US3655903A

US3655903A - Annular electron gun

Info

Publication number: US3655903A
Application number: US812263A
Authority: US
Inventors: Leonard F Roman; George H Elliott
Original assignee: Individual
Current assignee: Individual
Priority date: 1969-04-01
Filing date: 1969-04-01
Publication date: 1972-04-11
Anticipated expiration: 1989-04-11

Abstract

An annular electron gun adapted to produce a conical electron beam capable of vaporizing a source material situated at the apex of the beam. The gun comprises an annular filament, a truncated conical electrode disposed coaxially just below the beam, and an annular wire ring electrode suspended just above the beam in coaxial spaced relation with the conical electrode. The filament is maintained at a high negative potential, the electrodes at the same positive potential. Vaporized atoms of source material are ionized by the annular electrodes, and accumulate into an ion cloud within the cone of, but separated from, the electron beam.

Description

MTRQA XR 3,655,903

Ouucu MaIBS Patent [151 3,655,903 Roman etal. [451 Apr.ll, 1972 54] ANNULAR ELECTRON GUN Primary Examiner-Remand A. Gilheany [72] Inventors. Leonard F. Roman, 11018 Moorpark, Assistant Examiner Roy N'Envallh.

North Hollywood, Calif. 91602, George H. A" r R A M Elliott, 16501 Knollwood Drive, Granada ms Hills, Calif. 91344 ABSTRACT [22] Filed: 1969 An annular electron gun adapted to produce a conical elec- [21] Appl. No.: 812,263 tron beam capable of vaporizing a source material situated at the apex of the beam. The gun comprises an annular filament, a truncated conical electrode disposed coaxially just below the ..l3/3:I,6251b9[71/(2N1) beam, and an annular wire ring electrode suspended just above the beam in coaxial spaced relation with the'conical [58] Field of Search 1 3/31 219/121 250/495 electrode. The filament is maintained at a high negative poten- [56] References Citd tial, the electrodes at the same positive potential. vaporized atoms of source material are ionized by the annular elec- UNITED STATES PATENTS trodes, and accumulate into an ion cloud within the cone of,

but separated from, the electron beam. 3,040,112 6/1962 Smith, Jr ..l3/31 3 ,437,734 4/1969 Roman et al 1 3/31 10 Claims, 3 Drawing Figures 6| ELECTRODE t VOLTAGE VOLTAGE souizee sourace o i 1 I14 1 w 24 LEOA/fl/ZD R Roma/J 23 GEORGE H. ELL/arr Q5 INVIz'N'l'O/(S Patented A ril 11, 1972 3,655,903

2 Sheets-Sheet 2 ANNULAR ELECTRON GUN BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an annular electron gun device for vaporizing a source material, and more particularly,

' to an annular electron gun adapted to produce a conical beam devices have been suggested in the prior art for accomplishing vacuum deposition of thin layers of semiconductor or other materials onto a substrate. Such methods and devices find application in the production of semiconductor devices which employ very thin coherent, oriented layers of silicon or the like disposed on electrically insulating or semiconductive substrates. To permit the resultant structure to be used in the fabrication of transistors, diodes or other semiconductor devices, the deposited layers must be of high crystalline uniformity, free from undesired impurities.

Among the problems previously encountered in the production of such high purity, highly coherent depositions is the difficulty of providing a source of vaporized atoms of high purity. For example, if a silicon source material were vaporized merely by heating the crucible containing the silicon, considerable impurities from the crucible itself would diffuse into the source material. With such contamination, device grade deposition could not be accomplished.

More commonly, an electron beam focused on the surface of the source material has been used to accomplish vaporization of the source. Generally, a single electron gun was used, magnetic or other focusing means being used to direct the beam to a point on the source material surface. While such techniques accomplished source vaporization, the resultant vapor cloud often encompassed the electron beam. This complicated maintenance of beam current, and ionization and transportation of the vapor cloud away from the source, in some systems tended to distort focusing of the beam itself. This shortcoming of the prior art often necessitated the use of complex deflection devices to direct the electron beam around the vapor cloud.

1 A far more satisfactory approach, suggested in the abovementioned co-pending application, involves utilization of an annular electron filament. Electrons issuing from such a filament were focusable into a conical beam having an apex impinging at the surface of the source material. Separate positively charged ionization electrodes were provided to pull electrons from atoms of the vaporized material, the resultant positively charged ions accumulating into an ion cloud in the center of, but completely separated from, the electron beam cone.

