EP0125859B1 - Element - Google Patents
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- Publication number
- EP0125859B1 EP0125859B1 EP19840303057 EP84303057A EP0125859B1 EP 0125859 B1 EP0125859 B1 EP 0125859B1 EP 19840303057 EP19840303057 EP 19840303057 EP 84303057 A EP84303057 A EP 84303057A EP 0125859 B1 EP0125859 B1 EP 0125859B1
- Authority
- EP
- European Patent Office
- Prior art keywords
- display device
- cathode
- grid
- screen
- assembly
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
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- 229910052751 metal Inorganic materials 0.000 claims description 51
- 239000002184 metal Substances 0.000 claims description 51
- OAICVXFJPJFONN-UHFFFAOYSA-N Phosphorus Chemical compound [P] OAICVXFJPJFONN-UHFFFAOYSA-N 0.000 claims description 49
- 239000011159 matrix material Substances 0.000 claims description 38
- 239000000758 substrate Substances 0.000 claims description 31
- 239000000463 material Substances 0.000 claims description 22
- VYPSYNLAJGMNEJ-UHFFFAOYSA-N Silicium dioxide Chemical compound O=[Si]=O VYPSYNLAJGMNEJ-UHFFFAOYSA-N 0.000 claims description 20
- 239000004065 semiconductor Substances 0.000 claims description 17
- 229910052710 silicon Inorganic materials 0.000 claims description 13
- 239000010703 silicon Substances 0.000 claims description 13
- 238000000034 method Methods 0.000 claims description 12
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- 235000012239 silicon dioxide Nutrition 0.000 claims description 10
- 239000000377 silicon dioxide Substances 0.000 claims description 10
- WFKWXMTUELFFGS-UHFFFAOYSA-N tungsten Chemical compound [W] WFKWXMTUELFFGS-UHFFFAOYSA-N 0.000 claims description 10
- 229910052721 tungsten Inorganic materials 0.000 claims description 10
- 239000010937 tungsten Substances 0.000 claims description 10
- 239000011248 coating agent Substances 0.000 claims description 9
- 238000010438 heat treatment Methods 0.000 claims description 8
- 229910052581 Si3N4 Inorganic materials 0.000 claims description 7
- HQVNEWCFYHHQES-UHFFFAOYSA-N silicon nitride Chemical compound N12[Si]34N5[Si]62N3[Si]51N64 HQVNEWCFYHHQES-UHFFFAOYSA-N 0.000 claims description 7
- 239000000126 substance Substances 0.000 claims description 5
- PXHVJJICTQNCMI-UHFFFAOYSA-N Nickel Chemical group [Ni] PXHVJJICTQNCMI-UHFFFAOYSA-N 0.000 claims description 4
- 239000011261 inert gas Substances 0.000 claims description 3
- 238000005513 bias potential Methods 0.000 claims description 2
- 230000002301 combined effect Effects 0.000 claims description 2
- 239000007789 gas Substances 0.000 claims description 2
- 229910052759 nickel Inorganic materials 0.000 claims description 2
- 239000004020 conductor Substances 0.000 description 14
- XUIMIQQOPSSXEZ-UHFFFAOYSA-N Silicon Chemical group [Si] XUIMIQQOPSSXEZ-UHFFFAOYSA-N 0.000 description 12
- 238000010894 electron beam technology Methods 0.000 description 9
- 238000010276 construction Methods 0.000 description 6
- 238000005530 etching Methods 0.000 description 6
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- 230000008569 process Effects 0.000 description 3
- 239000000919 ceramic Substances 0.000 description 2
- 230000009467 reduction Effects 0.000 description 2
- PNEYBMLMFCGWSK-UHFFFAOYSA-N Alumina Chemical compound [O-2].[O-2].[O-2].[Al+3].[Al+3] PNEYBMLMFCGWSK-UHFFFAOYSA-N 0.000 description 1
- BVKZGUZCCUSVTD-UHFFFAOYSA-L Carbonate Chemical compound [O-]C([O-])=O BVKZGUZCCUSVTD-UHFFFAOYSA-L 0.000 description 1
- 230000000712 assembly Effects 0.000 description 1
- 238000000429 assembly Methods 0.000 description 1
- 150000004649 carbonic acid derivatives Chemical class 0.000 description 1
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- 230000009977 dual effect Effects 0.000 description 1
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- 230000008018 melting Effects 0.000 description 1
- 238000002844 melting Methods 0.000 description 1
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- 239000000203 mixture Substances 0.000 description 1
- 230000003647 oxidation Effects 0.000 description 1
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- XOLBLPGZBRYERU-UHFFFAOYSA-N tin dioxide Chemical compound O=[Sn]=O XOLBLPGZBRYERU-UHFFFAOYSA-N 0.000 description 1
- 229910001887 tin oxide Inorganic materials 0.000 description 1
Images
Classifications
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
- H01J17/00—Gas-filled discharge tubes with solid cathode
- H01J17/38—Cold-cathode tubes
- H01J17/48—Cold-cathode tubes with more than one cathode or anode, e.g. sequence-discharge tube, counting tube, dekatron
- H01J17/49—Display panels, e.g. with crossed electrodes, e.g. making use of direct current
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
- H01J1/00—Details of electrodes, of magnetic control means, of screens, or of the mounting or spacing thereof, common to two or more basic types of discharge tubes or lamps
- H01J1/02—Main electrodes
- H01J1/13—Solid thermionic cathodes
- H01J1/15—Cathodes heated directly by an electric current
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
- H01J2201/00—Electrodes common to discharge tubes
- H01J2201/28—Heaters for thermionic cathodes
- H01J2201/2803—Characterised by the shape or size
- H01J2201/2878—Thin film or film-like
Definitions
- This invention relates to an element which may be used either as an electron emitting device or as a light emitting device.
- one or more elements may be used as thermionic cathodes in a thermionic valve such as a cathode ray tube or as light emitting devices in, for example, a display.
- the physical phenomenon of thermionic emission is exhibited by a metal heated above a characteristic threshold temperature.
- a metal is heated such that the energy of electrons within the metal exceeds the value of the thermionic work function characteristic of the metal, electrons escape from the metal and form a space charge.
- a thermionic valve utilises the space charge to set up a current flow, in the form of electrons travelling in a vacuum, from a hot metal (a thermionic cathode) to a relatively positively charged anode.
- the flow of electrons may be altered by means of a control grid interposed in the electron flow.
- One such particular form is the cathode ray tube.
- the cathode ray tube is an image forming thermionic valve which has commercial application not only in the domestic market as the major component of television receivers but also in research and industry as the major component of oscilloscopes.
- a cathode ray tube comprises an evacuated glass envelope, one end of which is formed into a screen. At the opposite end of the tube is located an assembly for producing a stream of electrons ("a cathode ray").
- This assembly is commonly known as an electron gun and comprises a thermionic cathode, a control grid and at least one anode.
- the thermionic cathode produces a space charge of electrons, which electrons are accelerated away from the cathode by the positive charge of the accelerating anode. The speed and therefore the kinetic energy of the electrons may be controlled by a negatively biased control grid.
- a fine beam of electrons is required and further anodes are usually provided which create an anisotropic electrostatic field tending to focus the electron beam in a manner analogous to the focusing of light rays by a lens.
- the beam of electrons is subjected to a deflecting force.
