FR2803944A1 - ELECTRON GENERATING CATHODE AND ITS MANUFACTURING PROCESS - Google Patents

Abstract

The invention relates to a method for the production of an electron emitting cathode. The inventive method is characterized in that it is based on an emitting (20) material which is insulating or has a low electronic affinity and in that emission sites (261, 262) are defined artificially by localized creation of zones of high electronic affinity and/or conduction. Preferably, the density of the sites is predetermined, e.g. ranges between 10<8> and 10<13>/cm<2>.

Description

The invention relates to an electron-generating cathode and to an electron-generating device. It also relates to a method of manufacturing this cathode.

It is known that certain materials, in particular carbon-based materials, allow emission of electrons by application of a weak field, at most a few tens of volts per micrometer.

Although the origin of these emissions is not clearly established, it is commonly accepted that the emission comes from sites made up of very small zones, the largest dimension of which in the emission surface is from a few nanometers to 100. approximately nm, each site possibly corresponding to an abrupt transition between a material with high electronic affinity and a material with low electronic affinity.

It is recalled here that the electronic affinity of a material is a quantity characterizing the possibility of emission of electrons from the surface of the material. For carbon-based materials, the transitions between materials with low and high electron affinity are between an sp2 phase and an sp3 phase.

these transitions can also correspond to transitions between a conductive phase and an insulating phase; in the case of a carbon-based material, phase sp2 is conductive and phase sp3 is insulating.

According to a first model (Grâning et al. Applied Physics Letters 71, 2253, 1997) the sites would be conductive sp2 channels of size 10 to 100 nm in an insulating sp3 matrix and the electronic emission would come from a peak effect. According to a second model (J. Robertson et al., Diamond and related materials 7, 620, 1998) an emissive site would correspond to a surface variation, localized over a distance of approximately 10 nm, of the electron affinity. According to a third model (M.W. Geis et al. Nature, 393, 431, 1998) in diamonds electronic emission originates from triple metal / diamond / void junctions.

In order to increase the electronic emission, it is therefore sought to increase the density of emission sites. To date, the best results provide site densities of the order of 106 / cm2, ie 10-2 / @ tm2. These values are too low to obtain sufficient emission current densities, at least equal to 0.1 A / cm2. Moreover, the location of the emissive sites is not predictable, which can constitute a drawback for practical applications.

However, W. Zhu et al. (Appl. Phys. Lett. 75, p. 873 1999) have succeeded in producing carbon cathodes in the form of nanotubes with a density of emissive sites of the order of 107 / cm2, the current density obtained being order of 0.5 A / cm2; however, this value is still too low for this material to be usable in practice.

The invention overcomes these drawbacks. It makes it possible to achieve emission site densities of the order of 1012 / cm2 and therefore to increase the current densities by several orders of magnitude.

The cathode according to the invention is characterized in that it comprises a layer or a substrate made of an insulating or low electron affinity material and electron-emitting sites created artificially, these sites preferably having a predetermined density. between $ 10 and 1013 / cmz. These sites are created from areas with strong electronic affinity and / or conductors. It is believed (without the invention being limited to this interpretation) that the emission of electrons (the sites) takes place at the transition between the areas with high electronic affinity and / or conductors and the conductive layer or substrate or at low electron affinity. Thus, the emitting sites would be formed by the transition between the zones and the rest of the layer or substrate. However, in the following, for simplicity, the sites and areas will sometimes be referred to in the same way.

Thus, the invention departs from the routes explored until now, which consisted in naturally obtaining, by the choice of material, higher emitter site densities. The invention allows the material to be selected from a wide range. In general, it suffices that the material in which the artificially created emitter sites are provided is insulating or of low electron affinity.

In addition to the additional degree of freedom offered by the choice of material, the invention allows higher site densities.

In addition, the invention makes it possible, in certain cases, to control the locations of artificially created sites. Indeed, in a preferred embodiment, the artificially created locations are predefined.

According to one embodiment, the materials in which artificial emitter sites are created are chosen from the group comprising: monocrystalline or polycrystalline diamond, carbon with a structure similar to diamond (DLC, diamond-like carbon), a material with low electronic affinity. based on carbon such as ta-C or ta-C: N, a very weakly conductive amorphous material such as a-Si, aC: H: or a-SiC, a material with a large forbidden band such as the nitride of aluminum, AlN, or gallium nitrate, GaN, and an insulating material such as magnesium oxide MgO or titanium oxide TiO2. The invention also relates to a method for producing a cathode in accordance with the invention.