The present invention relates to an improved annular electron gun having combined accelerator and ionization elements. The invention permits effective vaporization of a source material, free from contaminants, with the production of an ion cloud permitting subsequent transportation of the vaporized material. The invention does not employ complex beam deflection schemes, but uses a minimum of components,

is easy to construct and may be readily supported within the vacuum chamber of a deposition apparatus.

SUMMARY OF THE INVENTION In accordance with the present invention, there is provided a novel annular electron gum incorporating combined accelerator and ionization elements. The apparatus is useful for the vaporization of a source material and the concomitant production of an ion cloud of the vaporized material to form a large ionized vapor source, such as in excess of 6 inches in diameter.

The inventive annular electron gun comprises a ring-shaped filament mounted within an annular channeled frame, and maintained at a high negative potential with respect to the crucible containing the source material. The crucible itself is positioned in coaxial, non-coplanar relation with the ring filament. Electrons issuing from the ring filament are focused into a conical beam, the apex of the beam occurring approximately at the center of the upper surface of the source material in the crucible.

Acceleration and focusing of the electron beam is accomplished by means of a pair of electrodes, annular in shape, and disposed respectively above and below the cone defined by the loci of the electrons travel. The lower electrode comprises a truncated conical member, somewhat in the form of a funnel, which extends radially inwardly and downwardly from below the position of the ring filament. A second electrode comprises a ring of wire having a diameter less than the inner diameter of the lower electrode, and suspended thereabove by means of support wires projecting from an annular electrode of generally inverted U-shaped cross-section. The inner lip of the upper electrode is disposed coaxially inwardly of and slightly above the ring filament. Appropriate adjustment of the spacing between the wire ring electrode and the funnel-shaped electrode accomplishes focusing of the electron beam.

Preferably the electrodes are maintained at a positive potential, typically on the order of +300 volts. This positive potential permits the electrodes to function both as accelerators and as ionizers, drawing electrons from atoms of vaporized source material. Since the resultant ions are charged positively, they are repelled from the positive electrodes, wherein the impacted ionized fresh source vapor forms an ion cloud within the cone of, but spaced from, the electron beam, while in the environment of a high vacuum.

Thus, it is an object of the present invention to provide an apparatus for effectuating vaporization of a source material.

Another object of the present invention is to provide an annular electron gun capable of producing a focusable conical beam of electrons.

It is another object of the present invention to provide an annular electron gun incorporating means for providing an acceleration field.

Yet another object of the present invention is to provide an electron gun including annular acceleration electrodes.

Still another object of the present invention is to provide a device incorporating an electron gun having combined accelerator and ionizer electrodes of annular shape.

It is a further object of the present invention to provide an electron beam apparatus for producing an ionized cloud of a source material, the cloud being separated from the electron beam.

Yet a further object of the present invention is to provide an apparatus for producing an ionized cloud in a high vacuum of a material contained in a crucible, the apparatus comprising an annular filament, and means for directing electrons issuing from the filament into a conical beam focused on the source material.

BRIEF DESCRIPTION OF THE DRAWINGS Still other objects, features and attendant advantages of the present invention will become apparent to those skilled in the art from a reading of the following detailed description of the preferred embodiment constructed in accordance therewith, taken in conjunction with the accompanying drawings wherein like numerals designate like parts in the several figures and wherein:

FIG. 1 is a side elevation view of a typical vacuum deposition apparatus in which the inventive annular electron gun may be incorporated;

FIG. 2 is a top plan view, partly broken away and in section, of the inventive annular electron gun, showing the combined accelerator and ionizer electrodes; and

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings, and particularly to FIG. 1 thereof, there is shown a side elevation view of an apparatus for the high vacuum deposition of evaporated materials, capable of utilizing the inventive annular electron gun for source material vaporization.

As evident in FIG. 1, a deposition apparatus incorporates a vacuum chamber 11 within which is situated the source material, the electron gun vaporization apparatus, an appropriate cylindrical electrostatically charged screen for transporting vaporized and ionized source material to a substrate, and a holder on which the target substrate is mounted. Vacuum chamber 11 is surrounded by a protective screen or grate 12 which offers protection in the event that the glass portion of vacuum chamber 11 should shatter. Screen 12 is retained in position about chamber 11 by means of fixtures 13, which fixtures partially may be attached to a flat lid 14 covering chamber 11. To permit orientation of the target within the deposition chamber, a shaft extending from the target holder projects through a passageway in lid 14, terminating in a vacuum seal cover 15.