- This may be an electrostatic force, in which case the beam is caused to pass between two mutually perpendicular pairs of plates.
- the plates may be charged in order to deflect the beam of electrons in one or both of two perpendicular axes. This method of deflecting the electron beam in a cathode ray tube is commonly used in oscilloscopes.
- the electron beam is caused to pass through the field of electromagnets capable of creating perpendicular electromagnetic fields.
- the electromagnets are usually located outside the glass envelope of the cathode ray tube.
- the application of current to the electromagnets allows deflection of the electron beam in one or both of two perpendicular axes. This method of deflection is commonly used in cathode ray tubes where a continuous raster scan is required (as, for example, in a television picture tube).
- the electron beam finally impinges upon a phosphor screen where the kinetic energy of the electrons comprising the beam is converted into light.
- the conventional miniature thermionic cathode structure is generally a coil of metal wire of high melting point.
- the coil is supported at either end by its electrical connection to the electron gun assembly.
- a miniature cathode is constructed from a wire of 20 microns in diameter and 10 millimeters in length.
- the cathode is commonly made of tungsten which requires to be heated to about 1100 K in order to produce a satisfactory flux of electrons.
- the operating temperature may be reduced to about 900 K by adding a coating which reduces the effective thermionic work function of the wire.
- a commonly used coating is a mixture of carbonates.
- a miniature thermionic cathode of the type described immediately above consumes a considerable amount of power at its operating temperature.
- a coated tungsten thermionic cathode of the dimensions given will consume about 50 mW at 900 K. Only a fraction of this power goes directly to provide the energy to liberate electrons from the cathode material. The majority of the power taken by the cathode is dissipated as heat by conduction and radiation.
- An object of the present invention is to reduce the power losses and also to provide a cathode of reduced overall size.
- Patents Abstracts of Japan, Vol. 7, No. 72 (E-166) (1217) describes the construction of a thermionic cathode comprising an alumina-ceramic substrate having an overlying glass glaze. A conductor and filaments are formed on the glass glaze such that the filaments overlie gaps in the glaze.
- Recent advances in solid state circuitry have provided the technology to produce layer structures of intricate, yet well defined, physical shape.
- This technology has been adapted to produce the element of the present invention.
- the element of the invention may be used as a thermionic cathode replacing the known cathodes in thermionic devices as described above, or as we have now discovered, by increasing the power supplied to the element, it may be used as an incandescent light source providing a light emitting device of small dimensions for use, for example, in a direct display device.
- an element for use as a thermionic cathode and/or light emitting device comprising a substrate, a metal layer and an electrically insulating support layer interposed between the substrate and the metal layer, wherein a part of the metal layer, having a cross-sectional area smaller than elsewhere in the metal layer, extends across a recess, characterised in that the substrate is of a semiconductor material and is provided with the said recess, across which extends a bridge structure comprising a part of the insulating support layer and the said part of the metal layer.
- an electrical current is passed through the metal layer, the current resistively heating the metal to a temperature at which thermionic emission and/or light emission occurs.
- the recess is formed such that the bridge structure is physically separated from the semiconductor substrate.
- the recess may be in the form of a hollow or groove formed in the surface of the semiconductor substrate, or a hole passing through the semiconductor substrate.
- the bridge structure may take the form of a narrow linear strip of the metal layer attached to a support layer of similar shape, a broad band of support material to which is attached a metal layer in the form of a sinuous strip or a region of support material having attached thereto a metal layer in the form of a sinuous strip, the region of support material overlying the recess and being supported by three or more support arms.
- the bridge structure is preferably formed such that is does not completely occlude the recess in the semiconductor material. In this way, when the element of the present invention is used as a thermionic cathode, in an evacuated envelope, a vacuum surrounds the bridge structure reducing conduction of heat from the heated metal layer. Energy losses due to conduction of heat to the semiconductor substrate are minimised due to the preferred construction of the bridge.
- the element When the element is used as a light-emitting device, it may be enclosed in an evacuated envelope or in an envelope containing a low pressure gas or an inert gas.
- the metal layer is preferably nickel or tungsten.
- the metal layer is provided with coating capable of reducing the effective thermionic work function of the metal layer.
- the metal layer of the bridge structure is about 5 microns wide and hence, in order to maintain mechanical stability of the metal layer, the support material must not only be physically strong but must also be capable of remaining mechanically stable at the temperatures required to cause the metal layer to exhibit thermionic emission. Where the metal layer is tungsten, the support layer must remain mechanically stable up to at least 1100 K.
- the semiconductor substrate is silicon.
- the support material is silicon dioxide or silicon nitride. These materials may be readily formed in a layer structure using processes known per se in the art.
- the bridge structure may be formed using the process of anisotropic etching, or chemical milling.
- the element of the present invention includes integrated circuitry upon the semiconductor substrate.
- the element may have circuit elements for providing cathode drive included on the semiconductor substrate.
- the substrate may include circuitry for providing a current supply to the element.
- a thermionic valve including a thermionic cathode the thermionic cathode comprising at least one element of the present invention.
- the thermionic valve is a cathode ray tube.
- the element of the present invention may be made in such a way that its overall size is of the order of 50 microns square and having a very low power consumption. It is therefore of great utility for small size, low power consumption cathode ray tubes.
- a display device comprising a thermionic cathode assembly provided with at least one element of the first aspect of the invention a screen coated with a substance capable of converting the kinetic energy of electrons impinging on the screen into light, a grid assembly interposed between the cathode assembly and the screen, and an anode for accelerating electrons from the cathode to the screen.
- the screen is a phosphor screen.
- the cathode assembly, grid assembly, screen and anode are enclosed in a vessel at reduced pressure, preferably substantially evacuated.
- the display device is further provided with cathode ray deflection means for deflecting a beam of electrons produced by the cathode to a predetermined point on the phosphor screen.
- the deflection means may be electromagnetic or electrostatic means, such as those known in the art.
- one or more focusing anodes may be provided.
- the cathode assembly may comprise a single element of the invention, to act as a thermionic cathode in the display device. If required, a number of elements may be closely packed to afford an increased flux density of thermal electrons. Multibeam display devices for use for example in dual beam oscilloscopes and three electron gun colour television tubes may be produced using either two or more cathode assemblies or one cathode assembly provided with suitably spaced elements of the invention.
- a plurality of elements of the invention may together form a matrix of elements, on a single wafer of semiconductor material.
- matrix refers to a number of elements of the invention arranged in a geometric pattern, preferably as one or more rows and/or columns most preferably linear rows and columns.
- Such a matrix may be formed using, for example the anisotropic etching process mentioned above.
- a display device comprising a thermionic cathode assembly provided with a matrix of elements of the invention, a screen coated with a substance capable of converting the kinetic energy of electrons impinging on the screen into light and an anode for accelerating electrons from the cathode assembly to the screen.
- the display device includes a grid assembly interposed between the screen and the cathode assembly.
- the screen is a phosphor screen.
- a particular and great advantage of this aspect of the invention is that a matrix of electron beams may be produced by a cathode assembly, each element in the matrix producing thermal electrons which may be accelerated towards a phosphor screen such that each electron beam thus produced forms a picture element (pixel) upon the screen.
- the need for deflection means is obviated.
- conventional cathode ray tubes the distance from the cathode to the screen must be sufficiently large for the deflection applied to the beam to produce a significant displacement of the point at which the beam impinges upon the screen. This has in the past limited the reduction in 'front to back' dimensions of cathode ray tubes.
