US3364370A - Tubular cell anode for sputter ion pumps - Google Patents

Description

Jan. 16, 1968 w. G. HENDERSON 3,364,370

TUBULAR CELL ANODE FOR SPUTTER ION PUMPS Filed Aug. 17, 1966 A fa/wey United States Patent O 3,364,370 TUBULAR CELL ANODE FOR SPUTTER ION PUMPS William G. Henderson, Lancaster, Pa., assigner to Radio Corporation of America, a corporation of Delaware Filed Aug. 17, '1966, Ser. No. 573,124 4 Claims. (Cl. 313-7) ABSTRACT F THE DISCLSURE An anode for a sputter ion pump has a plurality of uniformly sized tubular cells and at least one additional cell which has a larger diameter than the other cells and has a wall of scalloped configuration.

My invention relates to sputter ion pumps and particularly to an improved cellular anode structure for use in such pumps.

Sputter ion pumps are evacuating devices which may comprise an envelope having therein two flat parallel cathodes made of a reactive material, and a lcellular anode supported between the parallel cathodes. One or more magnets are mounted externally of the envelope. The combination of a relatively high voltage difference between the anode and the `cathodes and a magnetic field produced by the magnet, produces a pumping action. This pumping action involves emission of electrons from the cathodes. The magnetic iield produced by the magnet directs the electrons into helical paths oscillating back and forth within each anode cell thereby producing a relatively large number of collisions between the electrons and gas molecules. Such collisions ionize the gas molecules. The ionized or positive gas molecules are attracted to the cathodes with appreciable force so that they strike the cathodes at high velocity. The high velocity ions cause material to be sputtered from the cathodes which is deposited onto the anode surfaces. When the cathodes are made of a reactive metal such as titanium, the sputtered material deposited on the anode chemically combines with and adsorbs non-ionized gas molecules within the pump. This removal of molecules constitutes a pumping action and when the pump is connected to a chamber to be evacuated, gas molecules within the chamber are removed.

One important lconsideration in connection with an ion pump is the speed with which it evacuates a given chamber. Conventional vacuum pumps such as mechanical and diffusion pumps operate at relatively high speeds, only when the gas pressure in the pump is relatively high. Ion pumps, on the other hand, operate at highest speeds, only when the pressure within the pump is relatively low. However, with a given set of parameters for ion pump operation, a desirable speed of evacuation is effective, only within a relatively narrow pressure range. For example, where parameters such as magnetic field intensity, voltage difference between the cathodes and anode, and anode cell diameters are at a `fixed value, the pumping speed will reach a peak and then fall off appreciably as the press-ure is reduced further.

It is not convenient to modify any ion pump parameter referred to during pumping operation for increased pumping speed at such further reduced pressure. Therefore, when the peak speed falls olf with reduction in pressure, the choice is presented of continued pumping at a relatively low speed, or activating a second ion pump connected to the pumping system and having one or more modied parameters for peak pumping speed at a further reduced pressure. However, the provision of such addi- 3,364,370 Patented Jan. 16, 1968 tional ion pump is objectionable from the several standpoints of cost and space requirements.

Accordingly, it is an object of the invention to provide an ion pump capable of preserving a higher peak speed of evacuation throughout a wider pressure range than feasible heretofore.

A further object is to provide an ion pump having a wider pressure range at which the pumping speed is at a peak.

I have found that the cellular structure of an anode of an ion pump may be modified for increased speed of evacuation over an appreciably wider pressure range than heretofore feasible with a single conventional ion pump. The modification involves providing relatively small diameter and relatively large diameter cells in the anode structure. In such mixture of different diameter cells, the relatively small diameter cells exhibit greater effectiveness at a higher pressure region of a relatively wide pres sure range, while the relatively large diameter cells contribute to increased speed of evacuation at a lower pressure region of the range. In this way, a single ion pump having my improved cellular anode is effective for high speed evacuation throughout a pressure range of such width as to require heretofore two or more ion pumps in which one or more of the pump parameters referred to is modified. Eurthermore, my improved anode contributes to higher peak speeds of evacuation than those attainable with prior ion pumps.

