EP0376825A1 - Electron source of the field emission type - Google Patents

Electron source of the field emission type Download PDF

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Publication number
EP0376825A1
EP0376825A1 EP89403624A EP89403624A EP0376825A1 EP 0376825 A1 EP0376825 A1 EP 0376825A1 EP 89403624 A EP89403624 A EP 89403624A EP 89403624 A EP89403624 A EP 89403624A EP 0376825 A1 EP0376825 A1 EP 0376825A1
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EP
European Patent Office
Prior art keywords
electrode
electron source
potential
emissive
source according
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EP89403624A
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German (de)
French (fr)
Inventor
Bernard Epsztein
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Thales Electron Devices SA
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Thomson Tubes Electroniques
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J1/00—Details of electrodes, of magnetic control means, of screens, or of the mounting or spacing thereof, common to two or more basic types of discharge tubes or lamps
    • H01J1/02—Main electrodes
    • H01J1/30—Cold cathodes, e.g. field-emissive cathode
    • H01J1/304—Field-emissive cathodes
    • H01J1/3042—Field-emissive cathodes microengineered, e.g. Spindt-type
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J3/00—Details of electron-optical or ion-optical arrangements or of ion traps common to two or more basic types of discharge tubes or lamps
    • H01J3/02—Electron guns
    • H01J3/021—Electron guns using a field emission, photo emission, or secondary emission electron source
    • H01J3/022—Electron guns using a field emission, photo emission, or secondary emission electron source with microengineered cathode, e.g. Spindt-type
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J2201/00—Electrodes common to discharge tubes
    • H01J2201/30—Cold cathodes
    • H01J2201/319—Circuit elements associated with the emitters by direct integration

