GB2344686A - Field electron emission materials and devices - Google Patents

Abstract

To make a field electron emission material, insulator particles 401, perhaps with a textured surface, are partially embedded in a conducting substrate 402. This may be done by coating a conductive substrate with a mixture of insulating particles and solder, then heating and etching the solder, by coating a conductive substrate with a mixture of insulating particles and a chemical that can be reacted chemically to from a conducting matrix around the particles, then etching the matrix, or by pressing insulating particles into a soft conducting substrate. The material so formed may interface with a vacuum 405 to provide a MIV device. When an electric field 406 is applied between the conducting substrate 402 and a remote anode, the field penetrates the particles 401 and is concentrated in regions 407. At some critical field, a channel 403 is formed which facilitates the transport of electrons 404 injected from the conducting substrate 402 through the particles 401 into the vacuum 405. A layer may be formed between the substrate 402 and the particles 401 to facilitate electron injection. The material is particularly of use in field emission displays for which purpose it may incorporate a ballast resistor. Each insulating particle 401 may comprise a ceramic, oxide, carbide, nitride, boride, glass, silicate, aluminosilicate, carbon-based insulator, inorganic insulator, polymer, or highly cross-linked polymer. The conducting substrate 402 may comprise a metal, semiconductor, carbide, nitride, oxide, boride, or carbon-based conductor.

Description

FIELD ELECTRON EMISSION MATERIALS AND DEVICES This invention relates to field electron emission materials, and to devices using such materials.

In classical field electron emission, a high electric field of, for example, 3x109 V m~l at the surface of a material reduces the thickness of the surface potential barrier to a point at which electrons can leave the material by quantum mechanical tunnelling. The necessary conditions can be realised using atomically sharp points to concentrate the macroscopic electric field. The field electron emission current can be further increased by using a surface with a low work function. The metrics of field electron emission are described by the well known Fowler-Nordheim equation.

There is considerable prior art relating to tip based emitters, which term describes electron emitters and emitting arrays which utilise field electron emission from sharp points (tips). The main objective of workers in the art has been to place an electrode with an aperture (the gate) less than 1 um away from each single emitting tip, so that the required high fields can by achieved using applied potentials of 100V or less-these emitters are termed gated arrays. The first practical realisation of this was described by C A Spindt, working at Stanford Research Institute in California g. Appl. Phys. 39, 7, pp 3504-3505, (1968)). Spindt's arrays used molybdenum emitting tips which were produced, using a self masking technique, by vacuum evaporation of metal into cylindrical depressions in a Si02 layer on a Si substrate.

In the 1970s, an alternative approach to produce similar structures was the use of directionally solidified eutectic alloys (DSE). DSE alloys have one phase in the form of aligned fibres in a matrix of another phase. The matrix can be etched back leaving the fibres protruding. After etching, a gate structure is produced by sequential vacuum evaporation of insulating and conducting layers. The build up of evaporated material on the tips acts as a mask, leaving an annular gap around a protruding fibre.

An important approach is the creation of gated arrays using silicon micro-engineering. Field electron emission displays utilising this technology are being manufactured at the present time, with interest by many organisations world-wide.

Major problems with all tip-based emitting systems are their vulnerability to damage by ion bombardment, ohmic heating at high currents and the catastrophic damage produced by electrical breakdown in the device. Making large area devices is both difficult and costly.

In about 1985, it was discovered that thin films of diamond could be grown on heated substrates from a hydrogen-methane atmosphere, to provide broad area field emitters-that is, field emitters that do not require deliberately engineered tips.

In 1991, it was reported by Wang et al (Electron. Lett., 27, pp 1459-1461 (1991)) that field electron emission current could be obtained from broad area diamond films with electric fields as low as 3 MV m'. This performance is believed by some workers to be due to a combination of the negative electron affinity of the (111) facets of diamond and the high density of localise, accidental graphite inclusions (Xu, Latham and Tzeng : Electron. Lett., 29, pp 1596-159 (1993A although other explanations are proposed.

Coatings with a high diamond content can now be grown on room temperature substrates using laser ablation and ion beam techniques.

However, all such processes utilise expensive capital equipment and the performance of the materials so produced is unpredictable.

