US3074621A - Getter-ion pump - Google Patents

Description

Jan. 22, 1963 ENERGYSOURCE 8| CONTROL United States Patent 3,074,621 Patented Jan. 22, 1963 Of ce,

3,074,621 GETTER-ION PUMP Albert Lorenz, Hanan (Main), and Werner Blaske,

Ravolzhausen, Kreis Hanan (Main), Germany, assignors to W. C. Heraeus G.m.b.H., Hanau (Main), Germany, a firm Filed May 26, 1959, Ser. No. 815,944 Claims priority, application Germany May 30, 1958 6 Claims. (Cl. 230-69) The present invention relates to improvements in highvacuum pumps of the type known as getter-ion pumps, and more particularly to improvements in the evaporating mechanism for such pumps.

The name getter-ion pumps is derived from their function of evaporating suitable gettering substances and condensing the same on the walls of the pump housing, and in electrically ionizing the molecules of the gases to be evacuated, whereby a strong pumping action results. The gettering substances, usually in the form of a titanium wire, are evaporated in these pumps either continuously or intermittently and are then condensed and trapped on the inner wall surfaces of the pump, thus forming an active surface thereon. This depositing action is accelerated and increased by providing these pumps with suitable means lfOI' producing and emitting electrons which ionize the residual gases so that they will be propelled with considerable force upon and into the active surface. For this purpose and in order to render such ionization as complete and elfective as possible these pumps are also provided with suitable electrodes which are arranged so as to increase the distance of travel of the electrons as much as possible so that the latter will come into contact with as many gas molecules as possible and drive the same into the active surface of gettering substance in the inner walls of the pump housing where they will be trapped by chemical action and covered by subsequent layers of condensed gettering substance. The gases passing from the particular vessel to be evacuated into the getter-ion pump are therefore completely and continuously removed from the inside of the pump and thus also from the vessel to which the pump is connected so that the vacuum in both will be considerably increased. This manner of producing a high vacuum has the great advantage over that produced by oil diffusion pumps that the vacuum will be [free of any organic vapors.

An getter-ion pump as above described is usually used in cooperation with an auxiliary vacuum pump unit of a conventional type so that the latter will at first effect an initial evacuation of the particular vessel to be evacuated as well as of the getter-ion pump, whereupon the connection to the auxiliary pump unit is shut off and the evacuation is continued by the getter-ion pump. The final vacuum attained may thus be maintained for any desired length of time, for example, for several weeks or even months. In order to improve the efficiency of openation of these pumps, they are usually provided with a device which at first permits the evaporation of a greater amount of gettering substance so as to increase the initial vacuum produced by the auxiliary pump to as high a vacuum as possible without at first ionizing the gases. In the continued operation of the pump the gettering substance is then evaporated either continuously or intermittently either with or without an ionization of the residual gases, especially as soon as a slight increase in pressure is observed. In order to permit a continued operation of the pump for a long time, the same is usually provided with a coil of wire consisting of the respective gettering substance, preferably titanium, which will furnish an adequate supply for several weeks or months.

The evaporation of the gettering substance was carried out in the getter-ion pumps of prior designs by [feeding the getter wire to a small heated crucible which was mounted either within or outside of the area of the electrodes which were provided and arranged so as to increase the distance of travel of the gas ions and to eliminate the same. These prior pump designs have revealed certain disadvantages. The getter wire was fed to the evaporator crucible through a small guide tube. If this tube extended to a point close to the crucible, its front opening was easily clogged by a deposit of the evaporated metals, which then interfered with a continued feed of the getter wire. If, on the other hand, the guide tube was placed at a greater distance form the crucible, it easily occurred that the unguided end of the wire moved past the crucible so that an evaporation thereof did not occur. Either of these occurrences rendered the pump more or less or entirely inoperative and required an interruption of the evacuation and a release of the vacuum already attained since the pump housing had to be opened from the outside so that the deficiency could be corrected.

There have also been prior proposals to evaporate the end of a getter wire by means of electrons. The gettering substance is then freed of gases in a solid state by an electron bombardment but these gases do not impair the vacuum since they are trapped by the condensed gettering substance. The end of the getter wire is then heated by the impinging electrons to such a degree that it will evaporate, trap the residual gases while in a finely dispersed gaseous condition and, by forming large active surfaces, also trap and hold further gases or gaseous ions which are impinging on or are driven into these surfaces. The wires used for this purpose consisted of a composite of wires of a gettering substance which is easily vaporized and one which is vaporized only at very high temperatures. However, the pumps of this type require the provision of electrodes of some kind in the near vicinity of the evaporating end of the getter wire which then interfere with the condensation of the vapor of gettering substance on the larger, more remote surfaces of the pump housing. The vicinity of the electrodes therefore reduce the size of the active gettering surface and thus also the efiiciency of the pump and the degree of the vacuum attainable.

