US3249291A - Vacuum pump method and apparatus - Google Patents

Description

May 3, 1966 J- w ACKLEY I VACUUM PUMP METHOD AND APPARATUS Filed April 1, 1964 CONTROL CIRCUIT FIG. 2

CONTROL CIRCUIT INVENTOR v JAMES w. ACKLEY gh/h TOFINEY United States Patent 3,249,291 VACUUM PUMP METHOD AND APPARATUS James W. Ackley, Los Altos, Calif., 'assignor to Varian Associates, Palo Alto, Calif., a corporation of California Filed Apr. 1, 1964, Ser. No. 356,436 13 Claims. (Cl. 230-69) The present invention relates in general to vacuum pumps employing a subliming getter agent and more particularly to a pump of said class wherein the temperature of the subliming material is monitored for control purposes.

In conjunction with ion vacuum pumps, it is common to employ a getter agent, such as titanium, which is heated to a temperaturewhereby it is sublimed and residual active gases in the system to be evacuated are removed by the usual gettering mechanism. Sublimation of the getter agent has been attained successfully by direct thermionic, i.e., filament heating and by electron bombardment. Electron bombardment is usually preterred over filament heating because the former systems have longer life and the capability of evaporating greater quantities of getter at one time.

Typically, prior art pumps relying upon electron bombardment include an anode made of a getter substance or having a getter material deposited on it, and a cathode [for emitting electrons to bombard, hence heat the anode. Across the anode and cathode is connected a high voltage D.C. source. Normally the cathode power, hence temperature, is adjusted so that the diode including the anode and'cathode is operated in a temperature limited condition.

The temperature of the cathode is maintained substantially constant by deriving a control voltage in response to the current flowing in the anode circuit.

This control voltage is utilized to vary the temperature and electron emission of the cathode. 'If the cathode current is not controlled, the anode may become heated excessively and give off more getter material than is necessary, especially if the anode power is kept constant and the size goes down. Due to material evaporating therefrom the radiating surface area is reduced, there- [fore the temperature will increase. Of course, this is to be avoided because it shortens the life of the pump.

Monitoring anode current has not proven completely satisfactory for deriving the control voltage because sometimes identical currents can produce varying degrees of anode heating depending on the anodes heat radiation properties. According to the present invention, control of the gettering material temperature is attained by directly measuring the temperature of the material. In response to the temperature indication, the amount of energy applied to the material is varied to maintain the getter at a substantially constant temperature once steady state has been reached, and over the entire life of the unit regardless of the change in heat radiation properties, and change in size.

According to a preferred embodiment of the invention, a collector is positioned adjacent to the anode. The main anode cathode path is energized by a high voltage A.C. source so that the anode is bombarded by'electrons from the cathode only on alternate half cycles of the source, when the anode is positive with respect to the cathode. On the other half cycle the anode is a thermionic emitter. By monitoring electron emission from said anode only with the auxiliary electrode or collector, a current proportional to the anode temperature is derived. The anode temperature indication is utilized to control the cathode power and hence temperature so that getter evaporation remains substantially constant once steady state operation is reached. In this manner, the need for an expensive, high voltage D.C. source is comice pletely obviated. Instead, only a high voltage transformer is required because the pump functions essentially as a half wave rectifier. With the typical prior'art anode current monitoring system sufiicient control is not achieved with A.C. high voltage sources. Lack of control leads to a runaway condition, ie excessive electrons are caused to flow from anode to cathode during the half cycle when the latter is positive. This raises the temperature of the cathode, so that during the half cycle 'When it is negative, the cathode will emit more electrons.

-It is accordingly the object of the present invention to provide a new and improved vacuum pump, and method of operating same.

One feature of the present invention is the provision of a getter vacuum pump wherein the getter material temperature is adjusted in response to its own value.

Another feature of the resent invention is the provision of a newand improved getter vacuum pump which does not require a high voltage D.C. source.

These and other objects and feature-s of the present invention and a further understanding may be had by referring to the following description and claims, taken in conjunction with the following drawing in which:

FIG. 1 is a diagram, partly schematic and partly in block form, of apparatus embodying the present invention; and,

FIG. 2 is an alternate embodiment of the present invention.

Reference is now made to FIG. 1 wherein the system 11 to be evacuated is connected to a mechanical or prefera-bly a refrigerated sorpti-on fore-pump 12 by valve 13, conduits 14, 15 and the getter vacuum pump 16 of the present invention. The stainless steel envelope 17 of pump 16 includes a pair of openings 18, 19 which communicate with conduits 1 4, 15 respectively.