Definitions

  • the present invention relates to an electron source operating on the principle of field emission. Its object is to perfect such sources, particularly when they are produced by processes which are part of integrated circuit technology or of the field of film deposition in thin layers on a substrate, such as for example for the manufacture of MOS transistors.
  • the techniques already used for integrated circuits or in the field of thin-film films have made it possible to make significant progress in the manufacture of field emission electron sources.
  • These techniques make it possible in particular to obtain structures of very small dimensions which each implement a tip with a very small radius of curvature: the tip is made emissive under the influence of an electric field created using an electrode brought to a positive potential compared to the tip potential.
  • the structure comprising a point constitutes an elementary electron emitting device, capable of forming a microtube, of the triode type for example, or even an electron microchannel, and this elementary device can be used alone or combined with other such devices.
  • FIG. 1 schematically illustrates by way of example an elementary emitter device for field emission electrons, of known type.
  • the transmitter device 1 is formed on a substrate 2, partially shown but whose dimensions can allow the production of a plurality of transmitter devices 1 arranged side by side in a matrix arrangement for example.
  • the substrate 2 is made of a semiconductor material, for example silicon, but it could also be made of a conductive layer, aluminum for example.
  • the substrate 2 is dug so as to comprise a well 3 in the center of which there remains a protuberance 4, of conical shape; the well 3 is centered around an axis 5 intended to constitute the axis of an electron beam 6.
  • the protuberance or cone 4 is made of the same material as the substrate 2, its base is integral with the bottom of the well 3, its apex or point 7 being oriented towards the outside of the well 3 and located on the longitudinal axis 5.
  • the cone 2 could be metallic, as explained in the documents herein above, and that it could also be added to the substrate 2.
  • the insulating layer 9 carries a layer 11 of an electrically conductive material and which has an opening facing the well 3, so as to surround the latter.
  • the layer 11 thus constitutes, around the longitudinal axis 5, an annular electrode, intended for example to constitute an electrode whose function is that which is fulfilled by a Wehnelt electrode as used in particular in the electron guns of cathode ray tubes.
  • wehnelt electrode 11 Above the Wehnelt electrode 11 is deposited an electrically insulating layer 12, open facing the well 3, and which separates the Wehnelt electrode 11 from a second electrically conductive layer 13; this second electrically conductive layer 13 is also open facing the well 3 so as to form a second annular electrode 13 centered around the longitudinal axis 5.
  • the second electrode 13 is brought to a positive potential, of 100 volts for example, relative to a reference potential applied to the substrate 2, the Wehnelt electrode 11 being for example to a potential close to or equal to that of the substrate 2.
  • the tip 7 emits electrons under the influence of the electric field created by the potential of the second electrode 13 which thus constitutes an extracting electrode.
  • the electrons emitted by the tip 7 form an electron beam 6, which could possibly be further accelerated by means of additional electrodes; but also the electrode 13 could be replaced by an anode without opening for the passage of the beam, as described in French patent application No. 2, 568.394 already cited.
  • the dimensions of the structure of the emitting device 1 are of the order of a few micrometers: for example two or three micrometers for the diameter D of the well 3; of the order of one micrometer for the height H of the cone 4; and of the order of 0.06 micrometer for the radius of curvature (not shown) of the tip 7 which constitutes the emissive tip.
  • an electronic current whose average intensity can be of the order of 25 microamps, and which can even reach and exceed, at peak, 100 microamps.
  • each elementary emitting device as the barrel of a microtube, and to associate and combine a large number of them to form the equivalent of an integrated circuit, the semiconductor components thus being replaced by vacuum microtubes.
  • Such sources offer many advantages. Compared to cathodes and guns conventionally used, and in microwave tubes in particular, they have the following advantages in particular: - absence of heating and instantaneous operation; - possibility of modulating the current with a low modulation voltage and at low impedance, hence the possibility of very wide band operation; - overall current density much higher than what we know currently obtain by traditional means (currently it is at most of the order of 10 amperes / cm2).
  • the advantages over semiconductor components are: - possibility of much higher power per element; - absence of losses within the material; - significantly higher microwave performance; - insensitivity to ionizing radiation; - far superior immunity to electromagnetic impulses; - possible applications for visualization.
  • this technique can be exploited, since it has the following disadvantages in particular: - strong variation in emission from one emissive point to another, depending on the radius of curvature which is in practice not controllable; - non-linearity of the modulation characteristic; - significant random variation in time of the current emitted by a tip, due to the temporary presence on the tip of residual gas molecules which modify the output work. It may even happen that the output work is reduced to the point that the intensity of the current emitted by the tip is sufficient to melt the latter by the Joule effect. On the other hand, the random variation of the current results in considerable noise; - the electrons emitted by the point constitute a beam with strong divergence practically non-refocusable.
  • the present invention relates to an electron source formed by at least one elementary emitting device with field emission of a type similar to those described above, that is to say which can constitute a vacuum microtube, or a microcanon capable of being applied to the display of an elementary image point or even for example a microcanon associated with a large number of similar devices mounted in parallel in order to produce a macroscopic cathode.
  • the object of the invention is to improve the elementary electron emitting device so as to avoid the above-mentioned drawbacks, while retaining a high electronic emission capacity, and retaining to these elementary emitting devices the possibility of being produced by the techniques used in the field of integrated circuits or in that of thin-film films.
  • an electron source producing electrons intended to constitute at least one electron beam comprising at least one elementary electron emitting device of the field emission type, the emitting device comprising an electron emitting tip brought to a reference potential, an extractor electrode brought to a positive potential with respect to the reference potential, the extractor electrode comprising a hole for the passage of electrons emitted by the emissive point, is characterized in that the emitting device further comprises at least one control electrode provided with a hole for the passage of electrons, the control electrode being arranged downstream of the extracting electrode relative to the direction of propagation of the beam , the control electrode being at a negative potential with respect to the extraction electrode, and in that means for accelerating the beam are arranged downstream of the control electrode.
  • the control electrode By the presence of the control electrode and its arrangement, owing to the fact that it is located downstream of the extracting electrode, the control electrode has no action on the emission of electrons from the tip. emissive, but on the other hand it slows down the electrons which have passed the level of the extraction electrode, and also tends to act like an electrostatic lens by making the electrons converge towards the axis of the beam, and it also tends to reflect towards the 'extracting electrode the electrons which have high transverse velocities; these actions of the control electrode being more or less pronounced, in particular as a function of the value of the potential which is applied to it.
  • FIG. 2 is a schematic sectional view of an electron source 20 which may include one or a plurality of elementary electron emitting devices according to the invention; but, in the nonlimiting example described, only two transmitting devices 31, 31b are shown for the sake of clarity in FIG. 2.
  • the transmitting devices 31, 31b are formed from 'a substrate 32 in a semiconductor material, in silicon for example. These two transmitting devices are identical, also, to simplify the description, only the first transmitting device 31 is described.
  • the substrate 32 is dug so as to constitute a hole or well 22, centered on a longitudinal axis X1, intended to constitute the axis of an elementary beam F1 of electrons.
  • the well 22 is dug so as to retain, in the center and at the bottom of the latter, a conical protuberance 33 whose apex or point 34 is oriented towards the outside of the well and is located on the longitudinal axis X1 .
  • a layer 36 of an electrically insulating material is deposited on the surface 35 of the substrate 32.
  • the insulating layer 36 in turn carries a layer 37 of an electrically conductive material.
  • These layers 36, 37 are open opposite the well 22 according to the same diameter D as the latter, so that the electrically conductive layer 37 forms an annular electrode 27 centered around the longitudinal axis X1 or beam axis elementary, this electrode being intended to constitute an extracting electrode 27.
  • the electrons emitted by the emissive tip 34 are intended to form the elementary beam F1, the direction of propagation of the beam F1 being symbolized in the figure by an arrow 41.
  • the elementary transmitter device 31 of the invention shown in FIG. 2 does not include wehnelt electrodes, because, both in the device of the prior art and in that of the invention, such an electrode is not essential for operation, since: particularly in the prior art, the modulation of the electron beam can be accomplished by modulating the potential applied to the extracting electrode; in the elementary emitting device 31 of the invention, the modulation of the elementary beam F1 is obtained in a new and particularly advantageous manner, using the control electrode 29 disposed downstream of the retrieving electrode 27 with respect to in the direction of propagation 41 of the elementary beam.
  • a wehnelt electrode of the type shown in FIG. 1 could also be mounted in the emitter device 31 of the invention where it would be placed upstream of the extractor electrode 27 and brought to a potential close to the reference potential VR.
  • the distribution of potential or potential map between these elements determines a strong divergence of a primary beam 45 formed, between the emissive tip 34 and the extracting electrode 27, by the electrons emitted by the emissive tip 34: the presence of large transverse velocities is noted for a high proportion of the electrons emitted by this tip; this strong divergence from the emissive tip 34 is symbolized in FIG. 4 by a number n of trajectories L1, L2, ..., Ln of electrons, the number n of the trajectories represented being small for clarity of the figure .
  • the extracting electrode 27 is followed, in the direction of propagation of the beam, by the control electrode 29 which is brought to a negative control potential V2 relative to the potential V1 of the grid extractor 27.
  • the control electrode 29 For the electrons (symbolized by the paths L1 to Ln) emitted by the emissive tip 34, the influence of the control grid 29 is exerted particularly from the moment when these electrons reach the level of the extractor electrode 27.
  • the control electrode 29 being negative with respect to the extractor electrode 27, it slows down these electrons which constitute the primary beam 45.
  • the primary beam 45 by its presence, digs the potential map and, depending of the control potential V2 applied to the control electrode 29, it is partially reflected, as illustrated in FIG. 4 by paths L4, L5 which join the extractor electrode 27 and which show that the latter can capture electrons thus reflected.
  • a virtual cathode 46 symbolized by a cloud of points which only can pass through, to constitute the elementary electron beam F1, the electrons which deviate little from the longitudinal axis X1 either by a divergence, or by a convergence, too pronounced, or in other words a virtual cathode 46 that only electrons with sufficient longitudinal energy can pass, that is to say say whose transverse speed is low.
  • the virtual cathode 46 constitutes a reserve of electrons or an electron plasma, from which it follows that the intensity of the elementary electron beam F1 or useful beam is practically independent of the primary source represented by the emissive tip 34, and independent of the fluctuations in electron flow from this primary source 34; the intensity of the elementary beam or useful beam F1 only depends on the geometry of the virtual cathode 46 which is controlled by the potential V2 applied to the control electrode 29. A positive variation in the potential V2 applied to the control electrode 29 leads to increasing the intensity of the elementary electron beam F1 or useful beam, and even to causing the disappearance of the virtual cathode 46. It is therefore necessary, for the best functioning, that the average intensity of the primary beam 45 emitted by the emissive tip 34 has a value substantially equal to or greater than the average intensity of the useful beam F1 emitted by the virtual cathode 46.
  • the diameter D of the well 22 is of the order of 2 micrometers; the height H of the cone 33 is of the order of 1 micrometer; the radius of curvature (not shown) of the apex or emissive point 34 is of the order of 0.06 micrometer; the distance d1 between the bottom 50 of the well 32 and the extracting electrode 27 is of the order of 2 micrometers; the distance d2 between the base 50 and the control electrode 29 is of the order of 3 to 4 micrometers; the electrically conductive layers from which the electrodes 27, 29 are made have a thickness (not identified) of the order of 1 micrometer; on the other hand under these conditions, the substrate 32 being silicon, the reference
  • the control potential V2 applied to the control electrode 29 can be variable, so as in particular to modulate the useful beam F1.
  • a negative potential of a few volts applied to the electrode 29 is sufficient to block the useful beam F1.
  • the electrons which constitute the elementary beam or useful beam F1 can be re-accelerated by means of an auxiliary anode or an accelerating electrode, or a cathodoluminescent anode , or some other means of acceleration in itself conventional, already contained in the device (not shown) in which the source 20 can constitute a cathode, in a microwave tube (not shown) for example.
  • the elementary emitting device 31 can be of the triode type, that is to say that its structure can be limited to the emitting tip 34, the extracting electrode 27 and the control electrode 29, the upper part voltage of the microwave tube used to accelerate the beam.
  • auxiliary anode 60 disposed downstream of the control electrode 29.
  • the auxiliary anode 60 can be produced from an electrically conductive layer 59 which is separated from the control electrode 29 by an insulating layer 61, these two layers 59, 61 being etched so as to be open facing the well 22 and allowing the useful beam F1 to pass .
  • the cone 33 used to form the emissive tip 34 is formed from the substrate 32, made of silicon in the example, but in the spirit of the invention the substrate 32 could be another type, and furthermore the cone 33 could be made of a material different from that forming the substrate 32, an electrically conductive material such as, for example, tungsten, or molybdenum (as taught in the documents cited above), which would be attached to the bottom 50 of the well 22 and engraved to constitute the emissive point 34.
  • FIG. 2 shows that the resistor R1 is connected by one of these ends to the conductive layer 37 which serves to constitute the extracting electrode 27, and by the other end to the positive pole + a voltage generator G1 delivering the extraction voltage V1, the other pole of this generator being connected to the substrate 32 and forming the reference voltage VR.
  • the control electrode 29 is connected to the reference voltage VR via a modulation device M, to which a modulation signal SM can be applied, and which makes it possible to adjust the second control potential V2 applied to the control electrode 29; the modulation signal SM possibly being superimposed on the control potential V2.
  • a second voltage generator G2 delivers by a positive output + the third potential V3, of + 100 V for example, which is applied to the accelerating anode 60, the negative output - of this second generator G2 being connected to the reference voltage VR, that is to say to the substrate 32.
  • the elementary emitting device 31 is preferably produced (but not necessarily) by a technology specific to integrated circuits and to the field of films in thin layers, that is to say by using a substrate and successive deposits of insulating and conductive layers, and using etching techniques common in integrated circuit technology and films in thin layers. Consequently, the same substrate 32 can carry a large number (1 million for example) of elementary electron devices such as device 31, on a small surface.
  • the elementary emitting devices mounted on the same substrate can be linked together to form a complex circuit, in the same way as in the case of an integrated circuit. It is also possible, for example, to combine these elementary emitting devices by mounting them in parallel, so as to obtain the equivalent of a macroscopic cathode whose current, at peak, could reach 100 amps or more.
  • all the emissive tips 34 can be at the same reference potential VR; all the extracting electrodes 27 can be produced from the same electrically conductive layer 37 and are therefore interconnected, as may optionally be all the control grids 29 and all the accelerating anodes 60.
  • FIG. 2 wherein the substrate 32 carries the first and second elementary emitting devices 31, 31b.
  • These two transmitting devices 31, 31b belong for example to the same line which could include 1000 such transmitting devices; and a distance d4 of the order of a few micrometers to a hundred micrometers for example, between the longitudinal axis X1 of each of these emitting devices 31, 31b, can represent the pitch between two successive columns of such emitting devices, columns which extend in a plane perpendicular to that of the figure.

Abstract

The invention relates to an electron source (20) formed of at least one elementary electron emitter (31, 31b) comprising an emissive tip (34) having a very small radius of curvature and operating in accordance with the field emission principle. The invention applies particularly in the case of constructions employing technologies used for integrated circuits or in the area of thin-layered films, these technologies enabling a plurality of elementary emitters to be produced on one substrate. …<??>The aim of the invention is in particular to enable an electron beam (F1) to be produced, the intensity of which is independent of possible variations in electron emission by the emissive tip 34. …<??>The emissive tip (34) interacts with an extractor electrode (27), and according to a characteristic of the invention, a control electrode (29), having a negative potential relative to the extractor electrode (27), is arranged downstream of this latter in relation to the sense of propagation (41) of the beam (F1). …<IMAGE>…

Description

La présente invention concerne une source d'électrons fonctionnant selon le principe de l'émission de champ. Elle a pour objet de perfectionner de telles sources, particulièrement quand elles sont réalisées par des procédés qui relèvent de la technologie des circuits intégrés ou du domaine des dépôts de films en couches minces sur un substrat, comme par exemple pour la fabrication de transistors MOS.The present invention relates to an electron source operating on the principle of field emission. Its object is to perfect such sources, particularly when they are produced by processes which are part of integrated circuit technology or of the field of film deposition in thin layers on a substrate, such as for example for the manufacture of MOS transistors.