It is an object of the present invention to providea high-vacuum pump of the above-mentioned type, that is, a so-called getter-ion pump, in which the mentioned disadvantages of the previous pump designs are eliminated in a very simple. unobvious manner.

As distinguished from previous pump designs, the pump according to the invention does not involve the use of a heated element or of electrons which are active at only a short distance from their source in order to attain the desired evaporation of the gettering substance, but its operation essentially consists in evaporating the freely suspended or cantilevered end of a getter wire by focusing an electron beam from a considerable distance upon the end of the wire. Thus, the cantilevered end of a getter wire is evaporated within a large empty space, whereby the above-mentioned disadvantages will be entirely avoided which necessarily also occurred in all of the known methods of evaporating the gettering substance by means of electron rays.

For attaining this purpose, the present invention provides a suitable electron emitter of a type known as such within a separate chamber spaced from but communicating with the interior of the pump housing, and suitable electron-optical means also of a known type for focusing the electron beam upon a point within the pump housing through which the getter wire is fed so as substantially to intersect with the focal point of the electron beam. In order to increase the contact surface of the getter wire with the electron beam at the focal point thereof, it has been found advisable to feed the end of the getter wire at a relatively small angle to the axis of the beam, preferably at less than 30. If the evaporation of the gettering substance is to be carried out intermittently, the electron emitter is switched off during each interval of no evaporation, and the getter wire is during this period only fed for a short distance so that the end surface of the wire will remain within or close to the focal point of the electron beam.

The direction of feed of the getter wire is preferably opposed to the direction of flight of the electrons, and the getter wire is guided to a point close to the point of intersection with the electron beam by being passed through a small guide tube.

Although the axis of the electron beam may be disposed at any desired angle to the axis of the electrode system which is used for ionizing the residual gases, it has been found especially advisable to emit the electron beam in a direction past this electrode system and preferably in a direction at a right angle to the axis of this system. Furthermore, the getter wire is preferably fed into the actual pump housing from the side thereof opposite to the side from which the electron beam is emitted so as to avoid any screening effect by the guide tube of the getter wire upon the area,within which the ionizing electrodes are located.

The difiiculty of properly evaporating the thin cantilevered getter wires has been solved according to the invention by providing the getter Wire in the form of a composite of several wires which consist of the respective gettering substance, for example, titanium and of a metal which has a higher melting point, for example, tantalum. The higher-melting metal then forms a carrier for the molten gettering substance so that it will evaporate without any difficulties.

By means of such composite wires it is also possible to adjust or select the gettering substances in accordance with any special types of gases to be acted upon. It has been found advisable to apply in the composite wire not merely individual wires of one gettering substance but of several gettering substances and to combine them with other wires of higher-melting metals. Thus, by selecting a suitable combination of gettering substances, certain sorts of gases or gas combinations may be evacuated and eliminated in a very simple manner.

These and other objects, features, and advantages of the present invention will become further apparent from the following detailed description, particularly when read with reference to the accompanying drawing which shows diagrammatically a cross section of a vacuum pump according to the invention.

As illustrated in the drawing, the high-vacuum pump :according to the invention is removably connected by means offlanges 2 to any desired vessel 1 to be evacuated. It essentially consists of asteel vessel 3, the lower end of which also has a flange which may be either secured to a cover plate 4-, .as shown, or be connected through a tightly sealing valve, not shown, to a roughing pump of a known design, for example, a diflusion pump, for attaining a preliminary evacuation of the interior of the pump and the vessel 1.

Thepumping vessel 3 forms a housing whichcontains acylindrical electrode 5 consisting of a coarse-meshed rnetal screen which is connected to the positive potential of a high tension D.C. generator 20 having an output of, for example, 2000 volts, While the pumping vessel itself is connected to the negative potential of generator 20. Awire 6 is mounted withinvessel 3 at a point above thecylindrical electrode 5 and is heated electrically in the conventional manner to a high temperature so as to emit electrons which are adapted to dissociate and ionize the residual gas within the fieldless area ofelectrode 5. The positive potential ofelectrode 5 relative to thehousing 3 of the pumping vessel results in an acceleration of the ions into the active surface formed on the inner side of the Walls ofpump housing 3 by the layer of gettering substance which is deposited thereon by condensation.