Within envelope 17 is provided an anode 2-1 that is responsive to electrons emitted from a thermionic cathode 22, made of a refractory metal, for example, tungsten. Anode 21 is made of or coated with a getter material, -for example, titanium, that is heated to sublimation temperature when it is bombarded by electrons from a cathode or filament 22. Adjacent to and facing anode 21 is auxiliary electrode or collector 23, positioned and biased to receive electrons emitted from anode 21 only to the exclusion of electrons derived from cathode 22.

By receiving electrons only from anode 21, a signal indicative of the temperature of the subliming material may be derived from collector 23. This signal is derived by connecting collector 23 in series circuit with negative D.C. source 24, arnmeter 2S and variable load resistor 26. The voltage generated across resistor 26 is coupled across the input terminals of a conventional control circuit 27.

The A.C. output voltage of circuit 27, inversely related in amplitude to the variable signal amplitude developed across load 26, is app-lied to cathode 22 via transformer 28. In this manner, the voltage applied across the terminals of cathode 22, hence the cathode temperature, is controlled in response to the temperature of anode 21 in a manner whereby temperature of the anode remains substantially constant once steady state operation is attained.

To selectively control the cathode and hence the anode temperature, the value of resistor 26 is altered so that input voltage to circuit 27 is varied. It is necessary to adjust resistor 26 only once for any degree of sublimation rate desired.

High voltage A.C. is applied between anode 21 and cathode 22 by secondary winding 29 of the transformer 30, the primary winding 31 of which is connected to a 60 cycle line power source 32. On alternate half cycles of source 32 when anode 21 is positive, electrons are accelerated from cathode 22 to the anode 21. (Herein after, the half cycles of source 32 when anode 21 is positive is referred to as the positive half cycle of the source while the other half cycles are referred to as being negative.) During the negative half cycle, there is no electron emission from cathode 22. Instead, electrons are emitted by anode 21fbecause of its high temperature and the electron accelerating efiects of collector 23 and cathode 22, which are both .at positive voltages relative to the anode 21 during most of each negative half cycle of source 32. Since collector 23 is positioned adjacent to anode 21 so that it accurately monitors electron emission from the anode 21, the signal generated across resistor 26 is. a direct function of anode temperature.

In operation, valve 13 is initially opened and fore-pump 12 evacuates getter pump 16 and the system 11 to a vacuum of approximately 10" torr. and operation of pump 16 commences by connecting source 32 to primary winding 31. As. described above, on positive half cycles of source 32, electrons bombard anode 21, causing it to be heated. Virtually no electrons flow between electrodes 22 and 23 during the positive half cycle of source 32 because of the negative bias applied to collector 23 by DC. source 24, for example, 50-100 volts. Heating of anode 21 continues so that its temperature reaches the sublimation point of the getter mate rial. During sublimation the getter material is released from anode 21 as a vapor. The vapor condenses on the walls of envelope 17 as a coating. Eventually, a sufficient quantity of residual gases coming in contact with the coated walls is removed so as to reduce the pressure within the system down to on the order of 10- torr, which pressure is liimted by the partial pressures of the non-getterable gases such as helium, argon, neon, etc. If used in combination with a glow discharge sputter ion pump, for example, the type disclosed in US. Patent 2,993,638, dated July 25, 1961, even lower pressures may be attained.

When pump 16 is initially turned 'on, anode 21 is at a low temperature, hence emits very few electrons on the'negative half cycle of source 32. In consequence,

. the impedance between anode 21 and collector 23 is large and very little voltage is developed across resistor 26. The low voltage across resistor 26 is converted to a relatively largeA.C. potential by circuit 27 so that there is copious electron emission from cathode 22. Great emission from cathode 22 results in fast heating of anode 21 so that sublimation of the getter and pumping at high rates are both promoted. As anode 21 attains higher temperatures, there is greater electron flow between it and collector 23 during negative half cycle ofsource 32. Greater electron flow between anode 21 and collector 23 results in a large voltage across load 26, hence a decrease in electron emission from cathode 22, whereby the temperature of anode 21 is maintained substantially constant. It is important that the temperature of anode 21 does not become too great because excessive electrons are otherwise given off by it during the negative half cycle of source 32.- If'too many electrons are given up by anode 21 during each negative half cycle of source 32, the amount of getter material available on the anode is quickly depleted. Fast depletion of the getter material from anode 21 does not provide any benefit because it results in deposition of getter on envelope 17 at a higher rate than the getter is able to react with and absorb the residual gases in the system. This is to be avoided because it results in most of the sublimed getter being wasted without materially increasing the vacuum. Of course, controlled sublimation of the getter from anode 21 promotes long life of the pump.

