US3149774A

US3149774A - Getter ion pump method and apparatus

Info

Publication number: US3149774A
Application number: US85355A
Authority: US
Inventors: Robert L Jepsen
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1961-01-27
Filing date: 1961-01-27
Publication date: 1964-09-22
Anticipated expiration: 1981-09-22
Also published as: DE1414572B2; GB958423A; DE1414572A1; FR1312798A

Description

Sept. 22,1964 v jR.`| .YJEPsEN 3,149,774

GETTSR'ION PUMP METHOD AND APPARATUS Filed Jan.y 27, A1961 y l l l I o 5MM United States Patent O 3,149,774 GETTER N PUMP METHOE) AND APPARATUS Robert L. Jepson, Los Altos, Calif., assigner to Varian Associates, Palo Alto, Calif., a corporation of California Filed Jan. 27, 1961, Ser. No. 85,355 16 Claims. (Cl. 230-69) The present invention relates in general to glow discharge devices and more particularly to glow discharge devices having a high throughput capacity.

Heretofore electrical cold cathode gas discharge vacuum pumps have been built having for `their principle of operation the establishment of a glow discharge within the interior of au open ended tubular anode disposed between and spaced from two cathode .plates and having a magnetic eld threaded through the anode. Positive ions produced by the glo-w discharge are directed against the cathode plates and the impinging ions produce sputtering of a reactive cathode material. The sputtered material is collected upon the interior surfaces of the pump where it serves to entrap molecules in the gaseous state coming in Contact therewith. In this manner the gas pressure within a vessel enclosing the cathode and anode elements is reduced.

The operating performance of these pumps is determined largely by their pumping speed which is the volume rate at which gases Aare being removed from the vacuum system. The pumping speed is influenced by several `factors such as' the particular design and size of the pumping elements, the magnitude of ion current produced which is, in turn, dependent upon the applied voltage, etc. K

However, in certain types of vacuum applications the actual pumping speed of a vacuum pump is of less importance than the pumps throughput capacity which is determined by multiplying the pressure at which the pump is operating times its pumping speed `at that particular pressure. Throughput is therefore a measure of the pumpsactual molecular rate of gas removal. A prime example of an application in which throughput capacity is of greater importance than pumping speed is vacuum tube processing in which large quantities of cathode conversion gases are produced at certain times during the process thereby requiring a high molecular gas removal rate at these times while requiring a relatively low pumping speed to attain v the desired ultimate pressure in the tube.

ly Heretofore the maximum throughput obtainable with pumps of a given size has been limited by a number of considerations. Present pumps have a substantially con,- s'tant pumping speed in the low pressure region between about 108104 mm. Hg so that a pumps throughput will be directly proportional to operating pressure in this region. A maximum throughput will be reached, however, in the pressure range between l0*A1 and 103 mm. Hg for a number of reasons. It is in this pressure region that an impedance transition of the glow discharge takes place in present cold cathode pumps' whereby the impedance changes from `a high to a comparatively low value causing an increase in ion current and a reduction in voltage across the glow discharge. This reduction in voltage allows the glow discharge to become unconiined which iddfi Patented Sept. 22, i954 FPice greatly reduces the pumps pumping speed and thereby its maximum throughput.

Early attempts to enhance a pumps throughput capacity by increased voltages across the glow discharge failed to achieve satisfactory results. lt was not clear whether these failures resulted from fundamental properties of the cold cathode gas discharge or were rather due to effects arising from power dissipation heating of the pump electrodes. Nor was it known whether power dissipation heating of the pump electrodes produced any change in the fundamental properties of the cold cathode gas discharge, especially in the exceedingly complex impedance transition region. High power dissipation requirements in other types of electrical equipment can sometimes be met by cooling of various kinds and some cooling has been tried in the electrical vacuum pump tield. For example, the `application of cooling means to the exterior of electrical vacuum pump envelopes has been tried. These applications, however, are very inefficient because of the high insulation properties of the vacuum which exists between the cooling means and the pump electrodes.

Cooling has also been utilized in cathode emitter pumps although for entirely different reasons. The intense heat generated by thermal emitters results in extensive outgassing from the walls of these pumps preventing the attainment of high vacua. Cooling is primarily used, therefore, to prevent outgassing rather than to dissipate power produced by a glow discharge.

These early cooling attempts, being very inefficient, did not establish whether a dependent relationship existed between eicient cold cathode pump electrode cooling and the characteristics of the cold cathode gas discharge. Therefore, it was not apparent that the provision of an etiicient power dissipation means in a cold cathode vacuum pump to allow higher working voltages would necessarily produce greater maximum throughputs. It seemed quite possible that the use of adequate power dissipation means, although permitting higher pumping speeds, might also cause the glow discharge impedance transition to take place at lower pressures than was previously the case. The overall effect of such an occurrence could have resultedin a negligible increase or possibly even a decrease in aY maximum throughput obtainable in a given size pump.

