IMPROVED FIELD EMISSION BACKPLATE

FIELD OF INVENTION

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

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, e.g. because of the nonlinear 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, desirability 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 .

It is an object of at least one embodiment of at least one aspect of the present invention to provide a field emission backplate having a lower field emission threshold and/or higher uniformity of surface emission than that of the prior art, e.g. in the prior art the

threshold voltage was typically 10 V/μm, and it is desired or intended to reduce this to around 1 to 2 V/μm.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided a method of forming or manufacturing a field emission backplate, the method comprising: providing a backplate; and laser processing the backplate, wherein an output of the laser comprises a sloping or ramped leading edge.

It is believed by the present Inventors that the sloping leading edge serves to dehydrogenate the backplate material, e.g. amorphous silicon (Si: H) in a controlled manner. The leading edge slope may be used to control the initial rate of explosive release of hydrogen (dehydrogenation) in a controlled manner. The present Inventors have found that the shape of the laser output plays a role in determining the surface morphology and crystal fraction of the field emission backplate.

Preferably the output of the laser has a ramped or sloping trailing edge.

The output of the laser may comprise a pulse, e.g. at least one pulse.

In one implementation the laser output may have a mid-portion of substantially continuous intensity or

power between the leading edge and the/a trailing edge of the at least one pulse. In such implementation the laser output may be said to be "flat topped".

In another implementation the laser output may be substantially Gaussian. However, in advantageous implementations the laser output may be non-Gaussian.

It is presently believed by the present Inventors that:

(a) the leading edge of the laser output serves to dehydrogenate the backplate material and/or alter the subsequent structure of the material in a controlled manner;

(b) a peak energy of the laser output (e.g. of the mid-portion) serves to define or determine a final structure of the material;

(c) a higher peak energy provides for controlling the size of crystallites and/or providing a greater density of crystallites; and

(d) the trailing edge serves to stabilise the structure of the material and/or enhance field emission.

The present invention differs from the prior art in that in the prior art a square pulse or "top hat" laser output has been used. However, this prior art provides for immediate explosive dehydrogenation, not in a controlled manner.

The laser output may be symmetrical or asymmetrical, e.g. the leading and trailing edges may be the same or different .

The step of providing a backplate or backplate material may comprise providing a planar body of amorphous semiconductor based material upon a substrate.

The step of processing the material may comprise laser irradiating and/or crystallising at least a portion of the amorphous semiconductor based material, e.g. hydrogenated amorphous semiconductor based material .

Upon laser irradiating the amorphous semiconductor based material a plurality of emitter sites may be formed .

Preferably there are provided a plurality of pulses of laser radiation.

The/each laser pulse may be provided at a frequency of around 1 to 100 Hz, e.g. 5 to 50 Hz, e.g. around 20 Hz.

A length of the/each pulse may be around 1 to 100 nm, e.g. 5 to 50 nm, e.g. around 20 nm.

Adjacent pulses may be separated by greater than the thermal time constant of the backplate material .

A length of leading edge may be between 1 and 5 mm, e.g. around 3 mm.

A length of the trailing edge may be between 1 and 5 mm, e.g. around 3 mm.

A peak energy of the laser output may be between 100 and 500 mJ/cm ² , e.g. around 200 mJ/cm ² .

A length of the mid-portion may be between 1 and 10 mm, e.g. 4 mm.

The laser may be an eximer laser.

The step of providing a backplate material may comprise providing a backplate comprising hydrogenated amorphous silicon (Si:H) .

The backplate material may be provided on a substrate, e.g. a metallised substrate, e.g. aluminium, chromium, molybdenum or any other suitable metal or metallic material.

In the step of processing the at least one laser output may be caused to pass over or scan at least a part of the backplate material in a first direction and then in a second direction, the first and second direction preferably being substantially opposing or orthogonal directions to one another.

In the step of processing the laser output may also be caused to pass over or scan at least part of the backplate material in a third direction perpendicular to the first direction and then in a fourth direction, the third and fourth directions preferably being substantially opposing or orthogonal directions to one another.

The laser output may pass or scan at a speed or velocity of around 1 to 50 mm/sec, e.g. around 20 mm/sec.

Scanning may be provided by moving the backplate material relative to the laser output (or vice versa) .

The laser output (pulse or beam) may comprise a two dimensional output.

