US3176907A

US3176907A - Ion pump

Info

Publication number: US3176907A
Application number: US218917A
Authority: US
Inventors: Paul A Redhead
Original assignee: National Research Council of Canada
Current assignee: National Research Council of Canada
Priority date: 1962-08-23
Filing date: 1962-08-23
Publication date: 1965-04-06
Anticipated expiration: 1982-04-06
Also published as: GB1049125A

Description

April 6, 1965 P. A. REDHEAD 3,176,907

ION PUMP Filed Aug. 25, 1962 2 SheetsSheet l I/VVENTOR PAUL A. RED/{FAD PATENT AGE/VT April 6, 1965 P. A. REDHEAD ION PUMP 12 Sheets-Sheet 2 Filed Aug. 25, 1962 lNVE/VTO PAUL A. REDHE D PATENT AGE/VT United States Patent 3,176,907 EON PUW Paul A. Redhead, Ottawa, Gntario, (Zanada, assignor to National Research Council, Ottawa, flutario, Canada, a corporation of (Ianada Filed Aug. 23, 1952, Ser. No. 218,917 8 Claims. ($1. 230-69) 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 fields 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 difiYusion pumps. In recent years various types of ionization pumps have been developed. These pumps differ 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 fluid and therefore have one salient advantage over diffusion pumps which use a fluid 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 pump does sufier from a serious detect which might be called the historical effect, and which is one aspect of the general re-emission effect. When a first gas A is fed into the system and pumped away, and then a second gas 13 is introduced, it has been found that gas B is pumped but a small amount of gas A is evolved from the pump. This effect can be nullified in getter-ion pumps by covering the pumped gas with a layer of evaporated metal to prevent the gas from being re-emitted.

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

It is an object of 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 d sign of the pumping region.

Another object of 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 eificient 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 objects of the invention are realized by providing an ionization pump comprising a gas-tight enclosure defining two interconnected volumes, the first 3,l?fi,d? Fatented Apr. 8, 1965 said volume defining 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 evacuated, a metallic structure 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 strong magnetic field 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 region, said electric field being transverse to the said magnetic field.

In drawings which illustrate embodiments of the invention,

FIGURE 1 is a cross-sectional view of a getter-ion pump according to the invention,

FIGURE 2 shows section AA of FIGURE 1,

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

FIGURE 4 is a plural version of the pump capable of increased pumping speeds.

Referring to the drawings, FIGURE 1 shows a getterion pump according to the invention having an envelope indicated generally as 1, formed by an upper pole piece '7 and a lower pole piece 8, a generally cylindrical shaped outer barrier wall 13 made of stainless steel or other nonmagnetic material, a glass-to-metal seal 6 and tubular member 11 which would be connected to the system to be evacuated. This envelope defines 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 defining 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 field can be produced in the magnet gap. This requirement however must be consistent with the need to provide space for a discharge of sutficient size to he formed in the gap. The faces of the magnetic poles are covered with thin sheets 10 of a metal preferably titanium. Permanent magnets 14 which are magnetized along their length are positioned to contact pole pieces 7 and S which are preferably made of low reluctance mild steel to form a magnetic circuit and 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 19, a cathode. An anode in the form of a cylindrical ring 12 is positioned outwardly of magnet gap 3. The cathode 9 and 10 and

pole pieces

7 and 8 are operated at ground potential and anode 12 is operated at high positive potential with the result that an electric field is set up in the magnet gap region 3 generally at right-angles to the magnetic field mentioned above. An evaporator 26 is mounted on rod 5 having a heater winding 21 through which current can be passed by means of lead 22 which passes through glass seal 6. An accelerator electrode 23 in the form of two cylindrical tubes is positioned inwardly of cathode tubes 9. Rods 24 passing through the glass seal 6 hold the accelerator electrodes in position and provide an electrical connection so that the accelerator electrodes can be operated at a negative potential with respect to the cathode. A typical evaporator would comprise a sapphire rod on which is wound a tungsten heater wire, the heater wire in turn having a winding of titanium metal wound over its piece 7' by several bolts 31. A. gold wire 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 13 is welded or brazed to lower pole piece Cylindrical member 11 is connected by means of a glass=to.-metal' seal to the tubulation 19 of the external system. The gl'ass-to-metal seals can be standard Kovarglass seals or ceramic-metal seals.

FIGURE 2. is a cross-sectional view A-A through the pump shown in FIGURE 1 and shows a method of mounting the permanent magnets 1-4. Although twelve magnets are shown in this figure the number could be varied. quite. widely. If

pole pieces

7 and 8 shown in FIGURE 1, were constructed. of permanent magnet material a more efiicient device would be realized with the result that smaller or fewer permanent magnets would .be required. From this figure it will be seen that the device as shown, in FIG. 1 is, with the exception of the magnets, a figure of rotation and that. the discharge region (mag-net gap region) is an annulus completely encircling, the pumping region. 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. Itv can be seen that this results in a very efiicient device.