At the left side of thepumping vessel 3, as viewed in the drawing, there is an apparatus connected thereto for producing abeam 10 of electrons and for focusing the same upon a point 11 approximately at the center ofhousing 3. This electron emitter 7 is of a conventional construction and consists of a chamber 7 which is connected to and forms a lateral extension ofpump housing 3 and contains aheating filament 8, the lead wire of which is insulated from the walls of chamber 7. This electron emitter 7 further includes conventional electron-optical means, here indicated in the form of a field coil 9, for focusing thebeam 10 of electrons upon point 11.

From a point diametrically opposite to the electron emitter 7, thegetter wire 12 is passed substantially into the focal point of the electron beam at an angle relative to the axis of the beam. This angle should be relatively small, preferably less than 30, so that the electron beam will come in contact with a larger portion of thewire 12. The getter wire is guided by atube 13 which terminates at a point which is spaced only a short distance from the focal point 11, for example, less than 5 mm., and it is withdrawn from a coil ofwire 16 and passed into and throughtube 13 by means of a reciprocating feed member 14 which may, for example, be driven by asuitable mechanism 15 located at the outside of the vacuum pump, although it may also be disposed Within the pump and even at the point of or adjacent to the reciprocating member 14.

The drawing only illustrates the most important parts of the vacuum pump and their arrangement relative to each other, but not the details of the mechanical construction of these parts. As already indicated, theelectron source 6 in the form of a wire receives the same potential as thepump housing 3, while thecylindrical cathode 5 receives the opposite positive potential of at least several hundred volts. Thegetter wire 12 together with its associatedmeans 13, 14, 15, and 16 likewise receives the housing potential, while the electron emitter 7 for producing theelectron beam 10 receives a negative potential of a few thousand volts. In the particular embodiment illustrated, theelectron beam 10 extends in a direction transverse to the axis of thecylindrical electrode 5 and the wire-shaped electron source 6, and its focal point 11 is substantially determined by the focusing properties of the electron-optical system, that is, in this particular example, of the field coil 9. The focal point 11 is therefore a point in space which is quite accurately determined by the electron emitter 7 and by the focusing of the resulting electron beam.

The gettering substance which is evaporated at the point 11 is then deposited on the inner surfaces of the walls ofhousing 3. During the acceleration of the individual getter atoms, considerable amounts of the gas present will be bound which will then be immediately replaced by other gas molecules or atoms which difiuse from the vessel 1 to be evacuated into thepump housing 3. They are partly dissociated and ionized by the electrons which are produced by the wire-shaped electron source 6 and carry out oscillating movements in the field-free interior of thecylindrical electrode 5. These ions pass into the space between the positively chargedelectrode 5 and the oppositenegative housing walls 3 and are then accelerated by the field therein toward the inner surface ofhousing 3 and penetrate at a considerable energy into the gettering layer deposited thereon. As soon as this layer is substantially saturated, the vacuum will gradually increase to a slight extent and thereafter be restored by a renewed evaporation of gettering substance. This proceeding may be controlled in a known manner by means of avacuum gauge 17 and energy source andcontrol 18 for the emitter 7 which, in

combination with arelay system 19, controls the rate of feed of thegetter wire 12 as well as the operation of the electron emitter 7. The electron emitter 7 only needs to be turned on to deliver its full output when new getter wire has to be evaporated. Consequently, the electron emitter 7 does not have to be in operation for considerable periods of time or it may be heated to a lower degree and will therefore last for a long time. Thefeeding mechanism 14, 15 may be operated to feed thegetter Wire 12 for the required distance Whenever necessary, for example, during the periods when the electron emitter 7 is switched off.

If during the short period of the evaporation of the gettering substance there should be a considerable accumulation of metallic vapor wihtin the pump, there is a certain danger that this vapor may cause a gas discharge betweenelectrode 5 and pumphousing 3 or thewireshaped electron source 6. It is therefore advisable during the period of evaporation to switch off the current from theelectrode 5.

