FR2777113A1 - "ELECTRON TORCH" TYPE ELECTRON CANON - Google Patents

Abstract

The invention concerns an electron-emitting device comprising: a chamber (8) closed on one side by a membrane (6) capable of being traversed by an electron beam (4); a cathode with at least one microtip (2), for emitting an electron beam (4), characterised in that the cathode microtips are distributed by zones, according to a certain patterns, each zone comprising at least one microtip, the membrane having planar zones (26-1, 26-3) and one or several thicker reinforcing zones (26-2, 26-4, 26-6) separating the planar zones, each microtip zone being opposite a planar zone.

Description

"ELECTRON TORCH" TYPE ELECTRON CANON

  TECHNICAL FIELD AND PRIOR ART The invention relates to the production of an electron gun, in particular of the "electron torch" type, as well as various applications of this gun

with electrons.

  The article by L. Hanlon et al. "Electron window

  cathode ray tube applications ", J. Vac. Sci. Technol.

  B 4 (1), p. 305-309, 1986, describes a conventional cathode ray tube, in which a part of the slab where the electrons should strike the phosphorus of the screen is replaced by a thin membrane in BN or SiC. The membrane is thin enough for the beam electrons to pass through it. These electrons have: an energy of 20 keV or more, the thickness of the membrane is greater than µm, and its surface is of the order of mm2. 85% of the beam is transmitted and two applications (fluorescence and photocopying) are described. Mention is also made of metal membranes supported by a thicker mesh, to ensure the resistance to vacuum (embossed membrane). Because of their greater thickness, the transmission of these

membranes is less.

  In this first document, an electron gun, of the "TV cannon" type, produces the small diameter beam, and CRT (Cathodic Ray Tube) technology is

used in this realization.

  The article by J. Wieser et al. "Rare gas excimer light source ultraviolet vacuum", Rev. Sci. Instrum, 68 (3), p. 1360-1364 (1997) describes an experiment in which electrons emitted by a heated filament are accelerated to 10 or 20 keV, pass through a 300 nm window in Si3N4 and then excite a gas under pressure (Xenon), thus creating a

  UV radiation characteristic of this gas.

  In this second document, the strongest possible electron beam is sought so that the density of charges injected into the gas is high. This electron beam is produced by a heated filament. A significant part of this beam hits the membrane support and leads to heating of the support. The consumption of 10W in the filament suggests that several mA are emitted, which certainly leads to strongly heating the membrane and its support (at least lmAx20kV = 20W). Such a device is unsuitable for certain applications,

  especially for applications in biology.

  Furthermore, the dimensions of the first device are large, an electron gun measuring several centimeters. As for the second device, the distance of about 20 mm, necessary to be able to establish a potential difference of 20 keV between the filament and the membrane, leads to a burst of the electron beam emitted and a very low efficiency since only a fraction of the emitted beam passes through the membrane. Because of these losses of electrons in the membrane support as well as in the enclosure, the latter is not portal with bare hand without thermal protection. STATEMENT OF THE INVENTION The subject of the invention is firstly an electron emitting device comprising: - an enclosure, closed on one side by a membrane which can be traversed by an electron beam, - a cathode with at least a microtip, to emit

an electron beam.

  The use of a microtip cathode, which emits a directive beam, makes it possible to produce a device of small dimensions, presenting no problem of overheating, therefore portable with bare hands and

  insertable into a cryogenic assembly.

  Means for guiding an electron beam emitted by the cathode can also be provided. These

  means reinforce the directive character of the beam.

  In addition, the power consumption is low and reduced compared to the consumption of known torches, because there is no loss of power, neither in the enclosure, nor in the membrane or

its possible support.

  According to a particular embodiment, the microtip cathode has a distribution of microtip zones, according to a certain drawing, the membrane having flat and thin zones and thicker reinforcement zones, separating the flat and thin zones according to the drawing of distribution of microtip zones. Thus, the emissive parts of the cathode and the flat portions of the membrane, located between the

  reinforcement zones, correspond two by two.

  However, if each microtip zone corresponds to a flat zone of the membrane, it is possible that a flat zone corresponds to one or more microtip zones. Indeed, the reinforcements have only a mechanical action and do not necessarily follow the design of the microtip zones; Preferably, the means for guiding the electron beam include means for producing a magnetic field. This magnetic field can

  be adjustable in intensity and direction.

  The invention also relates to various

  applications of the electron source described above

  above. In particular, the electron source can be applied: - to optically pump a solid, light-emitting sample, - to excite a gaseous or liquid medium, for example a rare gas, in order to produce ultraviolet radiation, - to polymerize organic compounds, - to strike a target stuck against the membrane and produce X-rays,

  - to burn or treat biological tissue.

