EP1249028B1 - Electron generating cathode and method for the production thereof - Google Patents

Description

Description for the following Contracting States: AT, BE, CH, CY, ES, LI

The invention relates to an electron generating cathode and 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 an emission of electrons by application of a weak field, at most of a few tens of volts per micrometer.

Although the origin of these emissions is not clearly established, it is generally accepted that the emission comes from sites consisting of very small areas whose largest dimension in the emission surface would be a few nanometers to about 100 nm, each site may correspond to an abrupt transition between a high electron affinity material and a low electron affinity material.

It is recalled here that the electronic affinity of a material is a quantity characterizing the possibility of electron emission from the surface of the material. For carbon-based materials, the transitions between low and high electron affinity materials are between a sp ² phase and an sp ³ phase.

These transitions may also correspond to transitions between a conductive phase and an insulating phase; in the case of a carbon-based material, the sp ² phase is conductive and the sp ³ phase is insulating.

According to a first model (Gröning et al., Applied Physics Letters 71,2253, 1997) the sites would be sp ² conductive channels of size 10 to 100 nm in an insulating sp ³ matrix and the electron emission would come from a peak effect. . According to a second model (J. Robertson et al., Diamond and related materials 7, 620, 1998) an emitting site would correspond to a surface variation, localized over a distance of about 10 nm, of the electronic affinity. According to a third model (MW Geis et al., Nature, 393, 431, 1998) in diamond the electron emission originates from triple metal / diamond / vacuum junctions.

To increase the electronic emission, one therefore seeks to increase the density of emission sites. To date, the best results provide site densities of the order of 10 ⁶ / cm ² , that is to say 10 ^-2 / μm ² . These values are too low to obtain sufficient emission current densities, at least equal to 0.1 A / cm ² . Moreover, the location of the emitting sites is not predictable, which can be a disadvantage for practical applications.

However, W. Zhu et al. (Appl. Phys. Lett. 75, p. 873 1999) were able to achieve carbon cathodes form of nanotubes with an emissive site density of the order of 10 ⁷ / cm ^2, the current density obtained being the order of 0.5 A / cm ² ; but, this value is still too low for this material to be usable in practice.

The method of manufacturing an electron-emitting cathode as described in WO 96 25753 uses any electrode surface in which geometric irregularities are formed. These are the irregularities that emit the electrons. This method is limited by the density of irregularities that can be obtained and does not allow to obtain large current densities emitted by the cathode.

The invention according to claims 1-32 overcomes these disadvantages. It makes it possible to reach emission site densities of the order of 10 ¹² / cm ² and thus to increase by several orders of magnitude the current densities.

The cathode according to the invention is characterized in that it comprises a layer or a substrate made of an insulating material or with low electronic affinity and artificially generated electron emitter sites, these sites preferably having a predetermined density of between 10 ⁸ and 10 ¹³ / cm ² . These sites are created from areas with high electronic affinity and / or conductive. It is believed (although the invention is not limited to this interpretation) that the emission of electrons (the sites) is effected at the transition between the areas of high electron affinity and / or conductive and the conductive layer or substrate or at low electronic affinity. Thus, the emitter sites would be constituted by the transition between the zones and the rest of the layer or substrate. However, in what follows, for simplicity, we will sometimes designate the same sites and areas.

Thus, the invention departs from the routes explored so far, which consisted in naturally obtaining, by the choice of the material, densities of higher emitter sites. The invention makes it possible to select the material in a wide range. It is sufficient, in general, that the material in which artificially created transmitter sites are provided is insulating or of low electronic affinity.

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

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, diamond-like carbon (DLC), a low-affinity electronic material 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 bandgap such as aluminum nitride, AlN , or gallium nitride, GaN, and an insulating material such as MgO magnesium oxide or TiO ₂ titanium oxide.

The invention also relates to a method for producing a cathode according to the invention.

This method is characterized in that the emitting sites are made either by local modification of the conduction properties of an insulating material, or by local modifications of electronic affinity when using a material with low electronic affinity. The method does not require geometric modification of the surface of the cathode which can remain locally substantially flat. Unlike other embodiments using as angular areas emitter site including sharp points or sharp edges, the location of the sites is independent of the geometry of the surface of the cathode. It is possible to define mathematically a surface, in this case that of the cathode, locally substantially flat. It is a substantially continuous surface and does not have a derivative break at the scale of at least a few nanometers.

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

Advantageously, the electron or ion beam is produced in a vacuum chamber where there is a pressure of the type generally used in vacuum electronic tubes. This pressure is generally less than a few 133.10 ^-5 Pascals (10 ^-5 torr). It is recalled that the torr is a unit of pressure substantially equal to 133 Pascals. The torr also corresponds to the pressure exerted by a mercury column of height equal to 1 mm. In fact, the more the vacuum is pushed (the lower the pressure), the more precise the electron or ion beam will be. This makes it possible to increase the density of the electron emitting sites. This also makes it possible to control the density of the electron emitting sites.

Irradiation provides energy to the surface to modify its electronic affinity. In the case of an ionic irradiation, whatever the nature of the chosen ion, it is the energy that it brings to the surface which is determining.

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 10 ¹⁰ atoms / cm ² will induce 10 ¹⁰ sites / cm ² , the sites being separated by an average distance of 10 ^-5 cm, i.e. 100 nm.

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

Whatever the method used, it creates, in addition to local modifications on the surface, local changes in volume, namely that forming conductive channels.

For carbon-based materials of sp ³ type, the conductive channels created are sp ² type.

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

The conductive channels can also be made by doping by implanting doping atoms.

In the case of insulating materials, the conducting channels are for example the consequence of the creation of defects or localized deoxidation.

In one embodiment, the cathode is manufactured independently of the electron extraction grid. In another embodiment, the cathode is monolithic, i.e., this cathode and the extraction grid are manufactured simultaneously.

In both cases, the manufacture is either of series type or of parallel type. Serial type manufacturing consists of using a single beam sweeping the surface on which we want to create transmitting sites, this beam being activated at predefined locations. Parallel fabrication involves simultaneously producing a plurality of beams reaching predefined locations.

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

The invention also relates to a triode using a cathode according to the invention. It has been found that such a triode can be used at frequencies of the order of 10 GHz whereas up to now it has been possible to reach frequencies of 4 GHz.

