EP1153408A1

EP1153408A1 - Field emitter array with enhanced performance

Info

Publication number: EP1153408A1
Application number: EP00905117A
Authority: EP
Inventors: Georges Thomson-CSF Propriété Intel. FAILLON; Dominique Thomson-CSF Propriété Intel. DIEUMEGARD; Christian Thomson-CSF Propriété Intell. BRYLINSKY
Original assignee: Thales Electron Devices SA
Current assignee: Thales Electron Devices SA
Priority date: 1999-02-12
Filing date: 2000-02-11
Publication date: 2001-11-14
Anticipated expiration: 2020-02-11
Also published as: WO2000048220A1; EP1153408B1; DE60024784T2; DE60024784D1; KR20010102067A; FR2789801B1; US6522080B1; JP2002536808A; FR2789801A1

Abstract

The invention concerns a field emitter array capable of microwave modulation, comprising at least an array (R) of emissive tips, means for producing a microwave modulation signal including a semiconductor element (S) controllable in microwave modulation in the proximity of the tips and a short micro-line (L) for routing the modulation signal towards the array of tips which produces an impedance adaptation between the array of tips (R) and the modulation semiconductor element (S). The invention is useful for producing a compact field emitter array.

Description

IMPROVED PERFORMANCE FIELD-EFFECT CATHODE

The present invention relates to field effect cathodes (called "field emission array" in English, or FEA). These cathodes are already used in certain types of experimental high power electronic tubes, such as relativistic magnetrons, vircators ... but also in new tubes of more conventional types such as traveling wave tubes for radar or radar applications. telecommunication.

In this second case, the cathode is formed of at least one network of points comprising a substrate covered with a dielectric layer with cavities, each receiving a projecting emissive point, a grid placed on the surface of the dielectric layer surrounded at less partially the cavities.

To extract electrons from the tips, a potential difference is applied between the grid and the tips. The emission of electrons can be modulated in density by modulating the voltage applied to the grid.

From an electrical point of view, the grid and the substrate-spike assembly separated by the dielectric layer are equivalent to a high capacity of the order of 10 to 100 pF / mm ² and the corresponding conductance is of the order of a few tens mS / mm ² around 10 GHz.

Typically if we apply around 80 V between the grid and the substrate-tip assembly, we can extract a current of 1 μA / tip, the tips having a density of the order of 10 ⁶ to 10 ^{7 per} square centimeter

At frequencies from 10 to 100 kHz, the impedance presented by the grid to the modulator which supplies it, is essentially real and remains of a few tens of ohms, which makes it possible to use a reasonable power modulator.

Current developments relate to the operation of these cathodes, with a microwave field effect. The advantage of an electronic tube using such a microwave modulated cathode is that it can be very compact, that it can be constructed without a focusing device and that its efficiency is high. We can hope to obtain tubes whose operating principle will be comparable to that of IOTs (abbreviation in English language of Inductive Output Tube (tube with inductive output), but working with much higher frequencies

But if the grid is modulated in microwave, the impedance presented by the grid to the modulator which feeds it becomes very low because of the reactance of the capacity which is very low (0.1 to 1 Ω / mm ² towards 10 GHz for example), which requires a modulator with a bandwidth equivalent to that of conventional tubes, at very high power to obtain a satisfactory current intensity

The modulator is connected to the grid by a microwave transmission line, generally a microstrip line Another reason imposing on the modulator a high power is that the modulation signal applied to the grid is reflected at the transition between the transmission line and the wire rack

To this end, FIG. 1a shows a top view of a known type field-effect cathode. The cathode 1 comprises four networks 2 of sector-shaped tips grouped on the same electrically conductive support 50 Each network comprises a conductive substrate referenced 3 , a dielectric layer referenced 4 with cavities 5 in which emissive points 6 take place, the dielectric layer is surmounted by a grid 7 We also refer to FIG. 1 b

The power supply of each of the networks 2 is done using microstrip lines 8 each connecting a network of spikes 2 to a power modulator M located remotely We have schematically shown a modulator M per network of spikes 2 but only one not enough for all The microstrip lines 8 are long, they occupy a much larger surface than that of the tip networks 2 We cannot approach the modulator M very close to the tip networks 2 because it is much more bulky than the networks spikes

