US3601503A

US3601503A - Thin membrane ionization pump apparatus

Info

Publication number: US3601503A
Application number: US849607A
Authority: US
Inventors: Thomas W Snouse
Original assignee: Individual
Current assignee: Individual
Priority date: 1969-08-08
Filing date: 1969-08-08
Publication date: 1971-08-24
Anticipated expiration: 1988-08-24

Abstract

An ionization process vacuum pump. Ions produced in the pump are accelerated to energies sufficiently high to enable them to pass through a thin membrane and thus out of the system being evacuated. The membrane prevents return of low energy gas molecules from the high-pressure side of the pump. Embodiments relating to different methods for production of energetic ions are discussed.

Description

United States Patent Thomas W. Snouse 13781 Pierce Road, Saratoga, Calif. 95070 849,607

Aug. 8, 1969 Aug. 24, 1971 Inventor Appl. No. Filed Patented THIN MEMBRANE IONIZATION PUMP APPARATUS 3 Claims, 3 Drawing Figs.

U.S. Cl 417/48 Int. Cl r F04b 37/02 Field of Search 103/1 E; 230/69; 60/202; 417/48, 49, 58

[56] References Cited UNITED STATES PATENTS 3,221,197 11/1965 Coppola 230/69 X 3,328,960 7/1967 Martin 60/202 3,427,978 2/1969 Hanneman et a] 103/1 Primary ExaminerRobert M. Walker PA TENTED AUG24 I97! 3,601, 503

FIGURE I FIGURE 2 THIN MEMBRANE IONIZATION PUMP APPARATUS The present invention is related to ion pumping for the production of low'pressures and more specifically, to a new method for removal of the pumped gases from the volume being evacuated.

Vacuum pumps 'utilizing ionization processes leading to a pumping action have been known for many years. In general, they utilized the processes of gettering active gases (fresh surfaces of the getter material may be provided in several ways such as sublimation or by sputtering by the positive ions which are to be pumped); and of ion burial (ions are driven into the cathode and later buried by subsequent deposits of material sputtered from another cathode or sublimed from a getter source). The distinguishing characteristic of said pumps being the remanence of the gases pumped within the body of the pump.

The aforementioned pump suffer from several drawbacks due to this characteristic. One drawback is the re-emission of previously pumped. gas due to sputtering. This contributes to the decline of pumping speeds of sputter-ion pumps at pressures of l -l0 torr. This factor may also limit the ultimate vacua attainable with some types of pump.

Another drawback is the poor tolerance of some pumps for hydrogen. Although it is pumped well initially, hydrogen eventually diffuses into the getter material causing cracking,

spalling, and flaking which adversely affects the pump performance. I

Those pumps which form a getter film by sublimation have the inconvenience of requiring a cooled surface. Also, they may contribute unwanted gas molecules to the system as the getter material is sublimed. 7

Some pumps, in order to provide surfaces for the deposit of getter materialhave multicelled anodes. This causes uneven distribution of the ion current to the cathode which results in waste of cathode material and shortened operating life. The size of the individual anode cell also affects pump speed as a function of pressure and contributes to the aforementioned decline or pump speed at low pressures.

A further drawback is the poor ability of these pumps to pump noble gases; speeds for thesegases range from O to 24 percent of the speeds for air.

It is the object of the present invention to provide a new ionization-type vacuum pump which removes the pumped gases from the volume being evacuated.

Briefly stated, in accordance with one teaching of the present invention, there is disclosed a diode ionization pump which has a thin membrane as a cathode, thus allowing ions which are accelerated toward the cathode to pass through it and into a chamber where they are removed by an auxiliary vacuum pump of any of several typescommercially available.

It is another object of this invention to employ a thin membrane as a cathode in other types of ionization pumps to increase their pumping speed at low pressures and to alleviate other of their drawbacks as heretofore mentioned.

It is another object of this invention to provide a design for the location of an electron source in a thin membrane diode ionization pump to facilitate the ionization process.

It is another object of this invention to employ a highly transparent anode together with a magnetic field parallel to the impressed electric field as a means for increasing the path length of the electrons.

The features of this invention include a lower ultimate pres sure than other ionization pumps, higher speeds at low pressures, ability to pump hydrogen and noble gases for sustained periods of time without significant re-emission, freedom from pump produced contaminants or deposits, and the lack of a requirement for cooling.

