US3352482A

US3352482A - Ion sputter pumping collector

Info

Publication number: US3352482A
Application number: US486937A
Authority: US
Inventors: Alvin T Forrester; Perel Julius
Original assignee: Electro Optical Systems Inc
Current assignee: Electro Optical Systems Inc
Priority date: 1965-09-13
Filing date: 1965-09-13
Publication date: 1967-11-14
Anticipated expiration: 1984-11-14

Description

NOV. 14,` 1967 A T FORRESTER ET AL 3,352,482

ION SPUTIER PUMPING COLLECTOR 2 Sheets-Sheet l Filed Sept. 13, 1965 0f Mn 0 2 w. o M f m. f M r IMMNM a w F 6 f M C e K m i 0 8 Vo d. @6, w

Nov. 14, 1967 A. T. FORRESTER ET AL 3,352,482

10N SPUTTER PUMPING COLLECTOR Filed Sept. 13, 1965 2 Sheets-Sheet 2 60o/er United States Patent() 3,352 482 IoN SPUTTER PUMPING COLLECTOR Alvin T. Forrester, Los Angeles, and Julius Perel, Altadena, Calif., assgnors to Electro-Optical Systems, Inc., Pasadena, Calif., a corporation of California Filed Sept. 13, 1965, Ser. No. 486,937 Claims. (Cl. 230--69) ABSTRACT 0F THE DISCLOSURE Ion sputter pumping apparatus having an ion source for producing a collimated ion beam which is directed into the cells of a multicellular grid of reactive source material disposed in the beam path, and a collector electrode defining a plurality of elongate projections with a different one of the projections extending into each cell of the grid. A source of DC potential connected between the grid and collector electrodes establishes an electric eld therebetween for deflecting the ions of the beam into impingement with the surfaces of the grid means to sputter the reactive source material.

Background of the invention There are well known in the art, ionic vacuum pumps in which ions sputter a getter material, the residual gas molecules when contacting the getter material being removed from the gaseous state by absorption or adsorption. The operating principles of such apparatus have heretofore been predicated upon the establishment of a glow discharge within an envelope coupled to the chamber it is desired to evacuate. The glow discharge produces positive ions which are accelerated through the electric field existing near the surface of a reactive cathode member to bom-bard the cathode, whereby portions of the reactive cathode member are caused to be dislodged or sputtered. The sputtered particles of the cathode are condensed upon other portions and surfaces of the apparatus. Upon condensation, the cathode material for-ms a layer of material which will serve to entrap molecules in the gaseous state coming in contact therewith. It is through this entrapment mechanism that the pressure within the structure is reduced.

Summary of the invention The present invention is directed toward an improved ion sputter pumping system wherein a directed ion beam is utilized for sputtering, rather than ion bombardment of the cathode of a glow discharge, whereby the sputtering becomes independent of residual gas pressure or constituents. The ion beam is directed onto a metal surface target composed of getter material, the action of sputtering due to the ion bombardment causing continual distribution of fresh coatings of getter material upon all surfaces in line of sight with the bombarded metal. In order to obtain a greater degree of control of the pumping rate an electric field can be established between two electrodes. An applied voltage is used to deflect the ion beam, the voltage being applied between a grid target and a collector electrode, an electrode system which is quite unlike the cathode-anode system in a glow discharge. The applied voltage in the present invention system controls the angle of impingement of the ions on the surface of the grid, and also determines the fraction of the ion beam which strikes the grid, this determination being independent of the ion beam current. Control of the applied voltage also enables selective Variation of the sputtering yield, a larger number of sputtered atoms being available for pumping action as the angle of ion impingement is decreased from normal incidence. The

present invention technique is particularly advantageous in applications in which an ion beam is required for other purposes, such as in the testing of ion sources, for example. In such applications the ion pump becomes an integral part of the system with consequent economies and greater pumping speed than attainable with prior art ion sputtering pumps.

When using certain sources, the sputtered material can be damaging to the ion source. Therefore, when utilizing the present invention technique of pumping gases by the gettering action of material sputtered by a directed ion beam, it is desirable to minimize the amount of sputtered material which returns to the ion source. This desire can be achieved by orienting the surface of the metal to be sputtered parallel to the ion beam direction and radially away from the source of the ions, the electric field applied between the grid composed of getter metal and the adjacent collector electrode surface deflecting the ions toward the grid and causing ion impingement at small angles with the grid surface. The increased sputtering yield resulting from lower angles of ion impingement results in an increase of the number of sputtered atoms available for pumping action.