In the configuration described and illustrated, the conductive support 50 serves as a conductive plane for the microstrip lines 8 The insulation of the microstrip lines is referenced 8 2 and the conductive tape 8 3

Each microstrip line 8 is electrically connected to a network of points 2 by a conductor 9 secured on one side of the conductive tape 8 3 and on the other of the grid 7 of the network 2 of points The modulators M must generate a high-level microwave signal in particular because being located far enough from the networks of points 2, they are connected by lines which generate strong reflection on the grid side, and because reflections also occur. in point networks 2 because of the presence of points 6

The further one moves away from the microstrip line 8 when entering the network of tips 2, the weaker the signal and the smaller the current density produced by the tips. This leads to an inhomogeneous electron beam, detrimental to the proper functioning of '' an electronic tube Beyond 100 micrometers of propagation in the spike network 2, the modulation signal becomes ineffective

The shape in sector given to the networks of points 2, allows if one does not exceed a width of 50 to 100 micrometers, to improve the homogeneity of the beam But one is limited in current density because one cannot make approach a large number point networks without considerably increasing the surface it occupies because of the size of the microstrip lines coming from the modulator M

The object of the invention is to provide a cathode which does not have these drawbacks. The present invention provides a microwave effect field cathode, formed of at least one network of emissive points, capable of emitting electrons with a much higher current density than existing field effect cathodes This cathode has the advantage of requiring neither a conventional power modulator to control the emission of electrons, nor a high-level transmission line Conventional modulators are costly, power-hungry and pose cooling problems Transmission lines pose problems of differential signal phase delay and microwave loss

To achieve this, the present invention is a microwave effect field cathode, comprising at least one network of emissive tips, means for producing a microwave modulation signal and means for routing it to the network of tips, characterized in that that the means for producing the modulation signal comprise a microwave-controllable modulation semiconductor element located very close to the spike network, the means for routing the modulation signal to the network of tips being a short microline introducing a practically negligible disturbance which achieves an impedance adaptation between the network of tips and the semiconductor modulation element. The microline is a line, in particular of the microstrip or coplanar type, the conductive ribbon of which is connected at one of its ends to the network of spikes and at the other end to the semiconductor modulation element.

The modulation semiconductor element is of the transistor type, in particular MESFET, or of the diode type.

To carry out the impedance adaptation, the conductive strip of the microline can be configured in two sections linked together by a capacitor.

The microline can also have a polarization function and be connected to a polarization source.

At least one element taken from the array of tips, the modulation semiconductor element and the microline is discrete.

At least two elements taken from the network of tips, the modulating semiconductor element and the microline are integral with the same electrically insulating or semi-insulating support. The two elements can be mounted on one side of the support, the other side of which is coated with a conductive layer which serves as a ground plane.

It is possible to connect the microline to the array of tips and / or to the modulation semiconductor element by a wire link. However, to avoid transmission disturbances, it is advantageous to avoid wired links at the level of the spike network. The network of points comprises an electrically insulating or semi-insulating substrate with on one face a conductive or semi-conductive layer, emissive points in electrical contact with the conductive or semiconductive layer, a dielectric layer provided with cavities each housing one of the points, the dielectric layer being surmounted by a conductive grid which at least partially surrounds the cavities. The substrate is traversed by at least one metallized hole which contributes to electrically connecting the tips to the other face of the substrate. The metallized hole can be extended by a contact which is attached to an appropriate conductive surface of the support. The substrate and the dielectric layer can also be crossed by at least one metallized hole which contributes to electrically connecting the grid to the other face of the substrate. It is then possible to remove the wire connections associated with the tips and / or the grid. To remove one or more several wire connections at the level of the semiconductor modulation element, it is possible to use an element compatible with a transfer technique by microbossings

The microline can easily be produced in a form integrated into the electrically insulating or semi-insulating support even if the network of tips and / or the semiconductor modulation element are discrete

To obtain a compact and relatively inexpensive tip effect cathode, it is advantageous for the tip network, the microline and the modulating semiconductor element to be integrated on the same semiconductor substrate. Preferably, the semiconductor employee is semi-insulating such as silicon carbide

The microline can then have a ribbon which extends on one side to form a grid of the spike network and on the other side to form a contact of the modulating semiconductor element.