From the 1960s onwards, another group of workers has been studying the mechanisms associated with electrical breakdown between electrodes in vacuum. It is well known (Latham and Xu, Vacuum, 42, 18, pp 1173-1181 (1991A that, as the voltage between electrodes is increased, no current flows until a critical value is reached, at which time a small, noisy current starts flowing. This current increases both monotonically and stepwise with electric field until another critical value is reached, at which point it triggers an arc. It is generally understood that the key to improving voltage hold-off is the elimination of the sources of these prebreakdown currents. Current understanding shows that the active sites are either metal-insulator-vacuum (structures formed by embedded dielectric particles or conducting flakes sitting on insulating patches such as the surface oxide of the metal. In both cases, the current comes from a hot electron process that accelerates the electrons, resulting in quasi-thermionic emission over the surface potential barrier. This is well described in the scientific literature, e. g. Latham, High Voltage Vacuum Insulation, Academic Press (1995J.

Figure 1 shows the situation as described in the above publication, where a particle 101 is in contact with conducting substrate 100. There is an area of surface oxide 102 around the particle in some of the illustrations in the publication. At a critical electric field, a channel 104 forms and electrons 103 are emitted into the vacuum 105.

In this specification, by a"channel"or"conducting channel", we mean a region of an insulator where its properties have been locally modified-for example, by some forming process. In the example of a conductor-insulator-vacuum (e. g. MIVD structure, such a modification facilitates the transport of electrons from the back contact (between conductor/electrode and insulator), through the insulator into the vacuum.

In the example of a conductor-insulator-conductor (e. g. MIM) structure, such a modification facilitates the transport of electrons from the back contact, through the insulator to the other conductor/electrode.

It is to be appreciated that both types of emitting sites referred to in the above work are unwanted defects, occurring sporadically in small numbers, and the main objective in vacuum insulation work is to avoid them. For example, as a quantitative guide, there may be only a few such emitting sites per cm2, and only one in 103 or 104 visible surface defects will provide such unwanted and unpredictable emission.

Accordingly, the teachings of the abovework have been adopted by a number of technologies (e. g. particle accelerators) to improve vacuum insulation.

Latham and Mousa (z. Phys. D : Appl. Phys. 19,. pp 699-713 (1986J) describe composite metal-insulator tip-based emitters using the above hot electron process, and in 1988 S Bajic and R V Latham, Vournal of Physics D Applied Physics, vol. 21 200-204 (19882), described a composite that created a high density of metal-insulator-metal-insulator-vacuum (MIMIV) emitting sites. The composite had conducting particles dispersed in an epoxy resin.

The coating was applied to the surface by standard spin coating techniques.

Much later in 1995, Tuck, Taylor and Latham (GB 2304989) improved the above MIMIV emitter by replacing the epoxy resin with an inorganic insulator that both improved stability and enabled it to be operated in sealed off vacuum devices.

All of the inventions described above rely on hot electron field emission of the type responsible for pre-breakdown currents but, so far, no method has yet been proposed to produce emitters with a plurality of insulating particle MIV emitters on or embedded in a conducting surface in a controlled manner.

In Patent Application PCT/US90/05193, Chason describes a method of forming an emitter that consists of preformed emitter particles embedded in a conducting surface but specifies that they should be electrical conductors. In US5608283, Twitchell et al describe a similar arrangement which uses carbon-based"non-insulating"particles.

In European patent application 0 709 868, Sungho describes an emitter formed by growing diamond crystallites with a high defect density on a conducting substrate, or by producing a composite of said crystallites in a conducting matrix. The specification describes at some length the special properties of diamond"high structural integrity and negative electron affinity"that make it possible to produce an electron emitter based upon it. In co-pending European patent application 0 675 519 from the same company, Eom et al references the aforementioned document and specifies that the particles must be electrical conductors.

In US Patent 4 663 559, Christensen describes a cermet-based field emitter where the electrons are emitted through the insulator. The material is a nano-composite with particles of an average diameter of 3.5 nm. Because of the small diameter of the particles, the relatively open structure of the composite and the fact that it is used in the form of a layer which is several particle diameters thick, this emitter material requires extraction fields in the region of 500 MV/m. As such fields cannot be sustained between parallel electrodes, Christensen goes on to describe a microgun arrangement that uses a truncated conical electrode, with the emitter on the flat top, to enhance the macroscopic field Preferred embodiments of the present invention aim to provide cost-effective, broad-area, field-emitting materials based upon insulating particles in or on a conducting surface, that operate at electric fields sustainable between substantially parallel electrodes. Furthermore, such emitter materials may be economically deposited and patterned, and formed into devices incorporating the emitter materials. In particular, preferred embodiments of the invention seek to avoid, when possible, expensive, diamond-based materials.