This process is characterized in that the emitting sites are produced either by local modification of the conduction properties of an insulating material, or by local modifications of electronic affinity when a material with low electronic affinity is used.

according to one embodiment, the local modification is obtained by surface irradiation of zones of predefined locations with an electron or ion beam with a section between 1 and 100 nm.

Alternatively, heavy ions are used which provide artificial emitter sites with random positioning. In this case, the impact of each ion creates a site and the density of sites is then directly equal to the implanted dose. Thus, a dose of 1010 atoms / cm2 will induce 1010 sites / cm2, the sites being separated by an average distance of 10-5 cm, ie 100 nm.

Alternatively, the emitter sites are created using localized pulses of electric current. These pulses change the conduction properties of the layer or substrate where they are applied.

Whatever process is used, it creates, in addition to local surface modifications, local volume modifications, ie conductive channels are formed.

For carbon-based materials of type sp3, the conductive channels created are of type sp2.

In a very weakly conductive amorphous material, the conductive channels are crystalline channels. In this case, the conduction is improved by several orders of magnitude.

The conductive channels can also be produced by doping thanks to the implantation of doping atoms.

In the case of insulating materials, the conductive channels are for example the consequence of the creation of defects or of localized deoxidation. In one embodiment, the cathode is fabricated independently of the electron extraction grid. In another embodiment, the cathode is monolithic, that is to say that this cathode and the extraction grid are manufactured simultaneously.

In both cases, the production is either of the series type or of the parallel type. Serial type manufacturing consists of using a single beam scanning the surface on which it is desired to create emitting sites, this beam being activated at the predefined locations. Parallel manufacturing involves simultaneously producing a plurality of beams reaching predefined locations.

In the case of a monolithic embodiment, one or more other electrodes can be produced at the same time as the cathode and the extraction grid.

The invention also relates to a triode using a cathode in accordance with the invention. it has been found that such a triode could be used at frequencies of the order of 10 GHz, whereas until now it had been possible to reach frequencies of 4 GHz.

Other characteristics and advantages of the invention will become apparent with the description of some of its embodiments, the latter being made with reference to the accompanying drawings in which FIGS. 1 and 1a, 2, 3a to 3c, 4a to 4d, and 5a to 5c are diagrams illustrating methods, according to the invention, for manufacturing a cathode, this manufacturing being carried out independently of the extraction grid, FIGS. 6a to 6b, 7a to 7c, 8a to 8d, 9a to 9d and 10a to 10d are diagrams illustrating manufacturing processes, according to the invention, of cathodes and extraction grids, FIG. 11 illustrates a cathode according to the invention with a grid d 'extraction as well as another electrode, FIG. 12 is a diagram of a tube using a cathode according to the invention, FIG. 13 is a diagram of a tube similar to that of FIG. 12 but for a variant, FIG. 14 is a diagram of an electron gun comprising a cathod e according to the invention, FIGS. 15, 15a and 15b are diagrams of an oscillator tube comprising a cathode according to the invention, FIG. 16 is a diagram of a triode comprising a cathode according to the invention, and Figure 17 is a schematic of a triode for an alternative.

We will first of all describe in relation to FIGS. 1, 1a, 2, 3a to 3c, 4a to 4d, 5a to 5c several methods making it possible to manufacture an electron emission cathode, this manufacture being carried out independently of that of the electron extraction grid.

The production can be of the series type or of the parallel type. FIGS. 1, 1a and 2 illustrate production methods of the series type.

In the embodiment shown in FIGS. 1 and 1a, the starting point is an emitter material which, in general, is an insulating material or material with low electronic affinity, deposited in the form of a layer 20, of thickness 10. at 100 nm on a conductive substrate 22, for example of doped silicon or else any substrate (for example of glass) covered with a metal layer.

On layer 20, at predetermined locations, an electron or ion beam 24 is focused. Thus, at each location 261, 262, etc. on which the beam 24 has been focused, electron-emitting sites are created.

The areas irradiated by the beam 24 each have a substantially circular section with a diameter of 1 to 100 nm. The pitch between two successive zones is between 5 and 500 nm.