The most efiicient operation of the pump will therefore be attained if, first, the axis of theelectron beam 10 is disposed substantially at a right angle to the axis of the electrode system formed of theelectrodes 5 and 6, second, if the direction of feed of thegetter wire 12 is substantially opposed to the direction of emission of the electron beam 14 and at a relative small angle to the axis of the latter, third, if the electron emitter 7 as well as thefeed mechanism 14, 15, 16 of the getter wire are disposed outside of theactual pump housing 3 so as not to interfere with the flight of the electrons fromelectrodes 5 and 6 and so as to be able to focus the electron beam from a considerable distance upon the end of thegetter wire 12.. The most preferred embodiment of the invention therefore has a confi uration as diagrammatically illustrated in the drawing in which theelectrode system 5 and 6 is disposed within the lower part ofpump housing 3 opposite to the end thereof which is connected to the vessel 1 to be evacuated, and with its axis extending in a substantially vertical direction, and further in which the electron emitter 7 and the wire feed mechanism are disposed in chambers which project outwardly from the actual pump housing at points which are substantially diametrically opposite to each other and are located above theelectrode system 5, 6, and in which the electron emitter 7 is disposed so that theelectron beam 19 extends at a substantially right angle to the vertical axis of theelectrode system 5, 6, while the chamber containing thewire feed mechanism 14, 15, 16 preferably extends at an oblique angle in a downward direction toward the actual pump housing so that thegetter wire 12 will be fed at a small angle toward the axis of theelectron beam 10 and also at a direction substantially opposite to the direction of emission of the beam.

An getter-ion pump of the design as above described has proved to be very successful even during a very long continued operation and did not reveal any of the deficiencies of similar pumps of previous designs.

Although our invention has been illustrated and described with reference to the preferred embodiments thereof, we Wish to have it understood that it is in no Way limited to the details of such embodiments, but is capable of numerous modifications within the scope of the appended claims.

Having thus fully disclosed our invention, what we claim is:

1. An getter-ion pump comprising a pump housing having an inlet, means for feeding a getter wire toward substantially the center of said housing, a casing mounted on the housing and having its interior in communication with the interior of the housing through an aperture formed on the wall thereof, said casing having mounted therein means for generating an electron beam, and means for focussing said beam into the housing and through said aperture in a direction toward the free end of the getter wire for evaporating said free end.

2. An evaporation pump according to claim 1, in which the distance between the aperture and the intersection point of the electron beam with the getter wire is large compared with the diameter of said aperture.

3. An evaporation pump according to claim 1, wherein the direction of flight of said electron beam being substantially opposite to the feeding direction of the getter wire, said electron beam having its axis disposed at an angle of between 10 to 30 degrees relative to the line of said feeding direction of the getter wire, the intersection of said beam with said getter wire being substantially near the focal point of said electron beam.

4. An evaporation pump as claimed in claim 1, wherein said means for generating an electron beam comprises an electrically heated means, and means for considerably reducing the heating effect of said last named means at predetermined periods of the operation of said pump.

5. An getter-ion pump comprising a pump housing having an inlet, a pair of later-ally disposed casings mounted on said housing substantially diametrically opposite to one another, one of said casings being connected for communication with the housing through a relatively small aperture, means for generating an electron beam mounted in said one casing, means mounted on said one casing for focussing said electron beam through said aperture and substantially in the center of said housing, the other of said casings being connected for communication with the housing and containing a getter wire feeding means, said feeding means being adapted to feed the wire toward substantially the center of said housing, the direction of flight of said electron beam being substantially opposite to the feeding direction of the getter wire, said electron beam having its axis disposed at an angle of between 10 to 30 degrees relative to the line of said feeding direction of the getter wire.

6. An evaporation pump according toclaim 5, including an electrode system mounted Within the housing and having an electrically heated electron emitter and an electrode, said electrode system having an axis positioned substantially at a right angle with respect to the axis of said electron beam generating means and said focussing means, and a source of electric current connected to said electrode system for supplying the electrode system with electric energy.

References Cited in the file of this patent UNITED STATES PATENTS 2,727,167 Alpert Dec. 13, 1955 2,796,555 Connor June 18, 1957 2,850,225 Herb Sept. 2, 1958 FOREIGN PATENTS 795,551 Great Britain May 28, 1958

Claims (1)

1. AN GETTER-ION PUMP COMPRISING A PUMP HOUSING HAVING AN INLET, MEANS FOR FEEDING A GETTER WIRE TOWARD SUBSTANTIALLY THE CENTER OF SAID HOUSING, A CASING MOUNTED ON THE HOUSING AND HAVING ITS INTERIOR IN COMMUNICATION WITH THE INTERIOR OF THE HOUSING THROUGH AN APERTURE FORMED ON THE WALL THEREOF, SAID CASING HAVING MOUNTED THEREIN MEANS FOR GENERATING AN ELECTRON BEAM, AND MEANS FOR FOCUSSING SAID BEAM INTO THE HOUSING AND

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