• les figures 1 et 1a, 2, 3a à 3c,4a à 4d, et 5a à 5c sont des schémas illustrant des procédés, conformes à l'invention, de fabrication de cathode, cette fabrication étant effectuée indépendamment de la grille d'extraction,
• les figures 6a à 6b, 7a à 7c, 8a à 8d, 9a à 9d et 10a à 10d sont des schémas illustrant des procédés de fabrication, conformes à l'invention, de cathodes et de grilles d'extraction,
• la figure 11 illustre une cathode conforme à l'invention avec une grille d'extraction ainsi qu'une autre électrode,
• la figure 12 est un schéma d'un tube utilisant une cathode conforme à l'invention,
• la figure 13 est un schéma d'un tube analogue à celui de la figure 12 mais pour une variante,
• la figure 14 est un schéma d'un canon à électrons comportant une cathode conforme à l'invention,
• les figures 15, 15a et 15b sont des schémas d'un tube oscillateur comportant une cathode conforme à l'invention,
• la figure 16 est un schéma d'une triode comportant une cathode conforme à l'invention, et
• la figure 17 est un schéma d'une triode pour une variante.

Other characteristics and advantages of the invention will become apparent with the description of some of its embodiments, this being done with reference to the attached drawings in which:

• FIGS. 1 and 1a, 2, 3a to 3c, 4a to 4d, and 5a to 5c are diagrams illustrating cathode manufacturing methods according to the invention, this fabrication 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 methods of manufacture, according to the invention, of cathodes and extraction grids,
• FIG. 11 illustrates a cathode according to the invention with an extraction grid 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 cathode 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
• Fig. 17 is a diagram of a triode for a variant.

First of all, in connection with FIGS. 1, 1a, 2, 3a to 3c, 4a to 4d, 5a to 5c, several methods for producing an electron-emitting cathode will be described, this fabrication being carried out independently of that of the electron extraction grid.

The manufacture may be of the series type or of the parallel type. Figures 1, 1a and 2 illustrate series type manufacturing processes.

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

On the layer 20, at predetermined locations, an electron or ion beam is focused 24. Thus, at each location 26 ₁ , 26 ₂ , etc. on which the beam 24 has been focused, electron-emitting sites are created.

The zones 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 optimal field effect at each site, a step greater than or equal to twice the height of the conductive channels, that is to say the thickness of the layer 20, will preferably be chosen. Otherwise, there would be a reduction in the peak effect of each channel and thus 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 electron beam of irradiation, in one example, a beam of the type used is used. in a transmission electron microscope. 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 specific locations, is 5 nm, we obtain emitting site densities of 4.10 ¹² / cm ² .

If a 5 nm size beam of the type provided by the electronic maskers used for high-resolution electronic lithography, with a pitch of 20 nm, is used, the emissive site densities are then 2.5 × 10 ¹¹ / cm ² .

In the embodiment shown in FIG. 2, instead of using an electron beam or an ion beam, a tip 30 is used whose contact section with the layer 20 has a diameter less than 20. and this tip is associated with a generator (not shown) of electrical pulses 31 which produces the pulses at predetermined locations 26 ₁ , 26 ₂ . Each pulse modifies the electrical properties of the emitter layer 20, that is to say transforms the corresponding locations into conductive areas and / or high electron affinity.

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 20 nm thick, the pitch is 50 nm and the electrical pulses 31 have an amplitude of 10 V.

FIGS. 3a to 3c, 4a to 4d and 5a to 5c will now describe 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 made using electron or ion beams.

The first step of the method 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 step of 200 to 500 nm. The material constituting the protective mask 32 is for example a resin of the type commonly used in high resolution lithography processes. This material can also be a heavy metal (for example molybdenum or tungsten) in order to block the high energy electron or ion beams.

In 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.

Electron emitter sites 38 ₁ , 38 ₂ , etc. are thus obtained. at the openings 36 with a predefined density, that of the openings of the mask 32. With apertures 36 of diameter 50 to 100 nm and a pitch of 200 to 500 nm, the density of emitter sites can reach 10 ¹⁰ / cm ² .

These values are of course only given by way of example. It is indeed possible to use electronic lithography techniques allowing 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 transmitting sites is substantially greater.

During a last step, the protective mask 32 is eliminated, for example by etching. An electron-generating layer 20 is thus obtained (FIG. 3c) with emitter sites 38 ₁ , 38 ₂ , ... created artificially.

In a variant (not shown), in place of an electron or ion beam 34, irradiation with heavy ions (for example of antimony or xenon) or aggregates (by example C ₆₀ ) optionally multicharged, and therefore 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 emitting sites or conducting channels and / or high electron affinity are created along the trace 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 transmitter sites is controllable by the dose of heavy ions or aggregates.

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

For the implementation of this method, is deposited on the layer 20 an insulating mask 40 (Figure 4a) having openings of diameter between 50 and 100 nm with a pitch of 200 to 500 nm. This insulating mask is, for example, silica or 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 / μm, an applied voltage of 100 volts, the insulation thickness will be of the order of 300 nm.

After the deposition of the insulator 40, depositing a metal layer 42 (Figure 4b) of thickness about one micron. 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, the latter is applied to a pulse 44, for example 100 volts. This pulse is, in one example, applied for a duration of less than one second. The voltage and the duration are chosen so as to locally modify, at the right of the openings of the layer 40, the electrical properties of the material emitting the layer 20 (FIG. 4c).

Finally, during a last step, the layers 40 and 42 are removed, for example by etching, leaving the layer 20 with conducting channels 46 ₁ , 46 ₂ forming electron emitters.

In the variant shown in FIGS. 5a to 5c, instead of using an insulating mask 40 to define the sites, use is made of a network of conductive tips 50 ₁ , 50 ₂ , etc. formed on the surface 52 of a conductive substrate 54 (Figure 5a). This network of tips 50 ₁ , 50 ₂ , etc. is applied on the surface of the layer 20 at the same time as an electric pulse 56 (FIG. 5b) is applied to the conductive substrate 54.

As in the embodiment previously described, the voltage of the pulse 56 and its duration are chosen to locally modify, in the sites located at the right of the tips, the conduction properties of the material emitting layer 20. As in the previous case, we obtain a network 46 ₁ , 46 ₂ , etc. conductive channels (Figure 5c). The density is equal to the density of the peak network.

