GB2441813A

GB2441813A - Improved field emission backplate

Info

Publication number: GB2441813A
Application number: GB0615629A
Authority: GB
Inventors: Mervyn John Rose
Original assignee: QUANTUM FILAMENT TECHNOLOGIES
Current assignee: QUANTUM FILAMENT TECHNOLOGIES
Priority date: 2006-08-07
Filing date: 2006-08-07
Publication date: 2008-03-19
Also published as: GB0615629D0; WO2008017808A1

Abstract

An improved method of manufacture of a field emission backplate, and a backplate produced by the method, used in a field emission device. The backplate 5 is based on, for example, amorphous silicon such that voltage pulses or current stressing produces filamentary elements that contain conducting particles. A thin layer 25 of hydrogenated amorphous silicon is deposited on a thin film of metal 15. Patterned counter electrodes 30 are provided and an electrical signal supplied to permanently change the structure of layer 25. Laser radiation may also be used to provide an emitting structure.

Description

2441813

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IMPROVED FIELD EMISSION BACKPLATE

FIELD OF INVENTION

The present invention relates to a field emission backplate, to a field emission device such as a display device, and to an associated method of manufacture. In particular, though not exclusively, the invention relates to a field emission display device comprising a field emission backplate having a plurality of conducting particulates formed or provided within the backplate.

BACKGROUND TO INVENTION

Flat panel displays are of immense importance in electronics. Active Matrix Liquid Crystal Displays (AMLCD) have challenged the dominance of Cathode Ray Tube (CRT) technology. AMLCD devices are non-emissive and require complex lithography. Filters and matching spectral backlights are required to produce colour. Further, there are many light losses and inherent complexity in AMLCD devices because of the non-linear nature of liquid crystal materials. This results in a display that is less bright than CRTs with a smaller colour gamut and poorer viewing angle and contrast. Also, due to the non-emissive nature of such displays, inefficient use of input electrical power is made, often with over 70% of energy being lost as non-useful energy.

Field Emission Displays (FEDs) are also known. GB 2 378 569 A (The University Court of the University of Dundee) discloses a field emission backplate comprising a planar backplate substantially comprising an amorphous semiconductor based material, and a plurality of grown tips substantially comprising a crystalline semiconductor based material formed on the backplate member.

GB 2 378 570 A (The University Court of the University of Dundee) discloses a method of forming a field emission backplate comprising:

providing a planar body of amorphous semiconductor based material upon a substrate; and laser crystallising at least a portion of the amorphous semiconductor based material;

wherein upon crystallising the amorphous semiconductor based material a plurality of emitter sites are formed. GB 2 378 570 A also discloses a field emission backplate comprising a plurality of emitter sites formed by laser crystallisation of a planar body or thin film of amorphous semiconductor based material.

GB 2 389 959 A (The University Court of the University of Dundee) discloses a field emission device comprising a field emission backplate, the backplate being made substantially from semiconductor based material and comprising a plurality of emitter tips, the field emission device further comprising at least one

electro-luminescent material, the at least one material having a fluorescent material chemically attached thereto.

The content of the aforementioned documents is incorporated herein by reference.

Known field emission devices suffer from a number of disadvantages such as: ease of manufacture, predictability of manufacture, quality of manufacture, predictability of technical characteristics.

It is an object of at least one embodiment of at least one aspect of the present invention to obviate or at least mitigate one or more disadvantages in the prior art.

SUMMARY OF INVENTION

According to a first aspect of the present invention there is provided a method of forming a field emission backplate comprising the steps of:

at least partially forming a plurality of conductive or conducting particulates or particles in the field emission backplate by application of an electrical signal thereto.

The at least partial formation of the conducting particulates may be termed "conditioning" of the backplate.

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The field emission backplate may comprise a layer of amorphous semiconductor material.

Each conducting particulate may comprise a point or locality, e.g. of crytallisation, e.g. a "crystallite", 5 within the layer of amorphous semiconductor material. The conducting particulates may each comprise semiconductor material and/or metallic material.

The amorphous semiconductor material may comprise amorphous silicon or an alloy thereof, e.g. hydrogenated 10 amorphous silicon, Si:H.

The layer of amorphous semiconductor material may be provided on a substrate.