Vacuum chamber 11 (see FIG. 1) is supported by a base 16 which itself is supported on a suitable flooring l7. Projecting through base 16 are one or more conduits for connection to an external vacuum roughing pump system used to evacuate the interior of chamber 11. Additional conduits 19 may extend through base 16 to provide for the flow of a coolant for the source material crucible, as will be described in detail hereinbelow. Located about the lower periphery of base 16 are a plurality of magnets 20 which are employed in a conventional sputter pump high vacuum system.

To accomplish magnetic focusing of ions impinging on the target, there is provided a toroidal electromagnet 21 situated coaxially about the target area at the upper end of chamber 11. Electromagnet 21 is supported by a pair of stanchions 22. The height of electromagnet 21 can be adjusted by rotation of a pair of fixtures 23 attached to posts 24 which threadingly engage the interior of base stanchions 25. A control panel 26, located adjacent apparatus 10, displays a variety of suitable gauges, dials, recording equipment and the like.

As described in detail in the inventors co-pending application entitled APPARATUS AND METHOD FOR EFFECT- ING THE RESTRUCTURING OF MATERIALS, Ser. No.

559,l98, a substrate on which a material is to be deposited using apparatus 10 initially is attached to the target support member depending from cap 15. Source material, initially situated in a crucible near the bottom of the interior of vacuum chamber 11, is vaporized using an electron beam, the vapor cloud then being ionized by an appropriate ionizer. The present invention, described in detail in conjunction with FIGS. 2 and 3 hereinbelow, serves exceptionally well to perform the vaporization and ionization functions for apparatus 10. The ionized vapor cloud then is transported upwardly within chamber 11 (see FIG. 1) by an appropriate electrostatically charged screen, the ions impinging on the target at the upper end of the chamber. An annular filament electron source is provided just below the target area, the source supplying additional electrons to the target so as to neutralize the charge of the impinging ions. The impinging ions are focused toward the target by the field of toroidal electromagnet 21. This apparatus and technique provides extremely well ordered, coherent layers of deposited material atop the substrate. Semiconductor material so deposited is of device grade, useful for the fabrication of microelectronic components.

Referring now to FIGS. 2 and 3, note that the source material to be vaporized using the inventive electron gun is contained within a generally hemispherical recess 31 in a copper crucible 32. Preferably, crucible 32 is provided with a reflective surface liner 33 of polished, plated chrome or the like. By using such a hemispherical, polished chrome crucible interior, heat radiated from the surface center 30' of the source material toward crucible 32 is reflected back by liner 33 to region 30. Thus, the liner 33 aids in providing the intense local heat required to effect vaporization of the source material. Of.

course, as will be described hereinbelow, the impinging electron beam used initially toheat source material 30 is focused so as to impinge at region 30.

Crucible 32 itself is mounted on a base 34 provided with appropriate passageways 35 for recirculation therethrough of a coolant such as liquid nitrogen or cold water. This coolant, supplied via conduits 19 shown in FIG. 1, acts to maintain the crucible at a relatively cool temperature, preferably under 500 C. This cooling minimizes the possibility of copper or chrome atoms from crucible 32 or liner 33 diffusing into and contaminating source material 30. Crucible 32 (see FIG. 3) may be slidable horizontally with respect to base 34 on tracts 36. This mechanism permits alignment of crucible 32 so that the central source region 30 can be aligned coaxially with the I inventive annular electron gun and with the central axis of vacuum chamber 11. Further, use of tracks 36 permits intimate thermal contact through differential thermal expansion. More than one crucible may be mounted on base 34, an appropriate linkage or other mechanism (not shown) permitting the operator to position, when cooled, at selected one of the crucibles at the apex of the electron beam. By providing different materials in each crucible, as for example N-type silicon in one, P-type silicon or metal in the other, subsequent deposition onto the target of layers of different characteristics may be accomplished.

