GB2322001A

GB2322001A - Electron emitters e.g. for displays

Info

Publication number: GB2322001A
Application number: GB9800704A
Authority: GB
Inventors: Neil Anthony Fox; Wang Nang Wang
Original assignee: Smiths Group PLC
Current assignee: Smiths Group PLC
Priority date: 1997-02-05
Filing date: 1998-01-14
Publication date: 1998-08-12
Also published as: GB9800704D0

Abstract

An electron emitter device, such as for a display, has a substrate 21 supporting several electron emitters 20. A gate electrode layer 25 and a layer 24 of a magnetic material are supported on an insulating layer 23 on the substrate. The gate electrode layer 25 and magnetic layer 24 both have orifices 35 and 36 through which electrons can be emitted from respective electron emitters 20. The magnetic layer 24 produces annular regions of magnetic flux lines coaxial with the orifices, which act to confine electrons to the central part of the orifices 35 and 36. The emitters 20 may be silicon or metal spikes coated with a layer 33 of polycrystalline diamond, or may be provided by a surface of low or negative electron affinity, e.g. of diamond material.

Description

ELECTRON EMITTERS This invention relates to electron emitters and devices.

Electron emitter devices can be of various different kinds, such as having microscopic discontinuities, for example, spikes or wedges, or being of a material with a low or negative electron affinity. One way of controlling the production of electrons from an electron emitter device is by means of a gate electrode in a triode configuration. A negative potential applied to the gate electrode will reduce or prevent the emission of electrons from the device. The gate electrode enables accurate and rapid control of the electron production.

The gate electrode can, however, present a problem in that there will generally be some parasitic emitter to gate current flow, reducing the anode current flow of the device.

This emitter to gate current flow can, in severe cases lead to catastrophic failure of the device.

Electron emitters are used in many different applications, such as in displays. In some of these applications it can be desirable for the electrons to be focussed onto a target of small area, such as to reduce the spot size and increase the electron density. Up to now, it has not been easy to achieve a focussed beam of electrons from such electron emitters.

It is an object of the present invention to provide an improved electron emitter device.

According to one aspect of the present invention there is provided an electron emitter device having an electron emitter, a gate electrode located above the emitter and defining a region through which electrons are emitted from the device, and a magnetic device located in the region of the gate electrode, the magnetic device being arranged to increase the confinement of the electrons within a central part of the region.

The gate electrode is preferably located above the magnetic device, which may be provided by a layer of magnetic material having an orifice aligned with the region defined by the gate electrode. The emitter device may include a substrate with a layer of insulating material, the substrate supporting the electron emitter, and the layer of insulating material supporting the layer of layer of magnetic material. The combined thickness of the layer of magnetic material and the insulating layer is preferably about 1 micron. The layer of magnetic material preferably supports the gate electrode. The emitter device preferably includes a substrate supporting the electron emitter, the gate electrode and the magnetic device being both provided by respective layers, and the gate electrode layer and the magnetic layer being supported one above the other with said substrate. The layer of magnetic material may be formed by deposition, such as by magnetron ion sputtering. The emitter device may include a plurality of electron emitters and the magnetic layer may have a plurality of orifices aligned with respective ones of the electron emitters. The magnetic device is preferably of a bariurn ferrite compound. The or each emitter may include a Spindt tip structure and, or alternatively, may have a diamond surface.

According to another aspect of the present invention there is provided a display including an electron emitter device according to the above one aspect of the invention.

The display may include a layer of fluorescent material located above the electron emitter device such that electrons emitted by the device cause the fluorescent layer to emit optical radiation.

A display including an electron emitter device according to the present invention, will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a sectional side elevation of a part of the display; Figure 2 is a sectional side elevation of an electron emitter of the display, to an increased scale; Figure 3 is a sectional side elevation of a part of the electron emitter of Figure 2, showing the magnetic lines of flux; and Figure 4 is a sectional side elevation of an alternative emitter.