Depuis quelques années, les techniques déjà utilisées pour les circuits intégrés ou dans le domaine des films à couches minces ont permis d'effectuer d'importants progrès dans la fabrication de source d'électrons à émission de champ. Ces techniques permettent notamment d'obtenir des structures de très petites dimensions qui mettent en oeuvre chacune une pointe de très faible rayon de courbure : la pointe est rendue émissive sous l'influence d'un champ électrique créé a l'aide d'une électrode portée à un potentiel positif par rapport au potentiel de la pointe. La structure comportant une pointe constitue un dispositif émetteur élémentaire d'électrons, pouvant former un microtube, du type triode par exemple, ou encore un microcanon a électrons, et ce dispositif élémentaire peut être utilisé seul ou combiné à d'autres tels dispositifs.In recent years, the techniques already used for integrated circuits or in the field of thin-film films have made it possible to make significant progress in the manufacture of field emission electron sources. These techniques make it possible in particular to obtain structures of very small dimensions which each implement a tip with a very small radius of curvature: the tip is made emissive under the influence of an electric field created using an electrode brought to a positive potential compared to the tip potential. The structure comprising a point constitutes an elementary electron emitting device, capable of forming a microtube, of the triode type for example, or even an electron microchannel, and this elementary device can be used alone or combined with other such devices.

Le fonctionnement et les procédés de réalisation de source d'électrons à émission de champ, formés d'une pluralité de dispositifs émetteurs élémentaires sont connus notamment par des études menées au STANFORD RESEARCH INSTITUTE par C.A. SPINDT et publiées entre autres dans Applications of Surface Science, 2, pp. 149-163 (1979) et dans Applications of Surface Science 16 (1983) pp. 268-276, ainsi que dans Journal of Applied Physics, vol. 47, No. 12, December 1976, page 5248-5263.The operation and the methods of producing a field emission electron source, formed of a plurality of elementary emitting devices are known in particular by studies carried out at STANFORD RESEARCH INSTITUTE by CA SPINDT and published inter alia in Applications of Surface Science, 2, pp. 149-163 (1979) and in Applications of Surface Science 16 (1983) pp. 268-276, as well as in Journal of Applied Physics, vol. 47, No. 12, December 1976, page 5248-5263.

On peut citer également une demande de brevet français publiée sous le n° 2 568 394, qui mentionne les travaux de C.A. SPINDT, et qui décrit différents modes de fonctionnement et de réalisation de cathodes formées chacune d'une pluralité de micropointes qui émettent des électrons selon le principe de l'émission de champ. Chaque micropointe peut émettre un faisceau d'électrons qui bombarde une anode cathodoluminescente formant l'écran d'un dispositif de visualisation. On trouve aussi des exemples d'utilisation et de fabrication de micropointes, pour constituer des cathodes émission de champ, dans une demande de brevet français n° 80 26934 publiée sous le n° 2 472 264, et dans le document de brevet US 4,513,308.Mention may also be made of a French patent application published under No. 2,568,394, which mentions the work of CA SPINDT, and which describes different modes of operation and embodiment of cathodes each formed from a plurality of microtips which emit electrons according to the principle of field emission. Each microtip can emit an electron beam which bombards a cathodoluminescent anode forming the screen of a display device. There are also examples of the use and manufacture of microtips, for constituting field emission cathodes, in a French patent application No. 80 26934 published under No. 2,472,264, and in US patent document 4,513,308.

La figure 1 illustre schématiquement titre d'exemple un dispositif émetteur élémentaire d'électrons émission de champ, de type connu. Le dispositif émetteur 1 est constitué sur un substrat 2, partiellement représenté mais dont les dimensions peuvent permettre la réalisation d'une pluralité de dispositifs émetteurs 1 disposés côte à côte selon un arrangement matriciel par exemple. Le substrat 2 est en un matériau semi-conducteur, en silicium par exemple, mais il pourrait être également en une couche conductrice, en aluminium par exemple. Dans l'exemple représenté, le substrat 2 est creusé de sorte comporter un puits 3 au centre duquel subsiste une protubérance 4, de forme conique ; le puits 3 est centré autour d'un axe 5 destiné constituer l'axe d'un faisceau d'électrons 6. Ainsi, dans l'exemple représenté, la protubérance ou cône 4 est en un même matériau que le substrat 2, sa base fait corps avec le fonds du puits 3, son sommet ou pointe 7 étant orienté vers l'extérieur du puits 3 et situé sur l'axe longitudinal 5. Il est à noter que le cône 2 pourrait être métallique, comme expliqué dans les documents ci-dessus cités, et qu'il pourrait en outre être rapporté sur le substrat 2.FIG. 1 schematically illustrates by way of example an elementary emitter device for field emission electrons, of known type. The transmitter device 1 is formed on a substrate 2, partially shown but whose dimensions can allow the production of a plurality of transmitter devices 1 arranged side by side in a matrix arrangement for example. The substrate 2 is made of a semiconductor material, for example silicon, but it could also be made of a conductive layer, aluminum for example. In the example shown, the substrate 2 is dug so as to comprise a well 3 in the center of which there remains a protuberance 4, of conical shape; the well 3 is centered around an axis 5 intended to constitute the axis of an electron beam 6. Thus, in the example shown, the protuberance or cone 4 is made of the same material as the substrate 2, its base is integral with the bottom of the well 3, its apex or point 7 being oriented towards the outside of the well 3 and located on the longitudinal axis 5. It should be noted that the cone 2 could be metallic, as explained in the documents herein above, and that it could also be added to the substrate 2.

On trouve sur la surface 10 du substrat 2 une couche 9 électriquement isolante. La couche isolante 9 porte une couche 11 en un matériau électriquement conducteur et qui comporte une ouverture au regard du puits 3, de sorte à entourer ce dernier. La couche 11 constitue ainsi, autour de l'axe longitudinal 5, une électrode annulaire, destinée par exemple à constituer une électrode dont la fonction est celle qui est remplie par une électrode Wehnelt telle qu'utilisée notamment dans les canons à électrons de tubes à rayons cathodiques. Au-dessus de l'électrode Wehnelt 11 est déposée une couche électriquement isolante 12, ouverte au regard du puits 3, et que sépare l'électrode Wehnelt 11 d'une seconde couche électriquement conductrice 13 ; cette seconde couche électriquement conductrice 13 est également ouverte au regard du puits 3 de sorte à former une seconde électrode annulaire 13 centrée autour de l'axe longitudinal 5.There is an electrically insulating layer 9 on the surface 10 of the substrate 2. The insulating layer 9 carries a layer 11 of an electrically conductive material and which has an opening facing the well 3, so as to surround the latter. The layer 11 thus constitutes, around the longitudinal axis 5, an annular electrode, intended for example to constitute an electrode whose function is that which is fulfilled by a Wehnelt electrode as used in particular in the electron guns of cathode ray tubes. Above the Wehnelt electrode 11 is deposited an electrically insulating layer 12, open facing the well 3, and which separates the Wehnelt electrode 11 from a second electrically conductive layer 13; this second electrically conductive layer 13 is also open facing the well 3 so as to form a second annular electrode 13 centered around the longitudinal axis 5.

La seconde électrode 13 est portée à un potentiel positif, de 100 volts par exemple, par rapport à un potentiel de référence appliquée au substrat 2, l'électrode Wehnelt 11 étant par exemple à un potentiel voisin ou égal à celui du substrat 2.The second electrode 13 is brought to a positive potential, of 100 volts for example, relative to a reference potential applied to the substrate 2, the Wehnelt electrode 11 being for example to a potential close to or equal to that of the substrate 2.

Dans ces conditions, la pointe 7 émet des électrons sous l'influence du champ électrique créé par le potentiel de la seconde électrode 13 qui constitue ainsi une électrode extractrice. Les électrons émis par la pointe 7 forment un faisceau d'électrons 6, qui éventuellement pourrait être accéléré davantage à l'aide d'électrodes supplémentaires ; mais également l'électrode 13 pourrait être remplacée par une anode sans ouverture pour le passage du faisceau, comme il est décrit dans la demande de brevet français n° 2, 568,394 déjà citée.Under these conditions, the tip 7 emits electrons under the influence of the electric field created by the potential of the second electrode 13 which thus constitutes an extracting electrode. The electrons emitted by the tip 7 form an electron beam 6, which could possibly be further accelerated by means of additional electrodes; but also the electrode 13 could be replaced by an anode without opening for the passage of the beam, as described in French patent application No. 2, 568.394 already cited.