The present invention will be better understood and other advantages will appear in the description which follows and in the appended figures which represent

FIGS. 1 a, 1 b already described a top view and a partial sectional view of a known field effect cathode,

FIG. 2 a top view of an exemplary embodiment of a field effect cathode according to the invention,

FIGS. 3a, 3b of exemplary embodiments of field effect cathodes according to the invention in which the modulating semiconductor element is a transistor or a diode,

FIGS. 4a to 4e different steps for producing the network of tips of a field effect cathode according to the invention,

FIGS. 5a to 5h different steps for producing the semiconductor modulation element of a field effect cathode according to the invention,

FIGS. 6a, 6b of examples of electrical diagrams for mounting field effect cathodes according to the invention, - Figures 7a to 7d of new examples of cathodes according to the invention in which certain wire connections have been deleted;

- Figure 8 an example of cathode field effect monolithic according to the invention. The various components of the cathodes according to the invention are not shown to scale for the sake of clarity.

FIG. 2 schematically shows, in top view, a field effect cathode, modular in microwave according to the invention. The cathode comprises at least one conventional network of points R in itself, means S for producing a microwave modulation signal which controls the emission of electrons and means L for routing the signal to the network of points R.

According to the invention, the means S for producing the microwave modulation signal comprise a semiconductor modulation element placed just next to the network of tips R, while the means for routing it to the network of tips R are a short microline introducing a practically negligible disturbance. The microline does not only have an electrical connection role between the network of tips and the modulating semiconductor element. It also has an impedance matching function between the spike network and the modulating semiconductor element. Furthermore, it can also convey at least one bias voltage.

In this way, we can do without bulky and expensive conventional power modulator and high level line which posed problems. The same semiconductor modulation element S can control the transmission of several networks of spikes R.

The arrangement of R-point networks offers a very large number of possibilities. It is possible to concentrate on a small area a large number of networks of points R, which makes it possible to obtain increased current densities. Each network of points R can have optimal dimensions so that there is no or very little disturbance of the modulation signal in the network R of points which makes it possible to obtain much more homogeneous electron beams than by the past. Typical values for such a network of points R are of the order of 50 micrometers by 300 micrometers Propagation over a distance of the order of 50 micrometers does not produce a significant disturbance towards 10GHz

The semiconductor modulation element S which delivers the microwave modulation signal, may for example be a transistor or a diode. In the case of a MESFET transistor, its surface area is of the order of 500 micrometers by 200 micrometers with a active part pa significantly smaller, about 50 micrometers by 200 micrometers As for the microline L, it can have a length of about 100 micrometers or even several hundred micrometers without introducing any significant disturbance

In FIG. 3a there is shown in section, an example of a field effect cathode according to the invention. In this configuration, the network of tips R, the microline L and the semiconductor modulation element S are discrete and integral with the same dielectric support 100 In this example, the array of tips R, the microline L and the semiconductor element S for modulation are each connected by brazing on a conductive pad respectively 10R, 10L, 10S carried by one of the faces of the dielectric support 100 The solder is shown in thick blackened line This dielectric support 100 has an essentially mechanical role but it may be advantageous to place on its other main face a conductive coating 101 so as to produce a local ground plane

It is assumed that, in the example described, the microline L is a microstrip line. It could be envisaged that it is a coplanar line and in the cross-sectional figure it would have the same profile. The microstrip line L conventionally comprises a plane. conductor 10 1 or ground plane, then an electrically insulating or semi-insulating layer 12 then a conductive tape 11 The conductive plane 10 1 is attached to the conductive pad 10L of the dielectric support 100 The conductive plane '• O 1 and the conductive tape 11 can be made of nickel or an alloy based on titanium, gold, platinum for example The electrically insulating or semi-insulating layer 12 can be made of ceramic, silica or even silicon carbide for example

As will be seen later in FIGS. 6a, 6b, the strip 11 of the microline L may be discontinuous and formed of two sections connected together by a capacitor C relates, for example, between the two sections This capacitor C participates in the adaptation of impedance

The network of points R comprises an electrically insulating or semi-insulating substrate 13 with on one face a conductive or semi-conductive layer 13 1, emissive points MP in electrical contact with the conductive or semi-conductive layer 13 1, a dielectric layer 14 provided with cavities 15 each housing one of the points MP, the dielectric layer 14 being surmounted by a conductive grid G which at least partially surrounds the cavities 15 The other face of the substrate 13 is coated with a conductive coating 10 2 for the secure by soldering on the dielectric support 100