- a cathode ray tube of this embodiment by negating the need for deflection of the beam provides a tube of greatly reduced 'front to back' dimension relative to a conventional cathode ray tube.
- the matrix of elements forming the cathode assembly is arranged such that each element in the matrix is provided with an independent current flow.
- the matrix of elements may be addressed by a digital logic circuit and suitable drive circuits.
- the drive circuits cause a current to flow through each addressed element thereby heating the element by means of resistance heating.
- the hot element acting as a thermionic cathode, will then melt thermal electrons which may be accelerated towards the phosphor screen, past the grid.
- the grid is, in this embodiment, preferably provided with an even potential over its surface. Therefore light produced by electrons impinging upon the phosphor screen will be produced in a matrix of pixels corresponding to the elements which are addressed.
- An image may therefore be built up by addressing those elements in the cathode assembly which correspond to the pixels it is desired to produce on the phosphor screen.
- the low thermal mass of the individual elements in the cathode assembly ensures that the individual cathodes heat up and cool down very rapidly thereby allowing a rapid change in the image produced on the phosphor screen.
- the intensity of each pixel produced on the phosphor screen may be controlled by adjusting the current supplied to the relevant element in the cathode assembly. Preferably however the intensity of each pixel is adjusted by adjusting the potential difference between the grid and the cathode assembly.
- Colour images may be built up by providing a matrix of different phosphors on the phosphor screen. For example adjacent elements in the cathode assembly may correspond to phosphors producing red, green or blue light in response to energy supplied by impinging electrons.
- the matrix of elements is addressed such that each element in a row of the matrix is supplied with an equal current flow and the grid assembly comprises a number of independent grid elements each corresponding to a column of the elements forming the cathode assembly.
- Each grid element may be addressed separately such that the combined effect of addressing the elements in rows of the cathode assembly and the grid elements in columns of the grid assembly provides a method of selecting, for example, the intensity of each point produced by the matrix of elements.
- row and column are of course interchangeable in respect of the cathode assembly drive and the grid assembly drive.
- each element in the cathode assembly is provided with substantially equal current flow, the grid assembly forming a matrix of grid elements, to each element of which may be applied an independent bias potential.
- Each grid element corresponds to a thermionic cathode in the cathode assembly, whereby by addressing the grid, e.g. using digital circuitry and, if necessary, drive circuits, the kinetic energy of electrons produced by each cathode in the cathode assembly may be individually adjusted thereby adjusting the intensity of light produced on the phosphor screen.
- the display devices may be used in for example so called flat screen televisions or alphanumeric displays.
- a display device comprising one or more light emitting devices the or each light emitting device consisting of an element of the first aspect of the invention.
- the display device comprises a plurality of light emitting devices most preferably arranged to provide a matrix of light emitting devices.
- the light emitting devices are enclosed in a suitable envelope.
- a suitable envelope may include for example an evacuated envelope or an envelope having an inert atmosphere.
- Individual light emitting devices in the matrix may be addressed using, for example, digital circuitry and drive circuitry.
- the light emission from each light emitting device may be controlled by the current passed to each device.
- Each device has a low thermal mass which allows modulation of the emitted light intensity at a relatively high frequency.
- a matrix may be used for example to produce a flat screen television or an alphanumeric display.
- a silicon substrate 1 provides mechanical support for the element, which may be a cathode or a light emitting device.
- the substrate is of a type now well known in the electronics industry as a semiconductor substrate upon which integrated circuits are constructed.
- the silicon substrate 1 is provided with a recess 3 in the form of a depression etched into the surface of the substrate by anisotropic etching.
- a layer of silicon dioxide (or silicon nitride) 5 overlies the surface of the silicon and is so shaped as to form a narrow bridge 7 over the recess 3.
- a layer of tungsten metal 9 lies over the silicon dioxide layer such that the cross sectional area of the layer is at its lowest in that part of the layer overlying the silicon dioxide layer forming the bridge 7. This ensures that the maximum resistive heating of the metal layer is concentrated in the bridge region.
- the layer of silicon dioxide (or silicon nitride) 5 shown in Figure 1 is shaped to correspond to the shape of the layer of tungsten metal 9.
- the silicon dioxide layer may cover the surface of the silicon substrate 1 except for regions adjacent the bridge structure, which allow communication between the recess 3 and the surroundings.
- the element In use, the element, if it is to be a thermionic cathode, is placed within an evacuated envelope and a current is passed through the metal layer 9. Resistive heating of the metal layer in the bridge area occurs and the metal is heated to a temperature sufficient to cause thermionic emission. This will typically be 1100 K for a tungsten conductor or 900 K for a carbonate coated tungsten conductor.
- the recess 3 reduces the loss of energy by conduction into the substrate and the low cross sectional area made possible by the supported conductor, reduces conduction and consequent heat loss through the conductor itself.
- Figures 3 and 4 show a second embodiment of the first aspect of the invention in which the techniques of anisotropic etching have been used to form a silicon substrate 11, provided with a recess 13.
- a layer of silicon dioxide (or silicon nitride) 15 overlies the silicon substrate forming a bridge 17 over the recess 13 such that the recess is not completely enclosed.
- a conductor 19 of tungsten has been deposited upon the silicon dioxide (or silicon nitride) layer and has been etched to provide a narrow sinuous conductor portion 21 on the bridge 17.
- the recess is square in plan with an edge measuring about 50 microns.
- the narrow conductor portion has a width of about 5 microns.
- the advantage of an etched conductor pattern of this type is that the operating voltage of the element can be tailored to a convenient value.
- FIG. 5 shows a third embodiment of the first aspect of the invention.
- the third embodiment comprises a silicon substrate provided with a recess (these being substantially as described above).
- the silicon substrate is overlain with a support layer 75 of silicon dioxide or silicon nitride.
- the support layer 75 is provided with apertures 77 communicating with the recess formed in the silicon substrate, such that a suspended region 79 of the support layer 75 is formed.
- the region 79 is suspended above the recess by four arms 81, 83, 85 and 87 formed of the support layer 75.
- a conductor 89 has been deposited upon the support layer 75 and has been etched to provide a relatively narrow, sinuous conductor portion 91 on the suspended region 79 of the support layer 75.
- Connector portions 93 and 95 pass over opposed arms 81 and 83 in order to reduce mechanical distortion which may otherwise occur during resistive heating of conductor portion 91.
- Arms 85 and 87 may be provided with dummy strips 97 and 99 (shown in broken line outline) of the same material as the connector portions 93 and 95 in order further to reduce possible mechanical distortion. It will be appreciated that connector portions 93 and 95 could pass over different arms from those shown, and that the number of arms may be different, although at least three should be provided for adequate support.
- FIG. 6 shows a thermionic cathode of the present invention (shown generally at 25) in combination with a control grid assembly (shown generally at 27) and exemplifies a possible mounting technique for the thermionic cathode of the present invention.
- the control grid assembly 27 comprises a ceramic plate 29 having a metal coating on its front and rear faces.
- the ceramic plate 29 is provided with an aperture 31 passing through both the plate and metal coatings, perpendicular to the general plane of the plate.
- the thermionic cathode 25 is provided with electrically conductive protrusions 33, e.g.
- the rear metal coated face 37 of the grid assembly 27 therefore is at the same potential as the thermionic cathode in use.