The subs trate may be made from glass, or alternatively from a ceramic or metallic material. 15 Filaments or pathways, e.g. conductive or conducting filaments, may be provided between the conducting particulates, at least in use. Such filaments may provide a means for electron transport through the backplate to emitter sites on a surface of the backplate, 20 which surface may comprise a surface of the amorphous semiconductor material.

The filaments may be considered as spatial instabilities or spatio-temporal features, e.g. formed as a consequence of intense internal electric field 25 confinement between conducting particulates.

In use, upon application of an electric field across the backplate, the filaments may be formed and may provide a transport network for electrons to the emitter sites.

In use, electrons may move through the filaments between conducting particulates and to an emitter site. The electrons may move, in use, by electron transport or under certain conditions ballistically, e.g. if the dimension of the conducting particulates are sufficiently small, and/or the space between the conducting particulates is sufficiently small.

The emitter sites may be provided anywhere upon and or across the surface.

The electrical signal may comprise a voltage signal or a current signal.

In the case of a voltage signal, the voltage signal may comprise at least one voltage pulse, e.g. a square pulse or a sawtooth pulse, e.g. having a ramped leading edge.

In the case of a current signal, the current signal may provide current stressing.

The current signal may be a constant current.

In one embodiment, the electrical signal may be a sole means of forming the conducting particulates.

In an alternative embodiment the conducting particulates may be at least partially formed by another

means, either before or after, or during application of the electrical signal.

The other means may comprise laser irradiation of the backplate, e.g. exposure of the backplate to at least one pulse of a laser.

The laser may operate at a wavelength of around 525 nm to 540 nm, e.g. substantially 532 nm. The laser may be an excimer laser, e.g. a KrF laser. The laser may be a Nd:YAG laser. Laser irradiation may be carried out in air, vacuum or an inert gas atmosphere.

The method may also comprise:

depositing a thin film of the amorphous semiconductor based material upon a/the substrate;

forming the conducting particulates by locally crystallising a plurality of areas of the thin film amorphous semiconductor based material.

The method may also comprise the step of depositing the thin film of amorphous semiconductor based material by Plasma Enhanced Chemical Vapour Deposition (PECVD).

According to a second aspect of the present invention there is provided a field emission backplate formed by the method according to the first aspect of the present invention.

According to a third aspect of the present invention there is provided a field emission device comprising a

field emission backplate according to the second aspect of the present invention.

The device may be a vacuum device, wherein each emitter site acts as an electron emission source of the device, in use.

The device may further comprise a substrate, an evacuated space and a transparent window, wherein the field emission backplate is formed upon the substrate and the evacuated space is located between the field emission backplate and the transparent window.

The field emission device may alternatively comprise an electro-luminescent material light emitting material into which electrons from the tips are emitted, in use.

Such a field emission device may further comprise a substrate, the light emitting material, and a transparent window, wherein electrons from the tips are emitted into the light emitting material.

The light emitting material may be a light emitting polymer.

The electro-luminescent or light emitting material may have a fluorescent material chemically attached thereto.

The transparent window may be a thin film transparent metal.

One surface of the light emitting material may be disposed on a surface of the field emission backplate,

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and the transparent window may be disposed on another surface of the light emitting material.

Preferably the device is a display device.

The emitter or emission sites of the field emission backplate may be of a density of at least 100 per square micron.

According to a fourth aspect of the present invention there is provided a field emission backplate comprising a plurality of conducting particulates formed in the field emission backplate, the conducting particulates having been at least partially formed by application of an electrical signal to the backplate.

According to a fifth aspect of the present invention there is provided a field emission device comprising a field emission backplate according to the fourth aspect of the present invention.

It will be appreciated that features of any one aspect of the present invention hereinbefore described, may be shared with or be common with any other aspect of the present invention hereinbefore described, either solely or in combination.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the present invention will now be described by way of example only, with reference to the accompanying drawings, which are:

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Figure 1 (a)

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Figure 1 (b)

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Figure 1(c)

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Figure 1(f)

a schematic perspective view of a substrate in a first step in a method of manufacturing a field emission backplate according to a first embodiment of the present invention;

a schematic perspective view of the substrate of Figure 1(a) in a second step in the method of manufacture;

a schematic perspective view of the substrate of Figure 1 (a) in a third step in the method of manufacture;

a schematic perspective view of the substrate of Figure 1 (a) in a fourth step in the method of manufacture;

a planar view of the substrate of Figure 1 (a) in a fifth step in the method of manufacture; a schematic side view of the substrate of Figure 1 (a) in a sixth step in the method of manufacture;