Les dimensions de la structure du dispositif émetteur 1 sont de l'ordre de quelques micromètres : par exemple deux ou trois micromètres pour le diamètre D du puits 3 ; de l'ordre d'un micromètre pour la hauteur H du cône 4 ; et de l'ordre de 0,06 micromètre pour le rayon de courbure (non représenté) de la pointe 7 qui constitue la pointe émissive. On peut obtenir avec un tel type de dispositif émetteur, un courant électronique dont l'intensité moyenne peut être de l'ordre de 25 microampères, et qui peut même atteindre et dépasser, en pointe, 100 microampères.The dimensions of the structure of the emitting device 1 are of the order of a few micrometers: for example two or three micrometers for the diameter D of the well 3; of the order of one micrometer for the height H of the cone 4; and of the order of 0.06 micrometer for the radius of curvature (not shown) of the tip 7 which constitutes the emissive tip. One can obtain with such a type of transmitting device, an electronic current whose average intensity can be of the order of 25 microamps, and which can even reach and exceed, at peak, 100 microamps.

Il est possible d'associer un grand nombre de tels dispositifs émetteurs en parallèle, sous forme de matrice notamment, de sorte à obtenir ainsi l'équivalent d'une source d'électrons ou cathode macroscopique dont le courant peut être modulé par la tension des électrodes extractrices 13.It is possible to associate a large number of such emitting devices in parallel, in particular in the form of a matrix, so as to obtain the equivalent of a source of electrons or macroscopic cathode, the current of which can be modulated by the voltage of the extracting electrodes 13.

On peut également utiliser chaque dispositif émetteur élémentaire comme canon d'un microtube, et en associer et combiner un grand nombre pour former l'équivalent d'un circuit intégré, les composants semiconducteurs étant ainsi remplacés par des microtubes à vide.It is also possible to use each elementary emitting device as the barrel of a microtube, and to associate and combine a large number of them to form the equivalent of an integrated circuit, the semiconductor components thus being replaced by vacuum microtubes.

De telles sources offrent de nombreux avantages. Comparés aux cathodes et canons utilisés classiquement, et dans les tubes hyperfréquences en particulier, ils présentent notamment les avantages suivants :
- absence de chauffage et fonctionnement instantané ; - possibilité de moduler le courant avec une faible tension de modulation et à basse impédance, d'où la possibilité d'un fonctionnement à très large bande ;
- densité de courant global largement supérieur à ce que l'on sait obtenir actuellement par les moyens traditionnels (actuellement c'est au plus de l'ordre de 10 ampères/cm²).Such sources offer many advantages. Compared to cathodes and guns conventionally used, and in microwave tubes in particular, they have the following advantages in particular:
- absence of heating and instantaneous operation; - possibility of modulating the current with a low modulation voltage and at low impedance, hence the possibility of very wide band operation;
- overall current density much higher than what we know currently obtain by traditional means (currently it is at most of the order of 10 amperes / cm²).

Vis-à-vis des composants semiconducteurs, les avantages sont les suivants :
- possibilité de puissance par élément nettement supérieure ;
- absence de pertes au sein du matériau ;
- rendement nettement supérieur en hyperfréquences ;
- insensibilité au rayonnements ionisants ;
- immunité bien supérieure aux impulsions électro­magnétiques ;
- possibilités d'applications à la visualisation.The advantages over semiconductor components are:
- possibility of much higher power per element;
- absence of losses within the material;
- significantly higher microwave performance;
- insensitivity to ionizing radiation;
- far superior immunity to electromagnetic impulses;
- possible applications for visualization.

Malgré ces nombreux avantages, cette technique est peut exploitée, du fait qu'elle présente notamment les inconvénients suivants :
- forte variation d'émission d'une pointe émissive à l'autre, en fonction du rayon de courbure qui n'est en pratique pas contrôlable ;
- non linéarité de la caractéristique de modulation ;
- variation aléatoire importante dans le temps du courant émis par une pointe, due à la présence temporaire sur la pointe de molécules de gaz résiduels qui modifient le travail de sortie. Il peut même arriver que le travail de sortie soit diminué au point que l'intensité du courant émis par la pointe soit suffisante pour fondre cette dernière par effet Joule. D'autre part, la variation aléatoire du courant se traduit par un bruit considérable ;
- les électrons émis par la pointe constituent un faisceau à forte divergence pratiquement non refocalisable.Despite these numerous advantages, this technique can be exploited, since it has the following disadvantages in particular:
- strong variation in emission from one emissive point to another, depending on the radius of curvature which is in practice not controllable;
- non-linearity of the modulation characteristic;
- significant random variation in time of the current emitted by a tip, due to the temporary presence on the tip of residual gas molecules which modify the output work. It may even happen that the output work is reduced to the point that the intensity of the current emitted by the tip is sufficient to melt the latter by the Joule effect. On the other hand, the random variation of the current results in considerable noise;
- the electrons emitted by the point constitute a beam with strong divergence practically non-refocusable.

La présente invention concerne une source d'électrons formée d'au moins un dispositif émetteur élémentaire à émission de champ d'un type semblable à ceux ci-dessus décrits, c'est-à-dire pouvant constituer un microtube à vide, ou un microcanon susceptible d'être appliqué à la visualisation d'un point élémentaire d'image ou encore par exemple un microcanon associé à un grand nombre de dispositifs semblables montés en parallèle en vue de réaliser une cathode macroscopique.The present invention relates to an electron source formed by at least one elementary emitting device with field emission of a type similar to those described above, that is to say which can constitute a vacuum microtube, or a microcanon capable of being applied to the display of an elementary image point or even for example a microcanon associated with a large number of similar devices mounted in parallel in order to produce a macroscopic cathode.

L'invention a pour objet de perfectionner le dispositif émetteur élémentaire d'électrons de manière à éviter les inconvénients ci-dessus mentionnés, tout en conservant une capacité d'émission électronique élevée, et en conservant à ces dispositifs émetteurs élémentaires la possibilité d'être réalisés par les techniques utilisées dans le domaine des circuits intégrés ou dans celui des films à couches minces.The object of the invention is to improve the elementary electron emitting device so as to avoid the above-mentioned drawbacks, while retaining a high electronic emission capacity, and retaining to these elementary emitting devices the possibility of being produced by the techniques used in the field of integrated circuits or in that of thin-film films.

Selon l'invention une source d'électrons produisant des électrons destinés à constituer au moins un faisceau d'électrons, comprenant au moins un dispositif émetteur élémentaire d'électrons du type à émission de champ, le dispositif émetteur comportant une pointe émissive d'électrons portée à un potentiel de référence, une électrode extractrice portée à un potentiel positif par rapport au potentiel de référence, l'électrode extractrice comportant un trou pour le passage d'électrons émis par la pointe émissive, est caractérisée en ce que le dispositif émetteur comporte en outre au moins une électrode de commande munie d'un trou pour le passage d'électrons, l'électrode de commande étant disposée en aval de l'électrode extractrice par rapport au sens de propagation du faisceau, l'électrode de commande étant à un potentiel négatif par rapport à l'électrode extratrice, et en ce que des moyens pour accélérer le faisceau sont disposés en aval de l'électrode de commande.According to the invention an electron source producing electrons intended to constitute at least one electron beam, comprising at least one elementary electron emitting device of the field emission type, the emitting device comprising an electron emitting tip brought to a reference potential, an extractor electrode brought to a positive potential with respect to the reference potential, the extractor electrode comprising a hole for the passage of electrons emitted by the emissive point, is characterized in that the emitting device further comprises at least one control electrode provided with a hole for the passage of electrons, the control electrode being arranged downstream of the extracting electrode relative to the direction of propagation of the beam , the control electrode being at a negative potential with respect to the extraction electrode, and in that means for accelerating the beam are arranged downstream of the control electrode.