Accordingly, it is an object of the present invention to provide an improved electrical vacuum pumping system.

It is also an object of the present invention to provide improved ion sputter pumping systems and apparatus.

It is another object of the present invention to provide an ion sputter pumping system wherein a directed ion beam is utilized for sputtering.

It is a further object of the present invention to provide an improved ion sputter pumping system in which the impingement angle of the ions is controllable.

It is yet another object of the present invention to provide an improved ion sputter pumping system utilizing a grid and wherein the fraction of the ion beam which strikes the grid is readily determinable independent of the ion beam current.

It is a still further object of the present invention to provide an improved ion sputter pumping system wherein control of the applied voltage enables selective variation of the sputtering yield.

It is also an object of the present invention to provide an improved ion sputter pumping system capable of greater pumping speeds.

It is another object of the present invention to provide a sputter pumping system capable of directing sputtered particles away from regions where they might be harmful.

It is still another object of the present invention to provide an ion beam collector electrode configuration for an ion sputter pumping system wherein a directed ion beam is utilized for sputtering.

It is a further object of the present invention to provide a grid configuration for an ion sputter pumping system wherein a directed ion beam is utilized for sputtering.

The objects of the present invention are accomplished, in a presently preferred embodiment, by direc-ting an ion beam along a predetermined axis on which is positioned a collector electrode and a multicellular shielding grid, a source of electrical potential being connected between the collector electrode and the grid. The grid is in the form of a honeycomb-like structure of getter material and with its cell walls extending substantially in radial alignment with a point on the predetermined axis at which a minimum exposure to sputtered particles is desired. The collector electrode is disposed in close proximity to the grid, but electrically insulated therefrom. 'Ihe collector electrode deiines a plurality of elongate projections, each one of the elongate projections being centrally disposed within a different cell of the multicellular shielding grid. Connection of the source of electrical energy between the collector electrode and the grid creates a substantially radial electric field within each of the cells of the grid. -Positive ions entering each cell are `deiected by the radial field and strike the grid surfaces with energies in the range producing sputtering. The sputtered grid material is collected by the collector electrode projections and other line-of-sight surfaces which are maintained at low enough temperatures to cause condensation of sputtered particles. Proper shaping of the collector electrode projections provides an almost grazing trajectory by the ions of closest approach, together with a large collecting and trapping surface. l

The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof will be better understood from the following description considered in connection with the accompanying drawing in which a presently preferred embodiment of the invention is illustrated by Way of example. It is to be expressly understood, however, that the drawing is for the purpose of illustration and description only, and is not intended as a `definition of the limits of the invention.

Brief description of the drawing FIGURE. 1 is a perspective view, partially cut away, of the presently preferred collector electrode-grid structure;

FIGURE 2 is a graph showing collector current and grid current plotted `as a function of grid voltage;

FIGURE 3 is an enlarged view taken along the line 3 3 of FIGURE l, shown in combination with an electrical Aschematic diagram; and

FIGIJR'E 4 is a schematic view depicting arrangement of the present invention apparatus in an evacuable envelopef.

Referring now to FIGURE 1 there is shown the presently preferred embodiment of the ion beam target, cornprising a dish shaped collector electrode, generally indicated by the reference numeral 10, to which is insulatively mounted a multicellular shielding grid, generally indicated by the reference numeral 20. The collector electrode is fabricated from an electroconductive material, with copper being presently preferred. The collector electrode 1 0 consists of a spherical segment 11 of circular configuration. For reference purposes the central axis of this spherical segment is shown in FIGURE 3 of the drawing as a centerline.