While the invention has been described specifically in connection with A.C. applied across the anode-cathode diode of the pump, it is to be understood that the basic temperature monitoring concept is applicable to D'.C. electron bombardment and filament gettering systems, for

Valve 13 is then closed example, with the anode at ground potential. Also, it is possible to carry out the concepts of the invention by reading meter 25, having a scale calibrated directly as a function of the temperature of anode 21, and thereafter manually adjusting the voltage applied to cathode 22. With such a method, circuit 27, of course, need not be provided.

Referring now to FIG. 2 wherein like numeralsrefer to like parts, the cathode or filament 22 is mounted within a vacuum tight cylindrical anode sublimation cartridge 33. The cartridge 33 is made of, or coated with a reactive material, for example, titanium. The anode cartridge is maintained positive with respect to the filament 22, for example, v4-8 thousand volts, by DC. power supply 34. The collector 23 in this embodiment is maintained positive with respect to the anode 33, for example, 500-1000 volts, depending on the spacing-and symmetry of the electrodes. The power supplies34 and 24 may be replaced by transformers'without regard to phase relationships. In operation of this embodiment, electrons from cathode 22 striking anode cartridge 33, cause heatinguntil a temperaturev is reached at which getter material from the outer walls of the cartridge sublimes onto the inner walls of pump 16. Electron emission due to heating, from cartridge 33 to collector 23 permits monitoring of anode temperature in the same manner as described with respect to FIG. 1. The embodiment of FIG. 2 has the advantage that the filament 22' may be isolated from the remainder of the system, thus avoiding the possibility of filament burnout when there is a sudden rise in pressure. Also, collector 23 is completely shielded from any electron emission between filament 22 and collector 23.

Sincemany changes can be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a vacuum pump for evacuating a chamber, the combination comprising an electron emitter, an electrode responsive to electrons derived from said emitter, said electrode including a getter material that is heated to sublimation temperat-urein response to bombardment by said electrons, said emitter and said electrode being disposed in vacuum-tight communication with saidchamber to be evacuated; means for deriving a signalin response .to the temperature of said electrode; and meansfor controlling the quantity of electrons reaching said electrode from said emitter in response to said signal.

2. A method for maintaining the rate of sublimation of a gettermaterial in a pump at a substantially constant level, wherein said pump comprises an electron source and an electrode for releasing, by subliming, getter material in response to electron bombardment, comprising the steps of measuring the temperature of said electrode, and in response to said measurement step, adjusting the emission of electrons from said source so that the measured temperature remains substantially constant.

3. In a vacuum piunp for evacuating a chamber, the combination comprising an'electron emitter, an anode electrode responsive to electrons derived from said emitter, said anode electrode including a getter material that is heated to sublimation temperature in response to bombardment by said electrons, said anode electrode being heated in response to bombardment by said electrons to a temperature whereby electrons are emitted from it, and means for monitoring the quantity of electrons emitted only by said electrode, said emitter, said electrode'and said means for monitoring being disposed in vacuum-tight communication with said chamber to be evacuated.

'4. The pump defined in claim 3 wherein said getter.ma-

being biased to receive electrons only from said anode electrode.

6. In a vacuum pump for evacuating a chamber, the combination comprising an electron emitter, an electrode responsive to electrons derived from said emitter, said electrode including a getter material that is heated tosublimation temperature in response to bombardment by said electrons, said electrode being heated in response to bombardment by said electrons to a temperature whereby electrons are emitted [from it, said emitter and said electrode being disposed in vacuum-tight communication with said chamber to be evacuated, and means for deriving a signal in response to electrons emitted only by said electrode, and means responsive to said signal for controlling the quantity of electrons reaching said electrode from said emitter.

7. In a vacuum pump for evacuating a chamber, the combination comprising an electron emitter, an electrode responsive to electrons derived from said emitter, said electrode including a getter material that is heated to sublimation temperature in response to bombardment by said electrons to a temperature whereby electrons are emitted from it, said emitter and said electrode being disposed in vacuum-tight communication with said chamber to be evacuated, and means for deriving a signal in response to electrons emitted only by said electrode, and means responsive to said signal for controlling the power applied to said emitter and thereby control the quantity of electrons derived from said emitter.