- the thermionic cathode 25 is aligned such that, in use, electrons emitted by the thermionic cathode pass through the aperture 31 under the influence of a positive potential provided by an anode (not shown).
- the front metal coated face 39 of the grid assembly 27 forms a control grid which may be used to alter the flow of electrons from the cathode.
- the front metal coated face 39 would normally be negatively biased with respect to the cathode.
- the advantage of this mounting system is that the cathode and grid are maintained at a fixed spacing and alignment. This has attendant advantages not only in terms of the mechanical strength of the arrangement but also in terms of ease of manufacture.
- the display device comprises a cathode assembly 39, as phosphor screen 41, and a grid assembly 43 interposed between the cathode assembly 39 and the phosphor screen 41.
- the cathode assembly 39 comprises a silicon substrate 45 upon which is formed a plurality of elements of the invention (for example 47). Each element of the invention acts as a thermionic cathode.
- the thermionic cathodes 47 are arranged in an array or matrix.
- the matrix depicted in Figure 7 has only nine rows and twelve columns for the purpose of ready illustration. In practice the matrix may possess a large number of rows and columns limited only by physical constraints of manufacture.
- the grid assembly 43 comprises a perforate plate.
- the construction of the grid assembly is as shown in Figure 6, an insulating plate being provided with a metal coating (not shown) on its front and rear faces.
- the grid assembly is provided with a plurality of grid apertures (for example 49) capable of allowing passage of electrons.
- Each aperture 49 in the grid assembly 43 corresponds spatially with a thermionic cathode 47 in an equivalent row and column in the cathode assembly 39.
- the phosphor screen 41 comprises a transparent support material to the side facing the grid assembly of which is affixed a coating of a phosphor material capable of converting electron kinetic energy into visible light.
- the phosphor coating is either of one type, capable of generating the same colour of visible light over the surface of the phosphor screen 41 or is comprised of regions of a number of different phosphors each different phosphor being capable of generating a different colour of visible light.
- Such phosphor screens are well known in the art and are therefore not further described.
- the cathode assembly 39, the grid assembly 43 and the phosphor screen 41 are enclosed in an evacuated envelope (not shown).
- the envelope also enclosed an accelerating anode (not shown) disposed between the grid assembly 43 and the phosphor screen 41.
- the phosphor screen 41 may form part of the envelope.
- the transparent support material of the phosphor screen may comprise a transparent electrically conducting material and may form the accelerating anode.
- a suitable material is, for example, a coating of tin oxide.
- the accelerating anode is maintained at high positive potential relative to the cathode assembly 39.
- Each thermionic cathode 47 in the cathode assembly 39 is capable of being separately supplied with an electrical current by means of address logic and drive circuitry which may be located outside the envelope or may be included within the envelope, for example, upon the silicon substrate 45 of the cathode assembly.
- address logic and drive circuitry which may be located outside the envelope or may be included within the envelope, for example, upon the silicon substrate 45 of the cathode assembly.
- Electrons produced by each thermionic cathode 47 pass through the corresponding aperture 49 in the grid assembly 43 and onwards to the phosphor screen 41 where their kinetic energy is converted into visible light by the phosphor.
- each thermionic cathode 47 when supplied with current produces a point of light on the phosphor screen 41, each point of light forming a picture element or pixel of an image to be formed on the phosphor screen 41.
- An image may therefore be produced on the phosphor screen 41 by selectively addressing and supplying current to those thermionic cathodes in the cathode assembly 39 corresponding to the desired pixels of the image.
- the intensity of each pixel may be individually altered by varying the voltage between the grid 43 and the selected thermionic cathode.
- the thermionic cathodes may be sequentially addressed and a synchronous video signal may be applied to the grid to produce a picture.
- the thermionic cathodes are about 5 microns across and may be spaced as little as 10 microns apart. In this way a high definition image may be produced on the phosphor screen 41.
- the display device of the invention may be used to display a rapidly changing, high definition image, for example a television picture.
- each thermionic cathode will tend to spread as it passes from the cathode assembly 39 to the phosphor screen 41 and it is desirable to obviate the need for separate focussing anodes if possible although they may be included if necessary.
- the grid assembly 43 tends to reduce the cross sectional area of each electron beam. If however the cathode assembly and the phosphor screen are close together the requirement for a grid assembly may be obviated. Such a construction may be preferred for alphanumeric displays.
- FIG. 8 there is depicted a further embodiment of the display device of the invention.
- this embodiment resembles the display device described above with reference to Figure 7.
- the display device again comprises a cathode assembly 51, a phosphor screen 53 and a grid assembly 55 interposed between the cathode assembly 51 and the phosphor screen 53. Differences arise however in the addressing of the thermionic cathodes 57 of the cathode assembly 51 and in the construction of the grid assembly 55.
- the remaining common features are as described above with reference to Figure 7.
- each row of thermionic cathodes 57 is linked such that in use the same current flows in each thermionic cathode of a row.
- the grid assembly 55 comprises a number of electrically separate grid elements 59 each grid element 59 being common for a column of grid apertures 61.
- Each row of thermionic cathodes in use is separately supplied with an electrical current by means of address logic and drive circuitry.
- Each grid element 59 may be separately supplied with an electrical potential.
- By supplying current successively to each row of thermionic cathodes 57 and appropriate bias to each grid element 59 a scanned image may be formed upon the phosphor screen 53.
- the intensity of each pixel forming the image may be adjusted by varying either the current passing through each row of thermionic cathodes 57 or preferably by varying the negative potential applied to each grid element 59.
- the display device is used to produce a television picture, the grid elements 59 being scanned at line scan speed, the rows of thermionic cathodes being scanned at frame scan speed, pixel intensity being varied by the value of negative potential applied to each grid element 59.
- FIG. 9 there is depicted a further embodiment of the display device of the invention.
- this embodiment resembles the display device described above with reference to Figure 7.
- the display device again comprises a cathode assembly 63, a phosphor screen 65 and a grid assembly 67 interposed between the cathode assembly 63 and the phosphor screen 65. Differences arise however in the addressing of the thermionic cathodes 69 of the cathode assembly 63 and in the construction of the grid assembly 67.
- the remaining common features are as described above with reference to Figure 7.
- each thermionic cathode 69 is provided with the same current flow such that each thermionic cathode produces substantially the same flux of thermal electrons.
- the grid assembly 67 comprises a number of electrically separate grid elements 71 each grid element 71 being associated with a single grid aperture 73.
- Each grid element 71 may be separately supplied with an electrical potential by means of address logic and if necessary suitable drive circuitry. In this way the intensity of each pixel formed on the phosphor screen 65 may be independently varied by varying the negative potential applied to each grid element 71.
- a direct display device of the fourth aspect of the invention comprises a matrix of elements of the invention resembling for example the structure shown generally at 39 in Figure 7.
- the matrix of light emitting devices is enclosed in a suitable envelope filled with a low pressure inert gas and provided with a transparent window in front of the matrix.
- the individual light emitting devices comprising the matrix may be separately supplied with an electrical current by suitable address logic and drive circuits.
- the intensity of light produced by each light emitting device may thereby be independently varied such that an image may be produced by the composite light emitting devices forming the matrix.
- the preferred method of forming each light emitting device is anisotropic etching as described above.
- the low thermal mass of light emitting devices of the invention allows for a rapid modulation of the light intensity of the device. This enables a rapidly changing image to be produced by the direct display device.
