US3742343A

US3742343A - Ion gauges

Info

Publication number: US3742343A
Application number: US00085157A
Authority: US
Inventors: L Pittaway
Original assignee: US Philips Corp
Current assignee: US Philips Corp
Priority date: 1969-10-29
Filing date: 1970-10-29
Publication date: 1973-06-26
Anticipated expiration: 1990-06-26
Also published as: DE2052094A1; CA933238A; JPS497511B1; FR2066651A5; GB1336126A

Abstract

An ion guage comprising a conducting cylinder closed at one end; a pair of cathodes within the cylinder; a cylindrical grid between the cathodes and having a wire mesh peripheral walls relatively transparent to electrons, which grid is bounded at one end by a wire mesh wall and at the opposite end nearer the closed end of the conducting cylinder by an apertured metal wall; means adjacent to the aperture for extracting and converging ions, and an electrode for collecting extracted ions.

Description

United States Patent [191 Pittaway ION GAUGES [75] Inventor: Lawrence Graham Pittaway,

Crawley, Sussex, England [73] Assignee: U.S. Philips Corporation, New York,

[22] Filed: Oct. 29, 1970 [2]] Appl. N0.: 85,157

[30] Foreign Application Priority Data Oct. 29, 1969 Great Britain 53,007/69 [52] U.S. Cl. 324/33 [5]] Int. Cl. GOln 27/00, GOln 27/62 [58] Field of Search 324/33 [56] References Cited UNITED STATES PATENTS 3,619,684 I1/l97l Pittway ct al. i. 324/33 June 26, 1973 2,993,638 7/1961 Hall 324/33 3,32l,70l 5/1967 Crowell 324/33 3,387,175 6/1968 Lloyd 324/33 2,836,790 5/1958 Hickam 324/33 Primary Examiner-Stanley T. Krawczewicz Att0rney-Frank R. Trifari [57] ABSTRACT An ion guage comprising a conducting cylinder closed at one end; a pair of cathodes within the cylinder; a cylindrical grid between the cathodes and having a wire mesh peripheral walls relatively transparent to electrons, which grid is bounded at one end by a wire mesh wall and at the opposite end nearer the closed end of the conducting cylinder by an apertured metal wall; means adjacent to the aperture for extracting and converging ions, and an electrode for collecting extracted ions.

7 Claims, 4 Drawing Figures ION GAUGES This invention relates to ion gauges.

An ion gauge is frequently employed for measuring very low pressures in high vacuum apparatus. One form of ion gauge known as the Bayard-Alpert gauge which was proposed by R. T. Bayard and D. Alpert in the Review of Scientific Instruments 21, 1950 pages 571-2, has the advantages that it is relatively easy to outgas, it exhibits a high sensitivity and a good linearity and does not require a magnetic field. The gauge was developed in order to reduce the residual output current caused by the soft X-rays and desorbed ions generated by impact of electrons with the electron accelerating grid. The soft X-rays and desorbed ions tended to be intercepted by the collector electrode, the former causing a photo-electron emission current which together with the desorbed ion current, tend to swamp the ion output current at the lower pressure.

In Modulated Bayard-Alpert Gauge, P. A. Redhead, Review of Scientific Instruments 31, 1960 pages 343-344, a form of Bayard-Alpert gauge is described in which a modulator electrode is added to provide the output ion current with a modulated component which can be separated from the normal ion current which contains the X-ray emission and desorbed ion currents. In this way the lower limit to the measurable pressure can be further extended.

In Gauge manome'trique a collecteur exte'rieur pour pressions tresbasses by J. Groszkowski, Bulletin de IAcade'mie Polonais des Sciences, Volume 14, 1966, pages 169-177 (1023-1031), there is illustrated in FIG. 6(a) on page 175 (1029) a form of ion gauge in which the collector electrode is withdrawn from the interior of the ionisation space and hence out of the region permeated by the majority of the soft X'rays and desorbed ions produced by electron collision with the accelerator and thus allowing lower pressures to be measured than with the modulated Bayard-Alpert gauge. The gauge illustrated in FIG. 6(a) on page 175 of the above reference will be referred to herein as the Groszkowski gauge. Reference will now be made to FIG. 1 of the drawings which is a sectional diagram illustrating the Groszkowski Gauge. Two electron emissive cathodes 1 and 2 are mounted on either side of a cylindrical electron accelerating grid 3. The grid 3 is made up of a peripheral wall 4 comprising a helix of molybdenum wire, an end wall 5 comprising a sprial of molybdenum wire and an end wall 6 comprising a metal disc having an ion exit aperture 7 formed therein. The

walls

4, 5 and 6 are conductively joined together and connected to a source of biasing potential not shown. The cathodes 1, 2 and the grid 3 are mounted in a glass envelope 8 which can be connected via a tubular neck 9 to a vacuum system the gas pressure in which it is desired tomeasure.