Figure 1(g)

Figure 2

Figure 3(a)

Figure 3 (b)

Figure 3(c)

Figures 4(a) to 4(e)

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a schematic perspective view of the substrate of Figure 1(a) in a seventh step in the method of manufacture;

a schematic side view of a field emission device comprising a field emission backplate formed according to the method of Figures 1(a) to 1(g);

a schematic perspective view of a substrate in a first step in a method of manufacture of a field emission backplate according to a second embodiment of the present invention;

a schematic side view of the substrate of Figure 3 (a) in a second step in the method of manufacture;

a schematic side view of the substrate of Figure 3 (a) in a third step in the method of manufacture;

a series of side cross-sectional views showing a method of forming a field emission

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backplate according to a third embodiment of the present invention; and

Figures 5(a) to 5(c) a series of side cross-sectional views showing a method of forming a field emission backplate formed according to a fourth embodiment of the present invention including the use of a planarising agent.

DETAIELD DESCRIPTION OF DRAWINGS

Referring initially to Figures 1(a) to 1(g), there is disclosed a method of manufacturing a field emission backplate, generally designated 5, according to a first embodiment of the present invention. The backplate 5 is based on amorphous silicon or any other suitable thin film semiconductor that when conditioned with an electrical signal, such as voltage pulse(s) or current stressing, produces filamentary elements or devices that can be used as pixel elements in a field emission display, as will hereinafter be described in greater detail.

In a first step of the method, as can be seen in Figure 1(a), the device 5 is formed by providing a substrate 10, upon which is deposited a thin layer or

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film of metal 15, which can be patterned into strips or fingers 20, and which form a "cathode" metal. The thin film of metal 15 can, for example, be molybdenum, chromium, aluminium, or the like. The substrate 10 can be any suitable substrate, for example, glass, or alternatively, ceramic or plastic.

With reference to Figure 1(b), a thin film layer 25 of hydrogenated amorphous silicon (Si:H) is deposited on top of the thin film of metal 15, e.g. by Plasma Enhanced Chemical Vapour Deposition (PECVD). Alternatively, an unhydrogenated amorphous silicon layer is deposited, for example, by sputtering or any other suitable means. The layer 25 is typically deposited to a thickness of between 100 nm and 300nm. The layer 25 forms a pre-cursor of the cathode material.

Referring to Figure 1(c), the layer 25 is patterned, for example, by photolithography and etching techniques to coat the cathode metal electrodes formed by fingers 20, as required. The layer 25, comprising a cathode material, is then "conditioned" to complete manufacture of the field emission backplate 5. The conditioning step may be done in a number of ways. Two methods are described hereinbelow.

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METHOD 1

Referring to Figures 1(d) and 1(e), fourth and fifth steps of the manufacturing method comprise providing counter electrodes 30, comprising a thin film metal, e.g. aluminium, chromium, molybdenum, or the like. The counter electrodes 30 are fabricated by coating the substrate 10 with a uniform film to a thickness of typically between 50 nm to 250 nm (Figure 1(d)).

The counter electrodes 30 are then patterned typically using a process of lithography to form strips or fingers 31 orthogonal to the cathode metal fingers or electrodes 20. This results in a metal-insulator-metal diode configuration. A current voltage characteristic of such a diode configuration at this stage is determined by the type of top and bottom metal used, and the electronic nature of the thin film layer 25.

Referring to Figure 1(f), the backplate 5 is then subjected to a voltage pulse or number of pulses, which create current confinement means in the form of a permanent change in the structure of the material comprising the layer 25, and also in the electrical characteristics of backplate 5. The pulses can be in the range of nanoseconds to milliseconds, and after a delay time the voltage may be caused to suddenly drop, such that the current through the backplate 5 rises

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accordingly. A series resistor (not shown in Figure 1(f)) typically in the range of 1 kohms to 10 kohms is used to protect the backplate 5 during this process.