- Line and frame storage techniques may also be used to reduce the number of and/or speed of switching cycles for the light emitting devices.
- the direct display device may therefore be useful as a display for a television picture.
- a coloured display may be produced by providing colour filters in or on the window of the direct display device.
Landscapes
- Cathode-Ray Tubes And Fluorescent Screens For Display (AREA)
Description
- This invention relates to an element which may be used either as an electron emitting device or as a light emitting device. In particular one or more elements may be used as thermionic cathodes in a thermionic valve such as a cathode ray tube or as light emitting devices in, for example, a display.
- The physical phenomenon of thermionic emission is exhibited by a metal heated above a characteristic threshold temperature. When a metal is heated such that the energy of electrons within the metal exceeds the value of the thermionic work function characteristic of the metal, electrons escape from the metal and form a space charge. A thermionic valve utilises the space charge to set up a current flow, in the form of electrons travelling in a vacuum, from a hot metal (a thermionic cathode) to a relatively positively charged anode. The flow of electrons may be altered by means of a control grid interposed in the electron flow.
- The advent of semiconductor technology and circuit integration has led to a marked reduction in the general use of thermionic valves. Particular forms of thermionic valve are however much used in special applications.
- One such particular form is the cathode ray tube.
- The cathode ray tube is an image forming thermionic valve which has commercial application not only in the domestic market as the major component of television receivers but also in research and industry as the major component of oscilloscopes.
- In broad terms a cathode ray tube comprises an evacuated glass envelope, one end of which is formed into a screen. At the opposite end of the tube is located an assembly for producing a stream of electrons ("a cathode ray"). This assembly is commonly known as an electron gun and comprises a thermionic cathode, a control grid and at least one anode. The thermionic cathode produces a space charge of electrons, which electrons are accelerated away from the cathode by the positive charge of the accelerating anode. The speed and therefore the kinetic energy of the electrons may be controlled by a negatively biased control grid. A fine beam of electrons is required and further anodes are usually provided which create an anisotropic electrostatic field tending to focus the electron beam in a manner analogous to the focusing of light rays by a lens. After leaving the electron gun, the beam of electrons is subjected to a deflecting force. This may be an electrostatic force, in which case the beam is caused to pass between two mutually perpendicular pairs of plates. The plates may be charged in order to deflect the beam of electrons in one or both of two perpendicular axes. This method of deflecting the electron beam in a cathode ray tube is commonly used in oscilloscopes.
- In an alternative arrangement, the electron beam is caused to pass through the field of electromagnets capable of creating perpendicular electromagnetic fields. The electromagnets are usually located outside the glass envelope of the cathode ray tube. The application of current to the electromagnets allows deflection of the electron beam in one or both of two perpendicular axes. This method of deflection is commonly used in cathode ray tubes where a continuous raster scan is required (as, for example, in a television picture tube).
- The electron beam finally impinges upon a phosphor screen where the kinetic energy of the electrons comprising the beam is converted into light.
- The conventional miniature thermionic cathode structure is generally a coil of metal wire of high melting point. The coil is supported at either end by its electrical connection to the electron gun assembly. Typically, such a miniature cathode is constructed from a wire of 20 microns in diameter and 10 millimeters in length. The cathode is commonly made of tungsten which requires to be heated to about 1100 K in order to produce a satisfactory flux of electrons. The operating temperature may be reduced to about 900 K by adding a coating which reduces the effective thermionic work function of the wire. A commonly used coating is a mixture of carbonates.
- A miniature thermionic cathode of the type described immediately above consumes a considerable amount of power at its operating temperature. For example, a coated tungsten thermionic cathode of the dimensions given will consume about 50 mW at 900 K. Only a fraction of this power goes directly to provide the energy to liberate electrons from the cathode material. The majority of the power taken by the cathode is dissipated as heat by conduction and radiation. An object of the present invention is to reduce the power losses and also to provide a cathode of reduced overall size.
- Patents Abstracts of Japan, Vol. 7, No. 72 (E-166) (1217) describes the construction of a thermionic cathode comprising an alumina-ceramic substrate having an overlying glass glaze. A conductor and filaments are formed on the glass glaze such that the filaments overlie gaps in the glaze.
- Recent advances in solid state circuitry have provided the technology to produce layer structures of intricate, yet well defined, physical shape. In particular, it is possible to produce complex multi-layered structures, by known miniconduc- tor fabrication techniques (such as material oxidation, deposition and selective etching) which may, for example, operate as complete electronic circuits. This technology has been adapted to produce the element of the present invention.
- The element of the invention may be used as a thermionic cathode replacing the known cathodes in thermionic devices as described above, or as we have now discovered, by increasing the power supplied to the element, it may be used as an incandescent light source providing a light emitting device of small dimensions for use, for example, in a direct display device.
- According to a broad, first aspect of the present invention, we provide an element for use as a thermionic cathode and/or light emitting device comprising a substrate, a metal layer and an electrically insulating support layer interposed between the substrate and the metal layer, wherein a part of the metal layer, having a cross-sectional area smaller than elsewhere in the metal layer, extends across a recess, characterised in that the substrate is of a semiconductor material and is provided with the said recess, across which extends a bridge structure comprising a part of the insulating support layer and the said part of the metal layer.
- In use, an electrical current is passed through the metal layer, the current resistively heating the metal to a temperature at which thermionic emission and/or light emission occurs.
- The recess is formed such that the bridge structure is physically separated from the semiconductor substrate. As examples, the recess may be in the form of a hollow or groove formed in the surface of the semiconductor substrate, or a hole passing through the semiconductor substrate.
- The bridge structure may take the form of a narrow linear strip of the metal layer attached to a support layer of similar shape, a broad band of support material to which is attached a metal layer in the form of a sinuous strip or a region of support material having attached thereto a metal layer in the form of a sinuous strip, the region of support material overlying the recess and being supported by three or more support arms.
- The bridge structure is preferably formed such that is does not completely occlude the recess in the semiconductor material. In this way, when the element of the present invention is used as a thermionic cathode, in an evacuated envelope, a vacuum surrounds the bridge structure reducing conduction of heat from the heated metal layer. Energy losses due to conduction of heat to the semiconductor substrate are minimised due to the preferred construction of the bridge.
- When the element is used as a light-emitting device, it may be enclosed in an evacuated envelope or in an envelope containing a low pressure gas or an inert gas.
- The metal layer is preferably nickel or tungsten.
- Most preferably when the element is used as a thermionic cathode, the metal layer is provided with coating capable of reducing the effective thermionic work function of the metal layer.
- Preferably, the metal layer of the bridge structure is about 5 microns wide and hence, in order to maintain mechanical stability of the metal layer, the support material must not only be physically strong but must also be capable of remaining mechanically stable at the temperatures required to cause the metal layer to exhibit thermionic emission. Where the metal layer is tungsten, the support layer must remain mechanically stable up to at least 1100 K.
- Preferably the semiconductor substrate is silicon. Preferably the support material is silicon dioxide or silicon nitride. These materials may be readily formed in a layer structure using processes known per se in the art. The bridge structure may be formed using the process of anisotropic etching, or chemical milling.
- Preferably, the element of the present invention includes integrated circuitry upon the semiconductor substrate. For example when used as a thermionic cathode, the element may have circuit elements for providing cathode drive included on the semiconductor substrate. Similarly, when the element is used as a light emitting device the substrate may include circuitry for providing a current supply to the element.
