US3176906A - Ion pump - Google Patents

Description

PYI;A 6i 1965* P; A.r REDHEAD 31,176,906?

rom BUMP Aug.. 2.15,. 1962 2 Sheds-Sheet l PAUL A. REDHEAD PATENT AGENT United States Patent O M' 3,1%,9-35 IGN PUMP Paul A. Redhead, ttawa, @nto io, anada, assigner to National Research Council, @travi/a, Gntario, Canada, a body corporate ci Canada Filed Aug. 23, w62, Ser. N Ztl i3 Claims. (Qi. 23u-ne?) This invention relates to a cold cathode ionization vacuum pump and more particularly to a magnetron type ionization vacuum pump having a discharge region in which electric and magnetic lields are produced substantially at right angles The production of low pressures has been achieved in the past by the use of a combination of mechanical pumps and diiusion pumps. In recent years various types of ionization pumps have been developed. These pumps diier from the mechanical and diffusion pumps in that they do not remove gas from the vacuum system but rather by immobilizing the gas within the system by a combination of chemical adsorption and entrapping of the ions of gas. ionization pumps contain no lluid and Vtherefore have one salient advantage over diffusion pumps which use a uid e.g. oil or mercury which can reach the vacuum system causing contamination.

There are two general types of ionization pumps known and in use at the present time. In the first type, which is known as a sputter-ion pump, the positive ions formed in the discharge region bombard a metal surface, usually titanium and sputter metal atoms from this surface. The sputtered material condenses on other portions of the device provided for this purpose, entrapping and adsorbing the chemically active (other than inert) gases in the system. The second type of pump, which is known as a getter-ion pump contains a source of metal usually titanium, which is evaporated. This evaporated material condenses on surfaces provided in the system, entrapping and absorbing the chemically active gases.

The sputter-ion type of pump is in more widespread use in that it does not require a source of heater current for an evaporator which is the case with the getter-ion type.

All ionization pumps using a magnet-ic lield in use at the present time are of the parallel-ield type in that the electric and magnetic fields are parallel. Because of the geometry of parallel-held pump the magnets required to produce the magnetic iield are large and heavy.

It is an object ot the present invention `to provide an ionization vacuum pump which is light in weight, simple in design, and has greatly increased pumping speeds in relation to its size and weight.

It is another object of the invention to provide an ionization pump that has its pumping area separated from the discharge region allowing wide flexibility in the design of the pumping region.

Another object oi this invention is to provide an ionization pump in which the magnetic field structure forms part of or is in close proximity to the envelope thus achieving a lighter and more ellicient device.

Another object of this invention is to provide a pump Whose geometry is such that a tube of large diameter can be readily used to connect the pump to the system required to be evacuated, allowing high pumping speed.

These and other obiects of the invention are realized by providing an ionization pump comprising a gas-tight enclosure delining two interconnected volumes, the iirst said volume deuing a discharge region and the second said volume defining a pumping region, means for connecting the pumping region to a system that is to be evacusted,` a metallic structure mounted in the pumping "region to provide metallic atoms for the absorbing Patented Apr. d, 1%55 ICC of the gas molecules being pumped, a magnetic circuit `forming part of said enclosure and having poles forming a magnet gap region therebetween, said magnet gap region being said discharge region, a permanent magnet in said magnetic circuit to produce a strong magnetic lield in the discharge region, a cathode positioned adjacent the interconnection between the pumping region and the discharge region, an anode positioned adjacent the magnet gap and opposite the said cathode, and lead means to supply a potential to the anode to produce an electric field in the discharge regio-n, said electric lield being transverse to the said magnetic field.

In drawings which illustrate embodiments of the inventon,

FIGURE l is a cross-sectional view of a sputterion according to the invention, v

FGURE 2 shows section A-A of FGURE 1,

FIGURE 3 is a three-quarter View of a sputter-ion pump according to the invention showing the positioning of the permanent magnets,

FGURE 4 is a plural version of the pump capable of increased pump-ing speeds.