With reference to Figure 1(g), a structural change in the form of a permanent filament that contains conducting particulates 35 of the thin film semiconductor layer and/or fragments of top or bottom metal, or a combination of all three. The conducting particulates 35 may comprise crystallites.

In a final step of the manufacturing method, the counter electrodes 30 may now be removed by a chemical etching process specific to the metal chosen. This is typically the final step in the fabrication of the backplate 5.

With reference to Figure 2, there is illustrated a field emission device, generally designated 100, according to an embodiment of the present invention. In this device 100, the backplate 5 (or cathode backplate) is mounted on a holder 105 and placed in a vacuum chamber with a counter electrode 115 (i.e. an anode), which comprises strips of indium tin oxide, or other transparent conductor, and which are orthogonal to the cathode metal strips or fingers 20. The anode 115

strips are coated with a low or high voltage phosphor. Cathode 120 and anode 125 are held apart using spacers 130. Typically the spacers 130 are in the range of

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microns to millimetres. Between spacers 130 is evacuated space 110. The vacuum chamber is then evacuated to a suitable base pressure. Alternatively, a sealed vacuum device can be prepared.

The device 100 is now in a field emission display configuration. Application of an electric field across the cathode and anode results in a threshold flow voltage being overcome, and an emission current in excess of microamps measured. Energetic electrons are released from emitter or emission sites on a surface of the cathode 120 via the (permanent) filament or filaments. The electrons travel through the space 110 towards the phosphor typically in a conical distribution or orthogonal depending on the nature of filaments, and if moving ballistic. Such electrons induce light emission in the counter electrode anode 125 so as to form an activated pixel.

METHOD 2

In a second, modified method of manufacture, the following may occur. The first, second and third steps of the second method are common, with the first, second and third steps of Method 1, hereinbefore described.

In the second method, and with reference to Figure 3 (a) the cathode is formed using the method described in Method 1 above, but without deposition of the counter

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electrode or top metal 30. The cathode at such point is in an "unformed state".

With reference to Figure 3(c), the backplate 5' is mounted in a vacuum chamber 40' . In this case the counter electrode 30' is a metal probe 36' , or a metal counterplate 37' held at a distance above the cathode 120.

Application of a large electric field, in this case induces breakdown of the thin film semiconductor layer 25'. Such creates a permanent change in the cathode in a localised point forming a filament that may contain conducting particulates or crystallites of the thin film semiconductor material, fragments of top or bottom metal, or a combination of all three. The backplate 5' at this stage has now become stressed or "conditioned", and is now in a state similar to the cathode 120 described in Method 1 above.

It will be appreciated that the conducting particulates or crystallites 35 provide filaments therebetween. The filaments are typically of mixed phase comprising metal, particles, and crystallised amorphous semiconductor.

Typically a voltage applied may be in the range of 10 to 100 volts per micron, e.g. in the order of 1,000 volts for a layer 25 of 10 microns.

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If, as well as the electrical signal being applied, laser radiation is also employed for conditioning the backplate 5;5', then a laser power of between 50 and 700 mJ per cm2, for example, of the order of 200 mJ, may be used.

In a modification to the disclosed embodiments, an additional insulator layer could be included between the amorphous silicon layer 25,25'and counter electrode 30. The provision of such an insulator layer could improve the predictability and/or registration of the filaments produced by the conditioning methods.

In a preferred embodiment comprising laser radiation and electrical signal application conditioning, laser power of around, for example, 140 mJ, could be used so as to provide an emitting structure, which can then be finally conditioned using current stressing.

Referring now to Figures 4(a) to 4(e) there is shown a third embodiment of the field emission backplate 5a constructing a three terminal device having self-aligned gates for each tip 27a. This field emission backplate is constructed in a manner illustrated in Figures 4 (a) to (e) .

In Figure 4 (a) there is shown a backplate 5a formed of a substrate 10a, metal cathode layer 11a and a thin film of amorphous silicon 15a. The thin film silicon 15a is "conditioned" in the manner described hereinbefore

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with reference to Figures 1(a) to 1(g) or Figures 3(a) to 3 (c) .

The first step of forming the self aligned gates involves forming by deposition a thin SiN (Silicon Nitride) insulator 238a, using PEVCD, upon the exposed surface of silicon completely encapsulating each of the tips 27a as is illustrated in Figure 4(b).

