US3495127A - Ultra-high vacuum magnetron ionization gauge with ion-collector shield - Google Patents

Description

Feb. 10, 1970 J. M. LAFFERTY 3,495,127

ULTRA-HIGH VACUUM MAGNETRON IONIZATION GAUGE WITH ION-COLLECTOR SHIELD Filed Feb. 26. 1968 72) End ores and Shield His Attorney- United States Patent US. Cl. 315- 108 6 Claims ABSTRACT OF THE DISCLOSURE Magnetron ionization gauge having the capability of measuring ultra-high vacuum of the order of 10- torr with high sensitivity and conversion constant is formed with hollow cylindrical anode and electron-retaining end plate at either end of anode. Ion collector is at center of anode and extends the entire length thereof. Electronemitting cathode is located off-center between ion collector and anode Wall and is parallel to the longitudinal axis. An ion collector shield extends the entire length of the ion collector and is interposed between the ion collector and the electron-emitting filament. Device is biased well beyond cut-off to provide restricted curvilinear paths for ionizing electrons circulating around inside the anode.

The present invention is directed to megnetron ionization gauges and, more particularly, to such devices which are capable of operating in the ultra-high vacuum region and which are adapted to operate with a very high conversion factor to record extremely low pressures.

As is well known to the art, ionization gauges are devices wherein electrons are emitted from a filament and are caused to execute extended curvilinear paths between a cathode and an anode electrode. If gas molecules are present within the anode-cathode space, the collision of the electrons executing extended curvilinear paths causes the creation of positive ions, which are attracted to a negatively-biased ion-collector electrode. At low pressures the measure of collector-electrode ion current is indicative of the gas pressure within the device.

Magnetron ionization gauges are gauges of the above type wherein a magnetron-type discharge occurs, generally between a centrally-located cathode electrode and a surrounding cylindrical anode electrode. In general, if the voltage between cathode and anode electrodes and the value of the crossed magnetic field (generally longitudinal with respect to a cylindrical anode electrode) is properly chosen, electrons execute helicoidal or cycloidal paths around the cathode electrode and, when the device is biased to cut-01f the circulating electrons just fail to be incident upon the anode electrode, thus causing the creation of a very long traversal path for each electron, greatly increasing the probability of electron-molecule collisions.

In my US. Patent 2,884,550, issued Apr. 28, 1959, I disclose ionization gauges of the afore-mentioned type with improved operation and the ability to measure pressures of as low as 10- torr. Devices in accord with that invention generally comprise a hollow cylindrical anode electrode with a longitudinal cathode electrode which is heated to thermionic emission temperatures. Electrons emitted from the heated cathode execute cycloidal paths, circulating around the cathode, with the device biased to cut-Off, so that extended curvilinear paths are achieved. A pair of disc-shaped electron-containment end members are juxtaposed at either end of the cylindrical anode electrode, one of the end members being negatively biased with respect to the cathode and functioning as an ioncollector electrode. Improved sensitivity of the patented gauge over prior art devices is achieved by utilization of the closely juxtaposed end member as a collector electrode, by operating the device beyond cut-off and utilizing filament temperatures lower than had been used in the prior art to minimize out gassing of the gauge. This avoids the resultant bombardment of the anode with high-energy electrons and the consequent emission of photoelectric emission-stimulating soft X-rays which operate as a limiting factor upon the minimum pressure which may be measured.

While the invention of my aforementioned patent 2,8E 4,550 provided a great improvement over prior art ionization gauges, the limit of measurable currents is not sufficiently low enough for the demands of an expanding technology. It is, therefore, desirable that such gauges be improved so as to improve the minimum pressure measureably thereby. Additionally, the amount of ion current for the pressure detected is quite small in prior art devices, including devices of the aforementioned patent. This results in another practical limitation, in that the sensitivity of such devices is directly related to the conversion factor, that is, the ion current per unit of pressure measured. At exceedingly-low pressures of 10 torr, for example, the amount of ion current in the device of the aforementioned patent is of the order of 5x10- ampere. This is so low that the possibility of error in measurement is great. Accordingly, the need for highly sensitive current measuring apparatus of this type is great.

It is, therefore, an object of the present invention to provide magnetron ionization gauges with the capability of accurately measuring pressures as low as 10- torr.

Another object of the resent invention is to provide magnetron ionization gauges having high measureable currents per unit of pressure measured.