Details of the inventive annular electron gun clearly are shown in FIGS. 2 and 3. Referring thereto, it may be seen that electron gun 40 is suspended above crucible 32, in coaxial relation with the vertical axis of crucible recess 31, by means of support stanchions 41. The lower end of stanchions 41 are supported by insulative base members 42 extending upwardly from a mounting member 43 on which crucible base 34 also is mounted.

As best evident in FIG. 2, the entire electron gun apparatus 40 is of annular shape, generally being larger in diameter than crucible recess 31. The electron source itself comprises a ring filament 44 disposed concentrically within the hemispherical channel 45 of an annular channeled insulating member 46. Ring filament 44 is maintained in this position by means of insulative support standoffs 47. Electrical connection to ring filament 44 is provided via a pair of conductive posts 48 passing concentrically through two of standoffs 47. Annular insulating member 46 itself is attached to stanchions 41 by means of appropriate brackets 49 and screw fasteners 49a.

Ring filament 44 preferably is composed of thorium tungsten or the like, appropriate current being provided therethrough so as to cause electrons to issue from ring filament 44. Filament 44 is maintained at a high negative potential, typically 6,000 volts, with respect to ground, and hence with respect to grounded crucible 32, this potential being provided by a conventional electron gun voltage source 50. The current source 1 for ring filament 44 is not shown in the drawings, but is understood to be in accordance with conventional practice.

A first annular electrode 51 of generally T-shaped cross section is disposed below annular filament 44. Electrode 51 comprises a top portion 51a which projects downwardly and slightly outwardly from the lower lip 46a of annular channeled insulating member 46. The stem portion 51b of electrode 51 is of truncated conical shape, projecting inwardly toward the vertical axis of annular electron gun 40, and downwardly toward the upper surface of crucible'32. The inner diameter of truncated conical electrode portion 51b is somewhat larger than the diameter of crucible recess 31, as best evident in FIG. 2. Electrode 51 is supported from mounts 49 by means of insulative standoffs 52.

Disposed above annular filament 44 is a second electrode 53 of generally inverted-U-shaped cross-section. As evident in FIG. 3, electrode 53 is formed from two

annular members

54 and 55 each of inverted-L-shaped cross-section. Member 54 is disposed radially inwardly of member 55 and includes a downwardly depending portion 54a which covers the upper lip 46b of annular insulating member 46. The top portion 54b of member 54 is connected to the top portion 550 of member 55 by means of screw fasteners 56. Thus,

members

54b and 55a together form the top of inverted-U-shaped electrode 53. The downwardly depending portion 55b of member 55 is of sufficient length so that its lower edge is disposed in a plane below that of ring filament 44. Electrode 53 is supported from mounts 49 by means of insulative standoffs 57 and screw fasteners 58.

Suspended in the middle of electrode 53, and electrically connected thereto, is a wire ring electrode 59. Wire ring 59 has a diameter less than the minimum diameter of electrode 53, but greater than the inner diameter of truncated conical electrode 51b. As shown in. FIGS. 2 and 3, support for wire ring 59 is provided by a plurality of radially disposed support wires 60 extending from top member 54b. Support wires 60 may be bent, as at 600, the bend angle controlling the spacing between wire ring 59 and truncated conical electrode 51b. As will be described hereinbelow, this spacing effects the focusing of electrons issuing from ring filament 44.

Preferably,

electrodes

51 and 53 and wire ring 59 are maintained at the same positive potential. This potential, typically on the order of +300 volts, may be supplied by a conventional voltage source 61 (see FIG. 3).

In operation, the current flow through ring filament 44 causes electrons to issue therefrom. These electrons are accelerated toward source material 30 in crucible 32, in part due to the difference in potential between the filament and the crucible, and primarily, due to the action of the accelerating field existing between filament 44 and

electrodes

51, 53 and 59. This electric field causes the electrons issuing from filament 44 to be accelerated through the gap between

electrode members

51a and 54a, in a generally conical direction toward source material 30.

Generally, the electrons from any region of filament 44 will tend to spread somewhat, impinging on a more or less broad area of the surface of source material 30. A very narrow beam spread may be accomplished by adjusting the spacing wire ring electrode 59 with respect to truncated conical electrode 51b. This adjustment is carried out by changing the angle at which support wires 60 depend from member 5411, or changing the angle of bends 60a. When the electrode spacing is appropriately adjusted, the loci of electrons issuing from filament 44 define a thin walled cone, the electrons impacting at the cone apex adjacent the center 30 of source material 30.