Par la présence de l'électrode de commande et sa disposition, du fait qu'elle est située en aval de l'électrode extractrice, l'électrode de commande n'a pas d'action sur l'émission d'électrons par la pointe émissive, mais par contre elle ralentit les électrons qui ont passé le niveau de l'électrode extratrice, et tend en outre à agir comme une lentille électrostatique en faisant converger les électrons vers l'axe du faisceau, et elle tend aussi à réfléchir vers l'électrode extractrice les électrons qui ont de fortes vitesses transverses ; ces actions de l'électrode de commande étant plus ou moins prononcées, notamment en fonction de la valeur du potentiel qui lui est appliquée. Il en résulte une accumulation d'électrons en amont de l'électrode de commande et la création d'une cathode virtuelle située sensiblement au niveau de l'électrode de commande et sur l'axe du faisceau. Par suite, le faisceau d'électrons en aval de l'électrode de commande c'est-à-dire le faisceau utile émis par la cathode virtuelle, a une faible divergence, et son intensité ne dépend pratiquement que de la géométrie de la cathode virtuelle, cette géométrie étant contrôlée par la valeur du potentiel appliqué à la grille de commande.By the presence of the control electrode and its arrangement, owing to the fact that it is located downstream of the extracting electrode, the control electrode has no action on the emission of electrons from the tip. emissive, but on the other hand it slows down the electrons which have passed the level of the extraction electrode, and also tends to act like an electrostatic lens by making the electrons converge towards the axis of the beam, and it also tends to reflect towards the 'extracting electrode the electrons which have high transverse velocities; these actions of the control electrode being more or less pronounced, in particular as a function of the value of the potential which is applied to it. This results in an accumulation of electrons upstream of the control electrode and the creation of a virtual cathode located substantially at the level of the control electrode and on the axis of the beam. As a result, the electron beam downstream of the control electrode, that is to say the useful beam emitted by the virtual cathode, has a small divergence, and its intensity depends practically only on the geometry of the cathode. virtual, this geometry being controlled by the value of the potential applied to the control grid.

L'invention sera mieux comprise à l'aide de la description qui suit, fait à titre d'exemple non limitatif, et aux figures annexées, parmi lesquelles :

  • - la figure 1 déjà décrite montre un dispositif émetteur élémentaire d'électrons à émission de champ, selon l'art antérieur ;
  • - la figure 2 est une vue schématique en coupe, qui montre les éléments caractéristiques d'un dispositif émetteur élémentaire à émission de champ perfectionné conformément à l'invention.
The invention will be better understood with the aid of the description which follows, given by way of nonlimiting example, and in the appended figures, among which:
  • - Figure 1 already described shows an elementary electron emitting device with field emission, according to the prior art;
  • - Figure 2 is a schematic sectional view, which shows the characteristic elements of an elementary emitting device with improved field emission according to the invention.

La figure 2 est une vue schématique, en coupe, d'une source d'électrons 20 pouvant comporter un ou une pluralité de dispositifs émetteurs élémentaires d'électrons conformes à l'invention ; mais, dans l'exemple non limitatif décrit, seulement deux dispositifs émetteurs 31, 31 b sont représentés pour plus de clarté de la figure 2. Dans l'exemple non limitatif de la description, les dispositifs émetteurs 31, 31b sont formés à partir d'un substrat 32 en un matériau semi-conducteur, en silicium par exemple. Ces deux dispositifs émetteurs sont identiques, aussi, pour simplifier la description, seulement le premier dispositif émetteur 31 est décrit.Figure 2 is a schematic sectional view of an electron source 20 which may include one or a plurality of elementary electron emitting devices according to the invention; but, in the nonlimiting example described, only two transmitting devices 31, 31b are shown for the sake of clarity in FIG. 2. In the nonlimiting example of the description, the transmitting devices 31, 31b are formed from 'a substrate 32 in a semiconductor material, in silicon for example. These two transmitting devices are identical, also, to simplify the description, only the first transmitting device 31 is described.

Le substrat 32 est creusé de sorte à constituer un trou ou puits 22, centré sur un axe longitudinal X1, destiné à constituer l'axe d'un faisceau élémentaire F1 d'électrons. Le puits 22 est creusé de sorte à conserver, au centre et sur le fond de ce dernier, une protubérance de forme conique 33 dont le sommet ou pointe 34 est orienté vers l'extérieur du puits et se trouve situé sur l'axe longitudinal X1. Sur la surface 35 du substrat 32, est déposée une couche 36 d'un matériau électriquement isolant. La couche 36 isolante porte à son tour une couche 37 en un matériau électriquement conducteur. Ces couches 36,37 sont ouvertes en vis-à-vis du puits 22 selon un même diamètre D que ce dernier, de sorte que la couche électriquement conductrice 37 forme une électrode 27 annulaire centrée autour de l'axe longitudinal X1 ou axe du faisceau élémentaire, cette électrode étant destinée à constituer une électrode extractrice 27. Au-dessus de l'électrode extractrice 27, on trouve une autre couche isolante 38 qui sépare l'électrode extractrice 27 d'une seconde couche conductrice 39 ; ces deux dernières couches 38,39 sont ouvertes au regard du puits 22 de manière à laisser passer des électrons émis par la pointe émissive 34, et afin que la couche électriquement conductrice 39 constitue une électrode de commande 29 annulaire centrée autour de l'axe longitudinal X1. Les électrons émis par la pointe émissive 34 sont destinés à former le faisceau élémentaire F1, le sens de propagation du faisceau F1 étant symbolisé sur la figure par une flèche 41.The substrate 32 is dug so as to constitute a hole or well 22, centered on a longitudinal axis X1, intended to constitute the axis of an elementary beam F1 of electrons. The well 22 is dug so as to retain, in the center and at the bottom of the latter, a conical protuberance 33 whose apex or point 34 is oriented towards the outside of the well and is located on the longitudinal axis X1 . On the surface 35 of the substrate 32, a layer 36 of an electrically insulating material is deposited. The insulating layer 36 in turn carries a layer 37 of an electrically conductive material. These layers 36, 37 are open opposite the well 22 according to the same diameter D as the latter, so that the electrically conductive layer 37 forms an annular electrode 27 centered around the longitudinal axis X1 or beam axis elementary, this electrode being intended to constitute an extracting electrode 27. Above the extracting electrode 27, there is another insulating layer 38 which separates the extracting electrode 27 from a second conductive layer 39; these last two layers 38.39 are open to the well 22 so as to allow electrons emitted by the emissive tip 34 to pass, and so that the electrically conductive layer 39 constitutes an annular control electrode 29 centered around the longitudinal axis X1. The electrons emitted by the emissive tip 34 are intended to form the elementary beam F1, the direction of propagation of the beam F1 being symbolized in the figure by an arrow 41.

Le principe de fonctionnement du dispositif émetteur élémentaire 31 et semblable au fonctionnement du dispositif émetteur selon l'art antérieur montré à la figure 1, mais seulement en ce qui concerne la partie "extraction des électrons". En effet la pointe émissive 34 émet des électrons sous l'influence du champ électrique créé par un potentiel d'extraction V1 qui est appliqué à l'électrode extractrice 27 ; ce potentiel V1 étant positif par rapport à un potentiel de référence VR du substrat 32, c'est-à-dire de la pointe émissive 34.The operating principle of the elementary emitting device 31 and similar to the operation of the emitting device according to the prior art shown in FIG. 1, but only as regards the part "extraction of electrons". Indeed the emissive tip 34 emits electrons under the influence of the electric field created by an extraction potential V1 which is applied to the extracting electrode 27; this potential V1 being positive with respect to a reference potential VR of the substrate 32, that is to say of the emissive tip 34.

Comme il a été précédemment mentionné, par rapport à la représentation du dispositif émetteur de l'art antérieur représenté à la figure 1, le dispositif émetteur élémentaire 31 de l'invention montré à la figure 2 ne comporte pas d'électrodes wehnelt, car, aussi bien dans le dispositif de l'art antérieur et dans celui de l'invention une telle électrode n'est pas indispensable au fonctionnement, du fait que : particulièrement dans l'art antérieur, la modulation du faisceau électronique peut s'accomplir en modulant le potentiel appliqué à l'électrode extractrice ; dans le dispositif émetteur élémentaire 31 de l'invention, la modulation du faisceau élémentaire F1 est obtenue d'une manière nouvelle et particulièrement avantageuse, à l'aide de l'électrode de commande 29 disposée en aval de l'électrode extratrice 27 par rapport au sens de propagation 41 du faisceau élémentaire. Bien entendu, une électrode wehnelt du type représenté à la figure 1 pourrait également être montée dans le dispositif émetteur 31 de l'invention où elle serait disposée en amont de l'électrode extractrice 27 et portée à un potentiel voisin du potentiel de référence VR.As previously mentioned, compared to the representation of the transmitter device of the prior art shown in FIG. 1, the elementary transmitter device 31 of the invention shown in FIG. 2 does not include wehnelt electrodes, because, both in the device of the prior art and in that of the invention, such an electrode is not essential for operation, since: particularly in the prior art, the modulation of the electron beam can be accomplished by modulating the potential applied to the extracting electrode; in the elementary emitting device 31 of the invention, the modulation of the elementary beam F1 is obtained in a new and particularly advantageous manner, using the control electrode 29 disposed downstream of the retrieving electrode 27 with respect to in the direction of propagation 41 of the elementary beam. Of course, a wehnelt electrode of the type shown in FIG. 1 could also be mounted in the emitter device 31 of the invention where it would be placed upstream of the extractor electrode 27 and brought to a potential close to the reference potential VR.