FIGS. 6a and 6b, 7a to 7c, 8a to 8d, 9a to 9d and 10a to 10d will now describe a method for manufacturing a monolithic type cathode consisting of simultaneously producing, on the same substrate, the sources of electrons and the grid of extraction of these electrons.

As in the previously described embodiment (manufacture of electron generators independently of the extraction grid), one can either use a series process, or use a parallel process for the manufacture of electron emitting sites.

Figures 6a and 6b show a series type manufacturing process.

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

The part of the insulating layer 60 which is situated at the right of the openings 66 is then removed, for example by chemical etching, so as to create cavities 64 of greater diameter than the openings 66 so as to avoid any interference. between the beam and the insulator, the interaction can create parasitic loads.

Finally, an electron or ion beam 70 is centered on each opening so as to irradiate the center of the layer 20 at the right of the corresponding opening 66, which creates the emitting sites 72 ₁ , 72 ₂ , etc.

When the layer 20 is insulating or very weakly conductive, 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 FIGS. 7a to 7c, the fabrication of the sites is of the parallel type.

In this example, provision is made (as described with reference to FIG. 6a), on the layer 20 of emitting material, an insulating layer 60 and on this insulating layer, a conductive layer 62 in which apertures 74 are etched and under which one performs, by chemical etching, 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 adjacent openings 74 is between 200 and 500 nm.

Next (FIG. 7b), 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 to reach only the layer 20 in line with the openings 74. It should be noted that in this case, interest in limiting the diameter of the openings 74 to obtain a correct positioning of the emitter sites 80 ₁ , 90 ₂ on the layer 20.

If the diameter of the openings 74 used to create the sites 80 ₁ , 88 ₂ , etc. is insufficient for normal operation, in a last step (Figure 7c), these openings are enlarged to form openings 84 of larger diameter, for example between 100 and 200 nm.

As in the embodiment previously described, 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, self-aligned emitter sites are manufactured with the openings of the extraction grid.

For this purpose, during a first step (Figure 8a), on the layer 20 of emitting material, is deposited an insulating layer 86 on which is deposited a metal layer 88 for constituting the gate. This metal 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, emitting sites 94 ₁ , 94 ₂ , etc. for example spaced in a pitch of 200 to 1000 nm.

Irradiation using beam 92 also creates a modification of resin layer 90 which can be developed to obtain apertures 96 ₁ , 96 ₂ , etc. (FIG. 8b) at locations that naturally correspond to those of the transmitter sites 94 ₁ , 94 ₂ , and so on.

These openings 96 _1, ... 90 of the resin layer are utilized to produce openings 98 _1, ... in the metal layer 88 and cavities 100 ₁ of the insulating layer 86 below the openings 98 ₁ , ... (Figure 8c). The illumination of the resin is used to make the openings 98 ₁ , ... of the grid 88.

Finally (Figure 8d), the resin layer 90 is removed.

This method allows easy alignment of the sites 94 ₁ with the openings 98 ₁ of the electron extraction gate 88.

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

In 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 that provides a random positioning. In this case, the realization is of parallel type; it has the same advantage of self-alignment of the sites and openings of the grid.

FIGS. 9a to 9d illustrate a method of manufacturing parallel type transmitter sites by means of a voltage pulse.

According to this method, an insulating layer 104 is deposited on the emitter layer 20 on which a metal layer 106 is deposited (FIG. 9a). The insulating layer 104 is for example silica or silicon nitride. In the layers 104 and 106, openings 108 ₁ , 108 ₂ , 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 that will be applied thereafter. 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 / micron.

After the openings 108 ₁ , 108 _{2 have been} made, a layer 110 (FIG. 9b) is deposited on the layer 106 and in the openings 108i, and a pulse 112 (FIG. 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 conducting channels, as previously described.

Finally (Figure 9d), the metal layer 110 is removed and under the openings of the metal layer 106, constituting the electron extraction grid is formed by etching cavities 114 _i .

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

In the embodiment that will now be described with reference to FIGS. 10a to 10d, the emitting sites are made by doping a semiconductor material.

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

In the layer 126, there are formed, for example by lithography, openings 128 ₁ , 128 ₂ , etc. (Figure 10a) whose diameter is between 50 and 100 nm with a pitch of 200 to 500 nm.

After making the openings 128 ₁ , 128 ₂ , etc., the metal layer 126 with its openings 128 ₁ to 128 ₂ is irradiated with a parallel ion beam 130, which allows the production of doped channels 130 ₁ , 130 ₂ , etc. by ion implantation in the layer 122. These doped channels are p-type (Figure 10b).

Preferably, the openings 128 ₁ , 128 ₂ have a relatively small diameter, so as to control the locations of the doped areas 130 ₁ , 130 ₂ , etc. Under these conditions, it may then be necessary to enlarge these openings 128 ₁ , 128 ₂ , so as to make openings 132 ₁ , 132 ₂ of larger diameter (Figure 10c). In this case, an isotropic etching is used. For example, etching inducing a 50 nm decrease in the thickness of layer 126 will result in a 100 nm increase in apertures 132.

After the openings 132 ₁ , 132 ₂ have been made, cavities 134 ₁ are made, for example by etching, under the openings 132 ₁ formed in the insulating layer 124 (FIG. 10d).

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

The emitter layer 20 in which emitter sites 131 _i 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. previously. Thus, in the example shown in FIG. 11, on the gate 136 an insulator 140 has been deposited and the electrode 138 is formed on the insulator 140. The opening 142 _i of the electrode 138 is of a diameter substantially greater than the diameter of the opening 144 _i of the grid 136.

The structure shown in FIG. 11 is feasible with all the types of emissive sites previously described.

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

We will now describe in relation to FIGS. 12 to 17, electronic tubes using the cathode according to the invention.

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

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

The electrons generated by the sites 151 _i of the cathode 150 are also extracted from the cathode / gate space; they are also current modulated because of 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 consists essentially of a cavity 154, the bottom, thicker, collects most of the electrons.

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

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

In the variant shown in FIG. 13, there is provided a gate 158 comprising two parts lying along two distinct surfaces, namely an active part 160 at a distance d ₂ from the surface of the cathode 150 and a second part 162 comprising the parts non-active of the grid 158 at a distance d ₁ from the surface 150 greater than the distance d _2.

