US3452923A

US3452923A - Tetrode ion pump

Info

Publication number: US3452923A
Application number: US668839A
Authority: US
Inventors: Lawrence T Lamont Jr
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1967-09-19
Filing date: 1967-09-19
Publication date: 1969-07-01
Anticipated expiration: 1986-07-01
Also published as: GB1228207A; FR1586208A; DE1764917A1

Description

' A July 1, 1969 L. T. LAMONT, JR

TETRODE ION PUMP Filed Sept. 19, 1967 FIG.|

FIG.2

NUMBER OF IONS FIG"4 PER UNIT AREA NUMBER OF To ATOMS FROM NUMBER OF OPPOSITE ATOMS v CATHOOE PER RESPUTTERED UNIT AREA PER UNIT AREA L'FOURTH ELECTRODE INVENTOR. LAWRENCE T. LAMONT, JR. BY

ATTORNEY,

United States Patent US. Cl. 230-69 8 Claims ABSTRACT OF THE DISCLOSURE There is disclosed a four electrode getter-ion pump which provides a to increase in argon pumping speed which speed is apparently stable over the entire pressure range. The pump is essentially a one voltage triode with a fourth apertured electrode positioned between the sputter element and the anode assembly and to which is applied a negative potential of about -2 kv. The anode assembly and pump housing are maintained at ground potential. The sputter member is maintained at a high negative potential, as of 4 kv. The sputter member or cathode is also apertured but includes vanes vertically disposed such that ions which strike the sputter mem her do so at a very small angle of incidence. Those ions which fail to strike the vanes are buried in the surface around the aperture of the fourth electrode on a subsequent pass through the vanes. A net flux of sputtered material is shown to arrive at the area around the aperture of the fourth element, such that a greater number of argon atoms are permanently pumped.

Background of the invention The present invention relates in general to cold cathode ion pumps and in particular, to a novel tetrode ion pump for improved pumping of noble gases.

Heretofore, vacuum pumps having an anode and a cathode have had for their principle of operation the establishment of a magnetically confined electrical discharge in and around the space between the anode and the cathode and a strong externally applied magnetic field threaded longitudinally through the anode. Free electrons, accelerated toward the anode in a tortuous path as a result of the magnetic field, ionize the gas to be evacuated forming positive ions of both chemically active and inert or noble gas. Both types of ions are then accelerated toward the cathode where upon impact they cause removal or sputtering of sputter cathode material. A sizeable portion of the active ions combine with the sputtered material and are permanently pumped. The inert or noble ions, however, do not form compounds but are merely buried in the surface of the electrode they strike. Subsequent sputtering of the surface removes the material covering the noble ions resulting, as a consequence, in their re-emission. Re-emission also occurs when the surface in which the noble ions impact becomes saturated with noble ions. This is believed to result from the fact that noble ions, though buried, are loosely bound. When the rate or re-emission of noble gas atoms becomes equal to the rate of arrival of noble gas ions, the net pumping speed approaches zero.

Various proposals have been submitted for reducing the re-emission of noble ions resulting from sputtering and saturation. In one, an apertured electrode is placed between the anode and cathode and has impressed on it a voltage intermediate the voltage potential of the anode and cathode. Ions which fail to pass through the apertured intermediate electrode are buried in the surface of the intermediate electrode and material sputtered by other ions which pass through the aperture is deposited over the buried ions.

While satisfactory in principle, the permanent pumping of noble gas atoms in such pumps is limited to only one of the two available surfaces of the intermediate electrode, i.e., the opposing or inner surfaces nearest the anode. Further, the sputter cathode is subjected to high ion densities at a point along the axis of the anode resulting in severe localized deterioration of the cathode requiring the addition of a fourth electrode for defocusing or overfocussing the ion strains. Furthermore, the potential on the intermediate electrode should approximate the free space potential otherwise the discharge intensity may be adversely affected resulting in a reduction of ion density.

Another proposal suggests an irregular cathode surface, such as by projections raised from the cathode, for providing increased sputtering and reduction of spontaneous periodic pressure fluctuations, however, the latter effect, often called argon instability remains undesirably significant.