Compte tenu de la forme et des positions relatives de la pointe émissive 34 et de l'électrode extractrice 27, la distribution de potentiel ou carte de potentiel entre ces éléments détermine une forte divergence d'un faisceau primaire 45 formé, entre la pointe émissive 34 et l'électrode extractrice 27, par les électrons émis par la pointe émissive 34 : on note la présence d'importantes vitesses transverses pour une forte proportion des électrons émis par cette pointe ; cette forte divergence à partir de la pointe émissive 34 est symbolisée sur la figure 4 par un nombre n de trajectoires L1, L2, ..., Ln d'électrons, le nombre n des trajectoires représentées étant faible pour plus de clarté de la figure. On observe qu'à partir de la pointe émissive 34, certaines de ces trajectoires s'écartent peu de l'axe longitudinal X1 qui constitue l'axe d'émission, les trajectoires L1, L2 par exemple, alors que d'autres trajectoires L3, Ln s'en écartent fortement jusqu'à ce qu'elles atteignent sensiblement, le long de l'axe longitudinal X1, le niveau de l'électrode extractrice 27.Taking into account the shape and the relative positions of the emissive tip 34 and the extracting electrode 27, the distribution of potential or potential map between these elements determines a strong divergence of a primary beam 45 formed, between the emissive tip 34 and the extracting electrode 27, by the electrons emitted by the emissive tip 34: the presence of large transverse velocities is noted for a high proportion of the electrons emitted by this tip; this strong divergence from the emissive tip 34 is symbolized in FIG. 4 by a number n of trajectories L1, L2, ..., Ln of electrons, the number n of the trajectories represented being small for clarity of the figure . We observe that from the emissive point 34, some of these trajectories deviate little from the longitudinal axis X1 which constitutes the emission axis, the trajectories L1, L2 for example, while other trajectories L3 , Ln strongly deviate therefrom until they substantially reach, along the longitudinal axis X1, the level of the extracting electrode 27.

Selon une caractéristique de l'invention, l'électrode extractrice 27 est suivie, dans le sens 41 de propagation du faisceau, par l'électrode de commande 29 qui est portée à un potentiel de commande V2 négatif par rapport au potentiel V1 de la grille extractrice 27. Pour les électrons (symbolisés par les trajectoires L1 à Ln) émis par la pointe émissive 34, l'influence de la grille de commande 29 s'exerce particulièrement à partir de l'instant où ces électrons parviennent au niveau de l'électrode extractrice 27. L'électrode de commande 29 étant négative par rapport à l'électrode extractrice 27, elle ralentit ces électrons qui constituent le faisceau primaire 45. Le faisceau primaire 45, par sa présence, creuse la carte de potentiel et, en fonction du potentiel de commande V2 appliqué à l'électrode de commande 29, il est partiellement réfléchi, comme illustré sur la figure 4 par des trajectoires L4, L5 qui rejoignent l'électrode extractrice 27 et qui montrent que cette dernière peut capter des électrons ainsi réfléchis.According to a characteristic of the invention, the extracting electrode 27 is followed, in the direction of propagation of the beam, by the control electrode 29 which is brought to a negative control potential V2 relative to the potential V1 of the grid extractor 27. For the electrons (symbolized by the paths L1 to Ln) emitted by the emissive tip 34, the influence of the control grid 29 is exerted particularly from the moment when these electrons reach the level of the extractor electrode 27. The control electrode 29 being negative with respect to the extractor electrode 27, it slows down these electrons which constitute the primary beam 45. The primary beam 45, by its presence, digs the potential map and, depending of the control potential V2 applied to the control electrode 29, it is partially reflected, as illustrated in FIG. 4 by paths L4, L5 which join the extractor electrode 27 and which show that the latter can capture electrons thus reflected.

Il se crée alors, sensiblement au niveau de l'électrode de commande 29 et sur l'axe longitudinal X1, une cathode virtuelle 46 (symbolisée par un nuage de points) que seuls peuvent franchir, pour constituer le faisceau d'electrons élémentaire F1, les électrons qui s'écartent peu de l'axe longitudinal X1 soit par une divergence, soit par une convergence, trop prononcée, ou autrement dit une cathode virtuelle 46 que seuls peuvent franchir les électrons possédant une énergie longitudinale suffisante c'est-à-dire dont la vitesse transverse est faible.There is then created, substantially at the level of the control electrode 29 and on the longitudinal axis X1, a virtual cathode 46 (symbolized by a cloud of points) which only can pass through, to constitute the elementary electron beam F1, the electrons which deviate little from the longitudinal axis X1 either by a divergence, or by a convergence, too pronounced, or in other words a virtual cathode 46 that only electrons with sufficient longitudinal energy can pass, that is to say say whose transverse speed is low.

On peut considérer que la cathode virtuelle 46 constitue une réserve d'électrons ou un plasma d'électrons, d'où il résulte que l'intensité du faisceau d'électrons élémentaire F1 ou faisceau utile est pratiquement indépendante de la source primaire que représente la pointe émissive 34, et indépendante des fluctuations en débit d'électrons de cette source primaire 34 ; l'intensité du faisceau élémentaire ou faisceau utile F1 ne dépend que de la géométrie de la cathode virtuelle 46 laquelle est contrôlée par le potentiel V2 appliqué à l'électrode de commande 29. Une variation positive du potentiel V2 appliqué à l'électrode de commande 29 conduit à augmenter l'intensité du faisceau d'électrons élémentaire F1 ou faisceau utile, et même à entraîner la disparition de la cathode virtuelle 46. Il est donc nécessaire, pour le meilleur fonctionnement, que l'intensité moyenne du faisceau primaire 45 émis par la pointe émissive 34 ait une valeur sensiblement égale ou supérieure à l'intensité moyenne du faisceau utile F1 émis par la cathode virtuelle 46.We can consider that the virtual cathode 46 constitutes a reserve of electrons or an electron plasma, from which it follows that the intensity of the elementary electron beam F1 or useful beam is practically independent of the primary source represented by the emissive tip 34, and independent of the fluctuations in electron flow from this primary source 34; the intensity of the elementary beam or useful beam F1 only depends on the geometry of the virtual cathode 46 which is controlled by the potential V2 applied to the control electrode 29. A positive variation in the potential V2 applied to the control electrode 29 leads to increasing the intensity of the elementary electron beam F1 or useful beam, and even to causing the disappearance of the virtual cathode 46. It is therefore necessary, for the best functioning, that the average intensity of the primary beam 45 emitted by the emissive tip 34 has a value substantially equal to or greater than the average intensity of the useful beam F1 emitted by the virtual cathode 46.

Ceci est obtenu en ajustant les valeurs de certains paramètres tels que notamment la valeur des potentiels V1, V2 en fonction de la valeur de l'intensité désirée du faisceau utile 40, et en fonction de la dimension de la structure et des distances entre les électrodes. On indique ci-après, à titre d'exemple non limitatif, quelques valeurs qui peuvent être conférées à ces paramètres :
- Le diamètre D du puits 22 est d'ordre de 2 micromètres ; la hauteur H du cône 33 est de l'ordre de 1 micromètre ; le rayon de courbure (non représenté) du sommet ou pointe émissive 34 est de l'ordre de 0,06 micromètre ; la distance d1 entre le fonds 50 du puits 32 et l'électrode extractrice 27 est de l'ordre de 2 micromètres ; la distance d2 entre le fonds 50 et l'électrode de commande 29 est de l'ordre de 3 à 4 micromètres ; les couches électriquement conductrices à partir desquelles sont constituées les électrodes 27,29 ont une épaisseur (non repérée) de l'ordre de 1 micromètre ; d'autre part dans ces conditions, le substrat 32 étant du silicium, la tension de référence VR étant par exemple à 0 volt, le potentiel V1 appliqué à l'électrode extratrice 27 a une valeur d'environ + 100 volts et le potentiel V2 de commande appliqué à la grille de commande 29 a sensiblement une même valeur moyenne que le potentiel de référence VR : ceci pour une intensité du faisceau d'électrons primaire élémentaire F1 ou faisceau utile, en continu, de l'ordre de 50 micro-ampères.This is obtained by adjusting the values of certain parameters such as in particular the value of the potentials V1, V2 as a function of the value of the desired intensity of the useful beam 40, and as a function of the dimension of the structure and the distances between the electrodes . We indicate below, by way of nonlimiting example, some values which can be conferred on these parameters:
- The diameter D of the well 22 is of the order of 2 micrometers; the height H of the cone 33 is of the order of 1 micrometer; the radius of curvature (not shown) of the apex or emissive point 34 is of the order of 0.06 micrometer; the distance d1 between the bottom 50 of the well 32 and the extracting electrode 27 is of the order of 2 micrometers; the distance d2 between the base 50 and the control electrode 29 is of the order of 3 to 4 micrometers; the electrically conductive layers from which the electrodes 27, 29 are made have a thickness (not identified) of the order of 1 micrometer; on the other hand under these conditions, the substrate 32 being silicon, the reference voltage VR being for example at 0 volts, the potential V1 applied to the stripping electrode 27 has a value of approximately + 100 volts and the potential V2 control applied to the control gate 29 has substantially the same average value as the reference potential VR: this for an intensity of the elementary primary electron beam F1 or useful beam, continuously, of the order of 50 micro-amps .