Brief description of the figures

  In any case, the characteristics and advantages of the invention will appear better on

  light of the description which follows. This

  description relates to the exemplary embodiments,

  given by way of explanation and without limitation, with reference to the appended drawings in which: - Figure 1 represents a first mode of

realization of the invention.

  - Figure 2 shows an example of a cathode

with microtips.

  - Figures 3 and 4 show two more

  exemplary embodiments of the invention.

  Detailed description of embodiments of the invention A first example of embodiment of

  the invention is illustrated in FIG. 1.

  A microtip cathode 2 is disposed in an enclosure 8. It produces and directs an electron beam 4 towards a membrane 6, which closes the enclosure 8 on one of its sides. The cathode 2 is itself polarized at -10kV or -20kV relative to the enclosure 8 and to the membrane 6. Means can also be provided (for example permanent magnets) to create a magnetic field B, for example from 1000G to 2000G which can be steered, and possibly be adjustable in direction and intensity, so that

  the impact of the electrons takes place on the membrane 6 itself

  same, and not on enclosure 8 and the perimeter more

thick membrane.

  These means for guiding the electron beam may not be provided, for example in the case where the beam is not very intense (for example: a few pA which pass through the membrane for a few

tens of amps emitted).

  For a membrane thickness of 100 to 300 nm in light material (Si3N4, Si, SiC, diamond, etc.) the transmission of the membrane is high. The surface of the membrane is typically of the order of mm2. The dimensions of such a torch, with a single cathode, can be, for example: length of the enclosure 8: 60 mm, - diameter of this enclosure: 10-15 mm, material of the enclosure 8: conductive or ceramic , for example stainless steel or alumina, - membrane: extension of a few mm2 for the thinnest area (10Onm); material: Si3N4, Si, SiC, diamond, etc. - vacuum in enclosure 8: of the order of 10-embars, - cathode 2: emissivezlmm2 surface; emitted current: lmnA for 100V of extraction voltage, - transmitted current: 0.8 mA for a voltage

20kV acceleration.

  Cathode 2 consists of a microtip cathode, for example of the same type as that used and described in patent FR-B-2,679,653

  (EP-A-0 524 870), and which is illustrated in FIG. 2.

  This cathode essentially comprises, on a substrate 11, for example made of glass, a layer of silica 13 covered with a resistive layer 15, for example made of silicon. On the resistive layer 15 are located 1 or 2 cathode electrodes 16 intended for the supply

  microtips 17 which are for example made of molybdenum.

  An insulating layer 18 pierced with holes in which the microtips 17 are located separates the electrode or cathode electrodes 16 from the electrode or electrodes of the electron extraction grid 19 (which may be made of Niobium). The electron extraction grid 19 is perforated above each microtip 17 so as to allow the emission of electrons. For a single non-matrixed microtip cathode, there is at least one electrode for polarizing the tips 16 and one electrode for

polarize the grid 19.

  The microtips 17 being brought to a potential Vp, the gate electrodes 19 to a potential Vg, the electrons are emitted with an energy

initial kinetics -e- (Vg-Vp).

  For more details concerning this cathode, reference will be made to document FR-B-2 679 653 (EP-A-0 524 870), in particular as regards the choice

  materials from which such a cathode is made.

  A method of producing this cathode is for example described in patents FR-2,663,462 and FR-2,593,953, and essentially comprises the implementation of the techniques of depositing layers by evaporation and sputtering, photoengraving and etching reactive ion of the layers formed. The realization of a matrix structure for this cathode is possible, for example if one wants to have two or more transmitters on the same substrate

  cathode, as explained below.

  As illustrated in FIG. 3, in the case of a membrane 26 of larger surface, this can be structured and thicker in places in order to resist bending, that is to say the pressure difference between exterior and interior of the enclosure. The membrane then has flat areas 26-1, 26-3 and reinforcement areas 26-2, 26-4, 26-6. In this case, the microtip cathode 22 can itself be structured (possibly in the form of a matrix cathode, with zones independent of each other) so that the emitted electrons go to the thinned zones 26-1, 26-3 rather only in the thick areas of the membrane 26-2, 26-4, 26-6 (which would otherwise lead to heating of the membrane, due to a strong absorption of the electron beam

and, perhaps, when it breaks).

  The chambers 8, 28 are moreover connected to pumping means, not shown in FIGS. 1, 3 in order to produce a vacuum at a desired pressure,

  for example of the order of 10-7 to 10-8 rmbar.