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

Each active portion 160 of the gate is connected to the inactive portion 162 by a flared portion, particularly conical, 164. Preferably, the shape of these flared portions 164 is chosen to prevent the electron beams 170 diverge under the effect the space charge and for these beams to pass through the active parts of the grid at the desired locations. Thus, the flared portions 164 form a wehnelt, that is to say an electronic lens.

Figure 14 shows a single gate type electron gun. In this embodiment, the cathode 172 is formed on a concave spherical surface, and the gate 174 has a similar shape. The openings of the gate 174 are, as described above, to the right of the emitter sites of the cathode 172.

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

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

Finally, the gate 174 can receive control signals for modulating the emission of electrons. The modulation of the beam is then carried out in the electron generator, whereas usually the generation and modulation take place with two separate devices.

FIG. 15 represents an oscillator tube comprising a cathode according to the invention. This oscillator tube is of the monotron type. The electrons 182 leaving the cathode / grid space immediately return to a resonant cavity 180 where they are accelerated by a DC voltage V _KA , 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 2nπ <2nFt <(2n + 1) π, there is oscillation at the frequency F, as long as the cavity resonates at this frequency F , or at a frequency close to F.

In a variant, it is provided, as in the example of FIG. 12, that the electrons are accelerated in a grid / cavity space and pass through the cavity at a 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 relationship.

avec x = 1, y = 0 et z = k (entier), l'axe y étant parallèle aux faisceaux et les axes x, y et z étant perpendiculaires entre eux. Si la cavité est cylindrique (figure 15b), le mode sera, par exemple, un mode ${\mathrm{TM}}_{\mathrm{\theta rz}}^{\mathrm{O}}$

avec θ = 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 has components electrical field parallel to the beams and located where these beams pass. If the cavity is rectangular (Figure 15a), the mode will be, for example, a ${\mathrm{YOU}}_{\mathrm{X\; Y\; Z}}^{\otimes },$

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 ${\mathrm{TM}}_{\mathrm{\theta rz}}^{\mathrm{O}}$

with θ = 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 on a gate 186, as well as an adjustable voltage 188 on the anode 190. The adjustable voltage 188 makes it possible to adjust the speed of the electrons and therefore the time t, ie the frequency of operation of the oscillator tube.

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

The configuration of the triode is such that the electron beam 196 passes through 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 according to the invention is also used to constitute a broadband amplifying triode. In this case, the distance between the gate 202 and the cathode 204 is variable; the emitting surface of each transmitter site 206 of the cathode 204 and the density of these sites is a function of the cathode-grid distance at the location of these sites. When the gate-cathode distance is large, the sites are wide but widely 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 not very spaced and therefore dense. In this way, the current density emitted remains the same.

The variable distance between cathode 204 and gate 202 allows a wide band. The lowest distance corresponds to operation at the highest frequencies and the highest distance at the lowest frequencies.

The invention can also be used for display applications.

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

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

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

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

According to one embodiment, the distance separating the gate from the emitting surface (204) of the cathode is variable.

In this case, preferably, the density of transmitter sites on the cathode varies in the opposite direction of the distance to the gate.

In this embodiment, it is advantageous for the current density emitted to be constant regardless of the distance from the gate to the emitting surface.

Description for the following Contracting States: FR, GB, DE

The invention relates to an electron generating cathode and 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 an emission of electrons by application of a weak field, at most of a few tens of volts per micrometer.

Although the origin of these emissions is not clearly established, it is generally accepted that the emission comes from sites consisting of very small areas whose largest dimension in the emission surface would be a few nanometers to about 100 nm, each site may correspond to an abrupt transition between a high electron affinity material and a low electron affinity material.

It is recalled here that the electronic affinity of a material is a quantity characterizing the possibility of electron emission from the surface of the material. For carbon-based materials, the transitions between low and high electron affinity materials are between a sp ² phase and an sp ³ phase.

These transitions may also correspond to transitions between a conductive phase and an insulating phase; in the case of a carbon-based material, the sp ² phase is conductive and the sp ³ phase is insulating.

According to a first model (Groning et al., Applied Physics Letters 71,2253, 1997) the sites would be sp ² conductive channels of size 10 to 100 nm in an insulating sp ³ matrix and the electron emission would come from a peak effect. . According to a second model (J. Robertson et al., Diamond and related materials 7, 620, 1998) an emitting site would correspond to a surface variation, localized over a distance of about 10 nm, of the electronic affinity. According to a third model (MW Geis et al., Nature, 393, 431, 1998) in diamond the electron emission originates from triple metal / diamond / vacuum junctions.

To increase the electronic emission, one therefore seeks to increase the density of emission sites. To date, the best results provide site densities of the order of 10 ⁶ / cm ² , that is to say 10 ^-2 / μm ² . These values are too low to obtain sufficient emission current densities, at least equal to 0.1 A / cm ² . Moreover, the location of the emitting sites is not predictable, which can be a disadvantage for practical applications.

However, W. Zhu et al. (Appl. Phys. Lett. 75, p. 873 1999) were able to achieve carbon cathodes form of nanotubes with an emissive site density of the order of 10 ⁷ / cm ^2, the current density obtained being the order of 0.5 A / cm ² ; but, this value is still too low for this material to be usable in practice.

The method of manufacturing an electron-emitting cathode as described in WO 96 25753 uses any electrode surface in which geometric irregularities are formed. These are the irregularities that emit the electrons. This method is limited by the density of irregularities that can be obtained and does not allow to obtain large current densities emitted by the cathode.

The invention according to claims 1-31 overcomes these disadvantages. It makes it possible to reach emission site densities of the order of 10 ¹² / cm ² and thus to increase by several orders of magnitude the current densities.

The cathode according to the invention is characterized in that it comprises a layer or a substrate made of an insulating material or with low electronic affinity and artificially created electron-emitting sites, these sites having, preferably, a predetermined density of between 10 ⁸ and 10 ¹³ / cm ² . These sites are created from areas with high electronic affinity and / or conductive. It is believed (although the invention is not limited to this interpretation) that the emission of electrons (the sites) is effected at the transition between the areas of high electron affinity and / or conductive and the conductive layer or substrate or at low electronic affinity. Thus, the emitter sites would be constituted by the transition between the zones and the rest of the layer or substrate. However, in what follows, for simplicity, we will sometimes designate the same sites and areas.