In general, in order to obtain an optimum field effect on each site, a pitch greater than or equal to twice the height of the conductive channels, that is to say the thickness of the layer, will preferably be chosen. 20. Otherwise, there would be a decrease in the peak effect of each channel and therefore a decrease in the total current emitted.

To obtain an irradiated zone with a diameter of between 1 and 100 nm, an electron or ion beam of the same size is used.

To create the irradiation electron beam, in one example, a beam of the type used in a transmission electron microscope is used. Such a beam allows a diameter of the order of 1 nm with a current of 1 nA. In this case, if the pitch between two impacts, that is to say between two determined locations, is 5 nm, emissive site densities of 4.1012 / Cm2 are obtained.

If we use a beam of size 5 nm of the type provided by the electronic maskers used for high-resolution electron lithography, with a step of 20 nm, the emissive site densities are then 2.5x101l / cm2 .

In the embodiment shown in FIG. 2, instead of using an electron beam or an ion beam, a tip 30 is used, the contact section of which with the layer 20 has a diameter of less than 20. nm and this tip is associated with a generator (not shown) of electrical pulses 31 which produces the pulses at predetermined locations 261, 262. Each pulse modifies the electrical properties of the emitting layer 20, that is to say transforms the corresponding locations in conductive and / or high electron affinity zones.

In one example: the section of the end of the tip 30 has a diameter of 10 nm, the material of the layer 20 is an amorphous carbon layer with a thickness of 20 nm, the pitch is 50 nm and the electrical pulses 31 have an amplitude of <B> 10 </B> V.

A description will now be given in relation to FIGS. 3a to 3c, 4a to 4d and 5a to 5c, a method of manufacturing an electron-emitting cathode in which the sites are produced simultaneously, that is to say in parallel.

In the example shown in Figures 3a and 3c, the sites are produced using electron or ion beams.

The first step of the process consists in depositing a protective mask 32 (FIG. 3a) on the layer 20. This mask, with a thickness of 100 to 1000 nm, has openings 36 with a diameter of between 50 and 100 nm separated by a pitch. from 200 to 500 nm. The material constituting the protective mask 32 is for example a resin of the type of those commonly used in high resolution lithography processes. This material can also be a heavy metal (for example molybdenum or tungsten) in order to block high energy electron or ion beams.

During a second step (FIG. 3b), the layer 20 is exposed to a parallel beam 34 of ions or electrons through the openings 36 of the mask 32.

In this way, electron emitting sites 381, 382, etc. are obtained. in line with the apertures 36 with a predefined density, that of the apertures of the mask 32. With apertures 36 with a diameter of 50 to 100 nm and a pitch of 200 to 500 nm, the density of emitter sites can reach 1010 / cm 2.

These values are, of course, given only by way of example. Electronic lithography techniques can in fact be used which allow openings with a diameter of 10 nm with a minimum spacing between patterns of between 30 and 50 nm. In this case, the density of emitting sites is significantly higher.

During a last step, the protective mask 32 is removed, for example by chemical attack. An electron-generating layer 20 is thus obtained (FIG. 3c) with emitter sites 381, 382, ... created artificially.

In a variant (not shown), instead of an electron or ion beam 34, use is made of irradiation with heavy ions (for example antimony or xenon) or else aggregates ( for example C60) possibly multicharged, and therefore of very high energy, for example from 50 MeV to 1 GeV. In this case, the irradiation can be carried out directly on the layer 20 without using a mask 32. The emitter sites or conductive channels and / or high electron affinity are created along the trace of the ions or aggregates in the material. of the layer 20. In this case, the transmitting sites are not in predetermined positions. However, the density of such emitter sites can be controlled by the dose of heavy ions or aggregates.

In the embodiment shown in FIGS. 4a to 4d, the emitting sites are produced in parallel using an electric field.

For the implementation of this method, an insulating mask 40 is deposited on layer 20 (FIG. 4a) having openings with a diameter of between 50 and 100 nm with a pitch of 200 to 500 nm. This insulating mask is, for example, made of silica or of silicon nitride. Its thickness depends on the voltage that will be applied during the following steps. With a breakdown field of the order of 500 V / pm, an applied voltage of 100 volts, the insulation thickness will be of the order of 300 nm.

After the insulation 40 has been deposited, a metal layer 42 (FIG. 4b) of approximately one micron thickness is deposited. The metal of the layer 42 fills the openings of the insulating mask 40 and comes into contact, through these openings, with the layer 20.

