US3489336A

US3489336A - Getter-ion pump for producing and maintaining a high vacuum

Info

Publication number: US3489336A
Application number: US750442A
Authority: US
Inventors: Max-Josef Schonhuber
Original assignee: Individual
Current assignee: Individual
Priority date: 1967-08-25
Filing date: 1968-08-05
Publication date: 1970-01-13
Anticipated expiration: 1987-01-13
Also published as: NL6811185A; GB1188615A; CH479161A; FR1577681A; DE1539151B2; DE1539151A1

Description

Jan. 13, 1970 MAXJOSEF SCHDNHUBER (BETTER-ION PUMP FOR PRODUCING AND MAINTAINING A HIGH VACUUM 5 Sheets-Sheet 1 Filed Aug. 5, 1968 Fig.7

Fig.5

3, 1970 MAX-JOSEF SCHONHUBER 3,489,336

GETTER-ION PUMP FOR PRODUCING AND MAINTAINING A HIGH VACUUM Filed Aug. 1968 5 Sheets-Sheet 2 UL z0 3 14 14 A i E? 4 I Fig. 2

Jan. 13, 1970 MAXJOSEF SCHONHU-BER 3,489,336

GETTER-ION PUMP FOR ERODUCING AND MAINTAINING A HIGH VACUUM Filed Aug. 5, 1968 5 Sheets-Sheet 3 Jan. 13, 1970 MAX-JOSEF SCHONHU BER ,489,336

GETTERION PUMP FOR PRODUCING AND MAINTAINING A HIGH VACUUM I Filed Aug. 5, 1968 5 Sheets-Sheet 4 1970 MAX-JOSEF SCHONHUBER 3,489,336

GETTERION PUMP FOR PRODUCING AND MAINTAINING A HIGH VACUUM Filed Aug. 5, 1968 5 Sheets-Sheet 5 28 u u [1 H96 United States Patent 3,489,336 GETTER-ION PUMP FOR PRODUCING AND MAINTAINING A HIGH VACUUM Max-Josef Schiinhuber, 1 Seefeldquai, 8008 Zurich, Switzerland Filed Aug. 5, 1968, Ser. No. 750,442 Claims priority, application Switzerland, Aug. 25, 1967, 11,995/ 67 Int. Cl. FtMh 37/02; H01j 7/16 US. Cl. 230-69 16 Claims ABSTRACT OF THE DISCLOSURE The invention concerns a getter-ion pump for producing and maintaining a high vacuum with electrodes between which an electrical discharge occurs.

Up to now such ion pumps are known as evapor-ion pumps and particularly as sputter-ion pumps. With evaporion pumps getter material is vaporized by heating the getter metal wire ohmically and the vaporized material condenses on cooler surfaces. These surface films which are for the sorption of non rare gases temporarily very active can to a certain extent be used for pumping away also rare gases, but only when their atoms are ionized by the application of an electrical voltage and are transported as positive ions to the negatively charged surface condensate which constantly renews itself and where the ions then are buried. The sputter-ion pumps also operate with a discharge, a so-called low current high voltage Penning discharge. The electrons ionize the gas particles and then by means of an electrical field acting in the direction of the cathode the positive ions are accelerated to several thousand volts and shot into the cathode which consists of getter material, for instance titanium, or continuously sputter the cathode surface. Already in view of this cleanup mechanism it is understandable that also here the rare gas atoms, which do not combine chemically with the getter material, can only be pumped away at a much lower speed. In the long run, these noble gas atoms are finally pumped away, mainly in the inaccessible precipitation due to sputtering which is not subjected tofurther sputtering.

The electron paths are extended in order to increase the pumping effect over a larger pressure range. This is achieved by means of magnetic fields which give the electrons a circular movement and thus lengthen the path between the electrodes (anode and cathode). The anode is given a cellular construction, so that the electrons can travel several times along the anode surfaces. In addition to the magnetic field for the purpose of obtaining a considerable increase in pump speed it is, however, necessary here to connect a large number of cells and electrode pairs in parallel and also complicated cathode constructions have to be taken into account.

These pumps, quite apart from a heavy magnet and a continuously applied high voltage of many thousand volts, can thus only achieve an adequate pumping speed also for rare gases when used in conjunction with several pairs of electrodes operating in parallel or when several pump units are connected together in parallel. The surface of the cathode material can namely only be bombarded by 3,489,336 Patented Jan. 13, 1970 "ice the rare gases for a certain time and up to a definite degree of saturation. Due to sputtering of the cathode material during the pumping process, gas molecules which have been shot into the cathode are released again so that after a comparatively short operation time a state of equilibrium is more or less reached between the ingoing and outgoing ions. In the end therefore the rare gas atoms continuously buried in the inaccessible precipitation due to sputtering, determine to a great extent the pump speed for rare gases.