Thus, the invention departs from the routes explored so far, which consisted in naturally obtaining, by the choice of the material, densities of higher emitter sites. The invention makes it possible to select the material in a wide range. It is sufficient, in general, that the material in which artificially created transmitter sites are provided is insulating or of low electronic affinity.

in addition to the additional degree of freedom afforded by the choice of material, the invention allows densities of higher sites.

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, diamond-like carbon (DLC), a low-affinity electronic material 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 bandgap such as aluminum nitride, AlN , or gallium nitride, GaN, and an insulating material such as MgO magnesium oxide or TiO ₂ titanium oxide.

The invention also relates to a method for producing a cathode according to the invention.

This method is characterized in that the emitting sites are made either by local modification of the conduction properties of an insulating material, or by local modifications of electronic affinity when using a material with low electronic affinity. The method does not require geometric modification of the surface of the cathode which remains locally substantially flat. Unlike other embodiments using as angular areas emitter site including sharp points or sharp edges, the location of the sites is independent of the geometry of the surface of the cathode. It is possible to define mathematically a surface, in this case that of the cathode, locally substantially flat. It is a substantially continuous surface and does not have a derivative break at the scale of at least a few nanometers.

According to one embodiment, the local modification is obtained by surface irradiation of predefined location zones with an electron beam of section between 1 and 100 nm.

Advantageously, the electron beam is produced in a vacuum chamber where there is a pressure of the type generally used in vacuum electronic tubes. This pressure is generally less than a few 133.10 ^-5 Pascals (10 ^-5 torr). It is recalled that the torr is a unit of pressure substantially equal to 133 Pascals. The torr also corresponds to the pressure exerted by a mercury column of height equal to 1 mm. Indeed, the more the vacuum is pushed (the lower the pressure), the more precise the electron beam will be. This makes it possible to increase the density of the electron emitting sites. This also makes it possible to control the density of the electron emitting sites.

Irradiation provides energy to the surface to modify its electronic affinity. In the case of an ionic irradiation, whatever the nature of the chosen ion, it is the energy that it brings to the surface which is determining.

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 10 ¹⁰ atoms / cm ² will induce 10 ¹⁰ sites / cm ² , the sites being separated by an average distance of 10 ^-5 cm, i.e. 100 nm.

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

Whatever the method used, it creates, in addition to local modifications on the surface, local changes in volume, namely that forming conductive channels.

For carbon-based materials of sp ³ type, the conductive channels created are sp ² type.

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

The conductive channels can also be made by doping by implanting doping atoms.

In the case of insulating materials, the conducting channels are for example the consequence of the creation of defects or localized deoxidation.

In one embodiment, the cathode is manufactured independently of the electron extraction grid. In another embodiment, the cathode is monolithic, i.e., this cathode and the extraction grid are manufactured simultaneously.

In both cases, the manufacture is either of series type or of parallel type. Serial type manufacturing consists of using a single beam sweeping the surface on which we want to create transmitting sites, this beam being 6 activated at predefined locations. Parallel fabrication involves simultaneously producing a plurality of beams reaching predefined locations.

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

The invention also relates to a triode using a cathode according to the invention. It has been found that such a triode can be used at frequencies of the order of 10 GHz whereas up to now it has been possible to reach frequencies of 4 GHz.

• les figures 1 et 1a, 2, 3a à 3c,4a à 4d, et 5a à 5c sont des schémas illustrant des procédés, conformes à l'invention, de fabrication de cathode, cette fabrication étant effectuée indépendamment de la grille d'extraction,
• les figures 6a à 6b, 7a à 7c, 8a à 8d, 9a à 9d et 10a à 10d sont des schémas illustrant des procédés de fabrication, conformes à l'invention, de cathodes et de grilles d'extraction,
• la figure 11 illustre une cathode conforme à l'invention avec une grille d'extraction ainsi qu'une autre électrode,
• la figure 12 est un schéma d'un tube utilisant une cathode conforme à l'invention,
• la figure 13 est un schéma d'un tube analogue à celui de la figure 12 mais pour une variante,
• la figure 14 est un schéma d'un canon à électrons comportant une cathode conforme à l'invention,
• les figures 15, 15a et 15b sont des schémas d'un tube oscillateur comportant une cathode conforme à l'invention,
• la figure 16 est un schéma d'une triode comportant une cathode conforme à l'invention, et
• la figure 17 est un schéma d'une triode pour une variante.

Other characteristics and advantages of the invention will become apparent with the description of some of its embodiments, this being done with reference to the attached drawings in which:

• FIGS. 1 and 1a, 2, 3a to 3c, 4a to 4d, and 5a to 5c are diagrams illustrating cathode manufacturing methods according to the invention, this fabrication 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 methods of manufacture, according to the invention, of cathodes and extraction grids,
• FIG. 11 illustrates a cathode according to the invention with an extraction grid 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 cathode 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
• Fig. 17 is a diagram of a triode for a variant.

First of all, in connection with FIGS. 1, 1a, 2, 3a to 3c, 4a to 4d, 5a to 5c, several methods for producing an electron-emitting cathode will be described, this fabrication being carried out independently of that of the electron extraction grid.

The manufacture may be of the series type or of the parallel type. Figures 1, 1a and 2 illustrate series type manufacturing processes.

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

On the layer 20, at predetermined locations, an electron or ion beam is focused 24. Thus, at each location 26 ₁ , 26 ₂ , etc. on which the beam 24 has been focused, electron-emitting sites are created.

The zones 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 optimal field effect at each site, a step greater than or equal to twice the height of the conductive channels, that is to say the thickness of the layer 20, will preferably be chosen. Otherwise, there would be a reduction in the peak effect of each channel and thus 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 electron beam of irradiation, in one example, a beam of the type used is used. in a transmission electron microscope. 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 specific locations, is 5 nm, we obtain emitting site densities of 4.10 ¹² / cm ² .

If a 5 nm size beam of the type provided by the electronic maskers used for high-resolution electronic lithography, with a pitch of 20 nm, is used, the emissive site densities are then 2.5 × 10 ¹¹ / cm ² .

In the embodiment shown in FIG. 2, instead of using an electron beam, a tip 30 is used whose contact section 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 26 ₁ , 26 ₂ . Each pulse modifies the electrical properties of the emitter layer 20, that is to say transforms the corresponding locations into conductive areas and / or high electron affinity.

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 20 nm thick, the pitch is 50 nm and the electrical pulses 31 have an amplitude of 10 V.

