US3400882A - Ion pump - Google Patents

Description

Sept. 10, 1968 J. A. MCMANUS ION PUMP Filed June 24. 1966 INVENTOR JOHN A. McMANUS an/6 5 ATTORNEY United States Patent 3,400,882 ION PUMP John A. McManus, Oakville, Ontario, Canada, assignor to Mallory Battery Company of Canada Ltd., Ontario, Canada, a corporation of Canada Filed June 24, 1966, Ser. No. 560,139 Claims. (Cl. 230-69) ABSTRACT OF THE DISCLOSURE An apparatus for scavenging gases from a partially evacuated environment including a chamber having intake and exhaust ports. The intake port is connected to the partially evacuated environment. A high voltage electric field of the apparatus substantially converts surrounding gases to ions. An ion attracting means is disposed between the electric field and the exhaust port so as to attract ions thereto thereby sweeping neutral gas particles out of the exhaust port. The efficiency of the apparatus increases as the vacuum becomes harder.

The present invention relates to ion pumps and more particularly to the means and methods for providing an ion pump for sweeping neutral gas particles through an exhaust port.

In the prior art, there are two basic types of pumps for producing a high-vacuum in an enclosed chamber. The first and oldest type of pump is a diifusion pump and the second type is a gettering pump. Both the diffusion pump and gettering pumps are designed to further evacuate a chamber after said chamber has been preliminarily pumped to 0.00 1 mm. of mercury or less. The preliminary pumping is usually accomplished with a mechanical roughing pump.

The diffusion pumps utilize mercury or oil which is vaporized and passed through a jet nozzle which is surrounded by or in communication with a chamber connected to the vessel being evacuated. The vaporized mercury or oil flows through the nozzle as a high velocity stream with which molecules of gas from the aforesaid vessel are diffused and otherwise trapped. As this process continues and the vapor stream is recirculated, gas molecules are progressively diffused into the stream and thereby exhausted from the vessel.

One problem with diifusion pumps, as described above, is that oil and mercury vapors are continuously circulated through the chamber and often backstream into the vessel being evacuated. Thus, there are severe contamination problems for certain vacuum applications.

The getter pumps ionize gas to form a cloud of positively or negatively charged ions. These ions are immobilized within the chamber by electrostatic attraction and by a gettering action which involves the entrapment thereof by a metal such as titanium. Most getter pumps generate ions by placing a strong magnetic field between two electrodes at cathode potential. Because of the strong magnetic field, electrons which are emitted from one of the electrodes cannot reach the other electrode and are oscillated in a random manner. The oscillating electrons strike gas particles to knock loose other electrons, thereby ionizing the gas. The electrons that are knocked loose also oscillate to further ionize gas particles.

3,400,882 Patented Sept. 10, 1968 One disadvantage with getter pumps, as described above, is that the magnetic field must be created with a large permanent magnet or electro-magnet. Thus, the equipment is bulky and dilficult to adapt to many vacuum vessels.

Another disadvantage of getter pumps, as described above, is that the strong magnetic field. may prevent their application in electron-optical devices sensitive to magnetic fields, such as cathode ray tubes or electronic cameras, and in vacuum systems where magnetic material is being evaporated.

There is presented in this specification an ion pump for providing a high-vacuum condition without introducing contaminating vapors or stray magnetic fields. The ion pump of the present invention works by sweeping neutral gas particles out of an exhaust port. The sweeping action is caused by the rapid movement of ions which are electrostatically attracted to a grid or screen which is disposed over the exhaust port. The ions are generated by a high voltage electrical field means for generating positively and negatively charged ions. In the illustrative embodiment of this specification, the high voltage electrical field means is a conducting member having a plurality of sharply pointed conductors located thereon. When the conducting member is connected to a high voltage source, the electric field at the pointed conductors becomes so great that ionization of the surrounding air or gas occurs. By connecting the aforementioned grid or screen to the opposite side of the high voltage source, ions are attracted towards the grid or screen and the movement of ions will sweep neutral gas particles through the grid or screen and out of the exhaust port. Of course, the positively charged ions will be attracted to and held by the negatively charged member and the negatively charged ions will be attracted to and held by the positively charged member.

The ion pump of the present invention is relatively simple to construct as compared to present day ion pumps. Only two hermetically sealed electrical feedthroughs are required, one for the positively charged conducting member and one for the negatively charged conducting member.