After deposition of the layer 42, a pulse 44, for example of 100 volts, is applied to the latter. This pulse is, in one example, applied for a period of less than one second. The voltage and the duration are chosen so as to locally modify, in line with the openings of the layer 40, the electrical properties of the emitting material of the layer 20 (FIG. 4c). Finally, during a last step, the layers 40 and 42 are removed, for example by chemical attack, which leaves the layer 20 bare with conductive channels 461, 462 forming electron emitters.

In the variant shown in FIGS. 5a to 5c, instead of using an insulating mask 40 to define the sites, an array of conductive tips 501, 502, etc. is used. formed on the surface 52 of a conductive substrate 54 (FIG. 5a). This network of points 501, 502, etc. is applied to the surface of the layer 20 at the same time as applying an electrical pulse 56 (Figure 5b) to the conductive substrate 54.

As in the embodiment described above, the voltage of the pulse 56 and its duration are chosen to locally modify, in the sites located to the right of the points, the conduction properties of the emitting material of the layer 20. As in the previous case, we obtain a network 461, 462, etc. conductive channels (Figure 5c). The density is equal to the density of the network of points.

a description will now be given in relation to FIGS. 6a and 6b, 7a to 7c, 8a to 8d, 9a to 9d and 10a to lod, a method for manufacturing a monolithic type cathode consisting in simultaneously manufacturing, on the same substrate, the sources of electrons and the grid for extracting these electrons.

As in the embodiment described above (manufacture of electron generators independently of the extraction grid), it is possible either to use a series method, or to use a parallel method for the manufacture of the electron emitting sites.

FIGS. 6a and 6b represent a production process of the series type.

In this example, on the layer 20 of emitting material, an insulating mask 60 with a thickness of 100 to 500 nm is deposited; the mask 60 is made of silica or of silicon nitride. On the insulating layer 60, a conductive metal 62 with a thickness of between 50 and 300 nm is deposited. The metal layer 62 is etched, for example by lithography followed by chemical attack or reactive ionic attack, so as to create circular openings 66 with a diameter of 50 to 200 nm in this layer 62. Layer 62 is intended to constitute the grid of extraction.

The part of the insulating layer 60 located in line with the openings 66 is then removed, for example by chemical attack, so as to create cavities 64 of larger diameter than the openings 66 so as to avoid any interaction between the beam and the insulator, the interaction being able to create parasitic charges.

Finally, an electron or ion beam 70 is centered on each opening so as to irradiate the center of the layer 20 to the right of the corresponding opening 66, which creates the emitting sites 721, 722, etc.

When the layer 20 is insulating or very weakly conducting, the insulating layer 60 is not essential, that is to say that the gate 62 is deposited directly on the layer 20.

In the embodiments shown in Figures 7a to 7c, the fabrication of the sites is of the parallel type. In this example, there is provided (as described in relation with FIG. 6a), on the layer 20 of emitting material, an insulating layer 60 and on this insulating layer, a conductive layer 62 in which openings 74 are etched and under which there is an insulating layer. produces, by chemical attack, cavities 76 of the insulator 60. In this case, the openings 74 have a diameter of between 50 and 100 nm and the pitch between two neighboring openings 74 is between 200 and 500 nm.

Next (FIG. 7b), the conductive layer 62 is exposed to a parallel beam 78 of electrons or ions. The direction of the parallel beam is perpendicular to the face of the layer 62. Thus, the openings 74 constitute diaphragms which allow the beam 78 only to reach the layer 20 in line with the openings 74. It will be noted that, in this case, we have interest in limiting the diameter of the openings 74 to obtain a correct positioning of the emitting sites 801, 902 on the layer 20.

If the diameter of the openings 74 used to create the sites 801, 882, etc. is insufficient for normal operation, during a last step (FIG. 7c), these openings are enlarged so as to constitute openings 84 of larger diameter, for example between 100 and 200 nm.

As in the embodiment described above, if the material of the layer 20 is insulating or weakly conductive, the extraction grid 62 can be deposited directly on the layer 20.

In the example shown in FIGS. 8a to 8d, the self-aligned emitter sites are fabricated with the openings of the extraction grid.

To this end, during a first step (FIG. 8a), on the layer 20 of emitting material, an insulating layer 86 is deposited on which a metallic layer 88 intended to constitute the gate is deposited. This metallic layer 88 is covered by a resin layer 90.