A thin wire ion collector electrode 10 is also present within the envelope 8 and is mounted outside the grid 3 in the region facing the ion exit aperture 7. A glass shield 11 attached to the inner surface of the envelope 8 surrounds the collector electrode 10 and is provided with an aperture 12 arranged in the ion path from the ion exit aperture 7 to the ion collector electrode 10.

The ion collector electrode 10 is normally connected to earth via the input circuit of ion current measuring means. The cathodes 1 and 2, which conveniently are electron emissive filaments and of which one is connected to a source of heater current, are maintained at a potential positive with respect to earth, typically 210 volts. The electron acceleration grid 3 is maintained at a positive potential with respect to the cathodes 1 and 2 so that emitted electrons are accelerated towards the grid 3 and pass through the spaces between the turns of the helix 4 with sufficient kinetic energy to ionize gas present within the space 13 enclosed by the grid 3. Typically the grid 3 is maintained at a potential of volts with reference to the potential of the cathodes l, 2. The negative potential on the collector 10 with respect to the grid 3 is said to cause ions formed within the space 13 to be drawn out via the

apertures

7 and 12 towards the collector 10 to form the ion ouput current. Because the ion collector 10 is substantially shielded by the disc 6 from the peripheral wall 4 with which a proportion of the incident electrons collide generating soft X-rays, the X-ray photoemission current component of the collector current is maintained at a low level.

It is common nowadays to provide an ion gauge without an envelope, the gauge being mounted on a flange adapted for attachment to a vacuum system for measuring the pressure therein. Such a gauge is commonly called a nude gauge and will be referred to herein as such. Such a vacuum system normally presents a grounded electrical environment to the gauge and at least some walls of the system are normally made of a metal such as stainless steel.

A nude form of the Groszkowski gauge was constructed and tested employing the electrode dimensions given in the article referred to above, and surrounded by a grounded cylindrical metal screen to sim ulate a grounded environment. It was found however that as the potential of the grid 3 was increased positively with respect to the grounded cylindrical metal screen, the cathode being biased 5 volts positively with respect to ground and the collector being maintained at 200 volts negatively with respect to the cathode, the sensitivity of the gauge increased until the potential difference between grid 3 and the grounded metal screen was approximately 60 volts, after which the sensitivity fell away rapidly. In addition the sensitivity under the above conditions was found to be very dependent on the ionizing electron current.

It is an object of the invention to provide a nude form of ion gauge having an external collector electrode, in which the above difficulties are substantially overcome. It is a further object of the invention to provide an improved form of ion extraction and collection means for withdrawing ions from the electron acceleration grid of an ion gauge.

According to the present invention there is provided a nude ion gauge for use in a grounded environment including a source of electrons; an ionization space surrounded by a conducting surface including a peripheral region that has a plurality of small apertures and that is relatively transparent to electrons directed thereat from the electron source mounted exterior to the ionization space to ionize gas present in the space, said conducting surface having an end portion substantially opaque to the passage of soft X-rays and provided with an aperture through which ions formed in the ionization space by the electrons, can be extracted by the application of an extraction field; and means for collecting the extracted ions to provide a current dependent on the pressure of gas in the ionization space, the apertures in the electron transparent region being sufficiently small and of such a shape that the normal electric field present outside the ionization space is substantially prevented from penetrating the electron transparent region and from extracting a substantial proportion of the ions present in the ionization region via the electron transparent region. To ensure repeatable performance in a variety of different vacuum systems the gauge can be surrounded by a grounded conducting cylindrical screen.