The materials may be used in devices that include: field electron emission display panels; lamps, high power pulse devices such as electron MASERS and gyrotrons; crossed-field microwave tubes such as CFAs ; linear beam tubes such as klystrons; flash x-ray tubes; triggered spark gaps and related devices; broad area x-ray sources for sterilisation; vacuum gauges; ion thrusters for space vehicles; particle accelerators; ozonisers; and plasma devices and reactors.

According to a first aspect of the present invention, there is provided a method of forming a field electron emission material, wherein a plurality of insulating particles are embedded in an electrically conducting substrate, to provide emission sites at said particles. By embedded, it is meant that each such particle is as a minimum firmly fixed to, and may be partially buried in, the conducting substrate, provided that it at least partly protrudes from the substrate.

Preferably, each said particle comprises a ceramic, oxide, carbide, nitride, boride, glass, silicate, aluminosilicate, carbon-based insulator, inorganic insulator, polymer, or highly cross-linked polymer.

Preferably, said electrically conducting surface comprises a metal, semiconductor, carbide, nitride, oxide, boride, or carbon-based conductor.

Preferably, said particles are in a size range having a lower limit of 10,100 or 500 nanometres and an upper limit of 10,20,100 or 500 micrometres.

Preferably, the distribution of said sites over the field electron emission material is random.

Said sites may be distributed over the field electron emission material at an average density of at least 102 crri z.

Said sites may be distributed over the field electron emission material at an average density of at least 103 cl-1, 104 cm2 orlOs cm~2.

Preferably, the distribution of said sites over the field electron emission material is substantially uniform.

The distribution of said sites over the field electron emission material may have a uniformity such that the density of said sites in any circular area of lmm diameter does not vary by more than 20% from the average density of distribution of sites for all of the field electron emission material.

Preferably, the distribution of said sites over the field electron emission material when using a circular measurement area of 1 mm in diameter is substantially Binomial or Poisson.

The distribution of said sites over the field electron emission material may have a uniformity such that there is at least a 50% probability of at least one emitting site being located in any circular area of 4 ym diameter.

Preferably said emitter may be formed by coating a substrate with a mixture of insulating particles and particles of a low melting point conductor such as a solder and heating said layer to fuse the conductor around the insulating particles. A selective etch may be used to expose said insulating particles.

Preferably, said emitter may be formed by coating a substrate with a mixture of insulating particles and a chemical that can be reacted chemically to form a conducting matrix around the insulating particles. A selective etch may be used to expose said insulating particles.

Preferably, said chemical is a liquid bright metal such as gold.

Preferably, said emitter may be formed by pressing insulating particles into a soft conducting substrate.

The distribution of said sites over the field electron emission material may have a uniformity such that there is at least a 50% probability of at least one emitting site being located in any circular area of 10 ym diameter.

The invention extends to a field electron emission material produced by any of the above methods.

According to a further aspect of the present invention, there is provided a field electron emission device comprising a field electron emission material as above, and means for subjecting said material to an electric field in order to cause said material to emit electrons.

A field electron emission device as above may comprise a substrate with an array of emitter patches of said field electron emission material, and control electrodes with aligned arrays of apertures, which electrodes are supported above the emitter patches by insulating layers.

Said apertures may be in the form of slots.

A field electron emission device as above may comprise a plasma reactor, corona discharge device, silent discharge device, ozoniser, an electron source, electron gun, electron device, x-ray tube, vacuum gauge, gas filled device or ion thruster.

The field electron emission material may supply the total current for operation of the device.

The field electron emission material may supply a starting, triggering or priming current for the device.

A field electron emission device as above may comprise a display device.

A field electron emission device as above may comprise a lamp.

Preferably, said lamp is substantially flat.

Preferably said emitter may be connected to the electric driving means via a ballast resistor to limit current.

Preferably, said ballast resistor is applied as a resistive pad under each emitting patch.

Preferably, said ballast resistor is formed by selecting a conducting matrix of suitable resistivity.

Preferably, said emitter material and/or a phosphor is/are coated upon one or more one-dimensional array of conductive tracks which are arranged to be addressed by electronic driving means so as to produce a scanning illuminated line.

Such a field electron emission device may include said electronic driving means.