Although the illustrative embodiment of this specification is a cylindrically shaped chamber having oppositely disposed intake and exhaust ports, other configurations can readily be designed. This feature provides a great advantage over ion pumps which utilize magnets to generate ions because, in magnetic systems, the magnetic field must be placed in close proximity to the anode and cathode and in a proper relationship to assure random oscillation of the emitted electrons. Thus, the configuration of the pump is dictated by the location of the magnet with respect to the anode and cathode.

The ion pump of the present invention can be readily adapted to work with other types of ion pumps in a vac uum system. For instance, the pump at the present invention could be combined With a titanium sublimation pump or a titanium sputtering pump to provide a pumping system that is extremely efiicient in operation.

Other features of the present invention will become apparent as this specification progresses.

It is an object of the present invention, therefore, to provide an ion pump which sweeps neutral gas particles out of an exhaust port.

It is another object of the present invention to provide a vacuum pump which does not introduce contaminants or stray magnetic fields in the vacuum chamber.

It is a further object of the present invention to provide an ion pump for a vacuum system which is simple in structure as compared to present day ion pumps.

It is still another object of the present invention to provide an ion pump which includes a high voltage electrical field means for generating positively and negatively charged ions.

It is yet another object of the present invention to provide an ion generating means comprising a conducting member having a plurality of sharply pointed conductors located thereon and a means for connecting said conducting member to a high voltage source.

The present invention, in another of its aspects, relates to novel features of the instrumentalities described herein for teaching the principal object of the invention and to the novel principles employed in the instrumentalities whether or not these features and principles vmay be used in the said object and/or in the said field.

Other objects of the invention and the nature thereof will become apparent from the following description conlidered in conjunction with the accompanying drawings and wherein like reference numbers describe elements of similar function therein and wherein the scope of the invention is determined rather from the dependent claims.

For illustrative purposes, the invention will be described in conjunction with the accompanying drawings in which:

FIGURE 1 is a sectional view of the ion sweep pump of the present invention.

FIGURE 2 is a sectional view 2-2 taken from FIG- URE 1 to show the arrangement of the ion generating means.

FIGURE 3 is a sectional view 3-3 taken from FIG- URE l to show the ion collecting grid.

FIGURE 4 is a fragmentary sectional view showing the pointed conductors of the ion generating means.

Generally speaking, the present invention is an ion pump comprising a chamber having an intake port and an exhaust port, an ion generating means, and an ion collecting means. The ion generating means is a high voltage electrical field means for generating positively and negatively charged ions. In the illustrative embodiment the high voltage electrical field means is a conducting member having a plurality of pointed conductors attached thereto and a means for connecting said conducting member to high voltage source so as to ionize the gas particles in the area surrounding the pointed conductors. The ion collecting means is a conducting grid or screen located in said chamber between said ion generating means and said exhaust port. When the ion generating means is connectedto a negative voltage source and the ion collecting means is connected to a positive voltage source, negatively charged ions will be attracted to said ion collecting means and the movement of said negatively charged ions will sweep neutral gas particles through said ion collecting means and out of the exhaust port. The positively charged ions will be attracted to said ion generating means.

Referring now to the drawing, and particularly to FIG- URE 1, the components and structure of the present invention can be visualized in conjunction with the following description.

The ion pump of the present invention is enclosed in the [metallic cylindrically shaped container 10. There is an intake port 12 and an exhaust port 13 formed in opposite ends of the cylindrically shaped container 10. A coating of ceramic material 11 covers the interior of the ion pump chamber. The walls of the pump may be any suitable metallic material such as stainless steel. The ceramic coating 11, which is optional, may be any suit able ceramic for high vacuum environments, preferably a ceramic that is not porous and that has a glazed surface exposed to the interior of the vacuum chamber.

A combination supporting ring and feed through 16 is disposed in a groove 22 cut in the Wall of the container 10. The supporting ring and feed through can be fabricated of any suitable insulating material capable of handling up to 25,000 volts in a high vacuum environment. Preferably, the insulating material should not have a rough surface, should not be porous, and should not contain materials which will readily outgas in a high vacuum system. A conducting member 18 is annularly supported in the supporting ring 16 and is connected to the current carrier 17 which extends axially through the feed through portion of the supporting ring 16. Obviously, there must be a hermetic seal between the current carrier 17 and the feed through portion of the support ring 16 and between the feed through portion of the support ring 16 and the wall of the container 10.

It can be seen that there are a plurality of pointed conductors 19 on the conducting member 18. The pointed conductors 19, which are pointed towards the exhaust port 13, are disposed on two cross members of the conducting member 18 as is shown in FIGURE 2.