Summary of the invention The present invention avoids the disadvantages briefly described above with respect to prior known vacuum pumps and as more fullydescribed hereinafter, represents a considerable improvement in argon pumping speed and stability.

The tetrode pump to be described is essentially a one voltage triode with an apertured fourth element positioned midway between an apertured sputter element (cathode) and an anode assembly. The potential which is applied to the fourth electrode is approximately one half the cathode potential. Ions formed Within the discharge are energetically not capable of reaching either the anode or the walls of the pump which are maintained at ground potential and must, as a consequence, strike either the cathode or the fourth electrode. Some of the ions directly strike the inner surface of the apertured fourth electrode on the side nearest the anode, while others pass through the aperture and are buried in the outer surface nearest the cathode. A sizeable portion of those which pass through the aperture also pass through the cathode and strike the cathode on a subsequent pass. The ensuing sputtering produces a net flux of sputtered material which covers the ions buried in the inner and outer surfaces of the fourth electrode, resulting in their being permanently pumped.

It is also proposed to use a cathode and fourth electrode preferably constructed of tantalum which displays special sputtering characteristics in that the rate of sputtering increases with decreasing angle of incidence which the impinging ions make with the cathode surface on im pact. It has been observed that ions impacting normal to a tantalum surface display a much lower sputtering yield than those which impact at a small angle of incidence. As a consquence, ions which impact on the surface of the cathode after passing through the cathode tend to sputter greater amounts of material than do those ions which impact on the outer surface of the fourth electrode resulting in a net flux of sputtered material arriving at the fourth electrode.

Since fresh material being laid over the fourth electrode surface is resputtered at a lower rate, ion re-emission due to saturation and sputtering is greatly reduced, if not entirely eliminated. Thus, argon pumping speed due to increased ion burial is improved and is apparently stable over the entire pressure range. Referenced to the speed of air of the equivalent diode structure, triode pumps using the cathode structure of the instant invention have argon speeds of 20% to 25% where as the tetrode pump of the instant invention displays argon speeds of from 31% to 36% and as indicated, the speed is apparently stable over the entire pressure range.

Accordingly, a primary object of the present invention is an ion pump having an improved argon pumping speed which is stable over the entire pressure range.

Another object of the present invention is a tetrode ion pump in which a substantially non-sputtering fourth electrode is positioned between the anode structure and the sputtering element.

Another object of the present invention is a pump as above described wherein both the substantially non-sputtering fourth electrode and the sputtering element are transparent to ions.

Another object of the present invention is a pump as above described wherein the sputtering element is made of tantalum.

Other objects, features and advantages of the invention will become apparent in the detailed description when taken in connection with the accompanying drawings in which:

FIG. 1 is a diagrammatic sectional view of an ion pump embodying the present invention;

FIG. 2 is an exploded perspective view delineated by the line 22 of FIG. 1;

FIG. 3 is a fragmentary cross-sectional view taken along lines 33 of FIG. 1; and

FIG. 4 is a diagram of the ion and sputtered atom flux distribution in the pump of the instant invention.

Detailed description Referring to FIG, 1, there is shown diagrammatically two cells of a novel multiple cell ion pump incorporating the present invention which is especially suitable for pumping ditficult to pump noble gases, such as argon.

A stainless steel vacuum tight pump housing 1 provided with a gas port 2 encloses a pump assembly 3. An externally applied strong magnetic field H is formed by magnets 7 and is oriented to thread through pump assembly 3 along the longitudinal axis of a plurality of anodes 4 and through

fourth electrodes

5, 5 and cathodes 6, 6'. Preferably, the anode is made of stainless steel, and the fourth electrodes and cathode are preferably made of tantalum. Gas port 2 may be coupled in a vacuum tight manner to a device or chamber (not shown) desired to be evacuated. As is well known in the art, ionization of the gas which enters through gas port 2 is caused by collision of electrons with gas atoms or molecules. The electrons are accelerated in tortuous paths within the anode cells under the influence of the magnetic and electric fields. As shown in FIG. 1 the anode is maintained at zero or ground potential. The apertured