It has been found, however, that the use of novel cooling means in a cold cathode pump has resulted in a glow discharge impedance transition at even higher pressures than those at which the transition took place without such cooling. This phenomenon, together with the greater pumping speeds achieved by higher working voltages, has resulted in maximum throughput capacities some ten times larger than could be previously obtained with pumps of a given size.

It is, therefore, the object of the present invention to provide a novel, improved, compact cold cathode vacuum pump having high throughput capabilities, and to provide a novel method for cooling a cold cathode vacuum pump so as to greatly enhance its throughput capacities.

One feature of the present invention is the provision of cooling means for a cold cathode vacuum pump in intimate contact with its reactive cathode thereby allowing higher power dissipation and greater throughput capacities.

Another feature of the present invention is the provision of cooling means for a cold cathode vacuum pump cornprising an elongated heat-conducting member in intimate contact with its reactive anode over a substantial part of its length thereby allowing higher power dissipation and greater throughput capacities.

Another feature of the present invention is the provision of cooling means of the above types which allow a minimum gap between the pole faces of the pump.

Still another feature of the present invention is the provision for a novel method for cooling a cold cathode vacuum pump.

Other features and advantages of the present invention will become apparent upon a perusal of the specification taken in connection with the accompanying drawings wherein,

FIG. 1 is a plan view partly in cross section of a novel electrical vacuum pump of the present invention,

FiG. 2 is an end view partly in cross section of the structure of FIG. 1,

FIG. 3 is a graph showing throughput vs. pressure for the pump structure of FlG. 1,

FIG. 4 is a graph showing pumping speed vs. pressure for the pump structure shown in FIG. l, and

FiG. 5 is a plan view partly in cross section of another embodiment of a novel electrical vacuum pump of the present invention.

Referring now to FIGS. l and 2 a rectangular cupshaped member 1@ is closed off at its flanged open end by a rectangular closure plate 6 welded about its periphery to the flanged portion of cup-shaped member 16 thereby forming a rectangular vacuum compartment '7. The rectangular closure plate 6 is provided with a pair of apertures which are closed by a recessed cylindrical ,chamber S and an outwardly extending cylinder 9 circumferentially supporting a cup-shaped member 11 having its open end facing away from the closure plate e.

A rectangular cellular anode 12, of, for example, titanium, is carried within the vacuum compartment 7 upon the end of a conductive rod 13 which extends outwardly of the rectangular vacuum compartment 7 through an aperture in the recessed cylindrical chamber S. The conductive rod 13 is insulated from and carried by the vacuum compartment 7 through the intermediaries of

annular insulator frames

141, 15 and 16 and cylindrical insulator 17. The free end of the rod 13 provides a terminal for applying a positive anode voltage with respect to two substantially rectangular cathode plates 13 made of reactive material.

The cathode plates 1S are spaced apart at their corners by the bands 19 and are supported from the closure plate u by the U-shaped support 21 and `associated pin 22 so as to be mechanically locked in position substantially parallel to and spaced from the anode 12. In the embodiment shown, the side wall of the vacuum compartment "i opposite the closure plate 6 is apertured to receive the hollow conduit 23 which may be of any convenient inside diameter commensurate with the desired pumping speed. The hollow conduit tube communicates with any desired vacuum system and is provided with a suitable mounting ilange 24.

A horseshoe-shaped permanent magnet having a pair of pole pieces 26 is positioned with respect to the rectangular vacuum compartment 7 such that the magnetic held of the magnet 26 threads through the individual cellular elements of the anode 12 in substantial parallelism t0 the longitudinal axes thereof.

The continuous copper tube 2S is shaped to form a pair of rectangles which are brazed to the edges of the cathode plates 1S on their surfaces facing the anode 12 so as to be enveloped by the vacuum compartment 7. The positioning of the copper ture 25 on the anode side of the cathode plates 18 rather than between them and .the vacuum compartment 7 or outside the vacuum compartment makes it unnecessary to either enlarge the gap between the magnet pole pieces 26 or increase the overall size of the pump. This is especially important from the point of View of minimizing the size, weight, and cost of the magnet necessary to produce a magnetic iield of desired strength. The ends of the copper tube 25 eX- tend out of the vacuum compartment 7 through a pair of apertures in the cup-shaped member 11. The dimensions of the rectangles formed by the copper tube 25 are larger than those of the `anode 12 so as to be `outside the glow discharge paths between the anode and the cathode plates. Thus, the copper tube 25 provides a path for a cooling iluid in intimate contact with the reactive cathode plates '18. This intimate contact is extremely important since the high insulation properties of a vacuum will allow even the smallest separation'between coolant and the reactive cathode material to greatly reduce cooling efliciency.

