Working with ion getter pumps: everything you need to know
Ion getter pumps require a large magnetic field within an isolated chamber and use high voltages to pull electrons into the assembly. Pumping relies on the sputtering of getter materials inside a series of cells and by the implantation or burial of the ions produced.
The gas molecules pumped by chemisorption (gettered) and physisorption (ions) are now permanently “bound” and not able to “contribute” to the pressure inside the chamber.
The process is quite extensive and complex, so in this blog, we will explain how ion getter pumps work and how they are used.
Working principle of ion getter pumps
With ion getter pumps, initial pumping (usually managed by a turbomolecular pump combination) is used to remove the bulk gas until vacuum drops to approximately 10-4 mbar or lower.
After removing the bulk gas, a high voltage (of between 4,000 and 7,000 volts) is then applied through the element assembly. This “pulls” electrons into the cylindrical anode-tube assembly. The electrons are bound into tight spiral paths by a permanent magnet (of 0.12 Telsa field strength) located outside the vacuum chamber, thus forming a plasma discharge.
The ions created then bombard the titanium cathode plate. Pumping of the molecular/gas ions can then occur by implantation (physisorption). The bombardment causes the sputtering of Titanium atoms from the cathode lattice. This results in deposits on surrounding surfaces of sputtered film. This film produces pumping via gettering i.e. chemisorption of gas molecules.
Functional principles of ion getter pumps
There are three types of ion getter pump elements: the conventional diode (CV), the differential ion (DI or noble diode), and triode. Each type has its advantages and disadvantages, as listed below:
1. CV/Diode pump
The CV/Diode ion pump element provides the highest pumping speed for reactive gases and superior vacuum and electrical stability. It does not, however, provide long-term stability for the pumping of noble gases.
2. DI/Noble Diode
With slightly lower than CV/Diode ion pumping speeds, the DI/Noble Diode ensures stable noble gas pumping; retaining 80% of CV pumping speed. That said, it does use higher priced materials.
3. Triode pump
The triode pump element is a “mesh” configuration. It provides stable noble gas pumping, retains 80% of CV pumping speed and has a higher starting pressure. On the down side, ultra-high vacuum (UHV) pumping speed is reduced, electrical instability is common, and the manufacturing costs are higher.
What are the differences between conventional, differential and triode pumps?
The main difference between conventional, differential and triode pumps is the cathode material used.
In the case of the CV/Diode pump, the cathode material is made of titanium. The titanium cathode will react with getterable gases which can be pumped by chemisorption (e.g. N2, O2, H2, CO, CO2 water vapour and light hydrocarbons). Non-reactive noble gases are pumped mainly by ion implantation, which is why CV/Diode pumps have a significantly reduced pumping speed for noble gases.
For the DI/Noble Diode pumps, instead of titanium, the cathode material is made of tantalum. Tantalum is an extremely hard, high atomic mass material. As such, it reflects noble gas ions as neutral particles with much higher energy than titanium. This gives much higher implantation depth in the electrodes and physisorption (trapping).
Finally, the triode. The configuration of the triode is different to the CV and DI in that the rings are actually grounded, and it uses negative voltage titanium rings as the cathode.
A collector plate at anode potential is positioned behind the cathode. Often the inner wall of the pump vessel serves as the third electrode (at grounded potential). As a result, pumping speed and stability are higher. But over time titanium atoms will build up on these rings, creating some whiskers and reducing the space between the rings and the vacuum wall, introducing electrical instability.
For more details about the different pumping elements, watch the video below:
UHV Ion pumps: applications and advantages
Ion getter pumps, which operate in the 10-5 to 10-12 mbar range, are frequently used in general UHV systems such as molecular-beam epitaxy (MBE), surface analysis (e.g. scanning tunnel microscopes), other surface analysis instruments and in high-energy physics, such as colliders and synchrotrons.
As well as producing UHV pressures, ion getter pumps are:
absolutely hydrocarbon free,
operable at high temperatures,
highly resistant to radiation/magnetic fields,
without moving parts (and thus no vibrations).
Also, as no regeneration is required, they are low maintenance (with cathode replacement) and (unlike many vacuum pumps) can be used without inlet isolation valves. These advantages make ion getter pumps well-suited for high-precision apparatuses. Unfortunately, they can be poor at pumping noble gasses, require high voltage and magnetic field, and need a turbomolecular or other secondary pump to create the starting pressure.