The field emission material may be disposed in an environment which may be gaseous, liquid, solid, or a vacuum.

A field electron emission device as above may comprise a cathode which is optically translucent and so arranged in relation to an anode that electrons emitted from the cathode impinge upon the anode to cause electro-luminescence at the anode, which electro-luminescence is visible through the optically translucent cathode.

It will be appreciated that the electrical terms"conducting"and "insulating"can be relative, depending upon the basis of their measurement. Semiconductors have useful conducting properties and, indeed, may be used in the present invention as the conducting layer. In the context of this specification, each said insulating particle has an electrical resistivity at least 102 times (and preferably at least 103 or 104 times) that of the conducting material.

For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example, to Figures 2 to 6 of the accompanying diagrammatic drawings, in which: Figure 2 shows the electric field distribution in and around a spherical insulating particle on a conducting surface; Figure 3 illustrates preferred insulating particle morphologies; Figure 4a illustrates how rough particles produce local field enhancements that promote the formation of channels; Figure 4b illustrates how a layer that facilitates electron injection into the insulator may be disposed at the interface between the particle and the substrate; Figures 5a, 5b and 5c illustrate examples of devices using improved emitters ; Figure 6a shows how a resistive layer may be disposed between an emitter layer and a conducting substrate; and Figure 6b shows how a conducting layer may be formed in such a way so as to provide a resistive ballast.

The illustrated embodiments of the invention provide materials based upon a MIV emission process with improved performance and usability, together with devices that use such materials.

To enable the reader to better understand the illustrated embodiments of the invention, the background to the technology will be described. When located parallel to electrodes into a vacuum gap with an applied electric field, a planar slab of dielectric will reduce the field within it by a factor equal to its dielectric constant. However, for other shapes, this is not the case. Figure 2 shows the case for a spherical insulating particle (dielectric constant=4) 211 on a conducting cathode surface 210 in an evacuated space 213 bounded by a remote anode 212 biased so as to produce an unperturbed electric field of 10 MV/m. Using a finite element model, contours of equal electric field 214 are computed. The diagram is axisymmetric about centre line 217. The numbers within the contour lines are electric fields measured in megavolts/metre. It can be seen that, in this case, not only is the electric field not reduced by the expected factor of four, but modest enhancements in field occur at the base of the particle 216 and in the region just outside the particle 215. This field enhancement is increased further in particles with angular, rough, dendritic or fractal shapes. If such particles are partially embedded in a conducting layer, field enhancements sufficient to inject electrons from the conductor (210) into or near the conduction band of the insulator (211) can be reached at practical externally applied fields.

Figure 3 shows conceptually preferred shapes of insulating particles 300 that, when embedded in a conductor 302 on a substrate 301, produce an electron emitter.

Mechanisms for electron emission from metals (or other conductors) through insulating layers into vacuum have been proposed and, to a large degree, experimentally verified. The theory and experiments associated with this so called MIV emission is well described in Latham 1995, High Voltage Vacuum Insulation, Academic Press.

In Figure 4a, an exemplary insulator particle 401 with a textured surface is embedded in a conducting layer 402. The surface structure so formed interfaces with another medium 405. Said medium is preferably a vacuum. An electric field 406 is applied between the conductor 402 and some remote anode (not shown). The field penetrates the insulator 401 and is concentrated in regions 407. At a critical field, a channel 403 is formed.

By a"channel"or"conducting channel"we mean a region of the insulator 401 where its properties have been locally modified, usually by some forming process. In the case of a conductor-insulator-vacuum MU structure, such a modification facilitates the transport of electrons 404 from the back contact through the insulator 401 into the vacuum 405.

Some examples of preferred embodiments of the invention, including some examples of processes and formulations, will now be described.

Example 1 Insulating particles are mixed in a dispersion with particles of a low melting point material, such as a solder, to form an ink. A suitable substrate is then coated with a layer of the ink and subsequently heated to fuse the metal. Such coating may be performed by techniques such as spinning, spraying, blade coating, wire roll coating, screen printing, pad printing, ink jet printing and gravure printing. When required, the layer may be printed so as to form patterns for devices such as displays. After firing, the layer may be etched to selectively expose the insulating particles.

Example 2 Insulating particles are mixed with a liquid that can be reacted chemically to form a conducting matrix. One class of materials that meets this requirement are the so-called liquid bright golds that have been used for many years by the pottery and glass industries to decorate their wares.