Le potentiel de commande V2 appliqué à l'électrode de commande 29 peut être variable, de sorte notamment à moduler le faisceau utile F1. En effet, dans le cadre des conditions ci-dessus mentionnées, un potentiel négatif de quelques volts appliqué à l'électrode 29 suffit pour bloquer le faisceau utile F1. Une variation d'une dizaine de volts par exemple sur l'électrode de commande 29 suffit pour obtenir toutes les valeurs d'intensité IF du faisceau utile F1 : la caractéristique IF = f(V2) s'apparente à la caractéristique Ip = f (Vg) d'un tube de type tétrode et présente la même linéarité.The control potential V2 applied to the control electrode 29 can be variable, so as in particular to modulate the useful beam F1. In fact, under the above-mentioned conditions, a negative potential of a few volts applied to the electrode 29 is sufficient to block the useful beam F1. A variation of ten volts for example on the control electrode 29 is sufficient to obtain all the intensity values IF of the useful beam F1: the characteristic IF = f (V2) is similar to the characteristic I p = f (V g ) of a tetrode type tube and has the same linearity.

Il est à noter que, au-delà de la grille de commande 29, les électrons qui constituent le faisceau élémentaire ou faisceau utile F1 peuvent être réaccélérés au moyen d'une anode auxiliaire ou d'une électrode accélératrice, ou d'une anode cathodoluminescente, ou de quelque autre moyen d'accélération en lui-même classique, déjà contenu dans le dispositif (non représenté) dans lequel la source 20 peut constituer une cathode, dans un tube hyperfréquence (non représenté) par exemple. Dans un tel cas, le dispositif émetteur élémentaire 31 peut être du type triode c'est-à-dire que sa structure peut se limiter à la pointe émettrice 34, l'électrode extractrice 27 et l'électrode de commande 29, la partie haute tension du tube hyperfréquence servant à accélérer le faisceau.It should be noted that, beyond the control grid 29, the electrons which constitute the elementary beam or useful beam F1 can be re-accelerated by means of an auxiliary anode or an accelerating electrode, or a cathodoluminescent anode , or some other means of acceleration in itself conventional, already contained in the device (not shown) in which the source 20 can constitute a cathode, in a microwave tube (not shown) for example. In such a case, the elementary emitting device 31 can be of the triode type, that is to say that its structure can be limited to the emitting tip 34, the extracting electrode 27 and the control electrode 29, the upper part voltage of the microwave tube used to accelerate the beam.

Cependant il est possible également, comme il est représenté sur la figure 2, de réaccélérer les électrons au moyen d'une anode auxiliaire 60 disposée en aval de l'électrode de commande 29. L'anode auxiliaire 60 peut être réalisée à partir d'une couche électriquement conductrice 59 qui est séparée de l'électrode de commande 29 par une couche isolante 61, ces deux couches 59,61 étant gravées de manière à être ouvertes en vis-à-vis du puits 22 et laisser passer le faisceau utile F1.However, it is also possible, as shown in FIG. 2, to re-accelerate the electrons by means of an auxiliary anode 60 disposed downstream of the control electrode 29. The auxiliary anode 60 can be produced from an electrically conductive layer 59 which is separated from the control electrode 29 by an insulating layer 61, these two layers 59, 61 being etched so as to be open facing the well 22 and allowing the useful beam F1 to pass .

Dans l'exemple non limitatif de la description, le cône 33 servant à former la pointe émissive 34 est constitué à partir du substrat 32, en silicium dans l'exemple, mais dans l'esprit de l'invention le substrat 32 pourrait être d'un autre type, et en outre le cône 33 pourrait être réalisé en un matériau différent de celui formant le substrat 32, un matériau électriquement conducteur comme par exemple du tungstène, ou du molybdène (comme enseigné dans les documents précédemment cités), qui serait rapporté sur le fonds 50 du puits 22 et gravé pour constituer la pointe émissive 34.In the nonlimiting example of the description, the cone 33 used to form the emissive tip 34 is formed from the substrate 32, made of silicon in the example, but in the spirit of the invention the substrate 32 could be another type, and furthermore the cone 33 could be made of a material different from that forming the substrate 32, an electrically conductive material such as, for example, tungsten, or molybdenum (as taught in the documents cited above), which would be attached to the bottom 50 of the well 22 and engraved to constitute the emissive point 34.

Il est à noter enfin, que pour éviter la fusion par effet Joule de la pointe émissive 34, quand cette dernière émet un courant électronique de trop forte intensité, on peut diminuer la valeur du potentiel V1 appliquée à l'électrode extractrice 27, et ceci seulement tant que l'intensité du courant émis est trop forte. A cet effet, il est possible par exemple de monter une résistance R1 en série avec l'alimentation de l'électrode extratrice 27, c'est-à-dire entre cette dernière et le potentiel d'extraction V1. Il en résulte que si l'émission par la pointe émissive 34 devient trop forte, une forte proportion des électrons du faisceau primaire 45 sont réfléchis par la cathode virtuelle 46 et ces électrons retombent sur l'électrode d'extraction 36 par laquelle ils sont captés. Si la valeur de la résistance R1 est correctement choisie, le nouveau potentiel d'extraction V′1, appliqué à l'électrode extractrice 27 du fait de la présence de la résistance R1, diminue suffisamment pour ramener l'intensité de l'émission à une valeur convenable.Finally, it should be noted that, to avoid fusion by Joule effect of the emissive tip 34, when the latter emits an electronic current of too high intensity, it is possible to decrease the value of the potential V1 applied to the extracting electrode 27, and this only as long as the intensity of the emitted current is too strong. To this end, it is possible, for example, to mount a resistor R1 in series with the supply of the extraction electrode 27, that is to say between the latter and the extraction potential V1. It follows that if the emission by the tip emissive 34 becomes too strong, a large proportion of the electrons of the primary beam 45 are reflected by the virtual cathode 46 and these electrons fall on the extraction electrode 36 by which they are captured. If the value of the resistance R1 is correctly chosen, the new extraction potential V′1, applied to the extracting electrode 27 due to the presence of the resistance R1, decreases sufficiently to bring the intensity of the emission to a suitable value.

Cette configuration est illustrée schématiquement à la figure 2, qui montre que la résistance R1 est reliée par l'une de ces extrémités à la couche conductrice 37 qui sert à constituer l'électrode extractrice 27, et par l'autre extrémité au pôle positif + d'un générateur de tension G1 délivrant la tension d'extraction V1, l'autre pôle de ce générateur étant relié au substrat 32 et formant la tension de référence VR. D'autre part, dans l'exemple non limitatif décrit, l'électrode de commande 29 est reliée à la tension de référence VR par l'intermédiaire d'un dispositif de modulation M, auquel peut être appliqué un signal de modulation SM, et qui permet d'ajuster le second potentiel V2 de commande appliqué à l'électrode de commande 29 ; le signal de modulation SM pouvant être éventuellement superposé au potentiel de commande V2. Un second générateur de tension G2 délivre par une sortie positive + le troisième potentiel V3, de + 100 V par exemple, qui est appliqué à l'anode accélératrice 60, la sortie négative - de ce second générateur G2 étant reliée à la tension de référence VR, c'est-à-dire au substrat 32.This configuration is illustrated diagrammatically in FIG. 2, which shows that the resistor R1 is connected by one of these ends to the conductive layer 37 which serves to constitute the extracting electrode 27, and by the other end to the positive pole + a voltage generator G1 delivering the extraction voltage V1, the other pole of this generator being connected to the substrate 32 and forming the reference voltage VR. On the other hand, in the nonlimiting example described, the control electrode 29 is connected to the reference voltage VR via a modulation device M, to which a modulation signal SM can be applied, and which makes it possible to adjust the second control potential V2 applied to the control electrode 29; the modulation signal SM possibly being superimposed on the control potential V2. A second voltage generator G2 delivers by a positive output + the third potential V3, of + 100 V for example, which is applied to the accelerating anode 60, the negative output - of this second generator G2 being connected to the reference voltage VR, that is to say to the substrate 32.

Comme il a été précédemment mentionné, le dispositif émetteur élémentaire 31 est réalisé de préférence (mais non nécessairement) par une technologie propre aux circuits intégrés et au domaine des films en couches minces, c'est-à-dire en utilisant un substrat et des dépôts successifs de couches isolantes et conductrices, et en utilisant des techniques de gravure courantes dans la technologie des circuits intégrés et des films en couches minces. Par suite un même substrat 32 peut porter un grand nombre (1 million par exemple) de dispositifs élémentaires d'électrons tel que le dispositif 31, sur une faible surface.As previously mentioned, the elementary emitting device 31 is preferably produced (but not necessarily) by a technology specific to integrated circuits and to the field of films in thin layers, that is to say by using a substrate and successive deposits of insulating and conductive layers, and using etching techniques common in integrated circuit technology and films in thin layers. Consequently, the same substrate 32 can carry a large number (1 million for example) of elementary electron devices such as device 31, on a small surface.