.These and other objects and features of the present invention will be apparent to those skilled in the art by reference to the description of the drawings that follows, and in which:

FIG. I is a diagrammatic view of one embodiment of the present invention;

FIG. 2 is a diagrammatic view of a modification of the invention by inclusion of an electron source;

FIG. 3 is a diagrammatic view of another embodiment of the present invention.

Referring to FIG. 1 there is disclosed a vacuum pump incorporating the novel features of the present invention comprising: an evacuable envelope 11 adapted to contain gas molecules; a transparent anode 12 as a means to transfer energy to the ions and electrons participating in the ionization process, and ameans 13 for allowing energetic ions to pass from the discharge region 14 of the pump.

Envelope 1 l is rectangular both from the front and side and may be of any dimension consistent with the considerations of good vacuum conductance and of economy in the production of magnetic field in the discharge region 14. Envelope 11 is made of gas impervious material, for example, stainless steel. At the top it is apertured for receiving, in vacuumtight manner, a high-voltage feedthrough 15 which is in turn connected to the anode 12. The envelope 11 is adapted tobe connected in vacuumtight manner to a structure to be evacuated (not shown) through the intermediary of a high conductance passage 16 and a vacuum flange l7 welded thereto, and is also adapted to be connected in the same manner by means of conductance 18 and flange 19 to a backing vacuum pump (not shown). The working pressure of the backing pump is typically 10 to 10 decades higher than that of the membrane pump; commercial ion pumps or diffusion pumps would be suitable.

The cathode 13 is composed of any of several materials known to have a low vapor pressure at elevated temperature in vacuum and capable of being formed into self-supporting hole-free membranes of thicknesses 400'Angstrom' units (1.6 microinches) or less. Materials suitable for this are suggested to be, but not limited to, carbon or aluminum oxide, with aluminum oxide as the preferred choice. It is suggested that the aluminum oxide be formed on an integral grid of aluminum for strength and rigidity. This is done as follows. An aluminum member is shaped into a rectangular can with'an open top and then etched to form a multitude of small (about 3/16 inch square), thin (about 0.001 inch thick) areas on the sides 20 and bottom 21. These areas are closely and regularly placed to leave a gridwork of thicker aluminum as a supporting structure. The thinned areas are then treated by any of several methods familiar to those skilled in the art, e.'g., according to that of'L. Harris, J. Opt. Soc. Am; 45,27 (1955 to form an aluminum oxide membrane. The cathode 13 is joined electrically and physically in a vacuumtight manner to the envelope 11 near the top. This seal may beeffected in any of several ways, as for example, by means'of an expansible inside collar (not shown). The envelope 11 (and by extension, the cathode 13) is connected to ground: A magnet or magnets 22 providing a field strength of 1,000 gauss, more or less, are placed flush with the outside walls of the envelope 1 1.

The anode 12 is a single cell and is constructed of very fine wires strung at large spacing over a supporting framework. It is made as nearly transparent as possible to afford the longest possible electron paths. It is hollow for the same purpose and for the purpose of placing the maximum feasible volume between the cathode membranes 13 at a high potential such that the majority of the ions are formed inthis region.

The pump is operated in the following manner. It and the system to be evacuated are pumped to an intermediate pressure, typically 10" to l0 torr. by'means of other vacuum pumps such as suitable trapped mechanical and diffusion pumps, or by sorbtion and sputter ion pumps. The pump is bypassed (not shown) during pump down to avoid mechanical stress on the membrane-by the significant force differentials.

found at higher pressures. When the desired forevacuum is reached the bypass is closed and a high voltage, typically on the order of a positive potential of 15 to 20 kv. DC and produced by an external power supply (not shown), is applied to the anode 12 by means of the feed through 15. This voltage initiates a Penning-type discharge one result of which is the acceleration of ions toward the cathode. Most of these ions possess energies sufficient to enable them to pass completely through the membrane (A. Van Wijngaarden, and H. E. Duckworth, Can. J. Phys., 40,1749 (l962),.T. W. Snouse, J. Appl. Phys. Lttrs. 5,122, (1964). Thus the ions are completely removed from the pumping volume and are later disposed of by the backing vacuum pump. Gas molecules existing in the backing pump, on the other hand, are prevented from entering the membrane pump because their energy is insufficient to allow their direct passage through the membrane. Thus a net pumping action results, extending to very low pressures. The ultimate pressure will be limited by extinction of the discharge, or, if the backing vacuum is not good, by the slow diffusion of gases back through the membrane. This pumping mechanism is particularly suited to the lighter gases such as hydrogen and methane which are principal residual gases found in extreme high vacuum systems. It is not well suited to gases heavier than carbon dioxide and sources of such gases such as elastomeric seals and vacuum oils and greases should not be employed. Additionally, both sides of the cathode membrane are subject to erosion by the bombarding ions. Since the number of said ions is directly proportional to pressure, it is prudent, in terms of long membrane life, to operate the pump at the lowest pressures possible where membrane lifetimes on the order of a year in continuous operation are to be expected. Aside from this consideration, there is no reason why the pump could not be operated at much higher pressures.