Referring to the drawings, FIGURE l shows a sputter-ion pump according to the invention having an envelope indicated generally as l, formed by an upper pole piece '7 and a lower pole piece S, a generally cylindrical shaped outer barrier wall i3 made of stainless steel or other non-magnetic material, a glass-to-metal seal d and tubular member il which would be connected to the system to be evacuated. This envelope denes a first Volume shown generally as 2, to be referred to below as the pumping region. Pole pieces 7 and 8 have raised portions extending towards each other, forming north and south magnetic poles N and S and dening a second region (magnet gap) between them shown generally as 3. The poles should be relatively close to each other so that a strong magnetic ield can be produced in the magnet gap. This requirement however must be consistent with the need to provide space for a discharge of sumcient size to be formed in the gap. The faces of the magnetic poles are covered with thin sheets llt) oi a metal preferably titanium. Permanent magnets ld which are magnetized along their length are positioned to contact pole piece and 8 which are preferably made of low reluctance mild steel to form a magnetic circuit md provide a strong magnetic field in magnet gap 3. Cylindrical tubes 9 of sheet titanium are positioned adjacent the interconnection between pumping region 2 and magnet gap 3 and form, with sheets itl, a

cathode. An anode in the form of a cylindrical ring 121 is positioned outwardly of magnet gap 3. The cathode 9 and itl and pole pieces 7 and S are operated at ground potential and anode l2 is operated at high positive potential with the result that an electric iield is set up in the magnet gap region 3 generally at right-angles to the magnetic iield mentioned above. A sputter cathode d mounted on rod 5 extending through glass seal 6 is positioned centrally of pumping region 2 and generally facing slit if formed by tubes 9. Sputter cathode d is formed of a cylindrical sheet or solid rod of titanium. Anode i2 is held in position by rod l5 which passes through barrier wall i3 by means of glass-to-metal or ceramic-metal seal ld. Rod l5 also acts as an electrical lead for the anode and would be connected at 17 to a source offhigh direct current voltage. Barrier Wall 13 is sealed to upper pole pieces 7 by means of an insert ring 32 which is welded or brazed to barrier wall i3 and bolted to pole iece '7 by several bolts 3l. A gold wireY O-ring seal 33 is compressed by the tightening of bolts 31 providing the required seal between pole piece 7 and insert ring 32. Barrier wall i3 is welded or brazed to lower pole piece d. Cylindrical member il. is connected by means Ythe pumping region.

- region.

of a glass-to-metal seal to the, tubulation 19 of the external system. The glass-to-metal seals can be standard Kovar-glass seals or ceramic-metal seals.

FIGURE 2 is acrosssectional view^AA through the pump shown in FIGURE 1 and shows a method of mounting the permanent magnets 14. Although twelve magnets are shown in this figure the number could be varied yquite widely. If pole pieces 7 and 8 shown in FIGURE 1 were constructed of permanent magnet material a moreV efiicient device would be realized with the result that smaller or fewer permanent magnets would be ait/esce required.v From this figure it will be seen that the device y as Ashown in FIG. l is, with the exception of the magnets, a figure of rotation and that the discharge region (magnet gap region) is an annulus completely encircling The discharge region which acts as a source of positive ions is able to supply ions to converge on the sputter cathode from a 360 sector. It can be seen that this results in a very efficient device.

In operationa discharge is established in the region bounded by the pole faces, the anode and the cathode. This discharge region coincides generally with the magnet gap region 3 mentioned above. Stray electrons in Athe discharge region are attracted towards the anode cules and if the electrons have sufficient energy they will ionize the gas molecules and produce electrons and pos-itive ions. These positive ions are attracted towards the cathode and a large proportion will have sufiicient energy to pass through slit 16 in the cathode tubes into the pumping region. These positive ions strike the sputter cathode 4 where some will be trapped in the sputter cathode and others upon collision with the sputter cathode will disintegrate portions of the cathode. The disintegrated materialV from the cathode is sputtered over the interior of the cathode tubes where it condenses entrapping and absorbing further gas molecules. This 'gives rise to pumping action. the discharge region overlap to some extent with the -discharge extending some distance into the pumping region and pumping action taking place in the discharge It should be pointed out that the action described here as absorption also includes the phenomenon usually described as adsorption.

The crossed electric and magnetic fields produce a discharge region similar to that in a magnetron. Electrons on their Way toward the anode travel in spiral orbits which increases greatly the probability of their striking and ionizing a gas molecule. This results in highly increased efficiency. Typical values for the anode voltage Would be, for example, 2 to 7 kv. and for the magnetic flux density in the gap, 1000 gausses. v It should be realized however that these values could be varied quite widely and still obtain good pumping performance.

FIGURE 3 is a three-quarter View of a Version of the pump showing external features especially the method of positioning of magnets 14.

If very fast pumping speeds are required, a plural version of the pump could be built and FIGURE 4 shows an example Vof a sputter-ion pump thatrwould have parallel pumping action at three positions.