These materials form bright metallic layers from a paint that contains organometallic compounds-the so called resinate or bright golds, palladiums and platinums. The metallic layer is formed by applying a paint and then firing the object in air at temperatures between 480 C and 920 C at which point the organometallic compound decomposes to yield pure metal films 0.1 to 0.2 um thick. Traces of metals such as rhodium and chromium are added to control morphology and assist in adhesion.

Although little (or not) used, or known of, in the art today, such techniques have been used in the past by the electron tube industry. For example Fred Rosebury's classic text"Handbook of Electron Tube and Vacuum Techniques"originally published in 1964 (Reprinted by Amencan Institute of Physics-ISBN 1-56396-121-0) gives a recipe for liquid bright platinum. More recently, Koroda (US Patent 4, 098, 939) describes their use for the electrodes in a vacuum fluorescent display.

Insulating particles may be dispersed in such a liquid bright gold material to form an ink. A suitable substrate is then coated with a layer of the ink and subsequently heated to decompose the organometallic compounds, forming a layer of gold or other metal. After firing, depending on the chemistry of the situation, the metal is either left totally coating the particles and substrate, or the particles are partially exposed as the ink shrinks down in thickness. In the former case, a selective etch such as iodine in potassium iodide solution may be used subsequently to expose the particles.

Such a coating may be performed by techniques such as spinning, spraying, blade coating, wire roll coating, screen printing, pad printing, ink jet printing and gravure printing. When required, the layer may be printed so as to form patterns for devices such as displays.

Example 3 Insulating particles may be dispensed onto the surface of a malleable conductive material and then pressed into it using, for example, a roller.

Moving now to Figure 4b, wherein like references to those in Figure 4a have like meanings, a layer 410 is formed between the insulator particle and the surface of the conductor 402 to facilitate electron injection.

Such a layer can be formed in a number of ways: depositing a layer of material between the surface of conductor 402 and insulator 401, which layer of material has properties intermediate those of the conductor and insulator; or doping the surface of conductor 402 and insulator 401 with a material that segregates out as the layer 410 during subsequent processing; or reaction of the materials of conductor 402 and insulator 401; or forming the layer 410 as a region of high doping, high defect density or intermediate composition.

The layer 410 of material between the conductor 402 and insulator 401 may be formed by a gradual change in stoichiometry, composition or doping of the material of the layer, to reduce discontinuity.

The technical details and theory of such injection layers 410 are described in our co-pending UK patent application 98 16684.6, a copy of the specification and drawings of which is attached herewith. Techniques described therein to control current flow through the insulator and to enhance the probability of emission of electrons from the surface of the insulator 401 into the vacuum 405 or other medium may also be applied to embodiments of this invention.

In all the above-described embodiments of the invention, there is an optimum density of conducting particles that prevents the nearestneighbour emitters interacting. For approximately spherical particles, the optimum particle-to-particle spacing is approximately 1.8 times the particle diameter.

A primary purpose of preferred embodiments of the invention is to produce emitting materials with low cost and high manufacturability.

However, for less cost-sensitive applications, the very high thermal conductivity that may be achieved means that using diamond as the insulator can provide materials that can deliver the highest mean currents before catastrophic failure of the electro-formed channels.

A problem with all field electron emission displays is in achieving uniform electrical characteristics from pixel to pixel. One approach is to use electronics that drive the pixels in a constant current mode. An alternative approach that achieves substantially the same objective is to insert a ballast resistor of appropriate value between the emitter and a constant voltage drive circuit. This may be external to the device. However, in this arrangement, the time constant of the resistor and the capacitance of an associated conducting track array places a limit on the rate that pixels can be addressed. Forming the resistor in situ in the emitter layer enables low impedance electronics to be used to rapidly charge the track capacitance, giving a much shorter rise time.

Figure 6a and Figure 6b show two arrangements of such an in situ resistive ballasting arrangement. In Figure 6a, a conducting substrate 600 has a resistive layer 601 disposed between it and an emitter structure above. The emitter structure comprises conducting layer 602 and embedded insulating particles 603, wherein emitter channels 604 may be formed to form a source of electrons 605 into a medium (usually a vacuum) 606. A disadvantage of this arrangement is that the conducting layer 602 electrically connects the individual electron sources. Consequently, this method is only applicable if the cathode is divided into a plurality of small regions.