From the ,concave inner surface of the spherical segment 11 extends an array of .elongate projections 15, the projections 15 preferably being in radial alignment with the ion source. Theelongate projections 15 are fabricated from an electroconductive material, preferably the same material as the spherical segment 11. It has been found desirable to utilize a particular shape `for the spherical segments 15, `for reasons which will be hereinafter explained. A side view of one of the elongate projections 15 is shown in FIGURE 3 of the drawing, the elongate projection being of substantially circular cross-section throughout and formed by a .series of successive disc shaped segments of progressively smaller diameter. Thus, the great majority of ,the surface area of each of the elongate projections 15 is recessed with respect to the reference axis so as to provide a collecting surface which cannot be hit by incident ions while providing a curved tapered configuration. i

The collector electrode 10 (as well as the walls of the vacuum envelope encarsing the apparatus) is maintained at a low enough temperature to condense the sputtered and ionic particles, or at much lower temperatures, such as by liquid nitrogen cooling. As shown in FIGURE 4 an ion source 45, the collector electrode 10 and shielding grid are disposed within an evacuable envelope 50. A

cooling system, indicated as cooler 60, jackets that portion of the envelope containing the collector electrode 10 to provide the desired cooling. Suitable configurations of cooling apparatus will be apparent to those skilled in the art, and in the interest of clarity the cooling apparatus and vacuum envelope are indicated only generally in the drawing.

The grid 20 seen in FIGURE 1 comprises a honeycomb-like structure 21 defining an array vof hexagonal cells disposed within a generally hexagonal ame 22. The honeycomb structure 21 and the hexagonal frame 22 are fabricated from an electroconductive, reactive source material, with titanium being presently preferred. The array of hexagonal cells `defined by the honeycomb structure 21 is complementary to the array of elongate projections 15 of the collector electrode 10, and the grid surfaces defining the cell walls are oriented in radial alignment with the ion source. The grid 20 is spaced slightly away from the concave surface of the collector electrode and is mounted to the concave interior .surface of the collector electrode 10 by six insulative mounting feet 25, as indicated in FIGURE 1. The grid 20 is mounted to the collector electrode 10 so that each one of the elongate projections 15 is centrally disposed within a cell of the honeycomb structure 21.

The collector electrode and grid are energized from a suitable source of D.C. electrical potential, polarized so that the collector is the positive electrode and the grid is the negative electrode. FIGURE 3 of the drawing shows a battery 30 used as the source of the`D.C. potential. The negative terminal of the battery 30 is connected through a grid current ammeter 31 and an electrical lead 32 to' the grid 20. The battery 30 is shunted by a potentiometer 35 having a variable tap 36. A voltpmeterA 37 is connected between the potentiometer tap 36 and the electrical lead 32 (and hence to the grid 20) for the measurement of grid voltage. An electrical lead 38 interconnects the collector electrode 10 with the potentiometer tap 36. A collector current ammeter 39 is connected between the electrical lead 38 Vand a point of common potential generally indicated by the reference numeral 40. Adjustment of the potentiometer movable tap 36 enables control of the potential difference applied between the grid and collector electrodes.

In operation, a vcollimated ion beam is vemitted from the ion source 45, ,the beam being directed along .the reference axis toward the collector-grid structure. The ion beam is schematically indicated :by the dashed .line arrows in FIGURES 3 and 4 of the drawing, the ion beam consisting of positively charged particles which originate from a'source of potential vthat is positive with respect to the point of common potential 40. As the collimated ion beam nears the collector-grid structure the paths of the ions in the beam are influenced by the electrical lfield created by maintenance of a potential difference between the collector electrode 10 and the grid 20. The positively charged ions are repelled by the elongate projections 15 which are at a positive potential,V and are attracted by the negative grid l20. Positive ions entering each cell are deflected by'the radial field and impinge upon the titanium grid surfaces with energies sufficient to dislodge and sputter titanium particles. The sputtered titanium particles, indicated by the dotted line arrows in FIGURE 3 are collected by the `copper projections 15 and other line of sight surfaces which are at liquid nitrogen temperature. The shape of the elongate projections 15 is such as to provide an almost grazing trajectory by the ions of closest approach, while the serrated'surface of the projections formed by the succession of disc shaped segments of decreasing diameter provides a collecting surface whichcannot be hit by incident ions.

This particular collector electrode configuration and orientation, wherein the elongate projections .15 are spaced radially away from .the source of ions and with their sur-V faces to be sputtered in substantial alignment with the ion beam direction greatly reduces the amount of sputtered getter material which returns to the ion source. The present invention electrode configuration, in conjunction with the use of the disclosed electrical ion deflection technique, effectively shields the atom collecting surface from the ion beam to prevent further sputtering at that surface thereby minimizing the number of sputtered getter atoms that flow toward the ion source.