The second step of the process, the results of which are shown in Figure 4(c), involves a layer of metal 240a in this case chromium, being deposited on top of the SiN layer 238a by thermal evaporation.

The third step of the process, the plate arrangement is then etched by plasma means, in this case using CF (Freon) gas. This results in the top of each tip 27s losing its metal and the SiN insulator layer 238d being exposed, as is shown in Figure 4(d).

As is shown in Figure 4(c), the SiN insulator 238a is then etched leaving a supporting metal ring 241a around the exposed tip 27a, which acts as a gate.

The resultant emission backplate 5a can be used to form a field emission device 10a that is completely lithography free. Further electron emission is controllably limited to emission sites 26d on tips 27d.

Referring to Figure 5 (a) to 5 (c) , this process can be improved by applying a planarising agent 37b, that is a liquid which upon heating or solvent evaporation,

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becomes a thin planar film, to the backplate 5b after the second step of the process resulting in an arrangement as illustrated in Figure 5(a). This shows the planarising agent 237b coating the backplate 5b leaving the tips 27b standing proud.

The step of etching the arrangement by plasma means thus results in the arrangement shown in Figure 5(b).

The SiN insulator is then etched as before, leaving a space between the metal layer and the tip 27d as is shown in Figure 5(c). By utilising the planarising agent 237d in this way, the underlying silicon backplate structure is protected from corrosive etch effects. The planarising agent can then be removed, resulting in a metal gate surrounding each tip.

Devices such as those detailed in the embodiments are suitable for many display applications due to their having low power consumption and being relatively simple to fabricate. Emission being confined to the tips 27a;27b is beneficial to the provision of low voltage operation. Such devices may also be used as the cathodes for high power transistors for microwave amplifiers in the satellite and mobile communication markets.

It will be appreciated that the embodiments of the present invention hereinbefore described are given by way of example only, and are not meant to be limiting thereof in any way. Indeed, various modifications may be made to

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the disclosed embodiments without departing from the scope of the invention. For example, it will be understood that any of the disclosed embodiments may comprise one or more of the features provided in the 5 summary of invention.

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Claims

CLAIMS 21

1. A method of forming a field emission backplate comprising the steps of:

2. A method of forming a field emission backplate as claimed in claim 1, wherein the field emission backplate comprises a layer of amorphous semiconductor material.

3. A method of forming a field emission backplate as claimed in any preceding claim, wherein each conducting particulate comprises a point or locality within the layer of amorphous semiconductor material.

4. A method of forming a field emission backplate as claimed in any preceding claim, wherein the conducting particulates each comprise semiconductor material and/or metallic material.

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5. A method of forming a field emission backplate as claimed in claim 2 or either of claims 3 or 4 when dependent upon claim 2, wherein the amorphous semiconductor material comprises amorphous silicon or an alloy thereof such as hydrogenated amorphous silicon, Si :H.

6. A method of forming a field emission backplate as claimed in claim 2 or any of claims 3 to 5 when dependent upon claim 2, wherein the layer of amorphous semiconductor material is provided on a substrate.

7. A method of forming a field emission backplate as claimed in claim 6, wherein the substrate is made from glass, a ceramic material or metallic material.

8. A method of forming a field emission backplate as claimed in any preceding claim, wherein filaments or pathways such as conductive or conducting filaments are provided between the conducting particulates, at least in use.

9. A method of forming a field emission backplate as claimed in claim 8, wherein the filaments provide a means for electron transport through the backplate to emitter

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sites on a surface of the backplate, which surface comprises a surface of the amorphous semiconductor material.

10. A method of forming a field emission backplate as claimed in either of claims 8 or 9, wherein the filaments are spatial instabilities or spatio-temporal features optionally formed as a consequence of intense internal electric field confinement between conducting particulates.

11. A method of forming a field emission backplate as claimed in any of claims 1 to 8, wherein in use, upon application of an electric field across the backplate, the filaments are formed and provide a transport network for electrons to the emitter sites.

12. A method of forming a field emission backplate as claimed in any of claims 8 to 11, wherein in use, electrons move through the filaments between conducting particulates and to an emitter site.

13. A method of forming a field emission backplate as claimed in claim 12, wherein the electrons move, in use, by electron transport or under certain conditions

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ballistically, optionally if the dimension of the conducting particulates are sufficiently small, and/or the space between the conducting particulates is sufficiently small.