The substrate 13 when it is insulating can be for example glass, alumina, silica and when it is semi-insulating for example silicon carbide SiC The materials of the substrate 13 are chosen for their capacity to withstand high voltages , for example of the order of a few hundred volts without damage as well as high temperatures, of the order of 400 ° C. for example, these temperatures being reached when the cathode is mounted in a microwave tube which is steamed to obtain a good vacuum In general, all the materials used in the composition in the cathode according to the invention must be able to withstand steaming and must not degas under vacuum

The dielectric layer 14 can be made of silica SιO ₂ for example and the grid G and the tips MP of molybdenum for example

The modulation semiconductor element S is in the example of FIG. 3a a transistor More precisely, in this example it is a MESFET type transistor, but of course other types of transistors can be used II comprises a conductive layer 10 3 for the soldering then a substrate 16 of semiconductor material with semi-insulating properties, then a semiconductor coating 18 of type N, preferably produced in two layers 18 1, 18 2, the surface layer 18 1 or contact layer is doped N + and therefore more conductive than the base layer 18 2 or active layer doped N, then two ohmic contacts, one of drain Ds and one of source Ss and a Schottky contact of gate Gs between the contacts ohmic Ds, Ss We also represented in this example a passivation layer 21 on the coating 18, it can be made of silica for example.

The microline L is connected at one of its ends to the semiconductor modulation element S, in the example described at its drain Ds, at its other end to the network of spikes R, in the example at the grid G. The grid G of the network of tips R is brought to a bias voltage E1 and the tips MP to a ground potential. The source Ss of the semiconductor modulation element S is connected to a ground potential and the gate Gs receives an HF signal of microwave modulation which the semiconductor element will amplify. The connections described above can be carried out by wire cabling (known under the English name of wire bonding) with 20.1 gold wires for example.

In FIG. 3b, the semiconductor modulation element S is now a diode, it can for example be of the Guπn or IMPATT type. It comprises a first conductive layer K which forms its cathode and which will be soldered on the appropriate conductive pad 10S of the dielectric support 100. Its anode A is formed by a second conductive layer and these two conductive layers A, K are separated by a semiconductor layer 30. Its cathode K is connected to a ground and its anode A to one end of the microline L.

At the network of points R, FIG. 3b differs from FIG. 3a by the fact that the electrically conductive or semi-conductive layer 13.1 is obtained by doping on the surface of the semi-insulating layer 13 then produced in a semiconductor material with semi-insulating properties such as for example silicon carbide. In the example, the MP tips are also produced with the semiconductor material with semi-insulating properties made semiconductor by doping. The MP tips could of course have been made of an electrically conductive material such as molybdenum. As regards the microline L, it is now integrated in the dielectric support 100. Its strip 11 is a conductive pad carried by the dielectric support 100, on the face where the network of tips R and the semiconductor element S are attached. modulation. Its ground plane is formed by the conductive layer 101. It has a radiation leakage screen function. We find the ribbon 11 in two sections and the capacitor C.

The techniques used to make the network of tips R can be those conventional in the semiconductor industry. An exemplary embodiment is illustrated in FIGS. 4a to 4e. These figures illustrate the case of a discrete spike network such as that described in FIG. 3a.

We start from an electrically insulating or semi-insulating substrate 13. We assume in the example that it is made of glass for example. An electrically conductive layer 13.1 is deposited thereon, made of molybdenum for example by vacuum evaporation. The dielectric layer 14 is then deposited, which can be made of silica for example (FIG. 4a).

The grid G is then deposited for example in molybdenum (FIG. 4b). After a masking operation, for example by lithography, the conductive layer of gate G to form openings 17 is attacked by chemical etching or reactive ion etching (RIE), then the dielectric layer 14 to form the cavities 15 (FIG. 4c). . The openings 17 open into the cavities 15.

The deposition of the points MP produced, in molybdenum Mo for example, can be done by evaporation under vacuum, (FIG. 4d). By chemical attack, all that is above the grid G (FIG. 4e) is then removed, that is to say the resin 25 which was used in the masking operation and the excess metal of the points MP is finding on the resin 25 and referenced 26. The substrate 13 is metallized in 10.2 on its face opposite to that carrying the points MP so as to be able to be joined by a gold solder, for example the network of points R to the dielectric support 100. This step could have intervened before.