FIGS. 3a to 3c, 4a to 4d and 5a to 5c will now describe 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 made using electron beams.

The first step of the method 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 step of 200 to 500 nm. The material constituting the protective mask 32 is for example a resin of the type commonly used in high resolution lithography processes. This material can also be a heavy metal (for example molybdenum or tungsten) in order to block the high energy electron or ion beams.

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

Electron emitter sites 38 ₁ , 38 ₂ , etc. are thus obtained. at the openings 36 with a predefined density, that of the openings of the mask 32. With apertures 36 of diameter 50 to 100 nm and a pitch of 200 to 500 nm, the density of emitter sites can reach 10 ¹⁰ / cm ² .

These values are of course only given by way of example. It is indeed possible to use electronic lithography techniques allowing 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 transmitting sites is substantially greater.

During a last step, the protective mask 32 is eliminated, for example by etching. An electron-generating layer 20 is thus obtained (FIG. 3c) with emitter sites 38 ₁ , 38 ₂ , ... created artificially.

In a variant (not shown), in place of an electron beam 34, irradiation with aggregates (for example C ₆₀ ) optionally 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 emitting sites or conducting channels and / or high electron affinity are created along the trace 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 transmitter sites is controllable by the dose of heavy ions or aggregates.

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

For the implementation of this method, is deposited on the layer 20 an insulating mask 40 (Figure 4a) having openings of diameter between 50 and 100 nm with a pitch of 200 to 500 nm. This insulating mask is, for example, silica or 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 / μm, an applied voltage of 100 volts, the insulation thickness will be of the order of 300 nm.

After the deposition of the insulator 40, depositing a metal layer 42 (Figure 4b) of thickness about one micron. 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, the latter is applied to a pulse 44, for example 100 volts. This pulse is, in one example, applied for a duration of less than one second. The voltage and the duration are chosen so as to locally modify, at the right of the openings of the layer 40, the electrical properties of the material emitting the layer 20 (FIG. 4c).

Finally, during a last step, the layers 40 and 42 are removed, for example by etching, leaving the layer 20 with conducting channels 46 ₁ , 46 ₂ forming electron emitters.

In the variant shown in FIGS. 5a to 5c, instead of using an insulating mask 40 to define the sites, use is made of a network of conductive tips 50 ₁ , 50 ₂ , etc. formed on the surface 52 of a conductive substrate 54 (Figure 5a). This network of tips 50 ₁ , 50 ₂ , etc. is applied on the surface of the layer 20 at the same time as an electric pulse 56 (FIG. 5b) is applied to the conductive substrate 54.

As in the embodiment previously described, the voltage of the pulse 56 and its duration are chosen to locally modify, in the sites located at the right of the tips, the conduction properties of the material emitting layer 20. As in the previous case, we obtain a network 46 ₁ , 46 ₂ , etc. conductive channels (Figure 5c). The density is equal to the density of the peak network.

FIGS. 6a and 6b, 7a to 7c, 8a to 8d, 9a to 9d and 10a to 10d will now describe a method for manufacturing a monolithic type cathode consisting of simultaneously producing, on the same substrate, the sources of electrons and the grid of extraction of these electrons.

As in the previously described embodiment (manufacture of electron generators independently of the extraction grid), one can either use a series process, or use a parallel process for the manufacture of electron emitting sites.

Figures 6a and 6b show a series type manufacturing process.

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

The part of the insulating layer 60 which is located at the right of the openings 66 is then removed, for example by etching, so as to create cavities 64 of greater diameter than the openings 66 so as to avoid any interaction between the beam and the insulator, the interaction can create parasitic loads.

Finally, an electron beam 70 is centered on each opening so as to irradiate the center of the layer 20 at the right of the corresponding opening 66, which creates the emitter sites 72 ₁ , 72 ₂ , etc.

When the layer 20 is insulating or very weakly conductive, 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 FIGS. 7a to 7c, the fabrication of the sites is of the parallel type.

In this example, provision is made (as described with reference to FIG. 6a), on the layer 20 of emitting material, an insulating layer 60 and on this insulating layer, a conductive layer 62 in which apertures 74 are etched and under which one performs, by chemical etching, 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 adjacent openings 74 is between 200 and 500 nm.

Next (FIG. 7b), conductive layer 62 is exposed to a parallel beam 78 of electrons. 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 to reach only the layer 20 in line with the openings 74. It should be noted that in this case, interest in limiting the diameter of the openings 74 to obtain a correct positioning of the emitter sites 80 ₁ , 90 ₂ on the layer 20.

If the diameter of the openings 74 used to create the sites 80 ₁ , 88 ₂ , etc. is insufficient for normal operation, in a last step (Figure 7c), these openings are enlarged to form openings 84 of larger diameter, for example between 100 and 200 nm.

As in the embodiment previously described, 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, self-aligned emitter sites are manufactured with the openings of the extraction grid.

For this purpose, during a first step (Figure 8a), on the layer 20 of emitting material, is deposited an insulating layer 86 on which is deposited a metal layer 88 for constituting the gate. This metal layer 88 is covered by a resin layer 90.

The resin layer 90 is then scanned by an electron beam 92 located in predetermined locations which creates, through the layers 86, 88 and 90, emitting sites 94 ₁ , 94 ₂ , etc. for example spaced in a pitch of 200 to 1000 nm.

Irradiation using beam 92 also creates a modification of resin layer 90 which can be developed to obtain apertures 96 ₁ , 96 ₂ , etc. (FIG. 8b) at locations that naturally correspond to those of the transmitter sites 94 ₁ , 94 ₂ , and so on.

These openings 96 ₁ , ... of the layer 90 of resin are used to make openings 98 ₁ , ... in the metal layer 88 and cavities 100 ₁ of the insulating layer 86, under the openings 98 ₁ , ... (Figure 8c). The illumination of the resin is used to make the openings 98 ₁ , ... of the grid 88.

Finally (Figure 8d), the resin layer 90 is removed.

This method allows easy alignment of the sites 94 ₁ with the openings 98 ₁ of the electron extraction gate 88.

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

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

FIGS. 9a to 9d illustrate a method of manufacturing parallel type transmitter sites by means of a voltage pulse.