  In the two figures 1 and 3, the references 20, both denote a tank of the enclosure 8, 28, and the references 12, 32 a trap (or "getter") making it possible to absorb residual impurities, contained in the enclosure 8, 28 in order to maintain the basic vacuum when the pumping means have been disconnected. Means 14, 34 for supplying voltage supply the voltages necessary for

operation of cathodes 2, 22.

  Another exemplary embodiment is given in FIG. 4. A membrane 36 is produced on a reinforcing structure 37, and has a diameter D of approximately 3 mm. The microtip cathode 42 has as many emissive zones as there are planar zones

in the membrane 36.

  In this application, the cathode emits

  simultaneously multiple electron beams.

  Each zone of the microtip cathode can,

  for example, emitting a beam with a diameter of 100 µm.

  All the emissive areas of the cathode are produced on a substrate 44 of width L10Omm. The structure illustrated schematically further comprises means not shown in Figure 4 and already described above: queusot, getter, means for producing a magnetic field of direction and

  of defined or variable intensity.

  The microtip cathode can have any distribution of microtip areas, depending on the desired application. Whatever the distribution of the microtips, the membrane can be provided with a reinforcement mesh reproducing the design of the complementary part of this distribution, so that the cathode and membrane are two homologous, even homothetic, images. According to another variant, it is possible to have a slender-shaped membrane placed behind a cathode which is also slender, while having a uniform electron emission over the entire length of the membrane. This possibility does not exist with a “filament” type electron source, because the central part of it is always much hotter than its edges and, therefore, the emission of electrons is good

  stronger in the center than at the edges.

  Device applications according to

the invention will be described.

  According to a first application, the device can be used to generate an electron beam of 10 to 20 keV. This beam strikes a light-emitting sample, of the III-V semiconductor compound type, to generate blue or ultraviolet light (GaN or AlN type compound in quantum well structure). It is a "microtip laser", in which the light source is placed outside the vacuum, although as close as possible to the membrane. According to another application, the electrons produced can be injected into a gas or a liquid in order to induce a light excitation (for example in gaseous or liquid xenon (-110 C), the de-excitation of the excimers producing transitions for example at 170 nm). The device can be that of Wieser's article, the electron source being that

according to the invention.

  According to other applications, it is possible to use an electron beam produced by the source according to the invention, for: - polymerizing organic compounds, - striking a target with a material capable of producing X-radiation, for example a material such as Mo or Cr "stuck" against the membrane or close to it, and produce an X emission. The cooling of the target is then greatly facilitated by the fact that it is outside the vacuum of the chamber containing the electron source. Moreover,

  the anode is easily interchangeable.

  - "burn" a biological tissue, as a laser would do, but on a much smaller depth of penetration, hence possible applications in dermatology. Finally, there is a miniature electron source suitable for use in any application

  o the source must be protected from its environment.

  For example, this source can be placed in a high pressure plasma source, in order to inject

electrons to this one.

Claims (12)

  1. Electron emitting device comprising: - an enclosure (8, 28) closed on one side by a membrane (6, 26) which can be crossed by an electron beam, - a cathode with at least one microtip (2, 22), for   emit an electron beam (4, 24).   2. Device according to claim 1, the microtip cathode having a distribution of the microtips by zones, according to a certain drawing, each zone comprising at least one microtip, the membrane having planar zones (26-1, 26-3) and a or several thicker reinforcement zones (26-2, 26-4, 26-6) separating the flat zones, each zone of   microtip facing a flat area.   3. Device according to one of claims 1 or 2, further comprising means for guiding the   electrons emitted from the cathode, towards the membrane.   4. Device according to claim 3, the means for guiding the electron beam comprising   means for producing a magnetic field.   5. Device according to claim 4, the magnetic field being adjustable in intensity and in direction.   6. Device according to one of claims 1   at 5, the membrane and the cathode both being of slender shape.   7. Device for producing electromagnetic radiation, comprising: - a solid capable of emitting such radiation, under electronic impact,   - a device according to one of claims 1 to 6   to produce a beam of excitation electrons from the solid. 8. Device for producing X-rays, comprising a device according to claim 7, the solid being a target of a material placed facing the   membrane of the electron emitting device.   9. Device for producing ultraviolet radiation, comprising: a cell containing a rare gas, in the gaseous or liquid state, provided with an optical window transparent to the working wavelength, - a device emitting electrons according to one of claims 1 to 6.   10. Process for the polymerization of an organic compound comprising the production of an electron beam using a device according to one of   claims 1 to 6 and the polymerization of the compound   organic with this electron beam.   11. Method for treating biological tissue comprising the following steps: - producing an electron beam using a   device according to one of claims 1 to 6,   - treat the tissue using this electron beam.   12. Device for treating biological tissue comprising an electron emitting device according to one of claims 1 to 6.