With reference to Figure 1, the display has an emitter assembly 1 including several emitter arrays 2, only one of which is shown in the drawing. The display also includes an anode assembly 3 in the form of a transparent plate 4 having a transparent anode electrode 5 on its lower surface. A phosphor layer 6 is deposited on the lower surface of the anode 5 in discrete regions 7 located above corresponding ones of the emitter arrays 2. The emitter assembly 1 and the anode assembly 3 are sealed together around their outer edge, the space between the two assemblies being evacuated to low pressure. In this way, electrons emitted from each emitter array 2 will be incident on the respective phosphor region 7 directly above it to cause the region to fluoresce and produce optical radiation. Light produced by the phosphor region 7 is emitted through the plate 4. By appropriately addressing the anode and different ones of the emitter arrays, any desired image can be produced, which may be multicoloured if the phosphor regions 7 are chosen to fluoresce with different colours. As so far described, the display is conventional.

The emitter array 2 is of a novel construction comprising several emitter devices 20 of the kind shown in more detail in Figure 2. Each emitter device 20 is a multi-layer device formed on a common substrate 21, such as of silicon, which carries a metal layer 22 arranged as contact strips by which electrical connection can be made to the emitter devices.

Preferably, the metal layer contact strips 22 are of molybdenum. On top of the metal layer 22 there is an electrically-insulative layer 23, such as of silica.

The insulative layer 23 supports a layer 24 of a magnetic material such as a barium ferrite compound, the purpose of which will be described later. The magnetic layer 24 is deposited on the insulative layer 23 by magnetron ion sputtering. The combined thickness of the insulative layer 23 and the magnetic layer 24 is typically about 1 cm.

The upper layer of the emitter device 2 is provided by a metal gate electrode layer 25.

A recess 30 extends through the gate electrode layer 25, the magnetic layer 24 and the insulative layer 23 as far as the upper surface of the lower metal layer 22.

Each emitter device 20 includes an electron emitting structure 31 within this recess 30. This electron emitter 31 may be of various different forms but, in the present example, is a Spindt tip structure comprising a conical spike 32 about lllm high. The spike 32 may be of a metal, a semiconductor, or an insulating material. The spike 32 may be made in various different ways. One way is to deposit a sacrificial layer (not shown) of aluminium onto the gate electrode layer 25 leaving a hole through this aluminium layer into the recess 30.

Material, such as silicon or a metal, is then e-beam sputtered from a target at an angle of about 70" to the substrate surface through the holes into the recess 30 to build up the conical spike. The spike is then coated with a layer 33 of a polycrystalline diamond doped, such as with boron, to make it p-type, or phosphorus, to make it n-type. This coating may be carried out by ion sputtering or excimer laser ablation of a doped CVD (chemical vapour deposited) diamond target using the angled deposition method. The target material is preferably doped with boron or phosphorus by low energy implantation, at liquid nitrogen temperatures, into a high quality CVD diamond film that has been rapidly thermally annealed at 1 0000C for 10 minutes in a forming gas atmosphere. Alternatively, the diamond target could be doped in situ, during CVD growth, using diborane or phosphine gases or solid dopants. The process of coating the diamond onto the spike 32 is preferably performed in a hydrogen atmosphere at about 30 torr or less, with the substrate structure at an elevated temperature of about 500 550"C. A dc voltage is preferably applied to the spike 32 to enhance the coating process.

After the diamond coating 33 has been deposited, the sacrificial aluminium layer is lifted off.

The coated emitter structure 31 is aligned centrally of a circular orifice 35 in the gate electrode layer 25 and a similar orifice 36 in the magnetic layer 24, the tip of the coated spike 32 being located approximately level with the junction between the gate electrode and magnetic layers.

The process by which the magnetic layer 25 is deposited ensures that it acts as a permanent magnet with an axis of polarisation normal to the plane of the layer. In this way, in the region of the orifice 36, the magnetic lines of flux are directed outwardly of the outer surface of the magnetic layer 25 and are then deflected inwardly through the orifice, as shown in Figure 3. This produces an annular region of magnetic flux lines coaxial with the orifice, which acts to confine electrons produced by the emitter to the central region of the orifice 35 through the gate electrode 23 where they do not have to cross the magnetic lines of flux.

Confining the stream of electrons to the centre of the orifice 35 in this way has two advantages. It increases the collimation of the electrons emitted from each device, thereby reducing the spot size of the electron beam on the phosphor layer 6 and improving the symmetry of the spot. Also, because the electrons are kept away from the edge of the orifice 35 in the gate electrode 25, the parasitic emitter to gate current is reduced. This increases the anode current arriving at the phosphor layer 6.