According to this method, an insulating layer 104 is deposited on the emitter layer 20 on which a metal layer 106 is deposited (FIG. 9a). The insulating layer 104 is for example silica or silicon nitride. In the layers 104 and 106, openings 108 ₁ , 108 ₂ , 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 that will be applied thereafter. 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 / micron.

After the openings 108 ₁ , 108 _{2 have been made} , a layer 110 (FIG. 9b) is deposited on the layer 106 and in the openings 108 _i , and a pulse 112 is applied (FIG. 9c), for example of an amplitude 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 conducting channels, as previously described.

Finally (Figure 9d), the metal layer 110 is removed and under the openings of the metal layer 106, constituting the electron extraction grid is formed by etching cavities 114 _i .

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

In the embodiment that will now be described with reference to FIGS. 10a to 10d, the emitting sites are made by doping a semiconductor material.

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

In the layer 126, there are formed, for example by lithography, openings 128 ₁ , 128 ₂ , etc. (Figure 10a) whose diameter is between 50 and 100 nm with a pitch of 200 to 500 nm.

After making the openings 128 ₁ , 128 ₂ , etc., the metal layer 126 with its openings 128 ₁ to 128 ₂ is irradiated with a parallel ion beam 130, which allows the production of doped channels 130 _1, 130 _2, etc. by ion implantation in the layer 122. These doped channels are p-type (Figure 10b).

Preferably, the openings 128 ₁ , 128 ₂ have a relatively small diameter, so as to control the locations of the doped areas 130 ₁ , 130 ₂ , etc. Under these conditions, it may then be necessary to enlarge these openings 128 ₁ , 128 ₂ , so as to make openings 132 ₁ , 132 ₂ of larger diameter (Figure 10c). In this case, an isotropic etching is used. For example, etching inducing a 50 nm decrease in the thickness of layer 126 will result in a 100 nm increase in apertures 132.

After the openings 132 ₁ , 132 ₂ have been made, cavities 134 ₁ are made, for example by etching, under the openings 132 ₁ formed in the insulating layer 124 (FIG. 10d).

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

The emitter layer 20 in which emitter sites 131i are implanted can be made 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 gate 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 diameter substantially greater than the diameter of the opening 144i of the grid 136.

The structure shown in FIG. 11 is feasible with all the types of emissive sites previously described.

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

We will now describe in relation to FIGS. 12 to 17, electronic tubes using the cathode according to the invention.

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

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

The electrons generated by the sites 151 _i of the cathode 150 are also extracted from the cathode / gate space; they are also current modulated because of 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 consists essentially of a cavity 154, the bottom, thicker, collects most of the electrons.

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

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

In the variant shown in FIG. 13, there is provided a gate 158 comprising two parts lying along two distinct surfaces, namely an active part 160 at a distance d ₂ from the surface of the cathode 150 and a second part 162 comprising the parts not active grid 158, at a distance d ₁ of the surface 150 greater than the distance d ₂ .

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

Each active portion 160 of the gate is connected to the inactive portion 162 by a flared portion, particularly conical, 164. Preferably, the shape of these flared portions 164 is chosen to prevent the electron beams 170 diverge under the effect the space charge and for these beams to pass through the active parts of the grid at the desired locations. Thus, the flared portions 164 form a wehnelt, that is to say an electronic lens.

Figure 14 shows a single gate type electron gun. In this embodiment, the cathode 172 is formed on a concave spherical surface, and the gate 174 has a similar shape. The openings of the gate 174 are, as described above, to the right of the emitter sites of the cathode 172.

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

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

Finally, the gate 174 can receive control signals for modulating the emission of electrons. The modulation of the beam is then carried out in the electron generator, whereas usually the generation and modulation take place with two separate devices.

FIG. 15 represents an oscillator tube comprising a cathode according to the invention. This oscillator tube is of the monotron type. The electrons 182 leaving the cathode / grid space immediately return to a resonant cavity 180 where they are accelerated by a DC voltage V _KA , 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 2nπ <2nFt <(2n + 1) π, there is oscillation at the frequency F, as long as the cavity resonates at this frequency F , or at a frequency close to F.

In a variant, it is provided, as in the example of FIG. 12, that the electrons are accelerated in a grid / cavity space and pass through the cavity at a 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 relationship.

avec x = 1, y = 0 et z = k (entier), l'axe y étant parallèle aux faisceaux et les axes x, y et z étant perpendiculaires entre eux. Si la cavité est cylindrique (figure 15b), le mode sera, par exemple, un mode ${\mathrm{TM}}_{\mathrm{\theta rz}}^{\mathrm{O}}$

avec θ = 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 has components electrical field parallel to the beams and located where these beams pass. If the cavity is rectangular (Figure 15a), the mode will be, for example, a ${\mathrm{YOU}}_{\mathrm{X\; Y\; Z}}^{\otimes },$

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 ${\mathrm{TM}}_{\mathrm{\theta rz}}^{\mathrm{O}}$

with θ = 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 on a gate 186, as well as an adjustable voltage 188 on the anode 190. The adjustable voltage 188 makes it possible to adjust the speed of the electrons and therefore the time t, ie the frequency of operation of the oscillator tube.

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

The configuration of the triode is such that the electron beam 196 passes through 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 according to the invention is also used to constitute a broadband amplifying triode. In this case, the distance between the gate 202 and the cathode 204 is variable; the emitting surface of each transmitter site 206 of the cathode 204 and the density of these sites is a function of the cathode-grid distance at the location of these sites. When the gate-cathode distance is large, the sites are wide but widely 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 not very spaced and therefore dense. In this way, the current density emitted remains the same.

The variable distance between cathode 204 and gate 202 allows a wide band. The lowest distance corresponds to operation at the highest frequencies and the highest distance at the lowest frequencies.

The invention can also be used for display applications.

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

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

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

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

According to one embodiment, the distance separating the gate from the emitting surface (204) of the cathode is variable.

In this case, preferably, the density of transmitter sites on the cathode varies in the opposite direction of the distance to the gate.

In this embodiment, it is advantageous for the current density emitted to be constant regardless of the distance from the gate to the emitting surface.