FIG. 2 discloses an alternate embodiment of the present invention. A filament 27 or other electron source is connected by means of a vacuumtight feed through 28 to an external power supply (not shown) which is biased 500 to 2,000 volts negatively with respect to the anode 12. The addition of relatively low energy electrons with properly directed initial velocities from this electron source contributes to the ionization efficiency in the anode region. Therefore, this'embodiment has the advantage of easier starting at low pressures, and of increased pumping speed due to increased ionization in the anode region.

FIG. 3 disclosed still another embodiment of the present invention similar to the embodiment of FIG. 1 but modified in that the long electron path lengths necessary to maintenance of an electrical discharge at low pressure are obtained by confining the electrons in an electrostatic field rather than in the mixed field of the prior description. This is a modification of the orbitron pump (Herb, et al., U.S. Pat. No. 3,244,969) or of the electrostatic getter ion pump of D. G. Bills et al., (U.S.

Pat. No. 3,407,991 No getter material is used. The grounded envelope 11 is cylindrical, vacuumtight, with two suitable flanged vacuum apertures, 17 and 19. The anode 12 is axially located and maintained at a positive 15 to 20 kv. DC by an extemal power supply (not shown) by means of the high-voltage feedthrough 15. The cathode 13 is cylindrical but otherwise similar to that of FIG. 1, and is physically, in a vacuumtight manner, and electrically attached to the envelope 11 at

points

33 and 34. A filament 36, supplied with current by means of feedthrough 35, acts as source for electrons which orbit around anode 12. These electrons produce ions which are accelerated by the electrostatic field, striking and passing through the membrane as in the first embodiment. This embodiment has the advantage of eliminating the magnets whose bulk, cost, and attendant stray fields may be a negative factor in some applications.

Having thus described the several useful and novel features of the embodiments of this invention, it will become immediately apparent that the several worthwhile objectives for which they were developed have been achieved. Although but a few specific embodiments of the invention have been illustrated and described herein, it is realized that other configurations are likely to occur to those skilled in the art within the broad teaching thereof, hence, it is intended that the scope of protection afforded hereby shall be limited only insofar as said limitations are expressly set forth in the appended claims.

What is claimed is: l. A vacuum pump apparatus comprising:

a. an evacuable envelope adapted to contain gas molecules:

b. means for producing ionization in the pump, said means to consist of a highly transparent anode maintained at high potential, together with a coparallel magnetic field;

c. means for passing ions thus produced out of the pump, said means to consist of a very thin cathode together with a vacuumtight passage to an auxiliary pump.

2. The apparatus according to claim 1 wherein the ionization process is facilitated by the addition of a suitably biased electron source between the anode and cathode.

3. The apparatus according to claim 1 with these modifications:

a. a cylindrical vacuum envelope;

b. means for ionization, said means to consist of a central axially located anode at high potential together with an electron source, such configuration optimized to provide electron confinement in the electrostatic field of the pumping volume;

c. means for passing ions out of the pump as in claim 1.

Claims

1. A vacuum pump apparatus comprising: a. an evacuable envelope adapted to contain gas molecules: b. means for producing ionization in the pump, said means to consist of a highly transparent anode maintained at high potential, together with a coparallel magnetic field; c. means for passing ions thus produced out of the pump, said means to consist of a very thin cathode together with a vacuumtight passage to an auxiliary pump.

3. The apparatus according to claim 1 with these modifications: a. a cylindrical vacuum envelope; b. means for ionization, said means to consist of a central axially located anode at high potential together with an electron source, such configuration optimized to provide electron confinement in the electrostatic field of the pumping volume; c. means for passing ions out of the pump as in claim 1.