The laser output (pulse or beam) may have a first intensity profile across the two dimensional area, e.g. in a scanning direction.

The first intensity profile may comprise the leading edge and optionally a/the mid portion and/or a/the trailing edge.

The first intensity profile may be provided in a scanning direction of the laser output.

The laser output (pulse or beam) may have a second intensity profile across the two dimensional area, e.g. in a direction perpendicular to the scanning direction.

The second intensity profile may comprise a square wave, top hat, Gaussian or semi -Gaussian profile.

During laser irradiation and/or crystallisation, the laser output may be pulsed during relative movement between the laser output and the backplate material . Consequently the backplate material may be subjected to a number of shots or pulses of laser radiation.

The step of processing the backplate or backplate material may at least partially form a plurality of

conductive or conducting particulates or particles within the backplate or backplate material .

Preferably the step of providing a backplate material comprises: providing a planar body of (hydrogenated) amorphous semiconductor material upon a substrate .

Preferably the step of processing the material comprises: laser processing or laser irradiating and/or crystallising at least a portion of the amorphous semiconductor material, such that the plurality of conducting particulates are formed.

The method may comprise the step of: selecting a level of hydrogenation of the amorphous semiconductor material such that when laser irradiated a surface of the amorphous semiconductor material remains substantially flat or planar.

Alternatively, the method may comprise the step of: selecting a level of hydrogenation of the (hydrogenated) amorphous semiconductor material such that when laser irradiated a surface of the amorphous semiconductor material becomes substantially non-flat or roughened, e.g. comprising a plurality of tips, e.g. microtips.

The planar body of amorphous semiconductor based material may be provided by depositing a thin film of material upon a substrate, e.g. by Plasma Enhanced Chemical Vapour Deposition (PECVD) .

The semiconductor based material may be silicon or an alloy thereof.

The step of performing laser processing may be carried out at a wavelength of around 240nm to 540nm, e.g. 248nm to 308nm or 525nm to 540nm, e.g. substantially 248nm or 532 nm. The step of laser processing may use an excimer laser or Nd: YAG laser at a suitable wavelength thereof. This step may be carried out in air, vacuum or an inert gas atmosphere. The excimer laser may be a KrF laser.

It is desirable to create a fine grain structure, and explosive modification and/or crystallisation of the backplate material may provide such.

According to a second aspect of the present invention there is provided a method of manufacturing a field emission backplate comprising: providing a backplate or backplate material; processing the backplate or backplate material by exposing the material to or irradiating the material with laser radiation, wherein an output of the laser (radiation) has a ramped or sloping leading edge.

Preferably the field emission backplate formed according to either of the first or second aspects of the invention may find use as an element of a display device.

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

Accordingly to - a fourth aspect of the present invention there is provided a field emission backplate. The following optional features apply to the third and fourth aspects of the present invention.

The field emission backplate may comprise a plurality of emitter sites formed by laser irradiating and/or crystallisation of a planar body or thin film of amorphous semiconductor based material .

The field emission backplate may comprise a plurality of conductive or conducting particulates or particles, wherein the conductive or conducting particulates are provided within the backplate.

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, within the layer of amorphous semiconductor material .

The conducting particulates may each comprise semiconductor and/or metallic material.

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

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

The substrate may be made from glass, or alternatively from a ceramic or metallic material.

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, 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 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 particulate is sufficiently small.

In one embodiment the surface of the amorphous semiconductor material is substantially flat.

In an alternative embodiment the surface of the amorphous semiconductor material is substantially non- flat or roughened, comprising a plurality of tips, e.g. microtips .

The emitter sites may be provided anywhere upon or across the surface. For example, in the alternative embodiment the emitter sites may be provided on the tips and/or between adjacent tips.

According to a fifth aspect of the present invention there is provided a field emission device comprising a field emission backplate according to the third or fourth aspects 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 or (wide band-gap) light emitting material into which electrons from the emission sites 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 emission sites 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, 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.