Claims (32)

1. Process for fabricating an electron-emitting cathode, which starts with an emitter material (20) that is insulating or has a low electron affinity, characterized in that the emitter material (20) has a surface locally remaining substantially plane, in that emitter sites (26₁, 26₂) are artificially defined on the surface by locally creating zones that have a high electron affinity and/or are conducting, and in that the sites (26₁, 26₂) are produced by irradiating the surface of the insulating or low-electron-affinity material using an electron or particle beam (24).
2. Process according to Claim 1, characterized in that the irradiation is carried out in a vacuum chamber in which the pressure is generally of the type used in vacuum electron tubes.
3. Process according to Claim 2, characterized in that the pressure is less than a few 10^-5 torr (133 × 10^-5 pascals).
4. Process according to any one of the preceding claims, characterized in that the density of the sites is predetermined.
5. Process according to Claim 4, characterized in that the predetermined densities are between 10⁸ and 10¹³/cm².
6. Process according to any one of the preceding claims, characterized in that the insulating or low-electron-affinity material is chosen from the group comprising single-crystal or polycrystalline diamond, carbon having a structure similar to that of diamond, a carbon-based material having a low electron affinity such as ta-C, ta-C:N, an amorphous material having a very low conductivity, such as a-Si, a-C:H or a-SiC, or a large-bandgap material, such as aluminium nitride or gallium nitride, and an insulating material such as magnesium oxide MgO or titanium oxide TiO₂.
7. Process according to any one of the preceding claims, characterized in that the conducting or high-electron-affinity sites (26₁, 26₂) are produced at predefined locations.
8. Process for fabricating an electron-emitting cathode, which starts with an emitter material (20) which is insulating or has a low electron affinity, characterized in that the emitter material (20) has a surface that locally remains substantially plane, in that emitter sites (26₁, 26₂) are artificially defined on the surface by locally creating high-electron-affinity and/or conducting zones and in that the sites (26₁, 26₂) are produced by applying an electric current pulse (31, 44) that causes the conduction properties of the material to be locally modified.
9. Process according to any one of Claims 1 to 6, characterized in that the emitter sites (26₁, 26₂) produced are randomly positioned.
10. Process according to any one of the preceding claims, characterized in that the artificially created sites (26₁, 26₂) constitute, volumewise, conducting channels.
11. Process according to Claim 10, characterized in that the low-electron-affinity insulating material is of the sp³ type and the conducting channels are of the sp² type, the insulating or low-electron-affinity material being based on carbon.
12. Process according to Claim 10, characterized in that the conducting channels are crystalline, the emitter material being amorphous and very weakly conducting.
13. Process according to Claim 10, characterized in that the conducting channels are produced by the creation of defects.
14. Process according to Claim 10, characterized in that the conducting channels are produced by localized deoxidation, the emitter material being insulating.
15. Process according to any one of the preceding claims, characterized in that each site (26₁, 26₂) extends over a substantially circular zone with a diameter between 1 and 100 nm.
16. Process according to any one of the preceding claims, characterized in that the ratio of the height of a site to the pitch between two sites is 0.5 or less.
17. Process according to Claim 7, characterized in that a focused electron beam (24) is used to create the sites, said beam being applied sequentially to the surface (20) of the emitter material at predetermined locations.
18. Process according to Claim 7, characterized in that the locations of the sites (38₁, 38₂; 72₁, 72₂) are defined by means of a mask (32; 62) placed in front of the emitter material and in that this emitter material is irradiated using a parallel electron beam (34) through the holes in the mask.
19. Process according to Claim 8, characterized in that the positions of the sites (46₁, 46₂) are defined by a mask (40) having holes, which is placed near the surface (20) of the emitter material, in that a conducting layer (42) is applied in the holes of the insulating mask (40) and in that an electric pulse (44) is applied to this conducting material so as to create the sites (46₁, 46₂) in the emitter material.
20. Process according to Claim 8, characterized in that the sites are produced using a tip (30) applied at predetermined locations against the emitter material, a voltage (32) being applied to the tip (30) in order to produce the sites (26₁, 26₂) at the predetermined locations.
21. Process according to Claim 8, characterized in that the sites (46₁, 46₂) are produced using a set of tips (50₁, 50₂) applied against the surface (20) of the emitter material, a pulse (56) being applied to the set of tips (50₁, 50₂).
22. Process according to any one of the preceding claims, characterized in that an extraction grid (62) is produced simultaneously with the cathode, this grid (62) being formed by a metal layer (62) having apertures (66, 74) that are used to produce the sites (72₁, 72₂; 80₁, 80₂).
23. Process according to Claim 22, characterized in that, when the sites (72₁, 72₂) are produced using a parallel electron or ion beam (78) for the irradiation, apertures (74) of a first size are produced in the grid (62) and, after the irradiation, the diameter of the apertures (74) in the grid (62) is increased.
24. Process according to Claim 18 or 19, characterized in that the sites are produced using an electron or ion beam (92) applied on a resist layer (90) covering a metal layer (88) intended to form the grid, the beam modifying the resist (90) and creating the site (94₁, 94₂), the illumination of the resist (90) being used to produce the apertures (98₁) in the grid (88).
25. Process according to Claim 22, characterized in that an insulator (60) is provided between the emitter material (20) and the grid (62) and in that a cavity (64, 100₁) is formed in this insulator (60), in line with each aperture in the grid, facing each site (72₁, 72₂) of the emitter material.
26. Electron-emitting device comprising an emissive cathode (150) and a grid (152), the emissive cathode (150) having artificially created electron-emitting sites (151_i) that are separated from one another, characterized in that the electron-emitting sites (151_i) are created on a surface that locally remains substantially plane, in that the sites are produced by irradiating the surface using an electron or particle beam and in that the electron-extracting grid (152) has apertures facing the sites (151_i).
27. Device according to Claim 26, characterized in that the apertures in the grid lie on a surface (160) at a first distance (d₂) from the cathode (150) and in that those parts of the grid (158) that do not have apertures lie on a second surface (162) at a second distance (d₁) from the emitting surface, which is greater than the first distance.
28. Device according to Claim 27, characterized in that each part of the grid (158) having at least one aperture and associated with at least one emitter site is connected to the rest of the grid having no apertures via a conical part (164) designed to constitute a Wehnelt cathode.
29. Device according to any one of Claims 26 to 28, characterized in that the distance separating the grid (202) from the emissive surface (204) of the cathode can be varied.
30. Device according to Claim 29, characterized in that the density of emitter sites on the cathode varies inversely with the distance to the grid.
31. Device according to Claim 27, characterized in that the emitted current density is constant irrespective of the distance of the grid from the emissive surface.

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