- In a second aspect of the invention, we provide a thermionic valve including a thermionic cathode the thermionic cathode comprising at least one element of the present invention. Most preferably the thermionic valve is a cathode ray tube. The element of the present invention may be made in such a way that its overall size is of the order of 50 microns square and having a very low power consumption. It is therefore of great utility for small size, low power consumption cathode ray tubes.
- Preferably, therefore, we provide a display device comprising a thermionic cathode assembly provided with at least one element of the first aspect of the invention a screen coated with a substance capable of converting the kinetic energy of electrons impinging on the screen into light, a grid assembly interposed between the cathode assembly and the screen, and an anode for accelerating electrons from the cathode to the screen.
- Preferably the screen is a phosphor screen.
- The cathode assembly, grid assembly, screen and anode are enclosed in a vessel at reduced pressure, preferably substantially evacuated.
- Preferably the display device is further provided with cathode ray deflection means for deflecting a beam of electrons produced by the cathode to a predetermined point on the phosphor screen. The deflection means may be electromagnetic or electrostatic means, such as those known in the art. In addition one or more focusing anodes may be provided.
- In this preferred embodiment, the cathode assembly may comprise a single element of the invention, to act as a thermionic cathode in the display device. If required, a number of elements may be closely packed to afford an increased flux density of thermal electrons. Multibeam display devices for use for example in dual beam oscilloscopes and three electron gun colour television tubes may be produced using either two or more cathode assemblies or one cathode assembly provided with suitably spaced elements of the invention.
- A plurality of elements of the invention may together form a matrix of elements, on a single wafer of semiconductor material. The term "matrix" as used herein refers to a number of elements of the invention arranged in a geometric pattern, preferably as one or more rows and/or columns most preferably linear rows and columns. Such a matrix may be formed using, for example the anisotropic etching process mentioned above.
- In a third aspect of the invention we provide a display device comprising a thermionic cathode assembly provided with a matrix of elements of the invention, a screen coated with a substance capable of converting the kinetic energy of electrons impinging on the screen into light and an anode for accelerating electrons from the cathode assembly to the screen.
- Preferably the display device includes a grid assembly interposed between the screen and the cathode assembly.
- Preferably the screen is a phosphor screen.
- A particular and great advantage of this aspect of the invention is that a matrix of electron beams may be produced by a cathode assembly, each element in the matrix producing thermal electrons which may be accelerated towards a phosphor screen such that each electron beam thus produced forms a picture element (pixel) upon the screen. In this way the need for deflection means is obviated. In conventional cathode ray tubes the distance from the cathode to the screen must be sufficiently large for the deflection applied to the beam to produce a significant displacement of the point at which the beam impinges upon the screen. This has in the past limited the reduction in 'front to back' dimensions of cathode ray tubes. A cathode ray tube of this embodiment, by negating the need for deflection of the beam provides a tube of greatly reduced 'front to back' dimension relative to a conventional cathode ray tube.
- Preferably the matrix of elements forming the cathode assembly is arranged such that each element in the matrix is provided with an independent current flow. For example, the matrix of elements may be addressed by a digital logic circuit and suitable drive circuits. The drive circuits cause a current to flow through each addressed element thereby heating the element by means of resistance heating. The hot element, acting as a thermionic cathode, will then melt thermal electrons which may be accelerated towards the phosphor screen, past the grid. The grid is, in this embodiment, preferably provided with an even potential over its surface. Therefore light produced by electrons impinging upon the phosphor screen will be produced in a matrix of pixels corresponding to the elements which are addressed. An image may therefore be built up by addressing those elements in the cathode assembly which correspond to the pixels it is desired to produce on the phosphor screen. The low thermal mass of the individual elements in the cathode assembly ensures that the individual cathodes heat up and cool down very rapidly thereby allowing a rapid change in the image produced on the phosphor screen. The intensity of each pixel produced on the phosphor screen may be controlled by adjusting the current supplied to the relevant element in the cathode assembly. Preferably however the intensity of each pixel is adjusted by adjusting the potential difference between the grid and the cathode assembly. Colour images may be built up by providing a matrix of different phosphors on the phosphor screen. For example adjacent elements in the cathode assembly may correspond to phosphors producing red, green or blue light in response to energy supplied by impinging electrons.
- Preferably the matrix of elements is addressed such that each element in a row of the matrix is supplied with an equal current flow and the grid assembly comprises a number of independent grid elements each corresponding to a column of the elements forming the cathode assembly. Each grid element may be addressed separately such that the combined effect of addressing the elements in rows of the cathode assembly and the grid elements in columns of the grid assembly provides a method of selecting, for example, the intensity of each point produced by the matrix of elements. The terms row and column are of course interchangeable in respect of the cathode assembly drive and the grid assembly drive.
- In a further preferred embodiment, each element in the cathode assembly is provided with substantially equal current flow, the grid assembly forming a matrix of grid elements, to each element of which may be applied an independent bias potential. Each grid element corresponds to a thermionic cathode in the cathode assembly, whereby by addressing the grid, e.g. using digital circuitry and, if necessary, drive circuits, the kinetic energy of electrons produced by each cathode in the cathode assembly may be individually adjusted thereby adjusting the intensity of light produced on the phosphor screen.
- The display devices may be used in for example so called flat screen televisions or alphanumeric displays.
- In a fourth aspect of the invention we provide a display device comprising one or more light emitting devices the or each light emitting device consisting of an element of the first aspect of the invention. Preferably the display device comprises a plurality of light emitting devices most preferably arranged to provide a matrix of light emitting devices. Preferably the light emitting devices are enclosed in a suitable envelope. A suitable envelope may include for example an evacuated envelope or an envelope having an inert atmosphere.
- Individual light emitting devices in the matrix may be addressed using, for example, digital circuitry and drive circuitry. The light emission from each light emitting device may be controlled by the current passed to each device. Each device has a low thermal mass which allows modulation of the emitted light intensity at a relatively high frequency.
- A matrix may be used for example to produce a flat screen television or an alphanumeric display.
- The various aspects of this invention are now described, by way of example, with reference to the accompanying drawings, in which:-
- Figure 1 is a plan view of one embodiment of a single element of the invention,
- Figure 2 is a section on the line A-A of Figure 1,
- Figure 3 is a plan view of a second embodiment of a single element of the invention,
- Figure 4 is a section on the line B-B of Figure 3,
- Figure 5 is a plan view of a third embodiment of a single element of the invention,
- Figure 6 is a side elevation of a thermionic cathode in combination with a control grid assembly,
- Figure 7 is a schematic isometric view of a display device of the third aspect of the invention,
- Figure 8 is a schematic isometric view of a further embodiment of a display device of the third aspect of the invention, and
- Figure 9 is a schematic isometric view of yet a further embodiment of a display device of the third aspect of the invention.