The transistor can be produced in a known manner. An exemplary embodiment is illustrated in Figures 5a to 5h and the transistor obtained corresponds to that illustrated in Figure 3a. A more conductive coating 18 is deposited on a substrate 16 made of semiconductor material with semi-insulating properties (silicon carbide for example) (FIG. 5a). It is preferable to produce this coating 18 in two layers 18.1, 18.2, the layer 18.1 on the surface, N + doped is the contact layer and the layer 18.2 between the substrate 16 and the contact layer 18.1 is the active layer and is N doped These deposits in silicon SiC or in gallium nitride GaN, for example, can be obtained by epitaxy whether it is liquid (LPE), in vapor phase (VPE) or by molecular jet (MBE) or even by ion implantation.

By reactive ion etching into the substrate 16, a plate 19 or mesa is defined (FIG. 5b).

A trench 20 is made in the contact layer 18.1 in a central region of the plate 19 by reactive ion etching (FIG. 5c).

A passivation layer 21 is generally deposited next (FIG. 5d). It can be made of silica SiO ₂ or of silicon nitride Si ₃ N ₄ for example.

The ohmic contacts D _s and S _s are deposited after an etching operation in the passivation layer 21 up to the contact layer 18.1, preceded by a masking operation, for example by lithography (FIG. 5e). The two substantially identical ohmic contacts D _s and S _s are then preferably deposited at the same time, by spraying or evaporation in the etched locations. They are generally made of nickel. The resin 25 which was used in the masking operation is then removed (FIG. 5f). The deposition of the Schottky contact G _s is done separately, here again we proceed at the level of the trench 20, an etching operation in the passivation layer 21 to the active layer 18.2 preceded by a masking operation for example by lithograph (Figure 5g). The Schottky contact G _s, made of titanium for example, is deposited by spraying or evaporation in the etched location and then the resin 27 which was used in the masking operation is removed. A metallization operation (reference 10.3) is carried out on the substrate 16 on the face opposite to that carrying the contacts so as to be able to be joined by a gold solder, for example, the modulating semiconductor element to the dielectric support 100 ( Figure 5h).

In what has just been described, the electrical connections of the spike network R and of the modulation semiconductor element S are in the form of wires 20.1. It may be advantageous to reduce or even eliminate the number of wired links. With this in mind, it is possible to use a semiconductor modulation element compatible with an assembly known under the English name of "flip chip" or of "report by microbossages" in French. With regard to the network of points R, it can also be compatible with this type of assembly Figures 7a, 7b illustrate this configuration At the level of the network of spikes R, a wired link can have a disturbing effect on the electron emission diagram A wired link is equivalent to a parasitic inductance

From the point of view of the useful surface of the dielectric support 100, it is possible to reduce it by eliminating certain wire connections because it is also possible to delete certain conductive ranges, for example that of local mass for MP points. This reduction in surface is advantageous

The semiconductor modulation element S shown diagrammatically is of transistor II type has three pads a drain pad pd, a source pad ps, a gate pad pg each coming in electrical contact with an appropriate conductive pad of the dielectric support 100 More particularly the drain pad pd comes into contact with the ribbon 11 of the microline L, the gate pad pg comes into electrical contact with a conductive pad 70 through which the modulation signal to be amplified is brought, as for the source pad ps , it comes into contact with a conductive pad 71 connected to the local ground by a metallized hole 72 which passes through the dielectric support 100, for example The pads pd, pg, ps also have a mechanical role of maintaining the semiconductor element of modulation S on the dielectric support 100, the mechanical connection can be made by fusion between the pads and the conductive pads

We will now describe in more detail the network of tips R in which a contact 74 of tips MP is brought to the base of the network opposite the tips. This network of tips R could be used independently of the semiconductor element of modulation S and of the microline L In the example of FIG. 7a, there is the electrically insulating or semi-insulating substrate 13 pierced right through with at least one hole 73 This hole opens out at the level of the electrically conductive layer or semiconductor 13 1 support of at least one MP tip II is plumb with a MP tip We find the dielectric layer 14 comprising the cavities 15 and the grid G without modification compared to what is shown in FIG. 3a.