Les dispositifs émetteurs élémentaires montés sur un même substrat peuvent être liés entre eux pour former un circuit complexe, d'une même manière que dans le cas d'un circuit intégré. On peut également, par exemple, associer ces dispositifs émetteurs élémentaires en les montant en parallèle, de sorte à obtenir l'équivalent d'une cathode macroscopique dont le courant, en pointe, pourrait atteindre 100 ampères ou plus. Dans ce cas, toutes les pointes émissives 34 peuvent être au même potentiel de référence VR ; toutes les électrodes extractrices 27 peuvent être réalisées à partir d une même couche électriquement conductrice 37 et sont donc reliées entre elles, de même éventuellement que toutes les grilles de commande 29 et toutes les anodes accélératrices 60. Une telle réalisation est illustrée sur la figure 2 dans laquelle le substrat 32 porte le premier et le second dispositifs émetteurs élémentaires 31, 31b. Ces deux dispositifs émetteurs 31, 31b appartiennent par exemple à une même ligne qui pourrait comporter 1000 tels dispositifs émetteurs ; et une distance d4 de l'ordre de quelques micromètres à une centaine de micromètres par exemple, entre l'axe longitudinal X1 de chacun de ces dispositif émetteur 31, 31b, peut représenter le pas entre deux colonnes successives de tels dispositifs émetteurs, colonnes qui s'étendent dans un plan perpendiculaire à celui de la figure.The elementary emitting devices mounted on the same substrate can be linked together to form a complex circuit, in the same way as in the case of an integrated circuit. It is also possible, for example, to combine these elementary emitting devices by mounting them in parallel, so as to obtain the equivalent of a macroscopic cathode whose current, at peak, could reach 100 amps or more. In this case, all the emissive tips 34 can be at the same reference potential VR; all the extracting electrodes 27 can be produced from the same electrically conductive layer 37 and are therefore interconnected, as may optionally be all the control grids 29 and all the accelerating anodes 60. Such an embodiment is illustrated in FIG. 2 wherein the substrate 32 carries the first and second elementary emitting devices 31, 31b. These two transmitting devices 31, 31b belong for example to the same line which could include 1000 such transmitting devices; and a distance d4 of the order of a few micrometers to a hundred micrometers for example, between the longitudinal axis X1 of each of these emitting devices 31, 31b, can represent the pitch between two successive columns of such emitting devices, columns which extend in a plane perpendicular to that of the figure.

Claims (10)

1. Source d'électrons (20) produisant des électrons destinés à constituer au moins un faisceau d'électrons (F1), comprenant au moins un dispositif émetteur (31) d'électrons du type à émission de champ, le dispositif émetteur (31) comportant une pointe émissive (34) d'électrons portée à un potentiel de référence (VR), une électrode extractrice (27) portée à un potentiel d'extraction positif (V1, V′1) par rapport au potentiel de référence (VR), l'électrode extractrice (27) comportant un trou pour le passage des électrons (L1 à Ln) émis par la pointe émissive (34), caractérisée en ce que le dispositif émetteur (31) comporte en outre au moins une électrode de commande (29) disposée en aval de l'électrode extractrices (27) par rapport au sens de propagation (41) du faisceau, l'électrode de commande (29) étant porté à un potentiel de commande négatif (V2) par rapport à l'électrode extractrice (27), et en ce que des moyens (60) pour accélérer le faisceau (F1) sont disposés en aval de l'électrode de commande (29).1. Electron source (20) producing electrons intended to constitute at least one electron beam (F1), comprising at least one electron emitting device (31) of the field emission type, the emitting device (31 ) comprising an emissive tip (34) of electrons brought to a reference potential (VR), an extracting electrode (27) brought to a positive extraction potential (V1, V′1) with respect to the reference potential (VR ), the extracting electrode (27) comprising a hole for the passage of the electrons (L1 to Ln) emitted by the emissive tip (34), characterized in that the emitting device (31) further comprises at least one control electrode (29) disposed downstream of the extractor electrode (27) relative to the direction of propagation (41) of the beam, the control electrode (29) being brought to a negative control potential (V2) relative to the extraction electrode (27), and in that means (60) for accelerating the ceau (F1) are arranged downstream of the control electrode (29). 2. Source d'électrons selon la revendication 1, caractérisée en ce que la pointe émissive (34) et les électrodes extractrice et de commande (27, 29) sont portées par un même substrat (32), et en ce que les électrodes extractrices et de commandes (27,29) sont constituées par une succession de couches isolantes et conductrices (35,36,37,38,39) déposées sur le substrat (32) et gravées de manière à constituer un puits (22) dans lequel est située la pointe émissive (34).2. Electron source according to claim 1, characterized in that the emissive tip (34) and the extractor and control electrodes (27, 29) are carried by the same substrate (32), and in that the extractor electrodes and controls (27,29) consist of a succession of insulating and conductive layers (35,36,37,38,39) deposited on the substrate (32) and etched so as to constitute a well (22) in which is located the emissive point (34). 3. Source d'électrons selon la revendication 2, caractérisée en ce qu'un même substrat (32) porte une pluralité de dispositif émetteur (31, 31b).3. Electron source according to claim 2, characterized in that the same substrate (32) carries a plurality of emitting device (31, 31b). 4. Source d'électrons selon l'une des revendications 2 ou 3, caractérisée en ce que le substrat (32) est en un matériau semi-conducteur.4. Electron source according to one of claims 2 or 3, characterized in that the substrate (32) is made of a semiconductor material. 5. Source d'électrons selon l'une des revendications 3 ou 4, caractérisée en ce que toutes les pointes émissives (34), toutes les électrodes extractrices (36), toutes les électrodes de commandes (39) sont électriquement reliées entre elles.5. Electron source according to one of claims 3 or 4, characterized in that all the emissive tips (34), all the extracting electrodes (36), all the control electrodes (39) are electrically connected to each other. 6. Source d'électrons selon l'une des revendications précédentes, caractérisée en ce que le potentiel de commande (V2) appliqué à l'électrode de commande (29) est modulé par un signal de modulation (SM).6. Electron source according to one of the preceding claims, characterized in that the control potential (V2) applied to the control electrode (29) is modulated by a modulation signal (SM). 7. Source d'électrons selon l'une des revendications précédentes, caractérisée en ce que le potentiel de commande (V2) appliqué à l'électrode de commande (29) a une valeur moyenne sensiblement égale au potentiel de référence (VR).7. Electron source according to one of the preceding claims, characterized in that the control potential (V2) applied to the control electrode (29) has an average value substantially equal to the reference potential (VR). 8. Source d'électrons selon l'une des revendications précédentes, caractérisée en ce que les moyens pour accélérer le faisceau (40) comportent une anode auxiliaire (60) formée par une couche électriquement conductrice déposée sur une couche isolante (59) elle-même déposée sur l'électrode de commande (29).8. Electron source according to one of the preceding claims, characterized in that the means for accelerating the beam (40) comprise an auxiliary anode (60) formed by an electrically conductive layer deposited on an insulating layer (59) itself even deposited on the control electrode (29). 9. Source d'électrons selon l'une des revendications précédentes, caractérisée en ce que le dispositif émetteur (31,31b) comporte des moyens (R1) pour limiter l'intensité du courant électronique émis par la pointe émissive (34), quand cette intensité devient trop importante.9. Electron source according to one of the preceding claims, characterized in that the emitting device (31,31b) comprises means (R1) for limiting the intensity of the electronic current emitted by the emissive tip (34), when this intensity becomes too important. 10. Source d'électrons selon la revendication 9, caractérisée en ce que les moyens pour limiter l'intensité du courant électronique émis par la pointe émissive (34) comportent au moins une résistance (R1) montée en série entre l'électrode extractrice (27) et le potentiel d'extraction (V1) destiné à être appliqué à cette électrode extractrice.10. Electron source according to claim 9, characterized in that the means for limiting the intensity of the electronic current emitted by the emissive tip (34) comprise at least one resistor (R1) connected in series between the extracting electrode ( 27) and the extraction potential (V1) intended to be applied to this extracting electrode.
EP89403624A 1988-12-30 1989-12-22 Electron source of the field emission type Ceased EP0376825A1 (en)

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FR8817484A FR2641412B1 (en) 1988-12-30 1988-12-30 FIELD EMISSION TYPE ELECTRON SOURCE

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US5070282A (en) 1991-12-03
FR2641412B1 (en) 1991-02-15
FR2641412A1 (en) 1990-07-06
JPH02226635A (en) 1990-09-10

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