The supporting ring 16, conducting member 18, and pointed conductors 19 comprise the ion generating means 14.

There is an ion collecting means 15 comprised of a combination supporting ring and feed through 20, collecting grid 21, and current carrier 24. The supporting ring and feed through 20, which is also fabricated of an insulating material suitable for high voltages in high vacuum environments, is disposed in a groove 23 cut in the wall of the container 10. The collecting grid 21, which is disc shaped, is supported in the supporting ring 20 and is connected to the current carrier 24 which extends through the feed through portion of the supporting ring and feed through 20.

As is the case with the feed through system of the ion generating means 14, there must be a hermetic seal between the current carrier 14 and the feed through portion of the supporting ring 20 and between the feed through portion of the supporting ring 20 and the wall of the container 10.

Referring now to FIGURE 2, a sectional view showing the structure of the ion generating means 14 can be discussed.

The pointed conductors 19 are afi'ixed to the cross members 25 of the conducting member 18 as discussed previously. It can be seen that the circular portion of the conducting member 18 is the portion which is supported in the supporting ring 16 and which is connected to the current carrier 17.

Referring now to FIGURE 3, a sectional view showing the collecting grid 21 of the ion collecting means 15 can be discussed.

The gril 21 can be fabricated of any suitable conducting grid or screen material. The thickness of the grid 21 and size of the openings are determined by the requirements of the pump. It can be seen that the collecting grid 21 is connected to the current carrier 24 so that whatever voltage is applied to the current carrier will be present on the collecting grid.

Referring now to FIGURE 4, an exploded sectional view of a pointed conductor 19 can be discussed.

Each pointed conductor 19 is joined to a cross member 25 as is shown. It can be seen that the pointed conductors 19 taper down to the point 26 and that there are two pointed portions 27 and 28 extending up and away from the point 26. Since the ion generating means 14 of the present invention is based on the principle that current applied to a sharply pointed conductor produces an electrical field at the pointed ends, the actual size and shape of the pointed conductors 19 will depend on the pumping requirements and the amount of voltage to be applied to the conducting member 18.

With the above description of components and structure in mind, and by making reference to the drawing figures, the following analysis of operation will serve to convey the functional details of the present invention.

When a sufiicient and continuous current is applied to the pointed conductors 19, the electric field at the pointed ends becomes so great that ionization of the surrounding air or gas occurs. If the pointed conductors 19 are negatively charged, the negatively chargedions are repulsed with such a force that they collide with neutral gas particles causing further ionization of the surrounding air or gas. If enough ionization is present, a sort of ionization chain reaction occurs.

If the collecting grid 21 is positively charged and is close enough to the pointed conductors 19, all of the negatively charged ions will be attracted to said grid. As the negatively charged ions rush towards the positively charged grid 21, they will collide with neutral gas molecules giving directional flow through the grid. Thus, the gas will be swept out of the ion pump by the sweeping action of the moving ions. Of course, the positively charged ions will be attracted to the negatively charged conducting member 18.

By placing a continuously high negative voltage, up to 25,000 volts, with a low amperage on the conducting member 18, ionization will rapidly occur very close to the conducting member and the positive ions will have very little distance to travel. Therefore, there is little or no positive ion sweep or flow. By placing a continuously high positive voltage, up to 25,000 volts, on the collecting grid 21, the negatively charged ions will be attracted towards the grid with considerable speed to sweep the neutral gases through the grid 21. As the vacuum further develops, efficiency of the pump will increase because the proportion of gas collision ionization decreases while the proportion of electric field ionization increases. Thus, the positive ion flow due to gas collision will decrease and the negative ion flow rate will increase.

When the positively charged ions strike the conducting member 18 and the negatively charged ions strike the collecting grid 21, a considerable number of gas particles will be buried in the surface of said conducting member and said grid. This action, of burying gas particles, will tend to increase the efficiency of the ion pump of the present invention.

The efiiciency of the ion pump of the present invention can also be increased by increasing the number of pointed conductors 19 so as to obtain greater ion discharge over more area of the pump enclosure.

Placing one or more cathode-anode units, i.e. ion generating and collecting means, in the same vessel at predetermined distances to avoid possible interferences will increase the efiiciency of the pump. Sequence timing of these units may further increase the efliciency of the pump. Similarly, placing two ion pumps in series will provide a more efficient pumping system.