sputter cathode structures

6, 6 are maintained at a high negative potential, as of 4 kv. The apertured fourth electrodes 5, 5' are maintained at an intermediate negative potential, as of 2 kv. The anode 4 and housing 1 can be electrically interconnected but the fourth electrodes 5, 5' and the cathodes 6, 6' are insulated from the anode and the housing and from each other. The leads for the fourth electrodes and cathodes pass through conventional insulators 8. The positive ions are accelerated toward the apertured

fourth electrodes

5, 5 and the apertured sputter cathode structures 6, 6'.

It should be understood that the negative potential imposed on apertured

fourth electrodes

5, 5 is due to the aperture size and is independent of the potential on and geometry of the anodes 4 and cathode structures 6, 6'. The larger the aperture in fourth electrodes 5, 5' the less effect the potential on

fourth electrodes

5, 5 has on the discharge intensity, As a consequence, the potential imposed on

fourth electrodes

5, 5 may be adjusted for maximum argon pumping speed. The walls of pump housing 1 are maintained at ground potential thus forming a ground plane about the other pump elements.

Referring to FIGS. l-3, it will be seen that the diameter of the aperture of the

fourth electrodes

5, 5 is only about 75% of the diameter of the cells of anode 4.

Referring to FIG. 2, there is shown in greater detail a portion of cathode structure 6. An apertured plate 10 is fixed, as by spot welding, to a vane assembly 11. A plurality of vanes 12 are held in position normal to the aperture of plate 10 by a slotted bracket 13. The diameter of the apertures in plate 10 approximates the diameter of the cells of anodes 4. The vanes 12 are spaced apart such that at least three vanes traverse the aperture in plate 10. It should be understood that the spacing between the vanes 12 may be made smaller if desired such that more vanes traverse the aperture in plate 10. In general, the space between the vanes 12 as well as their height should be large relative to their thickness. It should be noted, however, that apertured plate 10 is not essential to the operation of the pump so long as the vanes are of the dimensions and spacing indicated.

In operation, ions formed within the discharge are energetically not capable of reaching either the anodes 4 or the walls of the pump housing 1 and must, as a consequence, strike either the sputter cathode structure '6, 6' or the

fourth electrode

5, 5. Since a large fraction of the ions is transmitted through the apertures and vanes 12 of cathode structure 6, 6' and strike and sputter vanes 12 on the return pass, material sputtered from said cathode structure 6 may be deposited on elements, such as auxiliary cathode 5 on the opposite end of anode 4 as well as on the underside of auxiliary cathode 5 nearest the sputtering cathode structure 6, Of those ions which do not strike sputter vanes 12 of cathode 6, some will strike the anode side of fourth electrode 5, and others will strike the cathode side of fourth electrode 5 after passing down and back up through vanes 12 without striking the vanes. As a result ions will be buried in both the upper and lower surfaces of fourth electrode 5. Ions buried on the lower surface of fourth electrode 5 are covered by material sputtered up from cathode 6, and ions buried on the upper surface of fourth electrode 5 are covered by material sputtered down from cathode 6'. The described burial and covering action occurs in similar manner for the fourth electrode 5'. Since a net flux of sputtered material arrives from the sputter cathode structures 6, 6' at the area of the

fourth electrodes

5, 5 wherein ions are buried, these ions will be permanently pumped.

Referring to FIG. 1 and FIG. 4, there is shown a flux diagram of the ions and sputtered tantalum atoms which illustrates the arrival of a net flux of sputtered tantalum atoms arriving in the area near the periphery of each aperture in the fourth electrode 5. The basis for the illustrated flux distribution is twofold. First, the

fourth electrodes

5 and 5 are held at a potential less than that of the sputter cathode structures 6 and 6'. Since, over the range of interest, the sputtering rate is a monotonically increasing function of the kinetic energy of the incident ion, the -2 kv. potential on the fourth electrodes 5, 5' results in less sputtering from these electrodes than from the 4 kv.

cathodes

6, 6. Second, the sputtering rate is proportional to the ion flux at the surface and the ion flux is found to be a monotonically decreasing function outwardly along the radius from the axis of each anode cell and is substantially azimuthally symmetric.