Further objects and features will become evident as the description continues.

ln the drawing, to which reference is now made for an embodiment of my invention, by way of example:

FlG. 1 is a side view, partly in section, of an ion pump having my novel anode;

FIG. 2 is a sectional View taken along the line 2 2 of FIG. 1 and shows my improved anode in more detail; and

FIG. 3 is a functional graph showing the pumping speed and the pressure range throughout which peak speed is effective of an ion pump having my improved anode, in comparison with prior ion pumps.

The ion pump shown in FIG. l comprises a casing 1t) made of a material through which a magnetic field is adapted to extend, such as stainless steel for example, and having a duct 12 adapted to be connected to a chamber to be evacuated. A permanent magnet having legs 14, 116 of opposite polarity and field intensity of about 120'() gauss embrace the casing 1t). The magnet may be held in fixed relation with respect to the casing itl by a frictional engagement therewith.

Within the chamber 10i and suitably supported adjacent to opposite Walls thereof, are two flat or planar parallel cathodes 18, Ztl. The two cathodes 18, 20` may be made of a chemically reactive metal such as titanium. Mounted between the parallel cathodes 18, 2l) within the casing 10 is a cellular anode 22 having a plurality of cell-defining tubes 24 made of stainless steel, for example. The tube axes are normal to the planes of the two cathodes 18, 20. Adjacent cells or tubes 24 are in mutually tangent engagement and may be mutually fixed at their regions of engagement by suitable means such as brazing or spot Welding. A relatively rugged lead-in 26 having a cross bar 28 fixed to one end of the cell array 22., extends through a wall of the casing 10 and is electrically insulated from the wall by a bushing 30 made of ceramic for example. The bushing 3i) is hermetically sealed to the leadin 26 and to the adjacent wall portion of the casing 10. The lead-in 26 may `be connected to a voltage source of 7000 volts positive with respect to ground. The cathodes 18 and 20 and the casing 10 may be connected to ground.

My improved cellular anode 22, as shown more clearly "T3 9 in FIG. 2, comprises one group of tubes 24 of uniform diameter and length. The diameter of each tube may be one-half inch and its length may be three-quarters of an inch. Tubes 24 define a relatively large opening or cell 32 which may have a diameter of approximately one and one-half inches. n the example under consideration, the anode opening 32 is provided by leaving vacant an area normally occupied by seven of the uniform diameter tubes or cells 24. I have found that the resultant scalloped wall configuration of the relatively large Opening 32 is of advantage during pump operation in that it contributes to the speed with which the pump performs its evacuating function.

In the example of my improved anode shown in FIG. 2, the anode 22 comprises thirty-two of the smaller diameter tubes or cells 24 and one relatively large diameter opening or cell 32. Where larger anodes are desired, the number of the smaller cells 2li may be increased and additional larger cells may be included to substantially preserve the ratio of small to large cells depicted in FG. 2, for best results. If desired, the larger opening of cell 32 may be defined by a relatively large diameter tube, but in this event some, although not a prohibitive, sacrifice in speed of operation of the pump is involved. As indicated before herein, an appreciable increase in pumping speed is obtained when the wall defining the larger opening 32 is scalloped as shown in FIG. 2.

The speed of evacuation of an ion pump having my novel anode 22, in relation to the speed of evacuation obtainable by prior ion pumps, is indicated by the three curves of the graph of FlG. 3. The highest pressure shown in the graph is l*3 torr, which represents approximately the lowest pressure conveniently obtainable with conventional mechanical pumps. Curve 34 shows the range of operation and the peak pumping speed of a conventional ion pump having fixed parameters of magnet field intensity, uniform diameter of all cells of the anode, and voltage difference between the anode and cathode, for satisfactory speed through a pressure range from l04 torr to 10ml torr. It will be noted that the speed of evacuation peaks at a pressure of about l0-5 torr and then falls off sharply. Therefore, for evacuation to a pressure below "'rl torr, a second ion pump must be employed having fixed parameters at least one of which is different from the parameters associated with the firstnamed pump, for satisfactory speed of evacuation from about 10-7 torr to about lO-l0 torr. The speed of evacuation of the second ion pump, as indicated by curve 36, peaks at a pressure of about l0'ha torr.

Curve 38 denotes the pumping speed of an ion pump having my improved anode structure 22 and incorporating both small and large cells therein. Curve 38 shows satisfactory pumping speeds through a relatively wide pressure range 0f from about 10-l torr to about l0"10 torr. Furthermore, not only is the peak speed of my improved ion pump appreciably higher than that of prior ion pumps whose speed is depicted by curves 34 and 36, but the peak speed extends through a pressure range that is more than twice as wide as that of the peak speed range of one conventional ion pump, as shown by curve 38 in relation to either curve 3d or 36.

