by the TSP. By choosing a number of such sources, most constituents of the residual gas, except for the noble gasses, can be targeted.
s and valves) and damage them.
Many chambers containing TSPs also have an ion pump
. Often the ion pump provides a good location for the TSP, and some manufacturers promote the use of combined TSP/ion-pumps. Furthermore, TSPs have been shown to be effective against the regurgitation effects of ion pumps .
in the gas is large and thus the probability of collisions within the gas is insignificant compared to the probability or collision with the chamber walls). Credit for the invention of titanium sublimation vacuum pumps has generally been given to Linus Pauling
. However, the team of brothers Benjamin and Jeremy Chubb built the first TSP in 1946 in the town of Glenview, IL. There is in Glenview a vacuum pump museum sponsored by The Oreck Foundation. There is a
large exhibit dedicated to the Chubb brothers and their many contributions to high vacuum technology.
Ultra high vacuum
Ultra-high vacuum is the vacuum regime characterised by pressures lower than about 10−7 pascal or 100 nanopascals . UHV requires the use of unusual materials in construction and by heating the entire system to 180°C for several hours to remove water and other trace gases which adsorb on the...
systems.
Principle of Operation
Its construction and principle of operation is extremely simple. It consists of a titaniumTitanium
Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....
filament through which a high current (typically around 40 Amps) is passed periodically. This current causes the filament to reach the sublimation temperature of titanium, and hence the surrounding chamber walls become coated with a thin film of clean titanium. Since clean titanium is very reactive, components of the residual gas in the chamber which collide with the chamber wall are likely to react and to form a stable, solid product. Thus the gas pressure in the chamber is reduced.
After some time, the titanium film will no longer be clean and hence the effectiveness of the pump is reduced. Therefore, after a certain time, the titanium filament should be heated again, and a new film of titanium re-deposited on the chamber wall. Since the time taken for the titanium film to react depends on a number of factors (such as the composition of the residual gas, the temperature of the chamber and the total pressure), the period between successive sublimations requires some consideration. Typically, the operator does not know all of these factors, so the sublimation period is estimated according to the total pressure and by observing the effectiveness of the outcome. Some TSP controllers use a signal from the pressure gauge to estimate the appropriate period.
Since the TSP filament has a finite lifetime, TSPs commonly have multiple filaments to allow the operator to switch to a new one without needing to open the chamber. Replacing used filaments can then be combined with other maintenance jobs.
The effectiveness of the TSP depends on a number of factors. Amongst the most critical are; the area of the titanium film, the temperature of the chamber walls and the composition of the residual gas. The area is typically maximised when considering where to mount to TSP. The reactivity of the new titanium film is increased at lower temperatures, so it is desirable to cool the relevant part of the chamber, typically using liquid nitrogen. However, due to the cost of the nitrogen and the need to ensure a continuous supply, TSPs are commonly operated at room temperature. Finally the residual gas composition is important - typically the pump works well with the more reactive components (such as CO
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
and O2
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
), but is very ineffective at pumping inert components such as the noble gasses. Therefore TSP must be used in conjunction with other pumps.
Other pumps which use exactly the same working principle, but using something other than titanium as a source are also relatively common. This family of pumps are usually called 'getter
Getter
A getter is a deposit of reactive material that is deliberately placed inside a vacuum system, for the purpose of completing and maintaining the vacuum. When gas molecules strike the getter material, they combine with it chemically or by adsorption...
pumps' or 'getters' and typically consist of metals which are reactive with the components of the residual gas which are not pumped by the TSP. By choosing a number of such sources, most constituents of the residual gas, except for the noble gasses, can be targeted.
Practical considerations
When mounting the TSP in the chamber, a number of important considerations must be made. First, it is desirable that the filament can deposit on a large area. However, one must take care that the titanium is not deposited onto anything it can damage. For example electrical feedthroughs containing ceramic insulators will fail if the titanium forms a conducting film which bridges the ceramic insulator. Samples may become contaminated by titanium if they have line-of-sight to the pump. Also, titanium is a very hard material, so titanium film which builds up on the inside of the chamber may form flakes which fall into mechanical components (typically turbomolecular pumpTurbomolecular pump
A turbomolecular pump is a type of vacuum pump, superficially similar to a turbopump, used to obtain and maintain high vacuum. These pumps work on the principle that gas molecules can be given momentum in a desired direction by repeated collision with a moving solid surface...
s and valves) and damage them.
Many chambers containing TSPs also have an ion pump
Ion pump (physics)
An ion pump is a type of vacuum pump capable of reaching up to 10−11 mbar under ideal conditions. An ion pump ionizes gases and employs a strong electrical potential, typically 3kV to 7kV, to accelerate them into a solid electrode...
. Often the ion pump provides a good location for the TSP, and some manufacturers promote the use of combined TSP/ion-pumps. Furthermore, TSPs have been shown to be effective against the regurgitation effects of ion pumps .
Common Misconceptions
Although there are titanium bases ion pumps, it is possible to falsely infer that the operation of these pumps is related. This has perhaps also fueled the misconception that a TSP operates in gas phase, i.e. the titanium atoms or clusters from the source collide with molecules from the residual gas, and form gaseous products which are heavier and therefore easier for ion-pumps and turbomolecular pumps to remove. Although this sounds plausible, TSPs are most effective when the residual gas pressure is sufficiently low for the gas to behave ballistically (i.e. the mean free pathMean free path
In physics, the mean free path is the average distance covered by a moving particle between successive impacts which modify its direction or energy or other particle properties.-Derivation:...
in the gas is large and thus the probability of collisions within the gas is insignificant compared to the probability or collision with the chamber walls). Credit for the invention of titanium sublimation vacuum pumps has generally been given to Linus Pauling
Linus Pauling
Linus Carl Pauling was an American chemist, biochemist, peace activist, author, and educator. He was one of the most influential chemists in history and ranks among the most important scientists of the 20th century...
. However, the team of brothers Benjamin and Jeremy Chubb built the first TSP in 1946 in the town of Glenview, IL. There is in Glenview a vacuum pump museum sponsored by The Oreck Foundation. There is a
large exhibit dedicated to the Chubb brothers and their many contributions to high vacuum technology.