This hole 73 is metallized internally and is extended opposite the points MP by a contact 74 taking the form of a conductive pad 740. It is this pad 740 which will contribute to the electrical connection of the points MP and to the connection mechanical of the network of points R on the dielectric support 100. This stud 740 is in electrical contact with a conductive pad 75 carried by the dielectric support 100, this conductive pad 75 being connected in the example to the local ground by any suitable means.

It can be envisaged that the contact 74 of tips is not in the form of a conductive pad as illustrated in FIG. 7b. In this new embodiment, the hole 73 is metallized at the level of its walls, this metallization 78 forms a bottom on the side of the points MP and opens out opposite the points by forming an overhang 741 which comes into electrical and mechanical contact with an appropriate conductive pad 75 of the dielectric support 100. This connection can be made by soldering. In the example, this conductive pad 75 is connected to the local ground by a metallized hole 76 which passes through, the dielectric support 100 to the local ground plane 101. The metallization 76 is not hatched so as not to overload the figure. .

In the example described in FIG. 7b, as many holes 73 have been represented as MP points and the electrically conductive or semi-conductive layer 13.1 support for MP points is discontinuous and takes the form of pellets each serving as a base for a point MP. In the examples of Figures 3 and 7a, there is shown a continuous layer 13.1 which forms a carpet under the tips.

The holes 73 must have a relatively small diameter if the density of the points is high in the network. The order of magnitude of their diameter is less than a micrometer. The realization of these holes is delicate. To avoid making holes that are too fine, the electrically conductive or semi-conductive layer 13.1, which in this variant is continuous from one tip to another, can be extended by an area 77 free of tip MP. This variant is illustrated in Figure 7c. We then pierce one or several holes 79 through the electrically insulating or semi-insulating substrate 13 and these holes may be less fine than those plumb with the points MP.

The metallization 80 of the holes is similar to what has just been described for FIGS. 7a or 7b and the contact 74 of points opposite the points takes the form of either a stud or an overhang. The electrical connection of the tip contact 74 can be similar to what is described in FIGS. 7a, 7b. The mechanical connection of the network of tips to the dielectric support 100 can be done as in the examples of Figures 3. Another very important advantage of bringing a contact of tips MP at the base of the network of tips R through the electrically insulating substrate 13 or semi-insulating is that one considerably increases the thickness of insulating material between the grid G and this contact of points. Consequently, the tip grid capacity is significantly reduced. In FIG. 3a, the thickness to be taken into account is that of the dielectric layer 14 comprising the cavities 15 while in FIG. 7a it is that of the dielectric layer 14 comprising the cavities 15 and that of the substrate 13 electrically insulating or semi-insulating. The orders of magnitude of the thicknesses are as follows: approximately 1 micrometer for the dielectric layer 14 comprising the cavities 15 and approximately 300 micrometers for the electrically insulating or semi-insulating substrate 13. The energy required to charge the grid-tip capacity can be reduced for the same emission of electrons.

It may also be advantageous to eliminate the wire connections of the grid G and to bring a contact 81 of the grid to the base of the network of points opposite the grid. Figure 7d illustrates this configuration. One or more holes 82 have been made from the grid to the base of the array of points, on the one hand through the dielectric layer 14 carrying the cavities 15 and on the other hand the electrically insulating or semi-insulating substrate 13. These holes are metallized and arrangements are made for the metallization 83 to be without electrical contact with the electrically conductive or semi-conductive layer 13.1 supporting the points MP which can then be discontinuous. We find the pellets as in Figure 7b. At the base of the network R of points, the metallization 83 ends with the contact 81 in the form of a stud or overhang, the two variants being shown in FIG. 7d.

In the example of FIG. 7d, one of the contacts 81 (the one in the form of a stud) comes into mechanical and electrical contact with the strip 11 of the microline L and the other (the one in the shape of an overhang) comes into contact mechanical and electrical with a conductive pad 84 carried by the dielectric support 100 and connected to the polarization source E1.

From the production point of view, the holes can be obtained by RIE etching. The electrically conductive layer 13.1 and / or the grid G can be produced in nickel which is not etched during etching, if the latter is carried out after the deposition of the dielectric layer 14 and of the grid G. The metallization of the holes can be made in several layers based on titanium, nickel, gold for example. The studs and overhangs can also be in these materials.

Reference is again made to FIG. 3a. The microline L does not only serve to electrically connect the semiconductor element S of modulation to the network of spikes R. It also has an adaptation function because the semiconductor element S of modulation and the network of spikes R generally have very different output impedances. The impedance of the semiconductor element can be of the order of a few ohms to a few tens of ohms while that of the network of tips of the order of an ohm or a tenth of an ohm.