According to another aspect of the present invention an ion gauge comprises a conducting member enclosing a region within which ions are formed from the collision of electrons with gas present in the region, the enclosing member has an end wall formed of a material substantially opaque to soft X-rays and contains an aperture through which the ions can be extracted and fed to an ion collector electrode situated outside the enclosure, the ion gauge includes in addition to the collector electrode a conducting extractor electrode and a reflector electrode, the extractor electrode being provided with means for feeding an ion extracting potential thereto and being so disposed adjacent the ion exit aperture in the end wall, that, on the application of extracting potential to the extractor electrode ions are extracted from the enclosure towards the collector electrode substantially without striking the the extractor electrode the reflector electrode is disposed adjacent the collector electrode and has means for applying a potential to reflector electrode to cause ions to be directed towards the collector electrode. At the ion exit aperture in the end wall the enclosing member can include a cylindrical extension located external to the enclosure which cylindrical extension co-operates with a cylindrical extractor electrode to form a converging electrostatic lens. The extractor electrode and a cylindrical reflector electrode can co-operate to form a second converging electrostatic lens. A grounded conducting metal screen can surround the gauge.

In order that the present invention may be clearly understood and readily carried into effect, embodiments thereof will now be described, by way of example, with reference to the figures of the accompanying drawings of which:

FIG. 1 is a sectional diagram ofa Groszkowski Gauge which is known in the prior art,

FIG. 2 is a longitudinal section of one form of gauge employing the invention,

FIG. 3 is a detail in longitudinal section illustrating an alternative embodiment of the invention, and

FIG. 4 is a longitudinal sectional diagram illustrating a nude form of the Groszkowski gauge embodying the invention.

Consideration will now be given to the use of a nude ion gauge in a vacuum system. By virtue of the electrically conducting nature of the walls of such a vacuum system, since they are normally constructed of stainless steel, such a nude gauge is usually surrounded by an earthed conductor formed by the said walls. The potential difference between the electron acceleration grid and the walls results in an electrostatic field therebetween said grid and said walls, which is known to affect the passage of ionizing electrons through the ionizing space formed within the electron acceleration grid, thus causing variations in the sensitivity of the gauge. For this reason it is desirable that the gauge should be arranged in a cylindrical electrically conducting screen to ensure repeatability in pressure measurement in various configurations of the vacuum system. Furthermore when a similar ion gauge is alternatively enclosed in a glass envelope and connected to a vacuum system via a glass tube, it is desirable and common practice for an electrically conducting screen to be formed by a layer of tin oxide deposited on the inside wall of the glass envelope, thereby defining the electrostatic field between the grid and the envelope and also shielding the gauge from electrical disturbances outside the glass envelope.

It is also desirable that the electron accelerator grid should be maintained at a potential of at least volts positively with respect to the grounded environment, to enable the electron accelerating potential between the grid and the electron emitting cathode to provide optimum ionization efficiency of the gas molecules in the said space by the ionizing electrons.

As has been described above, in attempting to employ the Groszkowski gauge in nude form in a grounded environment, it was found that the sensitivity fell rapidly on increasing the electron acceleration grid voltage beyond about 60 volts above ground and also that the sensitivity was very dependent on the ionising electron current.

These disadvantages were overcome in accordance with one aspect of the invention by replacing the helical wound grid wall 4 and the spiral wound grid wall 5 of prior art ion gauges with a closely woven wire mesh, conveniently of Tungsten, and denoted by 20 in FIG. 4 to which reference will now be made and which illustrates a nude form of exterior collector ion gauge employing the invention. The tungsten mesh employed in one embodiment had a mesh of 50 wires per inch and an electron transparency of 92 percent. The cathodes l, 2 and the grid 3 were arranged in a cylindrical conducting screen 14.

In the nude gauge shown in FIG. 4, the potential of the grid 3 was raised until it was approximately 300 volts positive with respect to the screen 14 which was connected to ground, and the sensitivity was found to increase until it was approximately twice the maximum value reached when using a helical wound grid.

It is thought that when the Groszkowski gauge is employed in nude form, the electrostatic field produced between the helical grid and the grounded environment penetrates the helical grid and as the grid potential is raised, ions formed within the ionization space 13 tend to be drawn out of the ionization space via the walls 4 and 5 (FIG. 1) by this field, instead of via the aperture 7 by the collector field, thus depleting the ion current flowing to the collector 10. By employing the invention, a form of electron transparent grid is provided having a plurality of relatively small apertures and this is effective in reducing penetration of the field outside the ionization space 13, which field would extractions in preference to the collector 10.