Figure 6b shows an alternative method wherein references 600, 603,604 and 606 have the same meanings as in Figure 6a. Here the particles are located in or on a resistive layer 611, which layer provides resistive ballasting on an emitter by emitter basis.

The field electron emission current available from improved emitter materials such as are disclosed above may be used in a wide range of devices including: field electron emission display panels; lamps; high power pulse devices such as electron MASERS and gyrotrons ; crossed-field microwave tubes such as CFAs ; linear beam tubes such as klystrons; flash xray tubes; triggered spark gaps and related devices; broad area x-ray sources for sterilisation; vacuum gauges; ion thrusters for space vehicles and particle accelerators.

Examples of some of these devices are illustrated in Figures 5a, 5b and 5c.

Figure 5a shows an addressable gated cathode as might be used in a field emission display. The structure is formed of an insulating substrate 500, cathode tracks 501, emitter layer 502, focus grid layer 503 electrically connected to the cathode tracks, gate insulator 504, and gate tracks 505.

The gate tracks and gate insulators are perforated with emitter cells 506. A negative bias on a selected cathode track and an associated positive bias on a gate track causes electrons 507 to be emitted towards an anode (not shown).

The reader is directed to our co-pending application GB 97 22258.2 for further details of constructing Field Effect Devices, in which embodiments of the present invention may be employed. A copy of the specification and drawings of GB 97 2258.2 is attached.

The electrode tracks in each layer may be merged to form a controllable but non-addressable electron source that would find applicat

Figure 5c shows a flat lamp using one of the above-described materials. Such a lamp may be used to provide backlighting for liquid crystal displays, although this does not preclude other uses such as room lighting.

The lamp comprises a cathode plate 520 upon which is deposited a conducting layer 521 and an emitting layer 522. Ballast layers as described herein may be used to improve the uniformity of emission. A transparent anode plate 523 has upon it a conducting layer 524 and a phosphor layer 525. A ring of glass fritt 526 seals and spaces the two plates. The interspace 527 is evacuated.

The operation and construction of such devices, which are only examples of the many applications of preferred embodiments of this invention, will readily be apparent to those skilled in the art.

An important feature of preferred embodiments of the invention is the ability to print an emitting pattern, thus enabling complex multiemitter patterns, such as those required for displays, to be created at modest cost. Furthermore, the ability to print enables low-cost substrate materials, such as glass to be used; whereas micro-engineered structures are typically built on high-cost single crystal substrates. In the context of this specification, printing means a process that places or forms an emitting material in a defined pattern. Examples of suitable processes are: screen printing, Xerography, photolithography, electrostatic deposition, spraying, ink jet printing and offset lithography.

Devices that embody the invention may be made in all sizes, large and small. This applies especially to displays, which may range from a single pixel device to a multi-pixel device, from miniature to macro-size displays.

In this specification, the verb"comprise"has its normal dictionary meaning, to denote non-exclusive inclusion. That is, use of the word"comprise" (or any of its derivatives) to include one feature or more, does not exclude the possibility of also including further features.

The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

The invention is not restricted to the details of the foregoing embodiment (s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims (38)