- Referring to the simple embodiment of the invention shown in Figures 1 and 2, a silicon substrate 1 provides mechanical support for the element, which may be a cathode or a light emitting device. The substrate is of a type now well known in the electronics industry as a semiconductor substrate upon which integrated circuits are constructed. The silicon substrate 1 is provided with a
recess 3 in the form of a depression etched into the surface of the substrate by anisotropic etching. A layer of silicon dioxide (or silicon nitride) 5 overlies the surface of the silicon and is so shaped as to form anarrow bridge 7 over therecess 3. A layer oftungsten metal 9 lies over the silicon dioxide layer such that the cross sectional area of the layer is at its lowest in that part of the layer overlying the silicon dioxide layer forming thebridge 7. This ensures that the maximum resistive heating of the metal layer is concentrated in the bridge region. - The layer of silicon dioxide (or silicon nitride) 5 shown in Figure 1 is shaped to correspond to the shape of the layer of
tungsten metal 9. In practice the silicon dioxide layer may cover the surface of the silicon substrate 1 except for regions adjacent the bridge structure, which allow communication between therecess 3 and the surroundings. - In use, the element, if it is to be a thermionic cathode, is placed within an evacuated envelope and a current is passed through the
metal layer 9. Resistive heating of the metal layer in the bridge area occurs and the metal is heated to a temperature sufficient to cause thermionic emission. This will typically be 1100 K for a tungsten conductor or 900 K for a carbonate coated tungsten conductor. Therecess 3 reduces the loss of energy by conduction into the substrate and the low cross sectional area made possible by the supported conductor, reduces conduction and consequent heat loss through the conductor itself. - Figures 3 and 4 show a second embodiment of the first aspect of the invention in which the techniques of anisotropic etching have been used to form a
silicon substrate 11, provided with arecess 13. A layer of silicon dioxide (or silicon nitride) 15 overlies the silicon substrate forming abridge 17 over therecess 13 such that the recess is not completely enclosed. Aconductor 19 of tungsten has been deposited upon the silicon dioxide (or silicon nitride) layer and has been etched to provide a narrowsinuous conductor portion 21 on thebridge 17. In this embodiment, the recess is square in plan with an edge measuring about 50 microns. The narrow conductor portion has a width of about 5 microns. The advantage of an etched conductor pattern of this type is that the operating voltage of the element can be tailored to a convenient value. - Figure 5 shows a third embodiment of the first aspect of the invention. The third embodiment comprises a silicon substrate provided with a recess (these being substantially as described above). The silicon substrate is overlain with a
support layer 75 of silicon dioxide or silicon nitride. Thesupport layer 75 is provided withapertures 77 communicating with the recess formed in the silicon substrate, such that a suspendedregion 79 of thesupport layer 75 is formed. Theregion 79 is suspended above the recess by fourarms support layer 75. Aconductor 89 has been deposited upon thesupport layer 75 and has been etched to provide a relatively narrow,sinuous conductor portion 91 on the suspendedregion 79 of thesupport layer 75. Electrical connection to theconductor portion 91 is made by way ofconnector portions arms Connector portions opposed arms conductor portion 91.Arms connector portions connector portions - Figure 6 shows a thermionic cathode of the present invention (shown generally at 25) in combination with a control grid assembly (shown generally at 27) and exemplifies a possible mounting technique for the thermionic cathode of the present invention. The
control grid assembly 27 comprises aceramic plate 29 having a metal coating on its front and rear faces. Theceramic plate 29 is provided with anaperture 31 passing through both the plate and metal coatings, perpendicular to the general plane of the plate. The thermionic cathode 25 is provided with electricallyconductive protrusions 33, e.g. of solder or conductive paste, which can be used to connect the thermionic cathode 25 to thegrid assembly 27 and which make electrical contact between the metal layer of thethermionic cathode 35 and the rear metal coatedface 37 of thegrid assembly 27. The rear metal coatedface 37 of thegrid assembly 27 therefore is at the same potential as the thermionic cathode in use. The thermionic cathode 25 is aligned such that, in use, electrons emitted by the thermionic cathode pass through theaperture 31 under the influence of a positive potential provided by an anode (not shown). The front metal coatedface 39 of thegrid assembly 27 forms a control grid which may be used to alter the flow of electrons from the cathode. In use, the front metal coatedface 39 would normally be negatively biased with respect to the cathode. The advantage of this mounting system is that the cathode and grid are maintained at a fixed spacing and alignment. This has attendant advantages not only in terms of the mechanical strength of the arrangement but also in terms of ease of manufacture. - Referring to Figure 7 there is depicted a display device of the invention. The display device comprises a
cathode assembly 39, asphosphor screen 41, and agrid assembly 43 interposed between thecathode assembly 39 and thephosphor screen 41. - The
cathode assembly 39 comprises asilicon substrate 45 upon which is formed a plurality of elements of the invention (for example 47). Each element of the invention acts as a thermionic cathode. Thethermionic cathodes 47 are arranged in an array or matrix. The matrix depicted in Figure 7 has only nine rows and twelve columns for the purpose of ready illustration. In practice the matrix may possess a large number of rows and columns limited only by physical constraints of manufacture. - The
grid assembly 43 comprises a perforate plate. The construction of the grid assembly is as shown in Figure 6, an insulating plate being provided with a metal coating (not shown) on its front and rear faces. The grid assembly is provided with a plurality of grid apertures (for example 49) capable of allowing passage of electrons. Each aperture 49 in thegrid assembly 43 corresponds spatially with athermionic cathode 47 in an equivalent row and column in thecathode assembly 39. - The
phosphor screen 41 comprises a transparent support material to the side facing the grid assembly of which is affixed a coating of a phosphor material capable of converting electron kinetic energy into visible light. The phosphor coating is either of one type, capable of generating the same colour of visible light over the surface of thephosphor screen 41 or is comprised of regions of a number of different phosphors each different phosphor being capable of generating a different colour of visible light. Such phosphor screens are well known in the art and are therefore not further described. - The
cathode assembly 39, thegrid assembly 43 and thephosphor screen 41 are enclosed in an evacuated envelope (not shown). The envelope also enclosed an accelerating anode (not shown) disposed between thegrid assembly 43 and thephosphor screen 41. Thephosphor screen 41 may form part of the envelope. The transparent support material of the phosphor screen may comprise a transparent electrically conducting material and may form the accelerating anode. A suitable material is, for example, a coating of tin oxide. - In use, the accelerating anode is maintained at high positive potential relative to the
cathode assembly 39. Eachthermionic cathode 47 in thecathode assembly 39 is capable of being separately supplied with an electrical current by means of address logic and drive circuitry which may be located outside the envelope or may be included within the envelope, for example, upon thesilicon substrate 45 of the cathode assembly. When a current is passed through thethermionic cathodes 47, they are heated to a temperature at which thermionic emission of electrons occurs. The electrons are accelerated away from thecathode assembly 39 by the high positive potential of the accelerating anode. Electrons produced by eachthermionic cathode 47 pass through the corresponding aperture 49 in thegrid assembly 43 and onwards to thephosphor screen 41 where their kinetic energy is converted into visible light by the phosphor. In this way eachthermionic cathode 47 when supplied with current produces a point of light on thephosphor screen 41, each point of light forming a picture element or pixel of an image to be formed on thephosphor screen 41. An image may therefore be produced on thephosphor screen 41 by selectively addressing and supplying current to those thermionic cathodes in thecathode assembly 39 corresponding to the desired pixels of the image. The intensity of each pixel may be individually altered by varying the voltage between thegrid 43 and the selected thermionic cathode. For example the thermionic cathodes may be sequentially addressed and a synchronous video signal may be applied to the grid to produce a picture. The thermionic cathodes are about 5 microns across and may be spaced as little as 10 microns apart. In this way a high definition image may be produced on thephosphor screen 41. Suitably the display device of the invention may be used to display a rapidly changing, high definition image, for example a television picture. - In the schematic arrangement depicted in Figure 7 considerable spacing of the
