METHOD OF PRODUCING A FIELD EMISSION CATHODE, A FIELD EMISSION CATHODE AND A LIGHT SOURCE

Abstract: A method of producing a field emission cathode for a light source, and including at least one field emitting body having a field emitting surface, wherein the method includes modifying said emitting surface so as to provide at least one electric field emitting irregularity in each said surface. The method is distinguished by at least one beam of laser light being brought to shape the body and to simultaneously contact the field emitting surface and thereby provide a modifying treatment to the surface of the body. The invention also concerns a field emission cathode so produced and a light source including such a field emission cathode.

Description: METHOD OF PRODUCING A FIELD EMISSION CATHODE, A FIELD EMISSION CATHODE AND A LIGHT SOURCE FIELD OF THE INVENTION This invention concerns a method of producing a field emission cathode for a light source according to the preamble of claim 1. It also concerns a field emission cathode so produced and a light source including such a field emission cathode. BACKGROUND OF THE INVENTION WO 96/25753 concerns field emission illuminating devices employing a cold cathode, and in particular a method according to the above. Field emission illuminating devices have major advantages over other types of illuminating devices such as fluorescent tubes, since the latter require complicated external electrical devices for the function and since they typically contain materials having negative environmental effects. In fluorescent tubes, gas discharges are employed for emitting radiation onto a fluorescent material that in turn emits visible light. Field emission illuminating devices, on the other hand, allow inclusion of environmental harmless materials and can function using simpler and more economic equipment . Further in WO 96/25753 it is envisaged that a cold field emission cathode for use in a field emission light source is provided with a surface geometry that facilitates the obtaining of local high electric field strengths for electron field emission. It is further envisaged to form a field emission cathode with an emitting surface having a particular topography facilitating electron field emission. It is further foreseen that the irregularities are trained in such a way that high mechanical and electrical durability as well as long working life and high energy emission per surface area unit is obtained. According to WO 96/25753 this is achieved by irradiating the emitting ends of a fibre with ions of a low work function material so as to lower the electron work function of the emitting ends. This irradiation step causes sharp irregularities in the emitting ends. A modifying step is then carried out in order to train the very high and sharp irregularities into a rounded-off shape which will result in emitting ends being effective and durable so as to result in a cathode having a long working life. The method according to the cited document results in a well functioning cathode, the manufacture thereof is however, complicated, time consuming and thereby relatively expensive. SUMMARY OF THE INVENTION It is a primary object of this invention to provide a method for the production of a cold field emission cathode for a light source, which does not suffer from the mentioned drawbacks of the previously known method. It is also a general object to provide a method for producing an effective and long-lasting cold field emission cathode for a light source. This object is achieved in a method according to the above through the features of the characterising portion of claim 1. This way manufacture of a cold field emission cathode for a field emission light source is radically simplified and thereby more economic. Contrary to what could have been expected, it is possible to avoid time consuming and complicated steps according to the prior art. The treatment which is carried out by bringing a beam of laser light into contact with the field emitting surface of the emitting bodies results in a thermo-chemical treatment ensuring better control that the desired result is obtained with respect to resulting dimensions and distribution of field emitting elements when the inventive method is carried out. This in contrast to the previous method where inherent weaknesses in the irradiating and modifying steps can not guarantee even results on separate parts of bundles of fibres, which in practice are subjected to the treatment. The surprising reason for the good result from this treatment is explained by the synergy-effect of the energy of the laser beam and along the length of the beam in combination with the slightly uneven structure of the material in the emitting bodies, where in practice different regions have different properties. These different regions respond differently to the laser treatment, which subsequently results in the creation of irregularities having the desired high and rounded topography. With respect to carbon fibres, that material possesses microstructures of oriented, more or less ordered phase, such as crystallites of graphite, providing high strength, elastic modulus, conductivity and chemicals stability in a matrix and surface skin of amorphous carbon. The invention envisages the modification of the material by laser treatment whereby the crystallites and the amorphous material responds in such a way to the laser