7 In operation a discharge is established in the region bounded by the pole faces, the anode and the catode. This, discharge region coincides generally with the magnet gap region 3 mentioned above. Stray electrons in the discharge region are attracted towards the anode which is at a. high positive potential with reference to the cathode. During their flight they strike gas molecules and if the electrons have sufficient energy they will ionize the gas moleculesand' produce electrons and positive'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. In the; pumping region the positive ions emerging from the Slit are accelerated by a'negative potential on the accel'erator electrodes 23. The evaporator 20 is operated at a positive potential so that no positive ions can reachit. The positive ions move radially inward, turn around, and finally strike the accelerator electrode. A heating current passed through windings 21- evaporates titanium from the surface of the evaporator. This evaporated titanium condenses onto the surface of the accelerator electrode entrapping positive ions and absorbing molecules. of the gas present. This gives rise to pumping action. The pumping region. and 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 region. 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 efliciency.

As mentioned above, the getter-ion type of pump has the advantage that titanium metal can be evaporated onto theaccelerator electrodes, before pumping a new gas, in sufiicient quantity to cover up any atoms of a gas pumped during a previous operation. This eliminates the: historical. eifect" mentioned above.

Typical operating levels of voltage and current in a getter-ion pump of the type described above would be, for example, +2 to +7 kv. on the anode, 2 kv. on the accelerator electrodes, the evaporator slightly positive i or at ground or floating, and. 7 to. 8 amperes through. the heating windings. The magnetic flux density in the gap would be approximately 1000 gausses. These values, of course, can be varied quite widely.

FIGURE 3 is a three-quarter view of a version of the pump showing external features especially the method ofture of these pumps could vary considerably within the scope of the invention. For example, the upper and lower pole pieces and the permanent magnets could be, made as a unitary structure out of magnetic material. The stainless steelbarrier wall could be in the form of a thin sheet in contact with a portion of the inner surface of this unitary structure. The anode structure positioning rod' could be 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, calcium, barium, etc., might be 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 con-.

nected to an external electrical potential source willcause I an electrical field to be established between the anode and cathode, a magnetic circuit arranged to established a magnetic field transverse 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 mag; netic field, an envelope defining 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 posir tioned to permit ready escape of positive ions from the glow-discharge zone i-ntothe sorption zone, a source of sorption metal positioned within said sorption zone, 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 glowdischarge zone from the sorption zone, and means to connect the said sorption zone to a structure to be evacuated. 2. An ion :pump comprising an anode and perforated cathode connected toa source of electrical potential so as to establish an electrical field between them, a magnetic circuit arranged to establish a magnetic field crossing the said electrical field generally at right angles, said crossed electrical and magnetic fields defining anannular glowdischarge ionization zone whose inner boundary is defined by said perforated cathode, an envelope defining a sorption'zone whose outer boundary is defined by the perforated cathode, a. source of sorption metal vapors positioned within the sorption zone, a substantial sorption area within the sorption zone on which sorpt-ionmetal vapors are condensed, the sorption area being outside of the glow-discharge zone, and means to connect said sorption zone to a structure to be evacuated. V 3. An ion pump as in claim 1 in which the source of sorption metal is an evaporator formed of a body sava e? of sorption metal and a heater winding to evaporate said sorption metal.

4. An ionization pump as in claim 2 in which said perforated cathode is defined by a plurality of cathode plates defining spaces therebetween.

5. 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 regions.

6. A cold cathode ionization pdmp 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 metallic structure mounted in the pumping region to provide metallic atoms for the absorbing of the gas molecules eing pumped, a magnetic circuit forming :part of said enclosure and having poles forming a magnet gap region therebetween, said magnet gap region being said dis charge 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 adjacent 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 electrical field in the discharge region, said electric field being transverse to the said mag netic field.

7. A cold cathode ionization pump of the ion-getter 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 metal evaporator formed of a source of metal and a heater winding to evaporate said source of metal 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 posi- No references cited.

LAURENCE V. EFNER, Primary Examiner.

WARREN E. COLEMAN, Examiner.

Claims

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 POTENTIAL SOURCE WILL CAUSE AN ELECTRICAL FIELD TO BE ESTABLISHED BETWEEN THE ANODE AND CATHODE, A MAGNETIC CIRCUIT ARRANGED TO ESTABLISHED A MAGNETIC FIELD TRANSVERSE 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, AN ENVELOPE DEFINING A SORPTION ZONE IN WHICH IONS AND MOLECULES ARE SORBED, THE CATHODE EXTENDING ALONG