FIGURE 2 of the drawing shows the grid and collector currents plotted as a function of grid voltage, this graph representing the actual results obtained from operation of the present invention embodiment using a cesium beam from an ion engine. The axis of abscissa is calibrated in l0() volt increments and represents the grid voltage as read from the voltmeter 37. The ordinate axis is calibrated in 20 milliampere units. The collector current readings at various grid voltages appears as the horizontal line S1, representing the current read by the ammeter 39. The grid current, as read from the ammeter 31, is represented by the line 52. As the grid voltage was made increasingly negative, the grid ion interception current increased while the collector current remained constant. The increase in grid current above the collector current is due in part to the secondary electron ow from the grid to the projections of the collector.

The graph of FIGURE 2 was obtained While operating the apparatus at a constant ion beam current. Repeated runs were made utilizing an ion beam current ranging from 42 to 130 milliamperes. Throughout this range, the current-voltage characteristics of the collector remained essentially the same as that shown in FIGURE 2. It is readily apparent from these results that the deflection voltage controls the fraction of the ion beam which strikes the grid surfaces, this control being independent of the ion beam current.

In the actual apparatus of the illustrated embodiment from which the test results were obtained, the spherical segment 11 had a radius of tive feet, with 126 elongate projections 15. The collector electrode was fabricated of copper and was liquid nitrogen cooled. The shielding grid was fabricated of titanium and the diameter of the hexagonal frame 22 was approximately two feet. The ion source was spaced about five feet from the spherical seg- Y ment 11. The grid can be constructed of any other active source material which will combine with gases such as oxygen, hydrogen, steam, carbon monoxide, carbon dioxide and with most other gases typically used in vacuum systems, other than inert gases, including mercury gases. The collector electrode can be fabricated from metals such as titanium, zirconium and molybdenum, and semiconductor materials doped to sufficient conductivity levels.

Improvement of the pumping ability of the present invention apparatus can be caused by outgassing of the reactive source material with prolonged operation. The pumping response was initially quit slow, primarily due to outgassing and due to the pumping effect of the sputtered source material, which in some tests lasted many minutes after the collector voltage was decreased to zero. The vacuum envelope in which the apparatus was disposed was pumped only by the present invention ion pumping mechanism for a period of several hours during which a minimum stable pressure of about 3.8)(10`6 torr. was obtained. The collector pumping ability improves wi-th pumping time the maximum pumping speed of the test apparatus was measured at about 200 liters per second.

In the tested embodiment, it became apparent that when titanium outgassing becomes negligible, the pressure will not vary appreciably as the grid current is varied, except at very low grid currents. The collector pumping at low grid currents depends upon the amount of freshly sputtered titanium available to pump from previous operations.

Thus, there has been described a novel ion pumping sputter apparatus particularly useful in conjunction with an ion beam, the present invention apparatus operating by gettering action of material sputtered by a directed ion beam. The present invention technique of using a collimated ion beam for sputtering provides two distinct advantages over the usual glow discharge sputtering techniques: first, the sputtering is independent of residual gas pressures or constituents, and the sputtering ions can be chosen; and second, it is integrated into the main vacuum chamber providing greater pumping speeds because of the elimination of pump throats invariably required for the isolation of other pumping systems.

By orientation of the surface of `the metal to be sputtered parallel to the ion beam direction yand radially away from the source of ions, the amount of spu-ttered material which returns to the ion source is minimized. Although an electric field is not necessary to perform the basic present invention sputtering technique, an electric eld can be established between a positive collector electrode and a negative grid electrode to deect ions toward the grid and cause impingement at small angles with the grid surface, thereby increasing the sputtering yield over that obtainable at or near normal incidence. With this configuration and mode of operation the collector surface is hidden from the ion beam.

Although the invention has been described with a certain degree of particularity, it is understood that the present disclosure has been madev only by way of example and that numerous changes in the details of construction and the combination and arrangement Iof parts may be resorted to without departing from the spirit and the scope of the invention as hereinafter claimed. For example, although the presently preferred embodiment uses a dishshaped surface to facilitate radial alignment of the collector electrode projections, the preferred radial alignment can also be achieved by proper angular mounting of the projections on a plane surface.