treatment that the desired topography of irregularities is obtained. Further, the electrical properties of carbon fibre surface skin, which is inferior to the ordered phase, can be improved by laser treatment. The fact that the body is shaped at the same time as it is surface modified provides several further advantages. Cutting the emitting body with laser radiation so as to trim its dimensions, its length etc. and simultaneously modify the emitting surface with the laser radiation provides a rational and economic method. This way mechanical or like cutting may be avoided, whereby the drawbacks of mechanical cutting such as deforming body surface portions are avoided. Laser cutting gives the advantage over mechanical shaping, such as mechanical cutting, that dust formation can be reduced to a minimum. In light sources including emitters produced according to the prior art, it has been discovered that the efficiency and the working life is reduces due to presence of dust emanating from mechanical trimming and cutting of the emitters. Further, an illuminating device including a field emission cathode manufactured according to the inventive method, where the emitting body is laser shaped and simultaneously the emitting surface modified will be more effective. The reason for this is that more current may be led through the cathode and thereby more light may be emitted from the light source than in a prior art light source in otherwise like conditions due to the surface being more even. A more lasting light source with extended working life therefore will result. By the emitting surface being more even, better electron emitting action will result. This in turn results in a greater current and subsequently more light. As a comparison, it could be mentioned that in the case of fibres, fibres treated according to this invention have shown to be able to allow a current being five times as great as a current through a device according to the prior art. This could be used so as to construct light sources having a longer working life than before and/or more intensive light sources than before. The provision of an even emitting surface also is an advantage because of the more even and pleasant light distribution in comparison with a prior art light source. The invention could be applied for different kinds of field emitting bodies such as strand fibres of the kind being used according to WO 96/25753, so called carbon nano tubes (CNT) , so called diamond like carbon (DLC) , porous carbon foam material such as reticulated vitreous carbon (RVC) . Materials to be subjected to the method are generally substantially carbon materials, but other similarly functioning materials may be used as the emitting body material. Other similar materials and other body forms are thus not excluded from this invention. Basically the invention is a one step process, where simultaneously cleaning, cutting, shaping and surface modification is carried out. In some instances, however, the laser cut and somewhat modified surface may need further laser treatment in order to obtain a more effective emitting surface. According to one aspect of the invention this may be achieved by contacting the pre-treated surface with further laser radiation, which may be of a different kind and/or intensity than the first laser radiation for cutting, so as to optimise the treatment. The method is also applicable to integral porous bodies such as carbon foam material bodies. Such a body is comprised by interconnected thin structures so as to be referred to as being an integral structure. It could for example be shaped with plane or cylindrical emitting surfaces by the laser cutting or shaping. The resulting surface is modified so as to gain a preferred electron emitting surfaces similar to what has been described above for fibres. A field emission cathode, manufactured this way is comparatively easy and inexpensive to manufacture. The invention also is also applicable to woven structures made from yarns containing carbon or a similar acting material. DE-A1-196 53 820 describes production of field emission surfaces, in particular for flat screen application. According to this document laser radiation is applied locally, for example through a masking technique, to a diamond or diamondlike carbon layer. This way, areas protruding above the neighbouring areas will be obtained something that is highly unwanted in respect of the present invention. Further advantages are obtained by this invention and will be clear from the following detailed description of embodiments given by way of examples and with reference to the drawings. BRIEF DESCRIPTION OF DRAWINGS Fig 1 shows a field emission light source according to the invention, Fig 2 shows in an enlarged scale an emitting surface of an emitting body, Fig 3 shows a field emission light source according to a second embodiment, Fig 4 illustrates diagrammatically a first embodiment of the method according to the invention, and Fig 5 illustrates diagrammatically a second embodiment of the method according to the invention. DESCRIPTION OF EMBODIMENTS In a preferred method according to the invention a field emission cathode is made from bundles of fibres which are commercially available as polyacrylnitril carbon fibres. Other suitable material containing carbon or similar material having a diameter in the range of few microns (μm) may be used equivalently. As is shown in Fig. 1, a