The ribbon 11 of the microstrip line will have a geometry which is suitable for carrying out this adaptation function between the network of tips R and the semiconductor modulation element S. The thickness of the insulating substrate 12 participates in this adaptation function.

It is arranged so that the thickness of the semiconductor modulating element S is of the order of or slightly greater than that of the network of tips R so as not to prevent the extraction of the electrons, or to deflect their trajectories. A maximum deviation of the order of ten micrometers is acceptable.

During the operation of the cathode, the voltages to be applied to the grid G of the network of points can be such that the microline L and possibly the network of points R are connected to polarization sources FIG. 6a represents, in top view, a cathode according to the invention The semiconductor element S of modulation is always a MESFET transistor Its source S _s is brought to ground, its gate G _s connected to a polarization source E3 receives the HF microwave modulation signal and its drain D _s is connected to a first end of the microline L which is represented as a microstrip line

The second end of the microline L is connected to the gate G of the network of points R The geometry of the ribbon of the line L into two sections 11 1, 11 2 connected together by a capacitor C allows the adaptation between the transistor S and the network of tips R The microstrip line L is connected to a source of polarization E2 on the side of its first end This polarization applies to the drain D _s of the transistor S The wire connections at the two ends of the microline L are referenced 20 1 The MP points of the point network are connected to ground

This connection is made by an extension of the electrically conductive or semi-conductive layer 13 1, without covering with a dielectric layer, this extension being described in FIG. 7c

In the example described, the grid G of the network of points R is connected to a polarization source E1

Decoupling means C, L1, L2, L3 have been introduced in a completely conventional manner for a person skilled in the art. For this purpose, there is a capacitor C between the gate G _s of the transistor S and the input of the modulation signal. HF microwave, an inductance L3 between the polarization source E3 and the gate G _s of the transistor S, an inductance L2 between the polarization source E2 and the microstrip line L (drain side D _s of the transistor S), an inductance L1 between the polarization source E1 and the grid G of the network of points R

Similarly, FIG. 6b illustrates a cathode according to the invention in which the modulating semiconductor element S is a diode. The only differences from the diagram in Figure 6a are at the diode connections The cathode K of the diode is connected to ground and the anode A at the first end of the microline L which is also connected to the polarization source E2 A synchronization signal SY can be injected on the anode A of the diode The signal SY of synchronization can be electrical and a decoupling capacitor C "is then placed between the anode A and the arrival of the synchronization signal SY The synchronization signal could be optical and in this case the semiconductor modulation element S would be a optical component such as a photodiode

Instead of the cathode according to the invention comprising a network of tips R and a semiconductor element S of discrete modulation, it is possible that the cathode according to the invention is monolithic

Reference is made to FIG. 8 which shows such a monolithic cathode. The electrical connections to the local ground or to a polarization source have not been shown for the sake of clarity, but they can be produced according to one of the methods described. previously The network of points R, the microline L and the semiconductor element S of modulation are integrated on the same semiconductor substrate 200 with semi-insulating properties such as silicon carbide for example From the point of view of heat dissipation, gives all satisfaction

This common substrate 200 has one of its main faces coated with a conductive layer 201 which serves as a local ground plane. On the other main face, an area I is determined for at least one network of points R, an area II for microline L and an area

III for the semiconductor modulating element S

The modulation semiconductor element S is produced in zone III and this can be done as illustrated in FIGS. 5, the substrate 200 then being equivalent to the substrate 16. The network of points R and this can be done as illustrated in Figures 4, the substrate 200 then being equivalent to the electrically insulating or semi-insulating layer 13

Microline L is produced in zone II and its structure is equivalent to that shown in FIG. 3b The substrate 200 corresponds practically to that referenced 100 in FIG. 3b

With such a configuration, the transistor drain, the ribbon of the microline and the grid of the network of points can be made in the same step in the same material.

Similarly, the passivation layer 21 of the modulating semiconductor element, made of dielectric material, can extend in zone II by covering the substrate 200 and in zone I by forming the dielectric layer comprising the cavities 15.