The application of a powerful magnetic field with a heavy magnet for lengthening the electron path as well as the necessary parallel connection of cells and electrodes of getter material necessitate a very considerable outlay. Newer methods working with different cathode getter materials having different sputtering rates can only to a slighter extent increase the pumping speed for rare gases.

It has now been observed that with a high current arc discharge it is possible to obtain a pumping effect with all gases which is several orders of magnitude better per surface unit whereby a high vacuum can thus also be obtained for rare gases within a minimum of time. In accordance with the invention it is therefore proposed that means are provided which when switching on the pump, produce a cathode spot and arc between the electrodes also in the high vacuum region, further means being also provided which enlarge the arc as regards the current and maintain and prolong it after ignition and divide the original cathode spot into several spots. The cathode spot which forms melts the metal of the electrodes, which can consist of titanium, zirconium, tantal or other getter materials, at local microscopically small points and vaporizes it, so that the locally necessary gas atmosphere is present even in the high and ultra-high vacuum range. The are discharge can then continue with only a small electrode voltage and ionize the gas particles which come from the volume that has to be evacuated and transport them to the cathode surface with an energy corresponding to the cathode fall. There, due to the continuous motion of the cathode spot, the metal parts which have temporarily become liquified in the spot immediately become solidified again thereby enclosing the gas molecules in the much deeper cathode surface layers than is possible with a highvoltage Penning discharge, for example sputter-ion pumps. On condition that the cathode spot (arc discharge) has an adequate surface for its motion and to enable it to subdivide into several spots (partial arcs), this being possible when the electrodes have a suitable form, a pumping speed is attained which is higher than that of other getter-ion pumps of the same size. Moreover, this result is achieved without additional magnetic fields and, as tests have shown, already with only a single pair of electrodes even when these do not consist of a complicated construction and an expensive getter material. Due to the ultra rapid pumping effect, even in the case of rare gases and to the same extent as with other gases, it has been verified by tests that a short pumping operation of only a few seconds is sufficient to reduce permanently the partial pressure of a rare gas of a vacuum plant to for instance 10- Torr, and also during disconnections of the pump the unrare gases in the active surface films caused by the cathode spot as a result of electrode vaporization are continuously adsorbed and absorbed so that the high vacuum is maintained even during longer disconnection intervals.

Also the total pumpable amount of gas, down to a point where there is an appreciable drop in pump speed, is incomparably greater than that with other getter-ion pumps of the same size because of the gases which are pumped away from a cathode spot are enclosed even in the lower surface layers. Therefore the proposed arc getter-ion pump when compared with sputter-ion pumps can also operate with pressures exceeding 10 to 10 Torr, that is in the fine or rough vacuum range, and is very suitable for the rapid evacuation of vacuum vessels of all sizes where a vacuum free of oil vapour and the like is required Without the use of any cooling traps. Also no water cooling of the pump is necessary in the case of short time pumping.

Constructional examples of the invention are shown in the accompanying drawings. FIG. 1 shows an embodiment with moveable ignition pin. FIGS. 2 to 4 are embodiments of the invention without an ignition pin. FIGS. 5 and 6 shows two corresponding electrical circuits.

In FIG. 1 the vacuum vessel is indicated by reference number 1. The getter-ion pump is mounted on this vessel. It is located in an insulating cylinder 2 which is provided With a metallic cover 3. The cathode 4 and the anode 5 between which the arc discharge occurs are located in the vessel where there is also an ignition pin 6 which is connected electrically with the anode 5. This pin consists of a magnetic material, whilst the electrodes are of a getter material. Furthermore, a coil 7 is also provided. The device is put into operation by applying a voltage to the anode by way of a bushing insulater 8. At the same time the coil 7 is connected to a voltage source not shown in the figure, whereby pin 6 is pulled upwards and short circuits the anode and cathode. Then coil 7 is disconnected and the pin is returned to its initial position either by means of a spring or due to its own weight. This causes an arc discharge to occur and due to the melting and evaporation of the getter material the necessary' carrier or neutral gas for a low voltage arc is formed. The surface of the electrodes is so large that there is sufiicient room to allow the cathode spot to wander. Anode 5 is connected electrically to vessel 1 and is thus at earth potential. The metallic and insulating parts of the vessel are connected together in a vacuum-tight manner by fused joints. The gas molecules are drawn from the vacuum vessel 1 into the are by way of openings 9 in anode 5. Screens provided on the insulation cylinder 2 to protect it against sputter are not shown. FIG. 2 shows a modified form of the invention where the length and width of the arc is increased due to the form of the electrodes. In the figure reference number 1 again indicates the wall of the vacuum vessel and 2 is the pump wall consisting of insulating material and provided with a metallic cover 3. The anode 5 is grid-shaped to facilitate the passage of the gas residue from the vessel to the pump. The cathode 4 is located only a short distance away from anode 4. A high voltage pulse is applied here so that an arc occurs between the electrodes.