The invention is beneficial for both low and high anode voltage devices, enabling operation at lower switching voltages. The lower switching voltages enable switching fields to be reduced to less than about lV/,um, thereby reducing the risk of arcing damage. The higher current densities can be more evenly distributed among the emitter devices in a display. The use of doped diamond limits the effects of surface states on the bulk properties of the diamond and gives increased resistance to contamination.

It is not essential for the electron emitter to be a spike or other discontinuity, it could instead be provided by a surface with a low or negative electron affinity, such as shown in Figure 4. In this device, similar components to those in Figure 2 have been given the same reference numerals primed. The electron emitter is provided by a layer 60' of diamond material on the metal contact layer 22' filling a recess 30' in the insulator layer 23'. The upper surface 61 of the diamond layer 60 is flat and level with the upper surface of the insulator layer 23'. The diamond material is highly twinned CVD diamond and the layer 60 is deposited by the biased enhanced growth method onto the molybdenum layer 22' at 850 950"C in a hydrogen/methane atmosphere containing up to about 12% methane. The deposited film produced is fine grained, high quality material possessing sub-micron crystal features giving enhanced electron emission. The conductivity of the layer 60 is improved by the selective ion implantation with boron or phosphorous impurities. The substitutional impurities are activated by excimer laser annealing at 194no.

It will be appreciated that the invention is not confined to displays but could be used in other applications where an electron emitter is used. The magnetic device used to focus the electron beam need not be a layer across the entire surface of the emitter array but could, for example, be in the form of separate magnets around each emitter. The magnetic device is preferably located below the gate electrode but, providing the lines of flux are effective to increase the confinement of the electrons to a central part of the gate electrode orifice, it could be located above the gate electrode.

Claims

1. An electron emitter device having an electron emitter, a gate electrode located above the emitter and defining a region through which electrons are emitted from the device, and a magnetic device located in the region of the gate electrode, wherein the magnetic device is arranged to increase the confinement of the electrons within a central part of the region.

2. An emitter device according to Claim 1, wherein the gate electrode is located above the magnetic device.

3. An emitter device according to Claim 1 or 2, wherein the magnetic device is provided by a layer of magnetic material having an orifice aligned with the region defined by the gate electrode.

4. An emitter device according to Claim 3 including a substrate with a layer of insulating material, wherein the substrate supports the electron emitter, and wherein the layer of insulating material supports the layer of magnetic material.

5 An emitter device according to Claim 4, wherein the combined thickness of the layer of magnetic material and the insulating layer is about 1 micron.

6. An emitter device according to Claim 2 and any one of Claims 3 to 5, wherein the layer of magnetic material supports the gate electrode.

7. An emitter device according to Claim 1 including a substrate supporting the electron emitter, wherein the gate electrode and the magnetic device are both provided by respective layers, and wherein the gate electrode layer and the magnetic layer are supported one above the other with said substrate.

8. An emitter device according to any one of Claims 3 to 7, wherein the layer of magnetic material is formed by deposition.

9. An emitter device according to Claim 8, wherein the layer of magnetic material is formed by magnetron ion sputtering.

10. An emitter device according to any one of the preceding claims including a plurality of electron emitters.

11. An emitter device according to Claim 10 and any one of Claims 3 to 9, wherein the magnetic layer has a plurality of orifices aligned with respective ones of said electron emitters.

12. An emitter device according to any one of the preceding claims, wherein the magnetic device is of a barium ferrite compound.

13. An emitter device according to any one of the preceding claims, wherein the or each emitter includes a Spindt tip structure.

14. An emitter device according to any one of the preceding claims, wherein the emitter has a diamond surface.

15. An electron emitter device substantially as hereinbefore described with reference to Figures 1 to 3 of the accompanying drawings.

16. An electron emitter device substantially as hereinbefore described with reference to Figures 1 to 3 as modified by Figure 4 of the accompanying drawings.

17. A display including an electron emitter device according to any one of the preceding claims.

18. A display according to Claim 17 including a layer of fluorescent material located above the electron emitter device such that electrons emitted by said device cause said fluorescence layer to emit optical radiation.

19. A display substantially as hereinbefore described with reference to Figures 1 to 3 of the accompanying drawings.

20. A display substantially as hereinbefore described with reference to Figures 1 to 3 as modified by Figure 4 of the accompanying drawings.

21. Any novel and inventive feature as hereinbefore described.