cathode assembly 39, thegrid assembly 43 and thephosphor screen 41 is shown. In practice thecathode assembly 39 and thegrid assembly 43 are in close proximity and may be in actual physical contact as depicted for one thermionic cathode and one grid assembly in Figure 6. Thegrid assembly 43 and thephosphor screen 41 are separated. However in order to minimise the cross sectional area of the pixels formed upon thephosphor screen 41 it is desirable to reduce the distance between thecathode assembly 39 and thephosphor screen 41 as much as possible. The electron beam produced by each thermionic cathode will tend to spread as it passes from thecathode assembly 39 to thephosphor screen 41 and it is desirable to obviate the need for separate focussing anodes if possible although they may be included if necessary. Thegrid assembly 43 tends to reduce the cross sectional area of each electron beam. If however the cathode assembly and the phosphor screen are close together the requirement for a grid assembly may be obviated. Such a construction may be preferred for alphanumeric displays. - Referring now to Figure 8 there is depicted a further embodiment of the display device of the invention. In general arrangement this embodiment resembles the display device described above with reference to Figure 7. The display device again comprises a
cathode assembly 51, aphosphor screen 53 and agrid assembly 55 interposed between thecathode assembly 51 and thephosphor screen 53. Differences arise however in the addressing of thethermionic cathodes 57 of thecathode assembly 51 and in the construction of thegrid assembly 55. The remaining common features are as described above with reference to Figure 7. In the embodiment of Figure 8 each row ofthermionic cathodes 57 is linked such that in use the same current flows in each thermionic cathode of a row. Thegrid assembly 55 comprises a number of electricallyseparate grid elements 59 eachgrid element 59 being common for a column ofgrid apertures 61. Each row of thermionic cathodes in use is separately supplied with an electrical current by means of address logic and drive circuitry. Eachgrid element 59 may be separately supplied with an electrical potential. By supplying current successively to each row ofthermionic cathodes 57 and appropriate bias to each grid element 59 a scanned image may be formed upon thephosphor screen 53. The intensity of each pixel forming the image may be adjusted by varying either the current passing through each row ofthermionic cathodes 57 or preferably by varying the negative potential applied to eachgrid element 59. In a particular embodiment the display device is used to produce a television picture, thegrid elements 59 being scanned at line scan speed, the rows of thermionic cathodes being scanned at frame scan speed, pixel intensity being varied by the value of negative potential applied to eachgrid element 59. - Referring now to Figure 9 there is depicted a further embodiment of the display device of the invention. In general arrangement this embodiment resembles the display device described above with reference to Figure 7. The display device again comprises a
cathode assembly 63, aphosphor screen 65 and agrid assembly 67 interposed between thecathode assembly 63 and thephosphor screen 65. Differences arise however in the addressing of thethermionic cathodes 69 of thecathode assembly 63 and in the construction of thegrid assembly 67. The remaining common features are as described above with reference to Figure 7. In the embodiment of Figure 9 in use eachthermionic cathode 69 is provided with the same current flow such that each thermionic cathode produces substantially the same flux of thermal electrons. Thegrid assembly 67 comprises a number of electrically separate grid elements 71 each grid element 71 being associated with asingle grid aperture 73. Each grid element 71 may be separately supplied with an electrical potential by means of address logic and if necessary suitable drive circuitry. In this way the intensity of each pixel formed on thephosphor screen 65 may be independently varied by varying the negative potential applied to each grid element 71. - In a final embodiment, the elements of the invention are employed as individual light emitting devices. A direct display device of the fourth aspect of the invention comprises a matrix of elements of the invention resembling for example the structure shown generally at 39 in Figure 7. The matrix of light emitting devices is enclosed in a suitable envelope filled with a low pressure inert gas and provided with a transparent window in front of the matrix. The individual light emitting devices comprising the matrix may be separately supplied with an electrical current by suitable address logic and drive circuits. The intensity of light produced by each light emitting device may thereby be independently varied such that an image may be produced by the composite light emitting devices forming the matrix. The preferred method of forming each light emitting device is anisotropic etching as described above. This advantageously provides a mirror surface behind each light emitting device reflecting most of the light produced by the incandescent element in a forward direction. The low thermal mass of light emitting devices of the invention allows for a rapid modulation of the light intensity of the device. This enables a rapidly changing image to be produced by the direct display device. Line and frame storage techniques may also be used to reduce the number of and/or speed of switching cycles for the light emitting devices.
- The direct display device may therefore be useful as a display for a television picture. A coloured display may be produced by providing colour filters in or on the window of the direct display device.
Claims (38)
Applications Claiming Priority (4)
Application Number | Priority Date | Filing Date | Title |
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GB838312720A GB8312720D0 (en) | 1983-05-09 | 1983-05-09 | Cathode |
GB8312720 | 1983-05-09 | ||
GB838332733A GB8332733D0 (en) | 1983-05-09 | 1983-12-08 | Element |
GB8332733 | 1983-12-08 |
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EP0125859A1 EP0125859A1 (en) | 1984-11-21 |
EP0125859B1 true EP0125859B1 (en) | 1987-09-09 |
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EP19840303057 Expired EP0125859B1 (en) | 1983-05-09 | 1984-05-08 | Element |
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EP (1) | EP0125859B1 (en) |
DE (1) | DE3466127D1 (en) |
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NL9100327A (en) * | 1991-02-25 | 1992-09-16 | Philips Nv | CATHODE. |
FI110727B (en) * | 1994-06-23 | 2003-03-14 | Vaisala Oyj | Electrically adjustable thermal radiation source |
FI102696B (en) * | 1995-02-22 | 1999-01-29 | Instrumentarium Oy | Dual radiation source assembly and measuring sensor |
GB9607862D0 (en) * | 1996-04-16 | 1996-06-19 | Smiths Industries Plc | Light-emitting assemblies |
FR2748810A1 (en) * | 1996-09-30 | 1997-11-21 | Commissariat Energie Atomique | Miniature source of infrared radiation, with coated metal microfilament |
US6124145A (en) * | 1998-01-23 | 2000-09-26 | Instrumentarium Corporation | Micromachined gas-filled chambers and method of microfabrication |
US20060006787A1 (en) * | 2004-07-06 | 2006-01-12 | David Champion | Electronic device having a plurality of conductive beams |
CN101471215B (en) * | 2007-12-29 | 2011-11-09 | 清华大学 | Production method of thermoelectron source |
CN101471210B (en) * | 2007-12-29 | 2010-11-10 | 清华大学 | Thermoelectron source |
Family Cites Families (3)
Publication number | Priority date | Publication date | Assignee | Title |
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US3715785A (en) * | 1971-04-29 | 1973-02-13 | Ibm | Technique for fabricating integrated incandescent displays |
US4020381A (en) * | 1974-12-09 | 1977-04-26 | Texas Instruments Incorporated | Cathode structure for a multibeam cathode ray tube |
DE3063978D1 (en) * | 1979-09-05 | 1983-08-04 | Tokyo Shibaura Electric Co | Flat display device |
-
1984
- 1984-05-08 DE DE8484303057T patent/DE3466127D1/en not_active Expired
- 1984-05-08 EP EP19840303057 patent/EP0125859B1/en not_active Expired
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