US3540812A

US3540812A - Sputter ion pump

Info

Publication number: US3540812A
Application number: US720864A
Authority: US
Inventors: William G Henderson; John T Mark
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1968-04-12
Filing date: 1968-04-12
Publication date: 1970-11-17
Anticipated expiration: 1987-11-17

Description

1970 w. G'. HENDERSON ETAL 3,540,812

SPUTTER ION PUMP 2 Sheets-Sheet 1 Filed April 12, 1968 INVENTOR WILLIAM QHEMOERsouf Jonu T. Mann AT TORNEY SPUTTER ION PUMP Filed April 12, 1968 2 Sheets-Sheet 2 INVENTOR.

WILLIAM G-HEMDEQsnu? Joan T. Mama A T70R05) United States Patent 3,540,812 SPUTTER ION PUlVIP William G. Henderson and John T. Mark, Lancaster, Pa., assignors to RCA Corporation, a corporation of Delaware Filed Apr. 12, 1968, Ser. No. 720,864 Int. Cl. F04b 37/02 U.S. Cl. 417-49 7 Claims ABSTRACT OF THE DISCLOSURE A sputter ion pump having an anode comprised of a plurality of open-ended tubular cells which have slots in the walls thereof and having a thermoemissive filament located within the pump envelope.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to sputter ion pumps and particularly to means for enhancing both pumping speed and the pressure range over which pumping speed is at a maximum, as well as initiation of pumping action.

Description of the prior art Sputter ion pumps are used for evacuation and may be comprised of an envelope which contains a pair of fiat, oppositely located cathodes and a honey-comb-like multicellular anode located between these cathodes. The cathodes are made of a sputterably chemically reactive material such as titanium, zirconium, etc. A relatively high voltage diiference between the anode and the cathodes causes emission of electrons from the cathodes. A magnetic field co-axial with the anode cells and produced by a magnet located externally to the envelope, drives these emitted electrons into spiral paths oscillating within the anode cells. These driven electrons collide with gas molecules, ionizing these molecules. The positive ions which are thus created are accelerated towards and strike the cathodes and sputter 01f some of the reactive cathode material. Hence, some evacuation comes about through the ionization of gas molecules and the subsequent entrapment or embedding of the positive ions on the cathodes. Some of the sputtered material is then deposited on the anode and other surfaces available within the envelope. Pumping action also results from a gettering effect of the sputtered reactive material deposited on the anode and other surfaces. Other portions of the sputtered material are attracted back to the cathodes with such force as to produce secondary emission of electrons from the cathodes, these secondary electrons contributing to the ionization process. These attracted portions also perform a gettering function to evacuate the gas in the envelope.

Such sputter ion pumps operate at high pumping speeds only when the pressure within the pump is relatively low. Where variables such as magnetic field intensity, voltage difference between anode and cathodes, and anode cell diameters are held constant, the pumping speed will start out at a relatively low value, gradually reach a maximum as electron emission and the rate of ionization increases, and then drop 013?, as pressure in the pump is reduced. Also, the pressure range over which the pump operates at maximum rate is relatively narrow. In U.S. patent, 3,364,370, issued to W. G. Henderson means is described for enhancing not only the pressure range at which the pump operates at maximum speed, but also the maximum pumping speed.

At pressures greater than approximately torr, the initiation of pumping action in sputter ion pumps is not difficult. though it may require a significant period of time 3,540,812 Patented Nov. 17, 1970 ICC to allow cathodic discharge of electrons to advance to a stage where appreciable pumping action takes place. However, at pressures less than about 10 torr, it is usually very difiicult, if not impossible, to initiate pumping action regardless of the amount of time allowed. Even though the pump has been started in the pressure range above 10 torr, pumped to a pressure level below 10- torr, and temporarily shut off, it is extremely difficult to restart the pump while the pressure remains below about 10 torr. Generally, as pressure decreases in a range below about 10* torr, it becomes more difiicult to initiate pumping action. This is due to the short supply of gas molecules at lower pressures and the lower probability that these molecules will be ionized where the only source of ionizing electrons is the aforementioned cathodes.

