| If a gas is ionized and the resulting positive ions are accelerated to a negatively charged plate, atoms of the gas are effectively removed from the system, and thus a pumping action is produced. The general term ion-pump includes those vacuum pumps in which gas molecules are pumped by being ionized and trans ported in the desired direction by an electric field. The ionization may be produced by collisions of the gas molecules with electrons emitted either from a hot filament or from a cold cathode discharge. The first type of pump is referred to as hot cathode ion-pump, while those belonging to the second type are known as cold cathode ion-pumps. Both the hot and cold cathode ion gauges act as pumps in this manner, and can be used to pump small closed systems, which have been previously evacuated to about 10-3 Torr. Pumping speeds for hot cathode gauges are typically of the order of 10-3 liter/sec, while cold cathode gauges can reach about 1 lit/sec. In order to increase the pumping efficiency, sorption and gettering phenomena are combined with ionization. Pumps which combine the action of an ion-pump and the sorption of the ions in a sorbent, are known as ion-sorption pumps. lon-sorption pumps in which a getter is continuously or intermittently vaporized and condensed on the trapping surface to give a fresh deposit of sorbent, are termed getter-ion-pumps. The vaporization is due to thermal evaporation in evapor-ion-pumps and to cathode sputtering in sputter-ion-pumps. Although ion-pumps without sorption or gettering were also constructed, commercial ion-pumps are either of the evapor-ion or of sputter-ion type. An electrical gas discharge, in which ions are formed, is basically capable of pumping gases, and one assumes that the formed ions are either bombarded into a metallic collector provided for the purpose or that these ions are trapped within the surface atoms of such a collector, due to a chemisorption effect. The pumping efficiency is expressed by the ratio i+/P between the ion current and the pressure of the gas in the device. The ion current i+ is proportional to the number of molecules entering the device per unit time, thus to the throughput Q. Therefore
i+/P = K(Q/P) = KS
and the pumping speed S is given by
S = b(i+/P) liter/sec
where b = 1/K is the constant of the pump expressed in Torr*liter/sec*Amp, i+ is the ion current (Ampere), P is the pressure (Torr). Unlike the diffusion pump, the ion-pumps (evapor-ion and sputter-ion) do not require a forepump to pump the collected gas up to atmospheric pressure, since the pumped gas is in effect trapped. However, an auxiliary pump is needed to
reduce the system pressure to the range of 10-3 - 10-4 Torr, where the ion-pumping action will commence. Mechanical or sorption pumps are usually used for this initial pumpdown. Evapor-ion pumps Evapor-ion-pumps combine the ion-pumping effect with the gettering process of evaporated active metal. The gettering effect is used both during evaporation (dispersal gettering), and in the form of a fresh film on a surface (contact gettering). The gas is ionized to ensure transport by electrical pumping of the gases (which are not gettered) to the getter-coated wall at which they are made to arrive with energies of a few hundred electron volts. At these energies, about 20% of the ions are retained, and embedded in the film as fresh getter is vaporized. The most widely employed getter. In these pumps is titanium. Sputter-ion-pumps The pumping action of a magnetically confined discharge observed by Penning (Penning and Nienhuis, 1949) was increased by using titanium cathodes (Gurenwitch and Westendrop, 1954). This action was used in multicell pumps, which give higher pumping speeds and attain lower pressures (Hall, 1958), and further improved by using combined titanium-zirconium cathodes (Hall, 1969).
The sputter-ion-pumps are designed such that an electrical discharge occurs between the anode and the cathode at a potential of several thousands of volts in a magnetic field of a few thousand gauss. Since the magnetic field causes the electrons to follow a flat helical path, the length of their path is greatly increased. A high efficiency of ion formation down to pressures of 10-2 Torr and less is assured by this long path length. The gaseous ions so formed are accelerated to the titanium cathode, where they are either captured or chemisorbed. Due to the high energies they are propelled into the cathode plate and sputter cathode material (titanium), some of which settles on the surfaces of the anode where it alsotraps gas atoms.
Sputter-ion pumps consist essentially of a stainless steel vessel, containing an anode of  honeycomb construction (see fig. 1), and a titanium cathode mounted opposite each end of the anode. A potential of about 3000 V is maintained between the electrodes and a magnetic field of about 1500 G is applied by external permanent magnets along the axis of the electrode system. Positive ions of system gas which are formed in the region of the electrodes are accelerated to the cathode and acquire sufficient energy to sputter titanium. The sputtered titanium condenses mainly on the open structure anode and in so doing pumps active gases by both dispersal and  contact gettering. Gas molecules which reach the anode by either of these processes are rapidly buried beneath succeeding layers of titanium and are thus permanently removed from the system. On the other hand, gas which reaches the cathode as positive ions has a high probability of being desorbed by succeeding ion bombardment. This is particularly so in the case of the inert gases since they can only be ion-pumped and then held at the cathode by the relatively weak forces of physical adsorption. Surprisingly, helium, for which normal sorption by any material is insignificant, is pumped quite well, apparently by being rather buried in the cathode material. Argon is most troublesome in this respect (fig. 2) and is the main factor governing the ultimate pressure attainable. The low pumping speed for argon is frequently of concern, since the atmosphere contains 1 percent argon. A further difficulty with ion-pumps is the care which must be taken when starting the pump at high pressures (max. 10-2 Torr). The ion current at high pressures is large and causes heating of the pump. If the pump has previously handled much gas the temperature rise leads to outgassing which in turn causes a larger ion current. Such a process rapidly becomes "run-away" leading to glowdischarge between the electrodesand a rapid rise in. system-pressure. Even if the evolution is not rapid the pumping speed is reduced giving what is termed "slow starting". These troubles can be largely overcome, by initially pumping to pressures of the order of 5 x 10-4 Torr before operating the sputter-ion-pump. | |