A cathode spot thus forms and the getter material melts and evaporates locally at a microscopically small point, so that a carrier or neutral gas for a low-voltage arc is produced between the electrodes. A low voltage of only a few volts thus prevails between the electrodes whereby, however, a current of several hundred amperes can flow. As a result, the arc continues to burn and spreads rapidly over the electrode surface due to the original cathode spot splitting up into several smaller cathode spots. The electrodes are prolonged by cylindrical parts, thus cathode 4 by means of a cylinder 10 which extends up to cover 3 for connection to an electrical lead. The actual enlarged electrode surface extends up to the metallic flange 11 which enlarges the cylinder at its upper end. Anode 5 is enlarged by means of cylinder 12. The actual arc discharge is thus between the outer surface of cylinder 10 and the inner surface of cylinder 12. The effective surface of the cylinders must be larger than the surface of the associated electrodes. It need not, however, be more than ten times, because an additional effect is not to be expected. The spacing between the electrodes is smaller than between the cylinders. Between flange 11 and the upper part of cylinder 12 the spacing is also smaller but not so small as between the electrodes, so

that the arc cannot come near to the cylinder wall 2. Flange 11 is therefore also as near as possible to cylinder wall 2, but must not come in contact with it. Cathode 4 and anode 5 can consist of getter material, for instance titanium, zirconium or tantal.

The method of operation is as follows: A high voltage pulse is first of all applied to cylinder 10. Since the distance between the anode and cathode is very small, a breakdown occurs there. Due to the high voltage of many thousand volts this is suificient to produce a cathode spot where the getter material at a microscopically small point melts and evaporates and forms a carrier or neutral gas for maintaining an arc. Directly after the voltage pulse, a lower voltage is applied. The voltage source must, however, be adequate to enable a current of several hundred amperes to flow, so that the arc is well supplied. The cathode spot can thus travel over the surface of the electrodes and reaches the cylinder surface where it is driven upwards and then burns with a greater spacing and over a wider surface, that is, longer and wider. A powerful pumping effect thus occurs which causes the rest of the gas to flow through the opening in the gridshaped electrode 5 into the arc. The metal which is liquified locally to a microscopic extent in the cathode spot due to the motion of the arc is rapidly solidified again, so that the gas molecules become frozen into the cylinder surface. This is not only the case with active gases which can form chemical compounds with the metallic electrode but also with rare gases. Therefore, it is now possible to evacuate in a simple manner large quantities of rare gases in vacuum vessels of any size, this being impossible hither-to with conventional pumps.

The effect can be increased, is desired, by cooling the cylinder. For this purpose the coolant can be supplied to the inside of the cylinders and discharged again as indicated by the arrows 13, 14 in FIG. 2. During a brief pumping time no cooling is, however, necessary.

With the arrangement shown in FIG. 2, the electrodes extend into the vacuum vessel. The ion-getter pump can also be constructed so that it is completely mounted on the vacuum vessel. Even in this case, wall 2 does not need to be entirely of insulation material down to the vessel, but can'also consist partly of metal:

FIG. 3 shows another embodiment of the invention where the surface of

electrodes

4 and 5 are enlarged. The associated

cylinders

10 and 12 must then each consist of two parts having different cross-sections. The entire extent of the breakdown surface is thus increased without the overall height of an arrangement becoming greater.

FIG. 4 shows a further constructional example of the invention where

cylinders

10 and 12 are bent back at the upper end 15. In this way it is easier to prevent metallic vapour from passing from the electrodes to the space between the insulation wall and the electrode cylinder.

The electrode need not be completely level. It can be curved at the edge, so that the spacing at the edge is considerably greater than between the level parts. This is indicated in FIG. 4 at the point 27.

The thickness of the electrodes, particularly of the cylinder, may not 'be less than 2 mm., in order to prevent them from melting.

The electrical diagram of connections is shown in FIG. 5. An A.C. network with

branches

16, 17 serves as a source of supply. The arrangement can of course also be fed from a direct-current source or a two-phase system in which case care must be taken to ensure that the direct voltage does not have any harmonics which are too high. The individual phases feed, by way of rectifiers 18 and 19, the electrical arrangement for the getterion pump. An iron core 20 carries the coils 21 and 22. Coil 21 is connected to branch 16 by Way of a switch 23 and a resistor 24. Coil 22 is connected by way of another switch 25 to the

electrodes

4 and 5 of the getterion pump. Branch 17 is also connected to the

electrodes

4 and 5 by way of rectifiers 19 and blocking rectifiers 26 which in the blocking direction stop the high voltage. Since anode 5 is earthed, negative pulses and negative potentials are applied to the getter-ion pump, the former being for igniting the arc and the latter for maintaining it.