Briefly stated, in accord with one embodiment of the present invention, I provide a magnetron ionization gauge including a cylindrical anode, a pair of electron containment end members therefor, a longitudinally-centered, semi-cylindrical ion-collector electrode along the length dimension thereof, an oif-centered, longitudinal electronemitting filament between said ion-collector electrode and the inner wall of the anode electrode, and a shield electrode member interposed along the axis of the anode electrode between the ioncollector electrode and the electron-emitting filament. In operation, the gauge is biased with volt ages and longitudinal magnetic field such as to cause the device to operate well beyond cut-01f, further minimizing the possibility of X-ray photo-emission. Additionally, a

more positive potential on the shield electrode than that which is applied to the ion-collector causes high-energy electrons in the circulating space charge to be intercepted by the shield electrode, thus reducing electron currents which would otherwise counteract the currents due to the attraction of positive ions thereto and rendering the gauge inaccurate. Because ions are collected along the entire length of the ion collector electrode, a very high conversion factor and sensitivity results.

The novel features believed characteristic of the present invention are set forth in the appended claims. The invention itself, together with further objects and advantages thereof, may best be understood with reference to the following detailed description, taken in connection with the appended drawing, in which;

FIGURE 1 is a vertical cross-sectional view of a magnetron ionization gauge constructed in accord with one embodiment of the present invention,

FIGURE 2 represents a plan sectional view taken along section line 22 of FIGURE 1, and

FIGURE 3 illustrates a similar plan view of a device constructed in accord with an alternative embodiment of the present invention.

In FIGURE 1, magnetron ionization gauge, represented generally as 10, includes an hermetically-scalable envelope 11, comprising a flanged, cylindrical member 12,

:losed at one end by an apertured metal end plate 13, 1nd at the other end by an apertured annular end plate [4 into which a tubulation 48, which connects gauge 10 o a system to be monitored, is fitted. Within envelope [1, the operative elements of the gauge include a hollow :ylindrical anode member 15, a pair of electron containnent end plates 16 and 17 juxtaposed at either end of, LIld not in contact with, anode 15, a thermionically-emit ing filament 18, offset from the longitudinal axis of anode l5, a semi-cylindrical ion-collector member 19 extending he entire length of anode electrode 15, and symmetrically ocated about the longitudinal axis thereof, and a shield :lectrode member 20, also of semicylindrical configuration, uxtaposed between ion collector 19 and thermionic filanent 18 and symmetrically located with respect to the ongitudinal axis of anode member 15.

Shield electrode member 20 is electrically and mechanially fastened to end members 16 and 17. Anode elecrode 15 is supported upon a pair of anode support memrers 21 and 22. Support member 21 is connected to a lead member 23, which passes through a lead-in bushing 24 .nd is hermetically sealed therethrough to the exterior of he device. Lead-in bushing 24 comprises a longitudinally- .pertured ceramic member 25, having a bore therein sufliient to accommodate lead member 23 without making ontact thereto. An inner, metallic flanged member 26 is lermetically sealed through an aperture in annular end late 14 and surrounds insulator 25 and is sealed thereto 'y conventional metal-ceramic technique at flanged end :7 thereof. At the other end of insulator 25, an end cap member 28 is sealed in conventional ceramic-to-metal lermetic seal to insulator 25 and is hermetically sealed as -y brazing, or otherwise, to lead member 23 as it exists tom the bushing.

Support member 22 is connected to a lead member .9 which passes through lead-in bushing 30 having an pertured insulator 31, an interior sealing flange 32, ealed thereto at flanged end 33, and an exterior sealing lange 34, connected thereto in hermetic seal and, similarly, n hermetic seal with lead member 29 as it leaves the mshing.

Ion-collector electrode 19 is supported upon lead memver 35, which passes through lead-in bushing 36, including nsulator 37, inner flanged seal 38, hermetically sealed insulator 37 at flanged end 39, and exterior flanged seal -0, which is in hermetic seal with the outer end of insulaor 37 and is brazed or otherwise hermetically sealed to ead member 35 as it passes through the bushing. End members 16 and 17 are interconnected with shield elecrode 20 and all are supported on lead support member 1 which is connected to a lead member 42, which is sealed hrough end member 14 at bushing 43, and by another .iametrically-opposed support 49, not shown.

Filament 18 is electrically and mechanically connected 3 the upper anode end member 16 and passes through 11 aperture in the lower end member 17 and is supported lpon support member 44. Support member 44 is conected to lead member 45, which is hermetically sealed hrough the envelope end member 14 through bushing 46.