The resin layer 90 is then scanned by an electron or ion beam 92 located at predetermined locations which creates, through the layers 86, 88 and 90, emitter sites 941, 942, etc. for example spaced at a pitch of 200 to 1000 nm.

The irradiation with the aid of the beam 92 also creates a modification of the resin layer 90 which can be developed to obtain openings 961, 962, etc. (FIG. 8b) at locations which naturally correspond to those of the transmitting sites 941, 942, etc.

These openings 961, ... of the resin layer 90 are used to make openings 981, ... in the metal layer 88 and cavities 1001 of the insulating layer 86, under the openings 981, .. (figure 8c). The illumination of the resin is used to make the openings 981, ... of the grid 88.

Finally (Figure 8d), the resin layer 90 is removed. This method allows easy alignment of the sites 941 with the openings 981 of the electron extraction gate 88.

As in the previously described embodiments, if the material of layer 20 is insulating or very weakly conductive, layer 88 is deposited directly on layer 20.

As a variant, the beam 92 is replaced by a multiplicity of localized beams. The electron or ion beam 92 can also be replaced by a multicharged heavy ion beam which provides random positioning. In this case, the realization is of the parallel type; it has the same advantage of self-aligning sites and grid openings.

FIGS. 9a to 9d illustrate a method of fabricating parallel type emitter sites using a voltage pulse.

According to this process, an insulating layer 104 is deposited on the layer 20 of emitting material on which a metallic layer 106 is deposited (FIG. 9a). The insulating layer 104 is for example made of silica or silicon nitride. In the layers 104 and 106, openings 1081, 1082, etc. are formed. with a diameter of 50 to 100 nm with a pitch of 200 to 500 nm.

The thickness of the insulating layer 104 depends on the voltage which will be applied next. Thus with a voltage of 100 volts, the thickness of the insulator is of the order of 300 nm if this insulator has a breakdown field of the order of 500 volts / gm.

After making the openings 1081, 1082, a metallic layer 110 is deposited on the layer 106 and in the openings 108i (FIG. 9b), and a pulse 112 is applied (FIG. 9c), for example with an amplitude of 100 volts. and lasting less than one second. In any case, the voltage and the duration are chosen so as to locally modify the electrical properties of the layer 20 in order to create conductive channels, as previously described. Finally (FIG. 9d), the metal layer 110 is eliminated and under the openings of the metal layer 106, constituting the electron extraction grid, cavities 114i are formed by chemical etching.

This method also allows correct alignment of the emitter sites with the openings of the gate 106.

In the embodiment which will now be described in relation to FIGS. 10a to 10d, the emitter sites are produced by doping a semiconductor material.

This process consists of starting with a single crystal p-type semiconductor substrate (or layer) 120, for example made of silicon or gallium arsenide, on which an intrinsic semiconductor layer 122 is formed by epitaxy or ion implantation. An insulating layer 124 which is covered by a metallic layer 126 is then deposited on the layer 122.

In the layer 126, for example by lithography, openings 1281, 1282, etc. are formed. (FIG. 10a), the diameter of which is between 50 and 100 nm with a pitch of 200 to 500 nm.

After making the openings 1281, 1282, etc., using a parallel ion beam 130, the metal layer 126 is irradiated with its openings 1281 to 1282, which allows the production of doped channels 1301, 1302, etc. . by ion implantation in the layer 122. These doped channels are of p type (FIG. 10b).

Preferably, the openings 1281, 1282 have a relatively small diameter, so as to control the locations of the doped zones 1301, 1302, etc. Under these conditions, it may then be necessary to enlarge these openings 1281, 1282, so as to produce openings 1321, 1322 of larger diameter (FIG. 10c). In this case, an isotropic type etching is used. For example, an etching inducing a reduction of 50 nm in the thickness of the layer 126 will result in an increase of 100 nm of the openings 132. After making the openings 1321, 1322, cavities 1341 are produced, for example by etching, for example. under the openings 1321 formed in the insulating layer 124 (FIG. 10d).

This process makes it possible to obtain doped monocrystalline channels which are self-aligned with the openings of the extraction grid.