The ion pump of the present invention can be constructed so that a positive voltage is applied to the ion generating means 14 and a negative voltage is applied to the ion collecting means 15.

The ion pump of the present invention is adapted to be used with any suitable roughing-pump or preliminary pump for reducing the pressure in the vacuum chamber to a residual of the order of 0.001 mm. of mercury or less. That is, the ion pump of the present invention, as is the case with most ion pumps, is a means for obtaining a very high-vacuum condition in a chamber which has already been roughly evacuated.

The ion pump of the present invention can also be adapted for use in a vacuum system with a titanium sublimitation pump or a titanium sputtering pump. A titanium sublimation pump works on the principle that evaporated titanium will collect gas particles and hold said particles to a surface upon which the titanium condenses.

In a titanium sputtering pump, usually referred to as a sputtering-ion pump, the positive ions formed in the discharge region bombard a titanium surface and sputter atoms from this surface. The sputtered material condenses on other portions of the pump, entrapping and adsorbing gases from thes ystem.

The pump of the present invention, as hereinbefore described in one embodiment, is merely illustrative and not exhaustive in scope. Since many widely different embodiments of the invention may be made without departing from the scope thereof, it is intended that all matter contained in the above description and shown in the accompanying drawing shall be interposed as illustrative and not in a limiting sense. I

What is claimed is:

1. An ion pump comprising: a chamber having an intake port and an exhaust port; a conducting member having a plurality of sharply pointed conductors attached thereto, said conducting member being insulatively mounted in said chamber; means for connecting said conducting :member to a negative voltage source so as to create an intense electric field in said pointed conductors, thereby producing negatively and positively charged ions in the space surrounding said pointed conductors, said positively charged ions being attracted to said conducting member; a collecting grid insulatively mounted in said chamber between said conducting member and said exhaust port; and means for connecting said collecting grid to a positive 'voltage source so as to attract said negatively charged ions thereto, thereby sweeping neutral gas particles through said grid and out of said exhaust port.

2. In combination, a partially evacuated environment and an apparatus for scavenging gases from said partially evacuated environment, the efficiency of said apparatus increasing as the vacuum of said environment becomes harder said apparatus comprising a chamber having an intake port and an exhaust port, said intake port connected to the partially evacuated environment, conducting means positioned in said chamber developing an intense electrical field substantially converting the surrounding gases to ions, and ion attracting means in said chamber disposed between said conducting means and said exhaust port attracting said ions from the vicinity of said electric field substantially preventing back flow of said ions, said attracted ions influencing the direction of movement of neutral gas particles thereby causing said neutral particles to flow out of said exhaust port further evacuating said environment.

3. The combination of claim 2, wherein said conducting means includes an annular conducting member and cross member having the extremities thereof connected to said con-ducting member so as to substantially span said chamber, and a plurality of pointed conductors each having at least one point.

4. The combination of claim 3, wherein each of said pointed conductors is tapered from said cross member toward the points of said pointed conductors.

5. The combination of claim 4, wherein each of said pointed conductors includes a plurality of pointed extremities extending acutely from said pointed conductors.

6. The combination of claim 5, wherein said conducting means is carried by support means retained by the side wall of said chamber, said conducting means connected to a source of voltage through an hermetic seal in said side wall of said chamber.

7. The combination of claim 6, further including a substantially continuous ceramic means covering the walls of said chamber.

8. The combination of claim 7, wherein said ion colleting means is a grid means extending across said chamber in spaced parallel relationship with said conducting means and connected to said source of voltage.

9. The combination of claim 8, wherein said source of voltage is a direct current voltage source.

10. A method of scavenging gases from an environment using the apparatus of claim 2 having an intake port and an exhaust port and a chamber separating said ports comprising the steps of connecting said intake port of said apparatus to said partially evacuated environment, developing an intense electric field in said chamber for substantially converting the surrounding gases of said partially evacuated environment to ions,

attracting said ions from the vicinity of said electric field to ion attracting means disposed in said chamber between said electric field and said exhaust port, considerable numbers of said ions burying themselves in said attracting means, and

sweeping neutral gas particles from said chamber and l5 out said exhaust port by attraction of said ions to said attracting means thereby further evacuating said References Cited environment.

UNITED STATES PATENTS Bennett 230-69 Bennett 230-69 Misson 1031 Freeborn 1031 Jorgenson 230,-69 Naundorf 1031 Jutila 1031 ROBERT M. WALKER, Primary Examiner.

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