8. A vacuum pump comprising an electron emitter, an electrode responsive to electrons derived from said emitter, said electrode including a getter material that is heated to sublimation temperature in response to bombardment by said electrons, said electrode being heated in response to bombardment by said electrons to a temperature whereby electrons are emitted by it, a high voltage AC. voltage source, means for coupling said source between said emitter and electrode so that electrons from said emitter bombard said electrode only during the half cycle of the source when the electrode voltage is greater than the emitter voltage, said electrode being an electron source during the other half cycle of said source, means for monitoring electron emission deriving only from said electrode, and envelope means for enclosing said emitter and said electrode.

9. The pump of claim '8 wherein said electron respon sive electrode is an anode, said means for monitoring includes a collector electrode within said envelope means positioned adjacent to said anode, and a DC. voltage source for biasing said collector electrode negatively relative to said electron emitter.

' 10. A vacuum pump comprising an electron emitter, an

'to sublimation temperature in response to bombardment by said electrons, said electrode being heated in response to bombardment by said electrons to a temperature whereby electrons are emitted by it, a high voltage AC. voltage source, envelope means adapted to be connected in vacuum tight communication with a chamber to be evacuated for enclosing said emitter and said electrode, means for coupling said source between said emitter and electrode so that electrons from said emitter bombard said electrode only during the 'half cycle of the source when the electrode voltage is greater than the emitter voltage, said electrode being an electron source during the other half cycle of said source, means for deriving a signal in response to electrons emitted only by said electrode, and means responsive to said sign-a1 for controlling the voltage applied to said emitter and thereby control the quantity of electrons deriving from said emitter.

11. In a vacuum pump for evacuating a chamber, the combination comprising an electrode including a getter material that is sublimed when the electrode attains a pre determined temperature, said electrode being disposed in vacuum tight communication with said chamber to be evacuated, means for heating said electrode to said temperature, means for deriving a signal indicative of the temperature of said electrode, and means for controlling said heating means in response to said signal.

1 2. A method for maintaining substantially constant rate of sublimation of a getter material in a pump comprising an electrode for releasing, by subliming, getter material in response to heating of the electrode, comprising the steps of measuring the temperature of the electrode, and in response to said measuring step, adjusting the temperature of the electrode.

13. A vacuum pump comprising an envelope means adapted to be connected in vacuum tight communication to a chamber to be evacuated, an electron emitter mounted within said envelope, an anode electrode responsive to electrons derived from said emitter mounted within said envelope, said anode electrode including a getter material that is heated to sublimation temperature in response to bombardment by said electrons, said anode electrode being heated in response to bombardment by said electrons to a temperature whereby electrons are emitted by it, means for coupling a high voltage AC. voltage source between said emitter and anode electrode so that electrons from said emitter bombard said anode electrode only during the half cycle of the source when the anode electrode voltage is greater than the emitter voltage, said anode electrode being an electron source during the other half cycle of said source, a collector electrode mounted within said envelope adjacent said anode electrode, and means for coupling a DC. voltage source for biasing said collector electrode negatively relative to said electron emitter.

References Cited by the Examiner UNITED STATES PATENTS 3,181,775 5/1965 Herb 230-69 MARK NEWMAN, Primary Examiner. WARREN E. COLEMAN, Examiner.

Claims (1)

1. IN A VACUUM PUMP FOR EVACUATING A CHAMBER, THE COMBINATION COMPRISING AN ELECTRON EMITTER, AN ELECTRODE RESPONSIVE TO ELECTRONS DERIVED FROM SAID EMITTER, SAID ELECTRODE INCLUDING A GETTER MATERIAL THAT IS HEATED TO SUBLIMATION TEMPERATURE IN RESPONSE TO BOMBARDMENT BY SAID ELECTRONS, SAID EMITTER AND SAID ELECTRODE BEING DISPOSED IN VACUUM-TIGHT COMMUNICATION WITH SAID CHAMBER TO BE AVACUATED; MEANS FOR DERIVING A SINGAL IN RESPONSE TO THE TEMPERATURE OF SAID ELECTRODE; AND MEANS FOR CONTROLLING THE QUANTITY OF ELECTRONS REACHING SAID ELECTRODES FROM SAID EMITTER IN RESPONSE TO SAID SIGNAL.

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