With the acceleration voltages indicated, approximately 6,000 volts on the filament 44, ground potential at the crucible and +300 volts on

electrodes

51, 53 and 59, sufficient electron energy is obtained to melt and vaporize a source material such as silicon. Of course, considerable heat is generated in the region 30 where the electron beam impinges. Part of this heat radiates through source material 30 if it is as heat transparent as silicon, and is reflected by plated chrome crucible liner 33 back to region 30', further heating the source material in region 30'.

As atoms of the source material are vaporized from the surface of source material 30, they tend to move above truncated conical electrode 51b but within wire ring electrode 59. Since

electrodes

51, 53 and 59 are of positive potential, electrons tend to be drawn from the vaporized source atoms, positively ionizing these atoms. The positive ions are repelled from

annular electrodes

51, 53, 54 and 59 and tend to be directed back into the ion cloud and through the fresh vapor in the middle ending up back in the source material thereby maintaining a stable ionized area slightly above annular electron gun 40. When electron gun 40 is incorporated in a vacuum deposition apparatus of the type shown in FIG. 1, ions from this large diameter ion cloud may be transported by means of an appropriate electrostatically charged screen for deposition onto a substrate at the top of chamber 11.

Thus, the inventive apparatus provides a highly efficient means for vaporizing a source material, utilizing a conically shaped electron beam which is readily focusable onto the source material, and which beam does not pass through, nor require complex beam deflection techniques to avoid passage through the vaporized cloud. Further, the same electrodes which accomplish acceleration of electrons issuing from a circular filament also act to ionize atoms of the vaporized source material, causing accumulation of a concentrated ion cloud in the center of, yet spaced from the conical electron beam.

While the invention has been described with respect to several physical embodiments constructed in accordance therewith, it will be apparent to those skilled in the art that various modifications and improvements may be made without departing from the scope and spirit of the invention. Accordingly, it is to be understood that the invention is not to be limited by the specific illustrative embodiments, but only by the scope of the appended claims.

What is claimed is:

1. An electron gun comprising:

an annular filament for discharging electrons;

means for accelerating the electrons issuing from said filament into a conical beam, said means comprising a truncated conical electrode having a flanged edge disposed coaxially just below said beam; and

a circular electrode of inverted U-shaped cross-section suspended just above said beam in coaxial spaced relation with said conical electrode so as to define a circular open area therebetween for passage of said beam; and

a circular wire carried outwardly from and coaxial with said circular electrode so as to partially occupy said open area for focusing the electrons.

2. An electron gun as defined in claim 1 further comprising:

means for producing a potential difference between said filament and said means for focusing; and

means for adjustably mounting said wire within said open area whereby said wire may be moved to a selected location with respect to proximity of said electrodes.

3. An annular electron gun adapted to produce a focusable cone-shaped beam of electrons, said electron gun comprising:

an annular filament for issuing said electrons;

a first annular electrode having a truncated conical portion disposed coaxially with and immediately below said filament;

a second annular electrode disposed immediately above said filament so as to define a circular area in cooperation with said first electrode occupied by said filament; and

a third annular electrode having a wire ring suspended coaxially within said defined area in adjustable spaced relation with said conical portion for focusing said beam.

4. An annular electron gun adapted to produce a focusable cone-shaped beam of electrons, said electron gun comprising:

an annular filament;

a first annular electrode having a truncated conical portion disposed coaxially just below said beam; and

a second annular electrode having a wire ring suspended coaxially just above said beam in adjustable spaced relation with said conical portion; and

said second electrode having a generally inverted U-shaped cross-section, a lower edge of one side of said U being disposed radially inwardly of said filament and immediately above said beam, the bottom of said U extending radially outwardly from said one side, said wire ring having a diameter less than that of said one side and being suspended inwardly thereof by a plurality of support wires.

5. An annular electron gun adapted to produce a focusable cone-shaped beam of electrons, said electron gun comprising:

an annular filament;

said first electrode having a generally T-shaped cross-section, the stem of said T forming said conical portion, an edge of the top of said T being disposed radially inwardly of said filament immediately below said beam.

6. An electron gun as defined in claim 4 wherein:

said first electrode has a generally T-shaped cross-section, the stem of said T forming said conical portion, an edge of the top of said T being disposed radially inwardly of said filament immediately below said beam.