When the Groszkowski gauge is employed in a glass envelope 8 not having a conducting internal coating, the inside walls tend to be charged by the electrons emitted by the cathode l or 2 (FIG. 1) until they reach cathode potential. The field distribution within the gauge environment is thus made relatively predictable but not necessarily compatible with the requirements for optimum sensitivity. Furthermore the walls of the glass shield 12 (FIG. 1) also become charged to approximately the potential of the cathodes l, 2 and the shield 12 therefore acts as an extractor electrode which draws ions through the aperture 7 from the ionisation region 13 towards the collector electrode 10, a considerable proportion of the said ions being intercepted by the said shield 12 before reaching the said collector 10. It is a further object of the invention to provide an external collector gauge having improved means for extracting ions from the ionisation space, which do not depend on the accumulation of electric charges on glass surfaces.

One embodiment of the invention is illustrated in longitudinal section in FIG. 2, to which reference will now be made.

Cathodes

21, 22 in the form of electron emissive filaments are connected to support and connection wires 41 mounted in a glass seal 45, and which project therefrom so that heating and biasing potentials can be fed thereto. The glass seal 45 is attached to and forms a seal with a metal ring 46 which latter is sealed to a conducting cylinder 44. A metal flange 43 welded to the cylinder 44 enables the ion gauge to be attached to a vacuum system to form a seal therewith. The glass employed for the seal 45 is chosen to have a coefficient of thermal expansion which is compatible with that of the ring 46.

The ionization space 13 is bounded by a conductive electron acceleration grid assembly 23 comprising a peripheral wall 24 and one end wall 25, both formed of a closely woven wire mesh, conveniently of Tungsten, and a further end wall 26, substantially opaque to soft X-rays, and formed of thin sheet metal, conveniently stainless steel. An aperture 27 is formed at the center of the wall 26 which is provided with a cylindrical conducting extension 47. The electron acceleration grid assembly 23 is conductively attached to a support and connection wire 48 which passes through the seal 45 and is attached thereto.

An axially symmetrical ion extractor electrode 49 formed of conducting material, conveniently stainless steel, is mounted so that the forward end projects into the rearward end of the extension 47. The diameter of the extractor electrode 49 is increased in a rearward direction beyond the end of the extension 47, and the electrode 49 is supported by a conductive connecting wire 50.

The extractor electrode 49 forms, together with the rearward cylindrical extension 47 of the end plate 26, a converging electrostatic lens system having a focal point located at the point F,.

A reflector electrode 51, cylindrical in form and hav ing an end wall 52, is mounted on a conducting support 53 which extends through the seal 45. The reflector 51, conveniently of stainless steel, is arranged behind the extractor 49 and'is coaxial with the extractor 49 and the cylindrical extension 47. The reflector 51 and the extractor 49 form together a further converging electrostatic lens having a focal point at the point F A collector electrode 30 inthe form of a wire, conveniently of Tungsten, is mounted on a conductive sup-.

port member 54 which extends through the seal 45 to ated towards the mesh wall 24 and penetrate the apertures therein with sufficient energy to ionise gas present in the space 13. The screen 44 is connected to ground and the cathodes ll, 2 are biased positively with respect thereto. The extractor 49 and the collector 30 are biased negatively with respect to the cathodes l or 2. The reflector 51 is biased positively with respect to the collector 30, to a potential equal to or greater than that of the grid 23. Electrons present within the grid 23 form a space charge which, together with the screening effect of the closely

woven mesh walls

24 and 25, produce an electric field within the ionizing region 13 which prevents positive ions formed therein from escaping via apertures in the

walls

24 and 25.

The potential on the extractor 49 sets up an ion extraction field which draws positive ions out of the ionisation region 13 towards the collector 30. The converging effect of the electron lens formed between the extractor 49 and the extension 47 causes the ions to follow paths which, in general, avoid collision with the extractor 49. The further converging lens formed between the extractor 49 and the reflector 51 further assists in directing ions away from the extractor 49 and towards the collector electrode 30.

The collector electrode 30 is so situated that soft X- rays generated by the collision of electrons with the peripheral grid portion 24 of the acceleration grid 23 are substantially shielded therefrom by the end wall 26.

By employing the form of construction shown in FIG. 2, the extraction field can be predictably maintained with changes in the environment in which the nude form of gauge is employed, and a good ion collection efficiency can be provided.

An alternative embodiment of the invention is shown in FIG. 3, to which reference will now be made. FIG. 3 is a sectional detail showing an alternative form of ion extraction and collection means which can be employed in an ion gauge otherwise as shown in FIG. 2. The end wall 26 of the electron accelerator grid 23 is provided with an aperture 27 and a rim 60 is formed surrounding the aperture 27. A cylindrical reflector electrode 61 having an end wall 62, formed conveniently from stainless steel, is conductively attached, for example by a welding process, to the rearward face of the wall 26.

