An ion pump (also referred to as a sputter ion pump) is a type of vacuum pump capable of reaching pressures as low as 10−11 mbar under ideal conditions. An ion pump ionizes gas within the vessel it is attached to and employs a strong electrical potential, typically 3–7 kV, which allows the ions to accelerate into and be captured by a solid electrode and its residue.
- How Ion Pumps Work
- Early History
- Penning trap
- Standard diode pump
- Noble diode pump
How Ion Pumps Work
The first ion pump was invented by Varian Associates, now Agilent. The original activity of Varian, co-founded in 1948 by Russell Varian, the inventor of the klystron, and his brother Sigurd, was in the field of microwave electron tubes. Robert Jepsen joined the company in 1951 and soon became director of the Klystron research group. His investigation about electronic vacuum pumping led in 1957 to the realization of the first sputter ion pump (SIP), later named VacIon pump, of which he was co-inventor. The pump was developed as an appendage pump for maintaining Ultra High Vacuum (UHV) in microwave power tubes after processing, but soon after honeycomb-shaped anodes and commercial VacIon pumps with speeds of thousands of liters per second were produced. Lewis Hall and John Helmer were members of the research team, involved in the choice of the most suitable cathode material and in the optimization of the ion pump design. Sherm Rutherford joined Varian in February 1959 in the Central Research Department (which, soon after, spun off the Vacuum Division). His activity, working for Robert Jepsen, was to study the behavior of the sputter ion pump (in terms of pumping speed and discharge intensity I/P) over a wide range of parameters, such as magnetic field, voltage, anode cell diameter, anode cell length and pressure. Renn Zaphiropoulos joined the Vacuum Division in 1959 and was engaged in scaling up the ion pump from the original appendage pump to a full range of large pumps up to 5000 l/s . His group worked on high voltage feedthroughs, magnets, control units, systems, flanges, valves, sorption pumps and titanium Sublimation Pumps (TSP). In 1960 the “slotted” titanium cathode, named “Super VacIon Pump”, was introduced on the basis of observations about noble gas pumping mechanism and unstable Argon pumping phenomena. In the same years triode pumps were invented by W.M. Brubaker (Consolidated Electrodynamics Corporation) and Varian, now Agilent, began to sell them in the late ‘60s under the name “Noble Ion Pumps”, then changed the name to “Triode Ion Pumps” in the early ‘70s.
The basic element of the common ion pump is a Penning trap. A swirling cloud of electrons produced by an electric discharge is temporarily stored in the anode region of a Penning trap. These electrons ionize incoming gas atoms and molecules. The resultant swirling ions are accelerated to strike a chemically active cathode (usually titanium). On impact the accelerated ions will either become buried within the cathode or sputter cathode material onto the walls of the pump. The freshly sputtered chemically active cathode material acts as a getter that then evacuates the gas by both chemisorption and physisorption resulting in a net pumping action. Inert and lighter gases, such as He and H2 tend not to sputter and are absorbed by physisorption. Some fraction of the energetic gas ions (including gas that is not chemically active with the cathode material) can strike the cathode and acquire an electron from the surface, neutralizing it as it rebounds. These rebounding energetic neutrals are buried in exposed pump surfaces.
Both the pumping rate and capacity of such capture methods are dependent on the specific gas species being collected and the cathode material absorbing it. Some species, such as carbon monoxide, will chemically bind to the surface of a cathode material. Others, such as hydrogen, will diffuse into the metallic structure. In the former example, the pump rate can drop as the cathode material becomes coated. In the latter, the rate remains fixed by the rate at which the hydrogen diffuses.
There are three main types of ion pumps: the conventional or standard diode pump, the noble diode pump and the triode pump.
Standard diode pump
A standard diode pump is a type of ion pump employed in high vacuum processes which contains only chemically active cathodes, in contrast to noble diode pumps.
Noble diode pump
A noble diode pump is a type of ion pump used in high-vacuum applications that employs both a chemically reactive cathode, such as titanium, and an additional cathode composed of tantalum. The tantalum cathode serves as a high-inertia crystal lattice structure for the reflection and burial of neutrals, increasing pumping effectiveness of inert gas ions. Pumping intermittently high quantities of hydrogen with noble diodes should be done with great care, as hydrogen might over months get re-emitted out of the tantalum.
Ion pumps are commonly used in ultra-high vacuum (UHV) systems, as they can attain ultimate pressures less than 10−11 mbar. In contrast to other common UHV pumps, such as turbomolecular pumps and diffusion pumps, ion pumps have no moving parts and use no oil. They are therefore clean, need little maintenance, and produce no vibrations. These advantages make ion pumps well-suited for use in scanning probe microscopy and other high-precision apparatuses.
Recent work has suggested that free radicals escaping from ion pumps can influence the results of some experiments.