7. An electron gun as defined in claim 6 further comprising:

means for producing a potential difference between said filament and said first and second electrodes,

8. An annular electron gun device for producing an ion cloud of a source material, said device comprising:

an electron gun as defined in claim 3;

means for holding said source material at the apex of said beam; and

voltage source means for maintaining said filament at a high negative potential and said first and second annular electrodes at a positive potential, whereby said electron beam vaporizes atoms of said source material, said vaporized atoms being ionized by losing electrons to said positively charged electrodes.

9. An annular electron gun device as defined in claim 8 wherein:

said means for holding comprises a crucible having a hemispherical inner surface covered with a reflective liner, said source material being disposed within said crucible.

10. The invention as defined in claim 3 including:

means for producing a potential difference between said filament and said electrodes; and

said potential difference being characterized by said electrodes being at the same positive potential and said filament being at a substantially high negative potential.

Claims

1. An electron gun comprising: an annular filament for discharging electrons; means for accelerating the electrons issuing from said filament into a conical beam, said means comprising a truncated conical electrode having a flanged edge disposed coaxially just below said beam; and a circular electrode of inverted U-shaped cross-section suspended just above said beam in coaxial spaced relation with said conical electrode so as to define a circular open area therebetween for passage of said beam; and a circular wire carried outwardly from and coaxial with said circular electrode so as to partially occupy said open area for focusing the electrons.

2. An electron gun as defined in claim 1 further comprising: means for producing a potential difference between said filament and said means for focusing; and means for adjustably mounting said wire within said open area whereby said wire may be moved to a selected location with respect to proximity of said electrodes.

3. An annular eLectron gun adapted to produce a focusable cone-shaped beam of electrons, said electron gun comprising: an annular filament for issuing said electrons; a first annular electrode having a truncated conical portion disposed coaxially with and immediately below said filament; a second annular electrode disposed immediately above said filament so as to define a circular area in cooperation with said first electrode occupied by said filament; and a third annular electrode having a wire ring suspended coaxially within said defined area in adjustable spaced relation with said conical portion for focusing said beam.

4. An annular electron gun adapted to produce a focusable cone-shaped beam of electrons, said electron gun comprising: an annular filament; a first annular electrode having a truncated conical portion disposed coaxially just below said beam; and a second annular electrode having a wire ring suspended coaxially just above said beam in adjustable spaced relation with said conical portion; and said second electrode having a generally inverted U-shaped cross-section, a lower edge of one side of said U being disposed radially inwardly of said filament and immediately above said beam, the bottom of said U extending radially outwardly from said one side, said wire ring having a diameter less than that of said one side and being suspended inwardly thereof by a plurality of support wires.

5. An annular electron gun adapted to produce a focusable cone-shaped beam of electrons, said electron gun comprising: an annular filament; a first annular electrode having a truncated conical portion disposed coaxially just below said beam; and a second annular electrode having a wire ring suspended coaxially just above said beam in adjustable spaced relation with said conical portion; and said first electrode having a generally T-shaped cross-section, the stem of said T forming said conical portion, an edge of the top of said T being disposed radially inwardly of said filament immediately below said beam.

6. An electron gun as defined in claim 4 wherein: said first electrode has a generally T-shaped cross-section, the stem of said T forming said conical portion, an edge of the top of said T being disposed radially inwardly of said filament immediately below said beam.

7. An electron gun as defined in claim 6 further comprising: means for producing a potential difference between said filament and said first and second electrodes.

8. An annular electron gun device for producing an ion cloud of a source material, said device comprising: an electron gun as defined in claim 3; means for holding said source material at the apex of said beam; and voltage source means for maintaining said filament at a high negative potential and said first and second annular electrodes at a positive potential, whereby said electron beam vaporizes atoms of said source material, said vaporized atoms being ionized by losing electrons to said positively charged electrodes.

9. An annular electron gun device as defined in claim 8 wherein: said means for holding comprises a crucible having a hemispherical inner surface covered with a reflective liner, said source material being disposed within said crucible.

10. The invention as defined in claim 3 including: means for producing a potential difference between said filament and said electrodes; and said potential difference being characterized by said electrodes being at the same positive potential and said filament being at a substantially high negative potential.