Each of bushings 24, 30, 36, 43, and 46 are so contructed that the lead wire passing therethrough makes 0 contact with the insulating member. Similarly, the Jner portions of the bushing seals are so constructed that 0 contact is made with the inner portion of the insulator lember and hermetic seal is made thereto only at the outer anged end. Both of these measures greatly increase the lsulating characteristic of the bushing, in that the bushing rovides a long insulating surface path between the lead assing therethrough and the metallic member of the ushing exposed within the device, to prevent covering f the surface of the insulator and the short circuiting f the electrode leads passing therethrough.

The materials from which the device is constructed re conventional. Thus, for example, cylindrical side wall rember 12 may conventionally be made for molybdenum,

4 stainless steel or any other suitable nonmagnetic refractory metal, conventionally used in such device. Insulators 25, 31, and 36 may conventionally be a high-density alumina ceramic, or other similar material conventional used for insulator bushings in electron discharge devices. Filament 18 is preferably a lanthanum boride emitter and is preferably coated upon a rhenium substrate, as is disclosed in my U.S. Patent No. 3,312,856, issued Apr. 14, 1967.

Dimensionally, anode electrode 15 may have an inner diameter of approximately one inch and a longitudinal length of approximately .one and one eighth inches. The circle described by the exterior configuration of semicylindrical ion-collector and shield electrode 19 and 20 may conveniently have an outside diameter of approximately 0.25 inch. Thermionic cathode 18 may conveniently be a 0.008 inch rhenium wire, lanthanum boride coated, and is geometrically located at approximately half the distance between shield electrode member 20 and the inner surface of anode electrode 15. Thus, for example, the center of the filament, in the dimensions mentioned above, would be three sixteenths inch from either of shield electrode 20 and anode electrode 15.

In FIGURE 2, a plan view of the device of FIGURE 1, taken along section lines 22 of FIGURE 1, illustrates this configuration. Sidewall member 12 concentrically encloses anode 15 which in turn, concentrically encloses ion-collector electrode 19 and shield electrode 20, the two of which are spaced from one another by approximately 0.050 inch and which, together, describe a circle which is concentric with the longitudinal axis of anode electrode 15. Filament 18 is located equidistant between the rear surface .of ion-collector shield 20 and anode 15 and is along a plane normal to the plane passing through the longitudinal axis and the spacing between members 19 and 20. Respective bushings 24, 30, 46, 43, and 49 are illustrated in dashed lines, since they would not normally be visible.

Voltage means are provided by a voltage supply, generally indicated as 50 and comprising a battery 51 and voltage dividing resistor 52. Anode electrode 15 may conveniently be operated at a potential of approximately 300 volts positive, with end plate members 16 and 17 and shield electrode 20 operated at a potential of zero volts, and ion-collector electrode at a potential of ap proximately volts. The upper end of filament 18 is connected to upper end plate member 16 at zero voltage. The lower end of the filament is connected to battery 51 through lead 45 causing a voltage of approximately 3 volts to be impressed on filament 18. A strong longitudinal magnetic field, represented by arrow H, of approximately 600 to 800 oersteds is applied within the anode to cause magnetron action. This field, together with the applied electric field is approximately four times that necessary to bias the device to cut-oif. An ion-current measuring meter 53 is connected in the ion-collector current and is used as an indicator of collector ion current and, hence, gas pressure.

In operation, electrons are emitted from thermionic filament 18. In accord with one feature of the invention, the filament is operated at a very low temperature of approximately 650 C. to 800 C. This is possible, due to the utilization of the lanthanum boride cathode which is an excellent electron emitter at relatively low temperatures. Initially, emitted electrons tend to be accelerated to the anode. Due, however, to the crossed electric and magnetic fields existing within the interaction space therein, the electrons execute extended curvilinear paths, generally cycloidal or helicoidal in shape and circulate about the cathode in a spiral path creating a negative space charge. Due to the very high magnetic field, electrons are precluded from approaching too close to the anode.

In the device of my prior patent, cited hereinbefore, the magnetic field and cathode-anode voltage is adjusted so that electrons miss the anode electrode and the device was said to be biased beyond cut-off, resulting in the maximum curvilinear path for the electrons increasing the probability of electron-gaseous molecule collisions and increasing the probability of the creation of positive ions. I have found, nevertheless, in accord with the present invention, that device operation may be improved by biasing far past cut-off by a substantial amount of, for example, approximately four times the field necessary for cut-off at a given electric field strength. Although this does decrease the portion of the interelectrode space that is swept by the electrons, I have found that this does not appreciably reduce the number of ions created, since the electron density is increased in the portion so swept. On the other hand, advantages are gained.