What is claimed is:

1. An ion sputter pump comprising:

(a) tubular grid means of reactive source material;

(b) collector means within said tubular grid means and electrically insulated therefrom;

(c) means for directing a collimated ion beam into said tubular grid means; and,

(d) means for connecting a source of DC potential between said tubular grid means and said collector means to establish electric field therebetween for deflecting the ions of said beam into impingement with the interior surface of said tubular grid means to cause sputtering of said reactive source material.

2. An ion sputter pump comprising, in combination:

(a) grid means defining a surface of reactive source material in radial alignment with a predetermined point on a predetermined axis;

(b) collector means adjacent said grid means for condensing material sputtered from said grid means;

(C) an ion source at said predetermined point for directing a collimated ion beam along said predetermined axis and between said grid means and collector means; and

(d) means for connecting a source of D C. potential between said grid means and said collector means to establish an electric field therebetween for deflecting the ions of said beam into impingement with the reactive surface of said grid means to cause sputtering of said reactive 'source material.

3. An ion sputter pump as defined in claim 2, wherein said grid means is of tubular configuration with its interior surface defining said surface of reactive source material, and wherein said collector means includes an elongate projection disposed within said tubular grid means.

4. An ion sputter pump comprising, in combination:

(a) a vacuum envelope enclosing ya volume it is desired -to evacuate;

(b) grid means disposed within said envelope, said grid means defining a surface of reactive source mater-ial in radial alignment with la predetermined point on a predetermined axis extending through said envelope;

(c) tubular collector means concentrically disposed on said predetermined axis within said envelope and adjacent said grid means;

(d) means for directing a collimated ion beam into said envelope, along said predetermined axis and between ysaid `grid means and said collector means;

(e) means for establishing an electric field between said grid means and said collector means for deflecting the ions of said beam into impingement with the surface of said grid means to cause sputtering of said reactive source material; and

(f) means for cooling said vacuum envelope and said collector means sufficiently to cause condensation of sputtered material thereon, whereby gases within said envelope will be absorbed and the envelope thereby evacuated.

5. Ion sputter pumping apparatus comprising, in combination:

(a) multicellular grid means of reactive source material defining a honeycomb-like structure, the grid surfaces defining the cell walls being in radial alignment with a predetermined point on a predetermined axis;

(b) collector electrode means disposed on said predetermined axis proximate said grid means and electrically insulated therefrom for condensing material sputtered from said grid means, said collector electrode means defining a plurality of similar electroconductive elongate projections, each of said projections being disposed within a different cell of said shielding grid means;

(c) means for directing a collimated ion beam along said predetermined axis toward said collector electrode means and into the cells of said shielding grid means; and

(d) means for connecting a source of D.C. potential between said grid means and said collector electrode means to establish an electric field therebetween for deflecting the ions of said beam into impingement with the surfaces of said grid means to cause sputtering of said reactive source material;

6. Ion sputter pumping apparatus as defined in claim 5, wherein said means for directing an ion beam comprises an ion source positioned on said predetermined axis at said predetermined point.

7. Ion sputter pumping apparatus as defined in claim 5, wherein the cells of said grid means are of uniform hexagonal cross-section.

S. Ion sputter pumping apparatus as defined in claim 5, wherein the elongate projections of said collector electrode means are of substantially circular cross-section and define a series of successive tapered discs of progressively smaller diameter.

9. Ion sputter pumping apparatus as defined in claim 5, wherein the elongate projections of said collector electrode means are of circular cross-section and taper inwardly toward their projecting ends.

10. lon sputter pumping apparatus comprising, in' combination:

y(b) collector electrode means defining a dish-shaped member having an electroconductive concave surface, said collector electrode means being concentrically disposed on said predetermined axis proximate said grid means and electrically insulated therefrom for condensing material sputtered from said grid means, said collector means further defining a plurality of similar electr'oconductive elongate projections extend- 8 ing from said concave surface toward said predetermined point, each of said projections being disposed within a different cell of said shielding grid means; (c) means for directing a collimated ion beam along said predetermined axis toward the concave surface of said collector electrode means and into the cells of said shielding grid means; and

(d) means for connecting a source of DC. potential between said grid means and said collector electrode means to establish an electric field therebetween for deflecting the ions of said beam into impingement with the surfaces of said grid means to cause sputtering of said reactive source material.