The emitting layer 20 in which emitter sites 131i are implanted can be produced not only simultaneously with an extraction grid 136, but also with at least one other electrode 138 (FIG. 11), according to methods similar to those described above. . Thus, in the example shown in FIG. 11, on the grid 136 an insulator 140 has been deposited and the electrode 138 is formed on the insulator 140. The opening 142i of the electrode 138 is of a substantial diameter. ment greater than the diameter of the opening 144i of the grid 136.

The structure shown in FIG. 11 can be produced with all the types of emissive sites described above.

The cold field emission cathode in accordance with the invention can be used to produce electron tubes operating at frequencies, in particular of the order of 10 GHz.

Electron tubes using the cathode according to the invention will now be described with reference to FIGS. 12 to 17.

Fig. 12 shows a single gate multibeam inductive output tube.

This tube comprises a cathode 150 in accordance with the invention with which is associated a planar grid 152 to which the input high frequency energy is applied. The distance d between the grid 152 and the cathode 150 can, according to the invention, be chosen relatively independently of the method of manufacturing the cathode. It can therefore be low enough for the electric extraction field to be high at the cathode, so as to extract the desired current; it can also be large enough (for example 5 to 10 microns) so that the grid / cathode capacitance is low and does not short-circuit, or disturb, the microwave energy injected into the grid.

Electrons generated by sites 151i of cathode 150 are also extracted from the cathode / grid space; they are also modulated in current due to the microwave field prevailing in this space. They are then accelerated by the electric field prevailing between this assembly and the anode block 154, which is essentially constituted by a cavity 154, the bottom of which, thicker, collects the major part of the electrons.

These electrons modulated in current and accelerated under a few hundred or a few thousand volts then give up their kinetic energy in the form of electromagnetic energy in the cavity 154.

The output power is extracted at a side end 156 of the cavity 154.

In the variant shown in FIG. 13, we see a grid 158 comprising two parts located on two distinct surfaces, namely an active part 160 at a distance d2 from the surface of the cathode 150 and a second part 162 comprising the parts non-active gate 158, at a distance d1 from the surface 150 greater than the distance d2.

This structure makes it possible, on the one hand, to further reduce the grid / cathode capacitance and to operate at even higher frequencies, and, on the other hand, to bring the active parts 160 of the grid closer to the cathode, in order to strengthen the extraction electric field and increase the current or reduce the "input" microwave energy injected into the grid.

Each active part 160 of the grid is connected to the inactive part 162 by a flared part, in particular conical, 164. Preferably, the shape of these flared parts 164 is chosen to prevent the electron beams 170 from diverging under the effect. of the space charge and so that these beams pass well to the desired places through the active parts of the grid. Thus, the flared parts 164 form a wehnelt, that is to say an electronic lens.

Fig. 14 shows a single grid type electron gun. In this embodiment, the cathode 172 is made on a concave spherical surface, and the grid 174 has a similar shape. The openings of the grid 174 are, as described above, in line with the emitting sites of the cathode 172.

The shape of the surfaces 172 and 174 makes it possible to create a converging electron beam and therefore with a high current density which can be used in a microwave tube of conventional structure, for example a traveling wave tube (PTT).

In this context, we are dealing with a gun with a single control grid 174, this grid not intercepting the beam. In other words, unlike conventional electron guns, it is not necessary to provide a grid forming a mask to delimit emissive zones. In addition, there is no parasitic emission.

Finally, the gate 174 can receive control signals making it possible to modulate the emission of the electrons. The modulation of the beam is then carried out in the electron generator, whereas usually the generation and the modulation are carried out with two separate devices.

FIG. 15 represents an oscillator tube comprising a cathode in accordance with the invention. this oscillator tube is of the monotron type. The electrons 182 leaving the cathode / grid space immediately enter a resonant cavity 180 where they are accelerated by a direct voltage VKA, applied between the bottom of the cavity and the cathode. If the residence time t of these electrons in the cavity is large and such that, approximately, 2nn <2nFt <(2n + 1) n, there is oscillation at the frequency F, provided that the cavity resonates at this frequency F , or at a frequency close to F.

As a variant, provision is made, as in the example of FIG. 12, for the electrons to be accelerated in a gate / cavity space and to pass through the cavity at constant speed, in the absence of interactions.