14. A method of forming a field emission backplate as claimed in any preceding claim, wherein the emitter sites are provided anywhere upon and or across a/the surface of the backplate.

15. A method of forming a field emission backplate as claimed in any preceding claim, wherein the electrical signal comprises a voltage signal or a current signal.

16. A method of forming a field emission backplate as claimed in claim 15, wherein in the case of a voltage signal, the voltage signal comprises at least one voltage pulse, such as a square pulse or a sawtooth pulse optionally having a ramped leading edge.

17. A method of forming a field emission backplate as claimed in claim 15, wherein in the case of a current signal, the current signal provides current stressing.

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18. A method of forming a field emission backplate as claimed claim 15 or claim 17, wherein the current signal is a constant current.

19. A method of forming a field emission backplate as claimed in any of claims 1 to 18, wherein the electrical signal is a sole means of forming the conducting particulates.

20. A method of forming a field emission backplate as claimed in any of claims 1 to 18, wherein the conducting particulates are at least partially formed by another means, either before or after, or during application of the electrical signal.

21. A method of forming a field emission backplate as claimed in claim 20, wherein the other means comprises laser irradiation of the backplate, such as exposure of the backplate to at least one pulse of a laser.

22. A method of forming a field emission backplate as claimed in claim 21, wherein the laser operates at a wavelength of around 525 nm to 540 nm, optionally substantially 532 nm.

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23. A method of forming a field emission backplate as claimed in either of claims 21 or 22, wherein the laser are an excimer laser, a KrF laser or a NdrYAG laser.

24. A method of forming a field emission backplate as claimed in either of claims 21 or 22, wherein laser irradiation is carried out in air, vacuum or an inert gas atmosphere.

25. A method of forming a field emission backplate as claimed in any preceding claim, wherein the method comprises:

26. A method of forming a field emission backplate as claimed in claim 25, wherein the method comprises the step of depositing the thin film of amorphous semiconductor based material by Plasma Enhanced Chemical Vapour Deposition (PECVD).

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27. A field emission backplate formed by the method of claims 1 to 26.

28. A field emission device comprising a field emission backplate according to claim 27.

29. A field emission device as claimed in claim 28, wherein the device is a vacuum device, wherein each emitter site acts as an electron emission source of the device, in use.

30. A field emission device as claimed in either of claims 28 or 29, wherein the device further comprises a substrate, an evacuated space and a transparent window, wherein the field emission backplate is formed upon the substrate and the evacuated space is located between the field emission backplate and the transparent window.

31. A field emission device as claimed in claim 28, wherein the field emission device may alternatively comprise an electro-luminescent material light emitting material into which electrons are emitted, in use.

32. A field emission device as claimed in claim 31, wherein the field emission device further comprises a

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substrate, the light emitting material, and a transparent window, wherein electrons from the tips are emitted into the light emitting material.

33. A field emission device as claimed in claim 32, wherein the light emitting material is a light emitting polymer.

34. A field emission device as claimed in any of claims 31 to 33, wherein the electro-luminescent or light emitting material has a fluorescent material chemically attached thereto.

35. A field emission device as claimed in claim 30 or any of claims 31 t 34, wherein the transparent window is a thin film transparent metal.

36. A field emission device as claimed in any of claims 31 to 34 or claim 35 when dependent upon any of claims 31 to 34, wherein one surface of the light emitting material is disposed on a surface of the field emission backplate, and the transparent window is disposed on another surface of the light emitting material.

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37. A field emission device as claimed in any of claims 28 t 36, wherein the device is a display device.

38. A field emission device as claimed in any of claims 28 to 36, wherein the emitter or emission sites of the field emission backplate are of a density of at least 100 per square micron.

39. A field emission backplate comprising a plurality of conducting particulates formed in the field emission backplate, the conducting particulates having been at least partially formed by application of an electrical signal to the backplate.

40. A field emission device comprising a field emission backplate according to claim 39.

41. A method of manufacturing a field emission backplate as hereinbefore described with reference to the accompanying drawings.

42. A field emission backplate as hereinbefore described with reference to the accompanying drawings.

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43. A field emission device as hereinbefore described with reference to the accompanying drawings.

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