In the above description a typical example of a method of producing these pumps has been shown. It should be realized that the invention-claimed in this application is directed to the novel arrangements of the magnetic circuit, the pumping region, the discharge region and the different electrodes positioned therein.V The method of manufacture of these pumps could vary considerably within the scope of the invention. For example, the upper andlower pole pieces and the permanent magnets Vcould be made as a unitary structure out of magnetic material. The stainless steel barrier Vwall could be in the form of a thinsheet in contact with a portion of the The pumping region and inner surface of this unitary structure. The anode structure positioning rod could bey taken through both the stainless steel sheet and the magnetic material by means of a glass-to-metal seal. In some cases, the barrier wall might be eliminated altogether. In ionization pumps, titanium has been the metal most generally used to absorb and entrap the gas molecules. Other suitable metals such as molybdenum, tantalum, tungsten, zirconium, iron, y calcium, barium, etc., might Vbe used. The present invention is not primarily concerned with the metal used.

What is claimed is:

1. An ion pump comprising a cathode and an anode spaced apart :to define therebetween a glow-discharge zone in which gas ionization is accomplished, electrical connections to said cathode and anode which When connected to an external electrical potentialV source will cause an electrical field to be established between the anode and cathode, a magnetic circuit arranged to establish a magnetic field Itransverse to the said electrical field, the glowdischarge zone having a long dimension transverse to the magnetic field which is at least several times greater than that dimension of the zone which is parallel to the magnetic field, a sorption zone in which ions and molecules are sorbed, the cathode extending along said long dimension, the cathode serving to separate the glow-discharge and sorption zones, the cathode being positioned to permit ready escape of positive ions from the glow-discharge zone into the sorption zone, a source of sorption metal positioned within said sorption zone,.said source being a sputter cathode positioned in said sorption zone Such that positive ions from the glow-discharge zone can strike its surface and disintegrate metal therefrom, means defining a substantial sorption area inthe sorption zone on which sorption metal is deposited, the sorption area being outside of the glow-discharge zone, and being substan-' tially greater than the area of cathode separating the glow-discharge zone from the sorption zone, `and means to connect said sorption zone to a structure to be evacuated.

2. An ion pump comprising an anode and perforated `cathode connected to a source of electrical potential so the glow-discharge zone can strike its surface and disintegrate metal therefrom, a substantial sorption area within the sorption zone on which sorption metal vapors areV condensed, the sorption area being outside of the glowdischarge zone, and means to connect said sorption zone to a structure to be evacuated.

3. A cold cathode ionization pump comprising in combination: means defining a glow-discharge region for ionizing gases therein, said means comprising anode means and cathode means, said anode and cathode means being arranged to provide an electric field in said glow-discharge region; magnetic field producing means for providing a magnetic field Itransverse |to said electric field; said glow-discharge defining means being arranged to define a long characteristic dimension for said region having atleast a major component transverse to said magnetic field and Va short characteristic dimension having at least aV major component parallel to said magnetic field; means defining a sorption region separate from said glow-discharge region and connected to it such that ions originating 'in said glow-discharge region can, enter directly into said Vsorption zone; a source of sorption metal positioned in said sorption region capable of disintegration by ion bombardment by ions entering said sorption region from said glow-discharge region; and means to connect said sorption zone to a structure to be evacuated.

4. A cold cathode ionization pump as in claim 3 in which said cathode means is arranged to provide openings therein for gas conductance.

5. A cold cathode ionization pump as in claim 3 in which said cathode means comprises a plurality of spaced cathode plates defining a gas conductance space therebetween.

6. An ionization pump as in claim 2 in which said perforated cathode is defined by at least two cathode plates defining an aperture therebetween.

7. An ionization pump as in claim 2 in which said perforated cathode is defined by a pair of cathode rings, said rings being coaxial and longitudinally spaced to define a gas conductance path between said ionization and sorption zones.

8. An ionization pump as in claim 3 in which said anode means comprises a plate and said cathode means comprises a second plate structure spaced from and parallel to said anode plate, said pump further comprising secondary cathodes consisting of members extending transversely from said cathode means towards said anode means.

9. An ionization pump as in claim 3 in which the magnetic tield producing means comprises a permanent magnet structure, said structure being horseshoe shaped in cross-section and spanning the far side of said anode means, such that said magnet struc-ture defines a partial envelope housing lthe ionization region.