In typical operation of this device, a positive potential of .4-10 kv. or more is applied to the anode 12 Via conductive rod 13. The vacuum compartment 7 and the cathode plates 18 are preferably operated at ground potential 4to reduce hazard to operating personnel. The applied potentials produce a region of intense electric iield between the cellular anode 12 and the cathode plates 18. This electric eld, acting in combination with the magnetic eld, produces a breakdown of gas within the pump resulting in a glow discharge within the cellular anode 12 and between the anode 12 and the cathode plates 18. The glow discharge results in positive ions being driven into the cathode plate 1S to produce ydislodgment of reactive cathode material which is thereby sputtered onto the nearby anode 12 to produce gettering of molecules in the gaseous state coming in contact therewith. In ythis manner, the pressure within the vacuum compartment 7 and, therefore, structures communicating therewith is reduced.

FIGS. 3 and 4 are graphs showing certain operating characteristics of the embodiment of the present invention shown in FG. l. Curve A is aplot of pumping speed in liters per second vs.,pressure in mm. Hg for acold cathode vacuum pump using cathode Acooling means as described above. Curve B is ya corresponding plot of throughput in micron liters per second vs. pressure in mm. kHg under the same operating conditions. Curves C and D are estimated curves of the same operating characteristics for the same pump without cathode cooling means and run with a power supply of appropriately lower current capacity to avoid over heating of the pump. it will be seen that the maximum throughput obtained with cathode cooling means is in the order of ten times greater than the maximum throughput obtained without cathode cooling means.

Referring now toFIG. 5 there is shown another embodiment of the present invention. The pump structure of this embodiment is substantially the same as that of the apparatus shown in FIGS. 1 and 2 ywith the exception ofthe particular cooling means utilized. In particular, the copper tubingfZS shown in FGS. 1 and 2 is replaced by the copper band 31 which is brazed or otherwise secured in intimate contactwith the outer edge of the anode 12 and the high voltage lead-in 13. The leadin insulator 32 outside the pump envelope has ns 33 which provide a greater surface for dissipating the heat which is conducted from the anode 12 by the lead-in 13. The width of the copper band 31 is the same or less than the width of the anode edge so that it does not affect the positioning of the anode or the path of the glow discharge. Thus, the copper band 31 and the lead-in 13 provide a cooling means in intimate contact with the anode 12.

The operation of this embodiment and the function of the cooling means is essentially the same as that described for the apparatus shown in FIGS. 1 and 2.

lt has been found in practice that cooling of the pump cathode has La much greater' eiiect on improved through- FTM put capacity than does cooling of the pump lanode which has made it possible to achieve desired maximum throughput capacities in most cases by cooling only the pump cathode. This fact has considerable signicance since cooling of the cathode does not present the many electrical insulation problems encountered in cooling the high voltage anode.

However, it may be desirable in certain circumstances to provide a pump with intimate anode cooling alone or with both anode and cathode cooling.

The high throughput capabilities of the intimately cooled cold cathode pump can be advantageously used in special vacuum applications such as the vacuum tube processing mentioned above. By controlling the process so that the large quantities of cathode conversion gas molecules are produced in the pressure region of about l0*3 mm. Hg at the pump (i.e., near the point of maximum throughput) the cooled pump will remove gas molecules during this period at the same rate as would a pump of about ten times its size without etiicient cooling. Furthermore, the moderate pumping speed of the cooled pump would be adequate to obtain the desired ultimate pressure in the tube during later periods of the vprocess when gas is being evolved at a much reduced rate. Thus, it will be apparent that the utilization of the intimately cooled cold cathode pump of the present invention will allow the use of much smaller pumps in certain vacuum applications than was formerly possible.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and and not in a limiting sense.

What is claimed is:

1. An electrical vacuum pump apparatus including, an anode electrode, a reactive cathode electrode spaced from said anode electrode, means for producing and directing a magnetic field between said electrodes, a vacuum compartment enclosing said anode and cathode electrodes, said vacuum compartment being adapted for connection to a vacuum system, means for applying a potential to establish a glow discharge between said anode land cathode electrodes so as to produce sputtering of said reactive cathode electrode, means for cooling said apparatus, and said cooling means being substantially enveloped by said vacuum compartment.

2. The apparatus according to claim 1 wherein said cooling means is in intimate contact with at least one of said electrodes.

3. The apparatus according to claim 1 wherein said cooling means is positioned between said anode electrode and said cathode electrode.

4. The apparatus according to claim 3 wherein said cooling means is in intimate contact with at least one of said electrodes.