11. Ion sputter pumping apparatus comprising, in combination:

(a) multicellular grid means of reactive source material defining a honeycomb-like structure, the grid surfaces dening the cell walls being in radial alignment with a predetermined point on a predetermined axis;

(b) collector electrode means having an electroconductive concave surface defining a spherical segment, said collector means being disposed on said predetermined axis proximate said grid means and electrically insulated therefrom for condensing material sputtered from said grid means, said collector electrode means further defining a plurality of similar electroconductive elongate projections extending from said concave surface toward said predetermined point;

(c) means for directing a collimated ion beam along said predetermined axis toward the concave surface of said collector electrode means and into the cells of said shielding grid means; and

(d) means for connecting a source of D.C. potential between said grid means and said collector electrode means to establish an electric field therebetween for defiecting the ions of said beam into impingement with the surfaces of said grid means to cause sputtering of said reactive source material.

12. lon sputter pumping apparatus comprising, in combination:

(a) multicellular grid means of reactive source material defining a honeycomb-like structure of hexagonal cells, the grid surfaces defining the cell walls being in radial alignment with a predetermined point on a predetermined axis;

(c) means for directing a collimated ion beam along said predetermined axis toward said collector electrode means to establish an electric field therebetween for defiecting the ions of said beam into impingement with the surfaces of said grid means to cause sputtering of said reactive source material.

13. Ion sputter pumping apparatus comprising, in combination:

(b) collector electrode means having an electroconductive concave surface defining a spherical segment,

said collector electrode means being disposed on said predetermined axis proximate said grid means and electrically insulated therefrom for condensing material sputtered from Said grid means, said collector electrode means further defining a plurality of similar electroconduetive elongate projections extending from said concave surface toward said predetermined point;

(d) means for connecting a source of D.C. potential between said grid means and said collector elect-rode means to establish an electric eld therebetween for dellecting the ions of said beam into impingement with the surfaces of said grid means to cause sputtering of said reactive source material.

14. Ion sputter pumping apparatus as defined in claim 13, wherein the output voltage of said source of D.C. potential is selectively variable.

15. Ion sputter pumping apparatus comprising, in combination:

(b) collector electrode means disposed on said predetermined axis proximate said grid means and elec` trically insulated therefrom for condensing material sputtered from said grid means, said collector electrode means delining a plurality of similar electroconductive elongate projections extending toward said predetermined point, said elongate projections being of substantially circular cross-section and tapered inwardly toward their projecting ends, each of said projections being disposed within a dilerent cell of said shielding grid means;

(d) means for connecting a source of D.C. potential between said grid means and said collector electrode means to establish an electric field therebetween for deilecting the ions of said beam into impingement with the surfaces of said grid means to cause sputtering of said reactive source material.

References Cited UNITED STATES PATENTS 2,850,225 9/ 1958 Herb 230-69 3,074,621 1/ 1963 Lorenz et al. 230-69 3,093,298 6/1963 Lafferty et al 230-69 3,251,536 5/1966 Connor 230-69 ROBERT M. WALKER, Primary Examiner.

Claims

1. AN ION SPUTTER PUMP COMPRISING: (A) TUBULAR GRID MEANS OF REACTIVE SOURCE MATERIAL; (B) COLLECTOR MEANS WITHIN SAID TUBULAR GRID MEANS AND ELECTRICALLY INSULATED THEREFROM; (C) MEANS FOR DIRECTING A COLLIMATED ION BEAM INTO SAID TUBULAR GRID MEANS; AND, (D) MEANS FOR CONNECTING A SOURCE OF DC POTENTIAL BETWEEN SAID TUBULAR GRID MEANS AND SAID COLLECTOR MEANS TO ESTABLISH ELECTRIC FIELD THEREBETWEEN FOR DEFLECTING THE IONS OF SAID BEAM INTO IMPINGEMENT WITH THE INTERIOR SURFACE OF SAID TUBULAR GRID MEANS TO CAUSE SPUTTERING OF SAID REACTIVE SOURCE MATERIAL.