In this variant, as in the embodiment shown in FIG. 15, when there is oscillation, the frequency is closely related to the residence time t, according to the above relation.

avec x = 1, y = 0 et z = k (entier), l'axe y étant parallèle aux faisceaux et les axes x, y et z étant perpendicu- laires entre eux. Si la cavité est cylindrique (figure 15b), le mode sera, par exemple, un mode

avec 6 = 0, r = 1 et z = 0, l'axe z étant parallèle aux faisceaux. The dimensions of the cavity and its coupling antenna 189 are such that the resonance mode exhibits significant electric field components parallel to the beams and located at the place where these beams pass. If the cavity is rectangular (figure 15a), the mode will be, for example, a mode

with x = 1, y = 0 and z = k (integer), the y axis being parallel to the beams and the x, y and z axes being perpendicular to each other. If the cavity is cylindrical (figure 15b), the mode will be, for example, a mode

with 6 = 0, r = 1 and z = 0, the z axis being parallel to the beams.

In this embodiment, provision is made to apply a bias voltage 184 to a gate 186, as well as an adjustable voltage 188 to the anode 190. The adjustable voltage 188 makes it possible to adjust the speed of the electrons and therefore the time t, ie the operating frequency of the oscillator tube.

Figure 16 shows a broadband triode using a cathode according to the invention. The configuration of this triode is similar to that of the tube shown in FIG. 15, but differs from it in that the assembly is of the amplifier type.

The configuration of the triode is such that the electron beam 196 crosses a maximum zone of the electric field at a given frequency f. The adjustment of the frequency f is obtained, in particular, by microwave short circuits 198.

In the variant shown in FIG. 17, the cathode in accordance with the invention is also used to constitute a broadband amplifying triode. In this case, the distance between the grid 202 and the cathode 204 is variable; the emitting surface of each emitting site 206 of the cathode 204 and the density of these sites is a function of the cathode-grid distance at the location where these sites are located. When the grid-cathode distance is large, the sites are wide but very spaced, the space between two sites corresponding to half a wavelength. On the contrary, when the grid-cathode distance is small, the sites are small but closely spaced and therefore dense. In this way, the emitted current density remains the same.

The variable distance between cathode 204 and grid 202 allows a wide band. The smaller distance corresponds to operation at higher frequencies and the higher distance corresponds to lower frequencies.

The invention can also be used for display applications.

The invention relates generally to a method of manufacturing an electron emitting cathode which is characterized in that it starts from an insulating or low electron affinity emitting material and in that the sites are artificially defined. emitters by localized creation of areas with high electronic affinity and / or conductors.

In one example, the artificially created sites constitute, by volume, conductive channels.

In this case, it is preferable for the ratio between the height of a site and the pitch between two sites to be less than or equal to 0.5.

The invention also relates to an electron emitting device comprising an emissive cathode and a grid which is characterized in that the emissive cathode has electron emitting sites separated from each other and created artificially and in that the electron extraction grid has openings in front of the sites.

according to one embodiment, the distance separating the grid from the emissive surface (204) of the cathode is variable.

In this case, preferably, the density of emitting sites on the cathode varies in the opposite direction to the distance from the grid. In this embodiment, it is advantageous for the emitted current density to be constant whatever the distance of the gate from the emissive surface.

Claims (9)