Instead of making a network of points R comparable to that of FIG. 3a with wire connections, it is possible that it is comparable to one of the examples of FIGS. 7, that is to say with the points connected to the plane. of mass 201 by at least one metallized hole passing through the substrate 200.

Such a monolithic field effect cathode is very advantageous because it is compact, its cost is reduced compared to that of a cathode with discrete elements because it uses less material and its production takes less time.

Claims

1. Microwave field effect cathode comprising at least one network of emissive points (R), means (S) for producing a microwave modulation signal with a microwave semiconductor modulation element which adjoins the network of points (R) and means (L) for routing the modulation signal to the spike network (R), characterized in that the means for routing the modulation signal to the spike network (R) are a microline (L) short which introduces a practically negligible disturbance and realizes an impedance adaptation between the network of points (R) and the semiconductor element (S) of modulation.

2. Field effect cathode according to claim 1, characterized in that the microline (L) is a line comprising a conductive ribbon (11) connected at one of its ends to the network of points (R) and at the other end to the modulating semiconductor element (S).

3. Field effect cathode one of claims 1 or 2, characterized in that the semiconductor element (S) of modulation is of transistor type or of diode type.

4. Field effect cathode according to one of claims 1 to 3, characterized in that the microline (L) has a conductive strip (11) in two sections (11.1, 11.2) connected together by a capacitor (C) .

5. Field effect cathode according to one of claims 1 to 4, characterized in that the microline (L) is connected to a polarization source (E2).

6. Field effect cathode according to one of claims 1 to 5, characterized in that the microline (L) is connected to the semiconductor element (S) of modulation and / or to the network of points (R) by a wired connection (20.1).

7 Field effect cathode according to one of claims 1 to 6, characterized in that at least one element taken from the network of points (R), the semiconductor element (S) of modulation and the microline ( L) is discreet

8 cathode with field effect according to claim 7, characterized in that at least two elements taken from the network of points (R), the semiconductor element (S) of modulation and the microline (L) are integral d '' the same electrically insulating or semi-insulating support (100)

9 field effect cathode according to claim 8, characterized in that the two elements are mounted on one face of the support (100) the other face of which is coated with a conductive layer (101) which serves as a ground plane

10 Field effect cathode according to one of claims 7 to 9, characterized in that the semiconductor modulation element (S) is compatible with a transfer technique by microbossings

11 Field effect cathode according to one of claims 7 to 10, in which the network of points (R) comprises an electrically insulating or semi-insulating substrate (13) with on one side a conductive or semi layer (13 1) -conductive, emissive tips (MP) in electrical contact with the conductive or semi-conductive layer (13 1), a dielectric layer () provided with cavities (15) each housing one of the tips (MP), the dielectric layer (14 ) being surmounted by a conductive grid (G) which at least partially surrounds the cavities (15), characterized in that the substrate (13) is crossed by at least one metallized hole which contributes to electrically connecting the points (MP) to the other side of the electrically insulating or semi-insulating substrate (13)

12 Field effect cathode according to one of claims 7 to 11, in which the network of points (R) comprises an electrically insulating or semi-insulating substrate (13) with a layer (13 1) on one face conductive or semi-conductive, emissive points (MP) in electrical contact with the conductive or semi-conductive layer (13.1), a dielectric layer (14) provided with cavities (15) each housing one of the points (MP), the layer dielectric (14) being surmounted by a conductive grid (G) which at least partially surrounds the cavities (15), characterized in that the substrate (13) and the dielectric layer (14) are crossed by at least at least one hole metallized (82) which contributes to electrically connecting the grid (G) to the other face of the substrate (13).

13. Field effect cathode according to one of claims

11 or 12, characterized in that the metallized hole (73, 82) is extended by an electrical contact (74).

14. Field effect cathode according to one of claims 8 to 13, characterized in that the microline (L) is integrated into the support (100) electrically insulating or semi-insulating.

15. Field effect cathode according to one of claims 1 to 7, characterized in that the network of points (R), the microline (L) and the modulating semiconductor element (S) are integrated on a same semiconductor substrate (200).

16. Field effect cathode according to claim 15, characterized in that the semiconductor substrate (200) is semi-insulating such as silicon carbide.

17. Field effect cathode according to one of claims 15 or 16, characterized in that the microline (L) has a ribbon which extends on one side to form a grid (G) of the network of points (R) and on the other side to form a contact (D _s ) of the semiconductor element (S) of modulation.