5. An electrical vacuum pump apparatus including, an anode electrode, a reactive cathode electrode spaced from said anode electrode, a vacuum compartment enclosing said anode and cathode electrodes, means for producing and directing a magnetic eld between said electrodes, means for applying a potential to establish a glow discharge between said anode and cathode electrodes so as to produce sputtering of said reactive cathode electrode, means for cooling said apparatus, said cooling means comprising an elongated heat-conducting member and being in intimate Contact with at least one of said electrodes over a substantial part of its length.

6. An electrical vacuum pump apparatus including, an anode member, a reactive cathode member disposed opposite said anode member and being spaced therefrom, a Vacuum compartment enclosing said anode and cathode members, means for producing and directing a magnetic field between said anode and cathode members, means for applying a potential to establish a glow discharge between said anode and cathode members so as to produce sputtering of said reactive cathode member, means for cooling said reactive cathode member, and said cooling means being in intimate contact with said reactive cathode member.

7. The apparatus according to claim 6 wherein said cooling means comprises tubing for circulating a cooling iiuid.

8. The apparatus according to claim 6 wherein said cooling means is positioned between said reactive cathode member and said anode member.

9. The apparatus according to claim 8 wherein said cooling means comprises tubing for circulating a cooling fluid.

10. An electrical vacuum pump apparatus including, an anode member subdivided into a plurality of cellular compartments, a reactive cathode member disposed opposite the open end of said cellular compartments and being spaced therefrom, a vacuum compartment enclosing said anode and cathode members, means for producing and directing a magnetic eld coaXially of said cellular compartments, means for applying a potential to establish a glow discharge between said anode and cathode members so as to produce sputtering of said reactive cathode member, means for cooling said reactive cathode member, and said cooling means being positioned between said cathode member and said anode member.

1l. The apparatus according to claim 10 wherein said cooling means is in intimate contact with said cathode member.

12. The apparatus according to claim 11 wherein said cooling means comprises tubing for circulating a cooling iluid.

13. An electrical vacuum pump apparatus including, an anode member, a reactive cathode member disposed opposite said anode member and being spaced therefrom, a vacuum compartment enclosing said anode and cathode members, means for producing and directing a magnetic field between said anode and cathode members, means for applying a potential to establish a glow discharge between said anode and cathode members so as to produce sputtering of said reactive cathode member, means for cooling said anode member, said cooling means comprising an elongated heat-conducting member and being in intimate contact with said anode member over a substantial part of its length.

14. An electrical vacuum pump apparatus including, an anode member subdivided into a plurality of cellular compartments, a reactive cathode member disposed opposite the open end of said cellular compartments and being spaced therefrom, a vacuum compartment enclosing said anode and cathode members, means for producing and directing a magnetic field coaxially of said cellular compartments, means for applying a potential to establish a glow discharge between said anode and cathode members, means for cooling said anode member, said cooling means comprising an elongated heat-conducting member and being in intimate contact with said anode member over a substantial part of its length.

l5. An electrical vacuum pump apparatus including, an anode member subdivided into a plurality of cellular compartments, a cathode member disposed opposite the open end of said cellular compartments and being spaced therefrom, a vacuum compartment inclosing said anode and cathode members, means for producing and directing a magnetic field coaxially of said cellular compartments, means for applying a potential to establish a glow discharge between said anode and cathode members, means for cooling said apparatus, and said cooling means being substantially enveloped by said vacuum compartment.

'16. An elecrical Vacuum pump apparatus including, an anode member subdivided ino a plurality of cellular comparments, a cathode member disposed opposie the open en'd of said cellular compartments and being spaced therefrom, a vacuum compartment enclosing said anode 5 and cathodemembers, means for producing and directing a magneic eld coaxially 0f said cellular compartments, means for applying a poential to establish a glow discharge between said anode and cathode members, means for cooling said cathode member, and said cooling means 10 being in intimate contac with saidcathode member.

O 6:9 References Cited in the le of this patent UNITED STATES PATENTS

Claims

1. AN ELECTRICAL VACUUM PUMP APPARATUS INCLUDING, AN ANODE ELECTRODE, A REACTIVE CATHODE ELECTRODE SPACED FROM SAID ANODE ELECTRODE, MEANS FOR PRODUCING AND DIRECTING A MAGNETIC FIELD BETWEEN SAID ELECTRODES, A VACUUM COMPARTMENT ENCLOSING SAID ANODE AND CATHODE ELECTRODES, SAID VACUUM COMPARTMENT BEING ADAPTED FOR CONNECTION TO A VACUUM SYSTEM, MEANS FOR APPLYING A POTENTIAL TO ESTABLISH A GLOW DISCHARGE BETWEEN SAID ANODE AND CATHODE ELECTRODES SO AS TO PRODUCE SPUTTERING OF SAID REACTIVE CATHODE ELECTRODE, MEANS FOR COOLING SAID APPARATUS, AND SAID COOLING MEANS BEING SUBSTANTIALLY ENVELOPED BY SAID VACUUM COMPARTMENT.