The method of operation is as follows. First of all

switches

23 and 25 are closed and a direct current then flows through winding 21. Switch 23 is opened for the ignition and then a high voltage pulse occurs on coils 21 and 22 and thus also at the electrodes. This pulse initiates the arc discharge in the pump and the circuit for branch 17 is thus closed, so that a high current can flow over the arc. Rectifiers 26 keep the high voltage pulse away fro-m rectifiers 19 of branch 17. When pumping is finished, switch 25 is reopened.

Switches

23 and 25 can also be operated automatically when a pressure supervising device switches in the arrangement when the vacuum deteriorates and switches it out again when the desired vacuum is reached.

A further electronic circuit is shown in FIG. 6 which operates as follows:

First of all

switches

29 and 38 are closed. When pushbutton 35 is actuated, condenser 36 is discharged on the primary winding of ignition coil 34 and this latter supplies a high voltage pulse to the

electrodes

4, 5 of the getter-ion pump, thereby igniting the arc and forming a cathode spot. As a result, the circuit of rectifier 31 is closed, so that a high current can occur which produces a powerful arc discharge with several cathode spots. The inductance 33 prevents the high voltage pulse from entering the rectifier 31. When the pumping operation is completed, switch 29 is opened again. Switch 29 and push button 35 can also be actuated automatically when a pressure supervising device is provided which switches in the apparatus when the vacuum deteriorates and disconnects it when the desired vacuum is attained. This automatic equipment is not shown in FIG. 6.

I claim:

1. Getter-ion pump for producing and maintaining a high vacuum by means of electrodes between which an electrical discharge occurs, characterized in that means are provided which when switching in the pump produces a cathode spot and are between the electrodes also in the high vacuum region, further means being also provided which enlarge the are as regards the current and maintain and prolong it after ignition and divide the original spot into several spots.

2. Getter-ion pump as defined in claim 1, characterized in that the one electrode is of the grid-type.

3. Getter-ion pump as defined in claim 1, characterized in that an ignition pin is provided which upon switching in connects at least part of the electrodes together and that a coil is provided at the upper end of said ignition pin which lifts it from a stationary electrode, so that an arc with cathode spot results.

4. Getter-ion pump as defined in claim 1, characterized in that the means for producing the cathode spot and are comprise a switching device which by way of a pulse transformer applies a voltage to the electrodes so that a cathode spot forms between the electrodes, and that a further switching device is provided which by way of the diodes applies a low voltage to said electrodes which enlarge the are as regards the current and maintain and prolong it and divide the original cathode spot into several spots, and that each electrode is connected to a cylinder which is arranged coaxially so that the external surface of one cylinder and the internal surface of the other cylinder form extended electrode surfaces.

5. Getter-ion pump as defined in claim 1, characterized in that additional means are provided which prevent mutual interference between the means for producing the are and those for maintaining the arc.

6. Getter-ion pump as defined in claim 5, characterized in that the means consist of blocking rectifiers (26).

7. Getter-ion pump as defined in claim 5, characterized in that the means consist of a coil (33).

8. Getter-ion pump as defined in claim 4, characterized in that the cylinders consist of two parts of different diameter, those having the larger diameter being connected to the electrodes.

9. Getter-ion pump as defined in claim 4, characterized in that the ends of the cylindrical parts remote from the electrodes are curved.

10. Getter-ion pump as defined in claim 4, characterized in that the circumferential surface of the cylindrical parts is not greater than ten times the surface of the associated electrode.

1.1. Getter-ion pump as defined in claim 4, characterized in that the spacing of the cylinders at their end remote from the electrodes is smaller than at any other point of the cylindrical parts.

12. Getter-ion pump as defined in claim 9, characterized in that the inner cylinder (10) at the end remote from the electrodes is provided with a flange (11) which is nearer to the outer cylinder (12) and the insulation wall (2) than the cylinders are with respect to each other.

13. Getter-ion pump as defined in claim 4, with a cooling system for the electrodes, characterized in that the cooling means are provided which cause the cooling medium to flow to the inside of the cylinders.

14. Getter-ion pump as defined in claim 1, characterized in that the electrodes are in the form of a plate with a curved edge, so that the spacing at the edge is greater than between the plate-shaped part.

.15. Getter-ion pump as defined in claim 1, characterized in that the electrodes and cylindrical parts have a thickness of at least 2 mm.

16. Getter-ion pump as defined in claim 1, characterized in that the electrodes are made of getter material.

References Cited UNITED STATES PATENTS 3,070,283 12/1962 Hall 230-69 3,198,422 7/1965 Kienel 23069 ROBERT M. WALKER, Primary Examiner U.S. Cl. X.R. 3137