light source is shown having a field emission cathode in the form of fibre bundles 1, said bundles being arranged in a matrix form and being arranged on a conductive substrate 17. In the same plane as the matrix, and is close proximity thereto, in the order of tenth of millimetres, and above the emitting ends of the bundles 1, there is provided a modulator electrode 12, having an aperture centred around each bundle. The substrate 17 and the modulator 12 rest on dielectric supports 18 inside an evacuated glass container with an upper boundary glass plate 15 and a lower boundary glass plate 16. Opposite the bundles 1 and the modulator, there is provided, on the inside of the upper boundary 15, an anode layer 13 and a luminescent layer 14. The anode layer 13, the modulator 12 and the substrate 17 have electrical terminals A, B, C, respectively, for application of voltages leading electrons from the bundles 1 via the modulator apertures to the luminescent layer 14 in connection with the anode layer 13. When electrons enter the luminescent layer 14, light is emitted escaping the transparent anode 13 and the glass container. The light source may also be constructed as a diode, i. e. without a modulator. It is important that the fibre bundle matrix arranged on the substrate 17 has an even surface and that the surface of each emitting body is provided with a number of irregularities shaped so as to easily reject electrons when the substrate is subjected to a potential. Further the surfaces should be provided with irregularities being shaped, rounded, such that no exaggerated deformation of said irregularities takes place during use. Such deformation could otherwise be essentially detrimental to the working life of the light source. According to the invention, the unit consisting of the substrate 17 and the fibre bundles is shaped and treated with laser light whereby the fibres are accurately cut to form an overall even surface, said surface by the laser treatment being provided with the desired rounded irregularities. If necessary, for perfecting the surface, an additional laser treatment through for example sweeping the cut surface with laser radiation, could be desired. Fig 2 shows a profile 10 of one fibre 9 after laser beam cutting and treatment. The emitting end profile has high but slightly rounded irregularities 11. Fig 3 shows a field emission light source 20 employing a field emission cathode 21 being made from carbon foam material. A modulator grid is indicated as 22 and the anode layer as 23, whereas the phosphorus layer is indicated with 24. The emission cathode 21, the modulator grid 22 and the anode layer as 23 are provided with terminals A, B and C respectively so as to subject these elements to appropriate potential. The surface 25 is evenly cut so as provide an emitting surface located substantially in one plane, and to the desired dimensions with the aid of laser radiation. If necessary the surface 25 is further treated with subsequent laser radiation in order to perfect the emitting surface. As has been indicated above, the method may be used for differently shaped and otherwise different field emission cathodes. In case of a circular cylindrical cathode for radial emission, either being made from fibres as is shown in WO 98/57344 or WO 98/57345 or from a circular cylindrical porous body of carbon foam material, laser cutting and/or treatment of the emitting surfaces may be applied by for example rotating the cathode body and simultaneously sweeping with the laser beam. This is illustrated in fig 4. A cylindrical body 30 having its peripheral surface provided with material to be shaped and modified is rotated around its axis. A laser 31 sweeps the surface of the body, by being pivotable around an axis, with laser radiation, interrupted line, so that the laser ray essentially follows the tangent of the cylindrical body 30. This way the body is shaped, whereby material protruding above a certain level is removed by the effect of the laser radiation, and modified according to the above explanation. An alternative method is illustrated in fig 5, where a plane body 32 is swept with laser radiation from laser 33. The sweep is obtained by sideways displacement of the laser with respect to the body at the upper surface, which is to be shaped and modified. The same effect is obtained as in the method illustrated in fig 4. Sweep of a laser ray may also be obtained by moving optical devices such as mirrors and lenses according to per se known methods. In respect of this invention it is thus important that the laser radiation is directed along the surface to be treated so as to be tangent to that surface in case of a curved surface or coincide with directions of the surface in case of it being plane. An advantage of the invention is that the resulting material could be controlled so as to purity and/or to addition of some wanted characteristics such that different properties may be given to the emitting surfaces. This is referred to as etching. This depending of the composition of the atmosphere surrounding the treated body during the treatment and on the beam intensity and spot size as well as the flow characteristic of the atmosphere. In an inert atmosphere, evaporation or sublimation of the amorphous carbon can be achieved. In the presence of oxygen or hydrogen, there may be catalytic graphitization of the amorphous carbon. In an atmosphere including