1. A method of manufacturing an electron emitting cathode, characterized in that one starts from an emitting material (20) which is insulating or has a low electronic affinity and in that the emitting sites (261, 262) are artificially defined. ) by localized creation of areas with high electronic affinity and / or conductors. 2. Method according to claim 1, characterized in that the density of the sites is predetermined. 3. Method according to claim 2, characterized in that the predetermined densities are between 108 and 1013 / cm2. 4. Method according to any one of claims 1 to 3, characterized in that the insulating material or with low electronic affinity is chosen from the group comprising monocrystalline or polycrystalline diamond, carbon with a structure similar to that of diamond, a material low carbon-based electronic affinity such as ta-C, ta-C: N, a very weakly conductive amorphous material such as a-Si, aC: H: or a-SiC or a wide bandgap material such as aluminum or gallium nitride, and an insulating material such as magnesium oxide MgO and titanium oxide TiO2. 5. Method according to any one of the preceding claims, characterized in that the conductive or high electron affinity sites are produced at predefined locations. 6. Method <B> according to </B> claim 5, characterized in that the sites are produced by irradiation of the surface of the insulating material or of low electron affinity with the aid of an electron beam or of ions. 7. Method according to claim 5, characterized in that the sites are produced by applying a pulse of electric current causing the local modification of the conduction properties of the material. 8. Method according to any one of claims 1 to 4, characterized in that the transmitting sites are produced with a random positioning. 9. The method of claim 8, characterized in that the sites are produced by irradiation of the insulating material or of low electron affinity using heavy ions or aggregates. <B> 10. </B> Method according to any one of the preceding claims, characterized in that the artificially created sites constitute, by volume, conductive channels. he. Process according to Claim 10, characterized in that the insulating material with low electron affinity is of the sp3 type and the conductive channels are of the sp2 type, the insulating or low electron affinity material being based on carbon. 12. The method of claim 10, characterized in that the conductive channels are crystalline, the emitting material being amorphous and very weakly conductive. 13. The method of claim 10, characterized in that the conductive channels are produced by doping. 14. Method according to .revendication 10, characterized in that the conductive channels are produced by creating defects. 15. The method of claim 10, characterized in that the conductive channels are produced by localized deoxidation, the emitting material being insulating. 16. Method according to any one of the preceding claims, characterized in that each site extends over a substantially circular zone with a diameter of between 1 and 100 nm. 17. Method according to any one of the preceding claims, characterized in that the ratio between the height of a site and the pitch between two sites is less than or equal to 0.5. 18. Method <B> according to </B> claim 5, characterized in that to create the sites use is made of a focused electron or ion beam (24) which is applied sequentially to the surface of the emitting material at locations predefined. 19. The method of claim 5, characterized in that the locations of the sites are defined using a mask (32; 62) disposed in front of the emitting material and in that this emitting material is irradiated with the. using an electronic or ion beam (34) parallel through the holes of the mask. 20. The method of claim 7, characterized in that the positions of the sites are defined by a mask (40) with holes arranged close to the surface of the emitting material, in that a conductive layer (42) is applied in them. holes in the insulating mask and in that an electrical pulse (44) is applied to this conductive material so as to create the sites in the emitting material. 21. The method of claim 7, characterized in that the sites are produced using a tip (30) applied at predetermined locations against the emitting material, a voltage (32) being applied to the tip to achieve sites at predetermined locations. 22. The method of claim 7, characterized in that the sites are produced using a set of spikes (501, 502) applied against the surface of the emitting material, a pulse (56) being applied to the assembly. points. 23. Method according to any one of the preceding claims, characterized in that an extraction grid is produced simultaneously with the cathode, this grid being constituted by a metal layer having openings which are used to produce the sites. 24. The method of claim 23, characterized in that the sites being produced using an electron or ion beam (78) of parallel type, for irradiation, openings of the grid of a first are made. dimension and after irradiation, the diameter of the grid openings is increased. 25. The method of claim 19 or 20, characterized in that the sites are produced using an electron or ion beam applied to a resin layer (90) covering a metal layer (88) intended to constitute the grid, the beam causing a modification of the resin and creating the site, the illumination of the resin being used to make the openings of the grid. 26. The method of claim 13, characterized in that the doped channels are produced by ion implantation. 27. The method of claim 26, characterized in that the doped channels (1301, 1302) are formed in an intrinsic semiconductor layer (122) covering a p-type semiconductor (120). 28. Method <B> according to </B> claim 23, characterized in that there is provided an insulator between the emitting material and the gate and in that a cavity (1001) is formed in this insulator, in line with each grid opening, opposite each site of the emitting material. 29. Electron emitting device comprising an emissive cathode and a grid, characterized in that the emissive cathode has electron emitting sites separated from each other and created artificially and in that the electron extraction grid comprises openings in front of the sites. 30. Device according to claim 29, characterized in that the openings of the grid are located on a surface (160) at a first distance (d2) from the cathode (150) and in that the parts of the grid not comprising opening are on a second surface (162) at a second distance (dl) from the emitting surface, greater than the first distance. 31. Device according to claim 30, characterized in that each part of the grid having at least one opening and associated with at least one emitter site is connected to the rest of the grid without opening by means of a conical part. (164) arranged to constitute a Wehnelt. 32. Device according to any one of claims 29 to 31, characterized in that the distance separating the grid (202) from the emissive surface (204) of the cathode is variable. 33. Device according to claim 32, characterized in that the density of emitter sites on the cathode varies in the opposite direction to the distance from the grid. 34. Device <B> according to </B> claim 33, characterized in that the emitted current density is constant whatever the distance from the grid to the emissive surface.