One advantage gained by the more restricted path of the electrons, due to the biasing far beyond cut-off, is that the probability of electrons impinging upon the anode is decreased. With a decrease in the incidence of electron bombardment of the anode, the creation of soft X-rays, resultant photoelectric emission at the ion collector, and erroneous indiciations of positive ion currents are decreased.

In the magnetron ionization gauges of my Patent 2,884,550, positive ions are collected by an end plate juxtaposed adjacent the end of the hollow anode. Ions attracted to the collector are attracted only insofar as the negative field due to the negative potential of the ion collector is effective wihin the anode space. As a practical matter, the positive potential on the anode limits the degree to which the negative field due to the ion-collector potential is effective. More specifically, due to the limitation imposed by the limited penetration of this ioncollector field into the anode cavity, only approximately one third of the ions created within the cavity are at tracted to the ion collector. In accord with the present invention, however, the ion collector extends the entire length of the anode spacing and positive ions created at any position along the longitudinal dimension of the anode space may be attracted thereto, greatly increasing the positive ion current. I have found that utilizing a device having the dimensions indicated hereinbefore, a magnetic field of 600 to 800 oersteds and the indicated poentials, that it has been possible to measure pressures of the order of torr with a conversion constant of approximately 0.5 ampere per torr. Additionally, increasing the length of the device of the present invention increases the ions collected and increases gauge conversion constant.

In accord with another feature of the present invention, the ionization gauges of the present invention are much more stable than prior art devices, including the gauges of my aforementioned Patent 2,884,550. In the structure of the gauge, as with any magnetron-type ionization gauge a statistically-predictable portion of the electrons executing curvilinear paths between the cathode and the anode electrodes achieve an abnormally high random energy, which is equivalent to a higher electron temperature. Due to these higher energy electrons and the magnitude of the energies which they may achieve, penetration of the negative repelling field of the ion collector thereby may occur. When an electron penetrates the negative repelling field of the ion collector and is deposited upon the ion collector, it negates the ion current caused by the collection of a positive ion. Thus, bombardment of the ion collector by electrons decreases the indicated ion current, even though the ions are created and collected. Under extreme conditions, in some instances, this effect mayeven drive the positive ion current to a negative value, thus rendering the gauge totally incapable of measurizing pressure.

In accord with the present invention, the foregoing effect is completely eliminated by the presence of shield electrode 20. Shield electrode 20, in accord with the present invention, is substantially positive with respect to the ion collector itself, but still negative with respect to the anode, and functions to attract high-velocity, high-energy electrons which otherwise might tend to penetrate the field of the ion collector. This effect has the result of essentially maintaining an upper limit to the energy which electrons may achieve in their curvilinear path within the anode electrode. The result of this performance, on the part of the electrode shield is greatly to increase the stability and reliability of the ionization gauges of the invention.

In addition to the foregoing function, the shield electrode 20 in FIGURE 1 of the drawing further shields the ion collector from positive ions thermionically emitted from the filament. In devices in accord with the present invention, the filament serves only the limited purposes of supplying, to the circulating space charge within the interaction space of the gauge, a sufficient number of electrons to replace electrons that are depleted therefrom by deposition upon the shield electrode, the end members, or the anode. By operation of the shield at zero potential, the filament is shielded from the negative field of the ion collector and positive ion emission is prevented.

FIGURE 3 of the drawing illustrates an alternative embodiment of the invention which has certain advantages over the illustrated embodiment of FIGURES 1 and 2. In the embodiment illustrated in FIGURE 3, positive ion collector 19 is substantially as in FIGURES 1 and 2, but the shield electrode 20 is a flat strip, passing through the longitudinal axis of the anode. Commensurate with this modification, filament 18 is moved closely in toward the positive ion collector, so that it is located on the theoretical circle which would be traced if the positive ion collector were a circular member. The advantage in this embodiment of the invention is that, in the configuration illustrated in cross-sectional view in FIGURE 2, the location of the source of electrons at the filament close to the anode and the perturbation of the collector anode electric field due to the semicylindrical shield electrode at a substantially more positive potential, causes the helical paths of the electrons as they circulate about the ion collector and shield electrode to approach more closely to the anode in the vicinity of the filament. This increases the probability of electron collisions with the anode and requires either a sacrifice of X-ray photoemission current limitation or a further increase in the magnetic field and reduction in sensitivity. The device as illustrated in crosssectional plan view of FIGURE 3 of the drawing tends to result in a more nearly symmetrical path for the electrons about the longitudinal axis of the anode, and achieves the advantages of the decrease in the X-ray photoemission current limit without sacrificing sensitivity by biasing the device so far beyond cutoff as to adversely affect sensitivity.