A cylindrical extractor electrode 69, formed conveniently of stainless steel, is mounted adjacent the aperture 27 inside the reflector 61 but is insulated therefrom and provided with connection to a source of bias potential. The diameter of the extractor electrode 69 is greater than that of the rim 60 of the aperture 27 so that it is at least partly screened by the rim 60 from ions passing through the aperture 27.

A collector electrode 30, conveniently a tungsten wire approximately 200 am in diameter and mounted as before on a conductive support 54, extends through an aperture in the wall 62 along the axis of the reflector electrode 6l. The arrangement is such that the collector is substantially shielded by the wall 26 from soft X-ray radiation emitted by electron collision with the peripheral wall 24 of the grid 23. A potential negative with reference to that of the grid 23 is applied to the ion extractor electrode 69 and similarly to the collector electrode 30. The extractor electrode 69 thus sets up an ion extraction field which draws positive ions out of the ionisation space 13 in the direction of the collector electrode 30 to which the positive ions are attracted by the field resulting from the potential supplied thereto The reflector 61, by being connected to the grid 23, tends to repel the positive ions towards the collector 30, and also screens the collector from any charged particles present in the vacuum space outside the region 13. Exterior to the reflector 61, the collector electrode 30 and support 54 are shielded as before by a surrounding glass tube 56.

The extremities of the extractor electrode 69 are shown to have

rims

71 and 72 which are desirable for purposes of rigidity. The extractor 69 can be modified by shortening it while maintaining a similar proximity to the wall 26.

Thus by employing the invention, a nude form of external collector ion gauge can be manufactured, the operation of which is substantially independent of the electric field conditions produced by the surrounding vacuum system. The use of extractor and reflector electrodes enables the various electrodes of such a gauge to be biased so that an optimum sensitivity can be obtained in a repeatable manner.

What is claimed is:

1. An ion gauge for measuring low gas pressures present in a high vacuum system, comprising a conducting cylinder closed at one end, said cylinder being operative at ground potential to screen said gauge from extraneous fields, at least one pair of cathodes radially spaced within said cylinder, said cathodes being operative at voltages that are positive relative to said grounded cylinder, a cylindrical grid positioned between said cathodes and having wire mesh peripheral walls that are relatively transparent to electrons from said cathodes when operated at positive voltages relative to said cathodes, said grid being bounded by a wire mesh wall at an end thereof opposite the closed end of said cylinder and by a metal wall having an aperture at the end thereof nearer the said closed end of said cylinder, the metal wall of said grid providing a shield so that only ions dependent upon the pressure of said gas are in proximity with the aperture of the said nearer end of said grid, means adjacent to the aperture of said grid for extracting and converging ions in proximity with the aperture of metal wall of said grid, and an electrode cooperating with said extracting means for collecting said extracted ions to provide a current dependent upon the pressure of gas within said grid.

2. An ion gauge as claimed in claim 1 wherein said pairs of cathodes are electron emissive filaments.

3. An ion gauge as claimed in claim 1 wherein said wire mesh peripheral walls and wire mesh end wall opposite the closed end of said grounded cylinder comprise closely woven tungsten wire.

4. An ion gauge as claimed in claim 1 wherein said extracting and converging means comprises a first and second electrostatic lens system.

5. An ion gauge as claimed in claim 4 wherein said first electrostatic lens system comprises a cylindrical conducting extension of the aperture of said metal end wall of said cylindrical grid, and an axially symmetrical extractor electrode adjacent said extension, said extractor electrode operating at negative voltages relative to said cathodes, and said second electrostatic lens system comprises said extractor electrode and a cylindrical reflector electrode adjacent to and axially aligned with and extractor electrode, said reflector electrode having an aperture for said collecting electrode and operating at a positive voltage equal to or greater than the voltages of said grid.

6. An ion gauge as claimed in claim 1 wherein said extracting and converging means comprises a rim surrounding the aperture of said grid, a cylindrical extractor electrode adjacent to but insulated from said rim, said extractor electrode having an inside diameter greater than the diameter of said rim to provide screening from extracted ions when operating at negative voltages relative to said cathodes, and a cylindrical reflector electrode surrounding said rim and said extractor electrode, said reflector electrode being fastened to the metal end wall of said grid.

7. An ion gauge as claimed in claim 1 wherein said collecting electrode is a tungsten wire.