1. CLAIMS: 1. A method of forming a field electron emission material, wherein a plurality of insulating particles are embedded in an electrically conducting substrate, to provide emission sites at said particles.
2. 2. A method according to claim 1, wherein each said particle comprises a ceramic, oxide, carbide, nitride, boride, glass, silicate, aluminosilicate, carbon-based insulator, inorganic insulator, polymer, or highly cross-linked polymer.
3. 3. A method according to claim 1 or 2, wherein said electrically conducting surface comprises a metal, semiconductor, carbide, nitride, oxide, boride, or carbon-based conductor.
4. 4. A method according to claim 1,2 or 3, wherein said particles are in a size range having a lower limit of 10,100 or 500 nanometres and an upper limit of 10,20,100 or 500 micrometres.
5. 5. A method according to any of the preceding claims, wherein the distribution of said sites over the field electron emission material is random.
6. 6. A method according to any of the preceding claims, wherein said sites are distributed over the field electron emission material at an average density of at least 102 cl 2.
7. 7. A method according to any of the preceding claims, wherein said sites are distributed over the field electron emission material at an average density of at least 103 cm~2, 104 cm2 orlOs cm~2.
8. 8. A method according to any of the preceding claims, wherein the distribution of said sites over the field electron emission material is substantially uniform.
9. 9. A method according to any of the preceding claims, wherein the distribution of said sites over the field electron emission material has a uniformity such that the density of said sites in any circular area of lmm diameter does not vary by more than 20% from the average density of distribution of sites for all of the field electron emission material.
10. 10. A method according to any of the preceding claims, wherein the distribution of said sites over the field electron emission material when using a circular measurement area of 1 mm in diameter is substantially Binomial or Poisson.
11. 11. A method according to any of the preceding claims, wherein the distribution of said sites over the field electron emission material has a uniformity such that there is at least a 50% probability of at least one emitting site being located in any circular area of 4 ym diameter.
12. 12. A method according to any of the preceding claims, wherein said emitter is formed by coating a substrate with a mixture of insulating particles and particles of a low melting point conductor such as a solder and heating said layer to fuse the conductor around the insulating particles.
13. 13. A method according to any of claims 1 to 11, wherein said emitter is formed by coating a substrate with a mixture of insulating particles and a chemical that can be reacted chemically to form a conducting matrix around the insulating particles.
14. 14. A method according to claim 13, wherein said chemical is a liquid bright metal.
15. 15. A method according to claim 14, wherein said chemical comprises gold.
16. 16. A method according to any of claims 12 to 15, wherein a selective etch is used to expose said insulating particles.
17. 17. A method according to any of claims 1 to 11, wherein said emitter is formed by pressing insulating particles into a soft conducting substrate.
18. 18. A method according to any of the preceding claims, wherein the distribution of said sites over the field electron emission material has a uniformity such that there is at least a 50% probability of at least one emitting site being located in any circular area of 10 ym diameter.
19. 19. A field electron emission material produced by a method according to any of the preceding claims.
20. 20. A field electron emission device comprising a field electron emission material according to claim 19, and means for subjecting said material to an electric field in order to cause said material to emit electrons.
21. 21. A field electron emission device according to claim 20, comprising a substrate with an array of emitter patches of said field electron emission material, and control electrodes with aligned arrays of apertures, which electrodes are supported above the emitter patches by insulating layers.
22. 22. A field electron emission device according to claim 21, wherein said apertures are in the form of slots.
23. 23. A field electron emission device according to any of claims 20 to 22, comprising a plasma reactor, corona discharge device, silent discharge device, ozoniser, an electron source, electron gun, electron device, x-ray tube, vacuum gauge, gas filled device or ion thruster.
24. 24. A field electron emission device according to any of claims 20 to 23, wherein the field electron emission material supplies the total current for operation of the device.
25. 25. A field electron emission device according to any of claims 20 to 23, wherein the field electron emission material supplies a starting, triggering or priming current for the device.
26. 26. A field electron emission device according to any of claims 20 to 22, comprising a display device.
27. 27. A field electron emission device according to any of claims 20 to 22, comprising a lamp.
28. 28. A field electron emission device according to claim 27, wherein said lamp is substantially flat.
29. 29. A field electron emission device according to any of claims 20 to 28, wherein said emitter is connected to an electric driving means via a ballast resistor to limit current.
30. 30. A field electron emission device according to claims 21 and 29, wherein said ballast resistor is applied as a resistive pad under each said emitting patch.
31. 31. A field electron emission device according to claim 30, wherein said ballast resistor is formed by selecting a conducting matrix of suitable resistivity.
32. 32. A field electron emission device according to any of claims 20 to 31, wherein said emitter material and/or a phosphor is/are coated upon one or more one-dimensional array of conductive tracks which are arranged to be addressed by electronic driving means so as to produce a scanning illuminated line.
33. 33. A field electron emission device according to claim 32, including said electronic driving means.
34. 34. A field electron emission device according to any of claims 20 to 33, wherein said field emission material is disposed in an environment which is gaseous, liquid, solid, or a vacuum.
35. 35. A field electron emission device according to any of claims 20 to 34, comprising a cathode which is optically translucent and is so arranged in relation to an anode that electrons emitted from the cathode impinge upon the anode to cause electro-luminescence at the anode, which electro-luminescence is visible through the optically translucent cathode.
36. 36. A method of forming a field electron emission material, substantially as hereinbefore described with reference to the accompanying drawings.
37. 37. A field electron emission material formed by a method according to claim 36.
38. 38. A field electron emission device, substantially as hereinbefore described with reference to the accompanying drawings.