The units of pumping speed shown in PEG. 3 are arbitrary and the curves depicted are intended merely to provide a comparison of the pumping speed of my novel ion pump with that of conventional ion pumps.

While the particular mixture of small and large diameter cells or tubes in the anode of my improved ion pump described in the foregoing represents the best form of my disclosed subject matter, it is feasible to modify the mixture and yet obtain satisfactory results. For example, any reduction short of parity in the difference in diameter between the small and large cells will still preserve the advantage of my pump in respect of the relatively wide pressure range through which my pump is effective. Such reduction in the diameter difference will have some adverse effect on the pump speed. However, even though the difference in diameter between the small and large cells is so small as to reduce the pumping speed below that obtainable with conventional ion pumps, my novel ion pump will still be advantageous in that it will be characterized by effectiveness through an appreciably wider pressure range than prior ion pumps.

On the other hand, if the difference in diameter between small and large tubes of my novel anode is increased to any appreciable degree above the values indicated in the foregoing for best results, there will be a tendency to separate the pressure region in which the small tubes are most effective, from the pressure region in which the large tubes are characterized by peak pumping speed. Such separation of the pressure regions in which each of the two groups of anode tubes produce peak pumping speed, will result in a dip in the midportion of curve 38 of FIG. 3. If this dip were of a magnitude so as to extend downwardly to the cross-over point of curves 34, 36, the peak pumping speed and effective pressure range of my ion pump would be substantially equivalent to the performance represented by curves 3d, 36, of a combination of two prior ion pumps. Even under these circumstances, my ion pump would possess advantage in that my single ion pump would perform a function requiring two prior ion pumps.

It will be appreciated, therefore, that the ion pump parameter in respect of anode cell diameter specified in the foregoing for best results, and exemplified by curve 38 of FlG. 3, may be modified without prohibitively reducing the pumping speed and pressure range throughout which the pumping speed is effective.

I claim:

1. In a sputter ion pump:

(a) a cellular anode comprising a plurality of tubular cells mutually joined to form a self-supporting structure,

(b) said structure comprising a plurality of uniform diameter cells and at least one cell of larger diameter than said plurality of cells,

(c) said at least one cell being defined by portions of the walls of said plurality of tubular cells adjacent to said at least one cell, said portions of the walls forming a scalloped contour for said at least one cell.

2. A sputter ion pump comprising:

(a) a casing,

(b) a cathode having a surface made of sputterable chemically reactive material supported on one wall of said casing,

(c) a cellular anode having tubular cells in parallel axial relation, the axis of said cells `being normal to said surface of said cathode, said cells comprising a first group having a relatively small diameter and a second group of one or more cells having a relatively large diameter, the cells in said first and second groups having uniform lengths,

(d) said first group of cells comprising individual cylindrical tubular structures and said second group of one or more cells being at least partially defined by the tubular structures of said first group of cells, whereby the walls of said second group of one or more cells are scalloped for increased pumping speed,

(e) means for causing electrons to be emitted from said cathode to said cellular anode, and

(f) means for providing a magnetic field extending into each of said cells, said magnetic field being adapted to elongate the paths of electron travel in said second group of cells to a greater degree than in the cells of said first group,

(g) whereby said cellular anode is adapted to provide a pumping speed extending through a wider pressure range and having a higher peak pumping speed than feasible with ion pumps having a cellular anode in which all of the cells are of uniform diameter.

3. An ion pump according to claim 2 and wherein the diameter of each of said smaller diameter cells is about one-half inch and wherein the diameter of said at least one larger diameter cell is about one and one-half inches for desired pumping speed through a pressure range of from about 10*3 torr to about 10-10 torr.

4. An ion pump according to claim 3 and wherein the ratio of the number of said smaller diameter cells to the number of said larger diameter cells is 32 to 1.

References Cited UNITED STATES PATENTS Zaphiropoulos et al. 230-69 Lloyd et al. 313-7 Jepsen 313--7 X Knauer 313--7 X Vanderslce 230-69 JAMES W. LAWRENCE, Primary Examiner. C. CAMPBELL, IR., Assistant Examiner.

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