SUMMARY OF THE INVENTION A sputter ion pump is disclosed that is characterized by one or more of several advantages. These advantages include a relatively high maximum pumping speed, a widened range over which the pump operates at maximum speed, a relatively high initial pumping speed, and a relatively easy starting of pumping action at relatively low pressures.

The pump includes a source of electrons in addition to the cathodes usually employed, and the anode is provided with a structure that provides increased electron access to the interior of the cells thereof.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a partly sectional cut-away view of a sputter ion pump having the improved structure of the invention;

FIG. 2 is a sectional view of the sputter ion pump of FIG. 1 taken on the line 22;

FIG. 3 is a partly sectional cut away view of another embodiment of the invention; and

FIGS. 4 and 5 are perspective views of individual anode cells usable with the invention.

PREFERRED EMBODIMENT An ion pump (FIGS. 1 and 2) may have an evacuable envelope 10 made of a material such as stainless steel, that is penetrable by a magnetic field. The envelope has an inlet duct 12 whereby connection may be made by suitable means to a chamber desired to be evacuated. Located externally to the envelope 10 is a permanent magnet 14, the magnet being arranged such that portions thereof (FIG. 1) of opposite polarity are situated on opposite sides of the envelope. Mounted on opposite internal walls of the envelope are two parallel cathodes in the form of cathode plates 16 (FIG. 1). These cathodes may be made of a sputterable chemically reactive material, such as titanium, zirconium, etc. Mounted between the parallel cathodes 16 is an anode 18 comprised of a plurality of openended tubular cells, which may be made of, for example, stainless steel. The longitudinal axes of the anode cells are normal to the planes of the two cathodes. The present invention may be used with any variety of cellular anodes and is not limited to use with an anode 18 (FIG. 2') having the cell arrangement set forth in U.S. Pat. 3,364,370 to W. G. Henderson.

Adjacent cells 20 of the anode 18 may be 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 conductor 22 having a cross bar 24 fixed to one end of the anode 18, extends through a wall of the envelope 10 and is electrically insulated from the wall by a lead-in bushing 26 made of ceramic, for example. The bushing is hermeti cally sealed to the lead-in conductor 22 and to the adjacent wall portion of the envelope. The lead-in conductor may be connected to a voltage source of, for example,

3 7000 volts positive with respect to ground while the cathodes and the envelope may be connected to ground.

There is provided a supplementary source of electrons in the form of a filament 28 made of a material that will emit electrons upon being heated. Such material may be, for example, tungsten and heating may be accomplished, for example, by an electrical power source of from about 2 to 36 watts. The filament 28 may extend from a lead-in conductor 30 to a tab 32 mounted on the interior wall of the envelope. The lead-in conductor is electrically insulated from the envelope by means of an insulative lead-in bushing 34. The filament is in proximity to the anode 18 such that emitted electrons are readily available for the ionization of gas molecules in the anode region. The supplementary source may also be a filament made of tungsten, for example, coated with a thermo emissive material, such as thorium oxide. Because at relatively low pressures thermionic emission from the supplementary source produces electrons more readily and quickly than does the field emission and glow discharge from the cathodes, a large supply of emitted electrons is available early in the pumping operation. The result is a facilitated start in pumping action, especially at pressures below about 10 torr, and a-more rapid achievement of maximum pumping speed. The emitting filament may be de-energized after electron emission from the cathodes has reached a satisfactory level. However, if the filament is allowed to continue in operation, the maximum pumping speed achievable will be relatively higher due to the additional electrons available from the filament for ionizing the gas molecules, the increased ionization resulting in increased sputtering. Also, the increased electron population from the continued operation of the filament extends the pressure range over which pumping speed is at maximum.