Thus, as shown in FIG. 4, the number of atoms sputtered from the fourth electrode per unit area decreases sharply with increasing distance from the axis of each of the anode cells whereas the sputtered atoms arriving from the opposite cathode decrease only slightly. The re sult is that the number of atoms of sputtered material arriving at the fourth electrode from the opposite sputter cathode is greater than the number leaving. A similar net buildup of sputter material occurs on the cathode side of the fourth electrode since the rate of sputtering due to ions impacting normal to the surface is less than the rate of material arriving from ions striking the sputter cathodes 6, 6' at a small angle of incidence. Radioactive xenon tracer techniques in conventional diode pumps indicate that maximum argon pumping is permanent in an area the inner radius of which is about the anode radius. Accordingly, the radius of the apertures of the fourth electrodes 5, 5' is about 75% of the radius of the anodes 4.

Argon .pumping is further improved by constructing the sputter cathode structure 6, 6' out of tantalum which exhibits unique sputtering yields, that is, the rate of sputtering or yield of tantalum decreases with increasing angle of incidence of the impinging ion. Since the angle of incidence of ions striking vanes 12 is relatively small, high yields are experienced. on the other hand, those ions which fail to strike vanes 12 and impact on the top or bottom surface of the fourth electrodes '5, '5' do so at a nearly normal angle of incidence. Consequently, the negative potential on auxiliary cathode 5 can be increased to increase pumping beyond that which would be possible if the sputter cathodes 6, 6' and fourth electrodes 5, 5' were made of a material which did not exhibit the angular dependence of sputter yields seen with tantalum.

It will now be obvious to those skilled in the art that other embodiments and arrangements are within the scope of the invention. Various anode geometries may be used, for example, rectangular instead of cylindrical. Further, the pump assembly 3 may 'be inserted directly in the chamber to be evacuated without the need for a pump housing 1, provided a grounded plate is provided adjacent outwardly of the

sputter cathodes

6, 6 to fix the potential.

Accordingly, the description and accompanying drawing are to be considered as illustrative only and shall not be construed as restricting the scope of the invention as hereinafter defined.

What is claimed is:

1. An ion pump apparatus comprising: an apertured anode member; an apertured electrode insulatingly disposed adjacent one end of said anode member; and an apertured sputter cathode insulatin-gly disposed adjacent to and outwardly of said apertured electrode and adapted to be maintained at a sufliciently high negative potential to insure sputtering of material therefrom.

2. An ion pump apparatus according to claim 1 Wherein the diameter of the aperture in said apertured electrode is approximately of the diameter of the aperture in said apertured anode member.

3. An ion pump apparatus according to claim 1 wherein said apertured electrode is adapted to be maintained at a negative potential intermediate the potential applied to said apertured anode member and said apertured sputter cathode.

4. An ion pump apparatus according to claim 1 comprising means for maintaining said apertured anode member at ground potential, means for maintaining said apertured sputter cathode at a high negative potential; and means for maintaining said apertured electrode at a potential intermediate the potential applied to said apertured anode member and said apertured sputter cathode.

5. An ion pump apparatus according to claim 3 wherein said apertured sputter cathode comprises one or more elongated members in a plane normal to the axis of aperture of said apertured anode member.

'6. An ion pump apparatus according to claim 5 wherein said elongated members comprise one or more elongated thin rectangular metal strips.

7. An ion pump apparatus according to claim 6 wherein said metal strips are made of tantalum.

8. An ion pump apparatus according to claim 6 wherein said metal strips and said apertured electrode are made of tantalum.

References Cited UNITED STATES PATENTS 3,292,844 12/1966 MacKenZie 230-69 FOREIGN PATENTS 1,289,986 2/1962 France.

ROBERT M. WALKER, Primary Examiner.

US. Cl. X.R. 313-7