10. A cold cathode ionization pump of the sputter-ion type comprising a gas tight enclosure defining two interconnected volumes, the first said volume being a discharge region and the second said volume being a pumping region, means for connecting the pumping region to a system that is to be evacuated, a sputter cathode mounted in the pumping region to provide metallic atoms for the absorbing of the gas molecules being pumped, a magnetic circuit forming part of said enclosure and having poles forming a magnet gap region therebetween, said magnet gap region being said discharge region, a permanent magnet in said magnetic circuit to produce a magnetic field in the discharge region, a cathode having a surface capable of absorbing gas molecules positioned ad jacent the interconnection between the pumping region and the discharge region, an anode positioned adjacent the magnet gap region and opposite the said cathode, and lead means to supply a potential to the anode to produce an electric field in the discharge region, said electric field being transverse to the said magnetic field.

11. A cold cathode ionization pump comprising in combination: means defining a glow-discharge region for ionizing gases therein, said means comprising an anode and a cathode arranged to provide an electrical field in said glow-discharge electric lield; magnetic field producing means for providing a magnetic lield transverse to said electric field, said magnetic iield producing means comprising a permanent magnet structure, horseshoe shaped in cross-section and partially encircling said anode, arranged to provide a portion of the envelope defining said glow-discharge region; said glow-discharge defining means being arranged to define a long characteristic dimension for said region having at least a maior component transverse to said magnetic field and a short dimension having at least a major component parallel to said magnetic tield; means defining a sorption region separate from said glow-discharge region and connected to it such that ions originating in said glowfdischarge region can enter directly into said sorption zone; and a sputter cathode mounted in the said sorption region to provide me- |tallic atoms for the sorbing of the gas molecules being pumped.

12. A cold cathode ionization pump as in claim ll in which the said sorption region comprises tubulation in Ithe central :region surrounded by said ionization region, the inner surface of said tubulation providing said sorption area and said tubulation connected to a system to be pumped to provide a high. conductance gas inlet passage for said pump.

13. An ionization pump as in claim 1 in which a plud rality of stacked glow-discharge zones are provided adjacent a common sorption zone.

No references cited.

LAURENCE V. EFNER, Primary Examiner.

WARREN E. COLEMAN, Examiner.

Claims (1)

1. AN ION PUMP COMPRISING A CATHODE AND AN ANODE SPACED APART TO DEFINE THEREBETWEEN A GLOW-DISCHARGE ZONE IN WHICH GAS IONIZATON IS ACCOMPLISHED, ELECTRICAL CONNECTIONS TO SAID CATHODE AND ANODE WHICH WHEN CONNECTED TO AN EXTERNAL ELECTRICAL POTENTIAL SOURCE WILL CAUSE AN ELECTRICAL FIELD TO BE ESTABLISHED BETWEEN THE ANODE AND CATHODE, A MAGNETIC CIRCUIT ARRANGED TO ESTABLISH A MAGNETIC FIELD TRANSVERSE TO THE SAID ELECTRICAL FIELD, THE GLOWDISCHARGE ZONE HAVING A LONG DIMENSION TRANSVERSE TO THE MAGNETIC FIELD WICH IS AT LEAST SEVERAL TIMES GREATER THAN THAT DIMENSION OF THE ZONE WHICH IS PARALLEL TO THE MAGNETIC FILED, A SORPTION ZONE IN WIXH IONS AND MOLECULES ARE SORBED, THE CATHODE EXTENDING ALONG SAID LONG DIMENSION, THE CATHODE SERVING TO SEPARATE THE GLOW-DISCHARGE AND SORPTION ZONES, THE CATHODE BEING POSITIONED TO PERMIT READY ESCAPE OF POSITIVE IONS FROM THE GLOW-DISCHARGE ZONE INTO THE SORPTION ZONE, A SOURCE OF SORPTION METAL POSITIONED WITHIN SAID SORPTION ZONE, SAID SOURCE BEING A SPUTTER CATHODE POSITIONED IN SAID SORPTION ZONE SUCH THAT POSITIVE IONS FROM THE GLOW-DISCHARGE ZONE CAN STRIKE ITS SURFACE AND DISINTEGRATE METAL THEREFROM MEANS DEFINING A SUBSTANTIAL SORPTION AREA IN THE SORPTION ZONE ON WHICH SORPTION METAL IS DEPOSITED, THE SORPTION AREA BEING OUTSIDE OF THE GLOW-DISCHARGE ZONE, AND BEING SUBSTANTIALLY GREATER THAN THE AREA OF CATHODE SEPARATING THE GLOW-DISCHARGE ZONE FROM THE SORPTION ZONE, AND MEANS TO CONNECT SAID SORPTION ZONE TO A STURCTURE TO BE EVACUATED.

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