From the foregoing, it may be appreciated that I have described improved magnetion ionization gauges having the capability of measuring ion pressures down to as low as 10- torr and having high conversion constants measured in amperes per torr of gas measured. This is achieved by utilizing a longitudinal ion collector along the axis of a hollow cylindrical anode electrode in lieu of a collector electrode at one end of the anode electrode. Conveniently, this ion-collector electrode is in the form of a semicylindrical member and provides for the establishment of a radial electric field between the collector and the anode which permits elongated curvilinear circulation paths for electrons therebetween. In further accord with the invention, I provide a shield electrode for the ion collector at a potential intermediate that of the ion collector and anode to remove, from the circulating electron space charge, abnormally-high energy electrons to preclude the bombardment of the ion collector electrode thereby and the consequent diminution of the apparent positive ion current. Additionally, I provide an off-center, low temperature filament, shielded from the ion collector to preclude erroneous ion currents and adapted to supply the necessary electrons to provide for ionizing electrons within the device.

While the invention has been set forth herein with respect to certain particular embodiments and examples, many modifications and changes will readily occur to those skilled in the art. Accordingly, by the appended :laims I intend to cover all such modifications and changes as fall within the true spirit and scope of the foregoing iisclosure.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An ionization gauge adapted to measure ultra-high vacua at high sensitivity and stability and comprising:

(a) an hermetically-scalable envelope adapted to be attached to a vacuum system;

(b) a hollow cylindrical anode-electrode within said envelope;

(c) a pair of end members disposed in spaced relationship to one another at opposite ends of and insulated from said anode-electrode;

(d) an ion-collector electrode disposed adjacent the longitudinal axis of said anode-electrode extending substantially the entire length thereof and parallel thereto;

(e) a thermionic filament disposed within said anodeelectrode space substantially parallel to the longitudinal axis thereof, extending substantially the entire length thereof, and offset from the longitudinal axis of said anode-electrode;

(f) a shield electrode disposed parallel to the longitudinal axis of said anode-electrode, on the opposite side of said axis from said ion-collector electrode, interposed between said ion-collector electrode and said thermionic filament, and extending substantially the entire length of said anode-electrode;

(g) bias means for applying a positive potential to said anode-electrode, a first negative potential to said ion-collector electrode, and a second substantially less negative potential to said shield electrode;

(h) means for providing a longitudinal magnetic field to said device sufficient, in connection with the radial electric field existing between said ion-collector electrode and said anode-electrode, to bias said device Well beyond cutoff so that the magnetic field applied is much greater than that just necessary to prevent electrons from reaching said anode electrode; and

(i) said applied potentials being of a magnitude as to cause electrons Within said anode space to describe elongated curvilinear paths about said ion collector and for electrons achieving abnormally-high energies to be attracted to said shield electrode and removed from said space.

2. The gauge of claim 1 wherein said ion-collector electrode is substantially semicylindrical in shape and is coaxial with the longitudinal axis of said anode.

3. The gauge of claim 2 wherein said shield electrode is substantially semicylindrical in shape and is coaxial with the longitudinal axis of said anode.

4. The gauge of claim 3 wherein said thermionic filament is located approximately half the distance between said ion-collector shield and the nearest surface of said anode and is along a plane perpendicular to a plane through said longitudinal axis separating said ion-collector electrode from said ion-collector shield.

5. The gauge of claim 2 wherein said ion-collector shield is flat and is along a plane including said longitudinal axis.

6. The gauge of claim 5 wherein said thermionic filament is located at the intersection of a plane passing through said longitudinal axis perpendicular to the plane of said ion-collector shield and an extension of the cylindrical surface of which said ion-collector electrode is a part.

References Cited UNITED STATES PATENTS 3,018,376 1/1962 Vanderschmidt 32433 X 3,244,990 4/1966 Herb et al 315-108 X 3,319,117 5/1967 Wheeler 315-108 X JAMES W. LAWRENCE, Primary Examiner P. C. DEMEO, Assistant Examiner US. Cl. X.R. 313--7; 324-33

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