nitrogen or air surface groups with bonded oxygen and/or nitrogen may be formed in a moderate flow atmosphere. High temperature erosion may be carried out under high flow atmosphere conditions. It is also possible, employing per se known processes, to add substances to the emitting surfaces so as to alter the properties thereof. The invention may be included in methods involving other per se known steps such as the ones referred to in the previously mentioned PCT documents. In case the carbon fibres are surface treated with for example a polymeric material, and if that surface treatment material needs to be removed, this could be obtained in some instances already during the laser treatment or otherwise in a heat treatment process of in any other suitable process. In principle any kind of laser may be used, having sufficient intensity and working in the visible light area or near that area. The light beam may be of circular or elliptic section or any other suitable section. The configuration of the laser beam may be adapted to the material to be modified. For the total process of cutting, etching and surface treatment, as an example, feed in the range of about 0,01 - 0,1 m/s may be used with beam intensities of 0,1 - 10 kW/mm2. Commercially available JAG and diode laser between about 100 - 1000 W may be employed. The beam may be pulsed or not pulsed. The shaping of the emitting body is achieved by the evaporation of unwanted material and local melting in the emitting surface of the material to be treated by the laser radiation. This way a cleaning effect will result. Small amounts of excess material as well as possible other non- stable material being present will be fixed by an etching effect or be evaporated. A surface thus cleaned, without loose particles, increases stability and working life of a resulting light source. The shaping may be to any desired shape. As examples flat and cylindrical bodies may be formed. Flat emitting surfaces (having the emitting portions along a plane) may be produced by a linear sweeping displacement sideways of a laser beam relative to a body (fig 5) or alternatively of a body to be shaped and modified relative to a stationary laser beam. Methods of making cylindrical emitting surfaces (having the emitting portions along a cylinder surface) other than the method mentioned above (fig 4) are as examples: either rotating the body to be shaped and modified relative to a stationary laser beam or rendering a laser beam a circular sideways sweep path relative to the body.

Claims: Claims : 1. A method of producing a field emission cathode for a light source, and including at least one field emitting body having a field emitting surface, wherein the method includes modifying said emitting surface so as to provide at least one electric field emitting irregularity in each said surface, characterised in that at least one beam of laser light is brought to shape the body and simultaneously to contact the field emitting surface and thereby provide a modifying treatment to the surface of the body. 2. The method of claim 1, characterised in that it is carried out on at least one field emitting body in the form of a fibre. 3. The method of claim 2, characterised in that each beam of laser light is arranged to cut each fibre simultaneously as the emitting surface is modified. 4. The method of claim 1, characterised in that it is carried out on an integral porous body. 5. The method of claim 4, characterised in that the body is produced from a carbon foam material. 6. The method of claim 1, characterised in that the body is formed from a carbon nano tube . 7. The method of claim 1, characterised in that the body is produced from a woven carbon yarn containing material. 8. The method of any of the claims 1 - 7, characterised in that the surface is further modified with further laser treatment. 9. The method of any of the previous claims, characterised in that the laser light is arranged to sweep the emitting surface. 10. The method of any of the previous claims, characterised in that laser light in or near the visible light area is used. 11. The method of any of the previous claims, characterised in that a laser beam intensity of about 0,1 - 10 kW/mm2 is used. 12. The method of any of the previous claims, characterised in that the laser beam is moved with a speed of about 0,01 - 0,1 m/s during the cutting and/or treatment. 13. The method of any of the previous claims, characterised in that the laser treatment is carried out under atmospheric conditions . 14. The method of any of the previous claims, characterised in that the laser treatment is carried out under inert atmosphere conditions. 15. A field emission cathode, characterised in that it is produced through a method according to any of the claims 1 - 14. 16. A light source comprising an evacuated container having at least one wall being provided with a luminescent layer and a conductive layer forming an anode, a field emission cathode and means for creating an electrical field for emission of electrons from the cathode, characterised in that it includes a field emission cathode according to claim 15.

Inventors: ANDERSSON CARL-HAAKAN, FORSBERG GUNNAR, ANDERSSON, CARL-HAAKAN, FORSBERG, GUNNAR
Applicants: ANDERSSON CARL HAAKAN, FORSBERG GUNNAR, LIGHTLAB AB, ANDERSSON, CARL-HAAKAN, FORSBERG, GUNNAR
Application year: 2000
Main classification: H01J1
IPC codes: H01J1, H01J9, H01J61, H01J63