Alternatively, the supplementary source of electrons may be a filament 38 (FIG. 3) electrically biasable with respect to ground. Such a filament 38 may also be made from thermoemissive material, such as tungsten, and may be supported at each end by and electrically connected to, electrical lead-in conductors 40. The lead-in conductors are electrically insulated from the envelope 42 by insulative lead-in bushings 44. The envelope 42 being at ground, the filament 38 may be biased with respect to the envelope. Such biasing may be, for example, over the range of from to 5000 volts with respect to ground. Negative biasing of the filament imparts additional energy to the electrons emitted from the filament, with the result that more gas molecules may be ionized by a single electron and that ionization will be carried on with greater efficiency.

While the improvement wherein a supplementary source of electronsIis illustrated in the foregoing as being used with the anode having the cell arrangement set forth in U.S. Pat. 3,364,370, it may be used with any variety of anode and is not restricted to the aforesaid anode.

The interiors of the respective anode cells may be made more accessible to electrons by slotting, or otherwise providing openings in, the cell walls. Where the walls of the individual anode cells 46 (FIGS. 2 and 3) are longitudinally slotted, as at 48, electrons that are emitted from the cathode (not shown) and/or filament (not shown) are able to enter the anode cells in increased numbers and thereby are more likely to collide with gas molecules present in the anode. Such enhancement of the probability of collisions between electrons and molecules results both in increased sputtering and consequent increased pumping speed, and in extension of the pressure range over which pumping speed is at maximum. It is not necessary that every cell be slotted since improvethan all the cells.

FIGS. 4 and 5 illustrate two types of slotted individual anode cells which may be used with the invention. A cell 50 may have a slot 52 extending over the entire length of the cell wall while another cell 54 may have slots 56 which extend over less than the entire cell length. Individual cells (not shown) may have both slots extending over the entire cell length and slots extending over only a portion of the cell length. It is not necessary that all the cells of an anode have slots of the same dimension, but cells having slots of different dimensions may be combined in the same anode. Cells slotted as described are not limited to use with the cell configuration (FIGS. 2 and 3) set forth in U.S. Pat. 3,364,370, but may be arranged in other configurations as well.

The aforementioned pump elements, that is, the thermoemissive filament, whether energizable or biasable, and the aforementioned slotted anode cells, are used in com bination in the same pump. The pumping improvements derived are significantly greater than the total of the improvements individually contributed by each element. Hence, it is desirable to take advantage of the interaction between these two pump elements to achieve significantly improved results.

We claim:

1. A sputter ion pump comprising:

(a) an outer envelope;

(b) a cathode made of sputterable chemically reactive material supported on the interior wall of said envelope;

(c) a multicellular anode disposed such that the longitudinal axes of the tubular cells thereof are normal to said cathode, said anode having a multiplicity of longitudinal slots in the cell Walls thereof, said anode and said cathode being adapted to operate in an electric field such that electrons are emitted from the cathode;

(d) means for providing a magnetic field, said magnetic field driving said emitted electrons into spiral paths in the region of said anode; and

(e) means within said envelope for providing electrons to supplement said electrons emitted from said cathode.

2. A multicellular anode as set forth in claim 1 wherein each of said slots extends over the entire length of one of said cell walls.

3. A multicellular anode as set forth in claim 1 wherein each of said slots extends over a portion, only, of the length ofone of said cell walls.

4. A sputter ion pump as set forth in claim 1 wherein said supplementary electron-providing means is a starter filament having a thermoemissive surface and is adapted to be biased negative to said anode.

5. A sputter ion pump as set forth in claim 4 wherein said filament is made of tungsten.

6. A sputter ion pump as set forth in claim 4 wherein said starter filament has one end mounted on an electrical lead-in through said envelope and the other end electrically connected to said cathode.

7. A sputter ion pump as set forth in claim 4 wherein said starter filament is mounted at opposite ends, on electrical lead-ins through said envelope.

References Cited UNITED STATES PATENTS 3,112,864 12/1963 Hall et al. 230-69 3,161,802 12/1964 Jepsen et a1. 230-69 XR 3,235,170 2/1966 Thoresen 230- 69 ROBERT M. WALKER, Primary Examiner U.S. Cl. X.R. 3137