BRIEF DESCRIPTION OF DRAWINGS

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

Figure l ( a) a schematic side view of a substrate in a first step in a method of manufacture of a field emission backplate according to a first embodiment of the present invention;

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

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

Figure 2 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 (c) ;

Figure 3 (a) a schematic side 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;

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

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

Figure 4 (a) a schematic side view of a substrate in a first step in a method of manufacture of a field emission backplate according to a third embodiment of the present invention;

Figure 4 (b) a schematic side view of a substrate of Figure 4 (a) in a second step in the method of manufacture ;

Figure 4(c) a schematic side view of the substrate of Figure 4 (a) in a third step in the method of manufacture ;

Figure 4 (d) a schematic side view of the substrate of Figure 4 (a) in an alternative third step in the method of manufacture;

Figures 5 (a) to 5 (e) a series of side cross-sectional views showing a method of forming a field emission backplate according to a fourth embodiment of the present invention;

Figures 6 (a) to 6 (c) a series of side cross-sectional views showing a method of forming a field emission backplate formed according to a fifth embodiment of the present invention including the use of a planarising agent;

Figure 7 a side view of a first profile of a first laser output capable of being used in the preceding embodiments ;

Figure 8 side views of first profiles of second and third laser outputs capable of being used in the preceding embodiments;

Figures 9 (a) to (g) side views of first profiles of fourth to eighth laser outputs capable of being used in the preceding embodiments; and

Figures 10 (a) to (c) perspective views from the side and above of first to third profiles of eleventh to thirteenth laser outputs capable of being used in the preceding embodiments .

DETAILED DESCRIPTION OF DRAWINGS

Referring initially to Figures 1 (a) to 1 (c) , there is shown a sequence of diagrams showing steps in a method of manufacture of a field emission backplate, generally designated 5a, according to a first embodiment of the present invention. The backplate 5a comprises a substrate 10a, which may be made of any suitable material, e.g. glass, ceramic or metal. Upon the substrate 10a is deposited a cathode metal layer 11a, e.g. molybdenum, chromium, aluminium, or the like. Upon the cathode metal layer 11a is deposited an amorphous semiconductor layer 15a. In this instance the amorphous semiconductor comprises hydrogenated amorphous silicon, which is preselected to have a relatively high hydrogen content, for example, of the order of 18%.

With reference to Figure 1 (b) , the backplate 5a is exposed to at least one pulse of laser irradiation, as will hereafter be described in greater detail, with reference to Figures 7 to 10 (c) , so as to form a

plurality of conductive or conducting particulates or particles 20a within the amorphous silicon layer 15a. The conducting particulates 20a can typically comprise semiconductor material, e.g. crystallised semiconductor material, and metallic material, e.g. from metal layer 11a.

Referring to Figure l(c), in use, when an electrical signal is applied across the backplate 5a, electronic transportation occurs between conducting particulates 20a due to filaments 21a therebetween. Electron transportation towards an electron emission surface 25a of the backplate 5a allows electrons to be emitted from emission sites 26a anywhere on the surface 25a.

It is noted in this embodiment that the surface 25a is substantially flat or planar. It is believed that this is an effect of preselection of the hydrogen content of the amorphous silicon layer 15a.

In summary, in this first embodiment the layer 15a of thin film amorphous silicon of a high hydrogen content is provided on the cathode metal layer 11a or thin film metal. The backplate 5a is then processed, for example, with at least one pulse from an excimer laser (as will be hereinafter described in greater detail with reference to Figures 7 to 10 (c) ) , resulting in changes in the internal structure (e.g. production of conducting particulates 20a), whilst maintaining a planar surface 25a. In use,

application of an external electric field induces a high internal electric field inducing filamentary conduction within the layer 15a or film, and subsequent field emission of electrons.

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 5a (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. 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 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 120 and anode 125 results in a threshold flow voltage being overcome, and an emission current in excess of microamps measured. Energetic electrons are released from surface 25a of the backplate 5a (cathode 120) at the emitter sites 26a, and travel through the space 110

towards the phosphor typically in a conical distribution, or orthogonal depending on the nature of the filament, and if moving ballistic. Such electrons induce light emission in the counter electrode anode 125 so as to form an activated pixel .

Turning now to Figures 3 (a) to 3 (c) , there is shown a series of diagrams showing steps in a method of manufacturing a field emission backplate 5b according to a second embodiment of the present invention. Like parts are indicated by the same numerals as in Figures 1 (a) to 1 (c) , but suffixed with the letter "b" instead of the letter "a".

With reference firstly to Figure 3 (a) , in this case the amorphous semiconductor (silicon) 15b layer is preselected to have a relatively low hydrogen content, for example, around 10%.

Referring now to Figure 3 (b) , in this case laser irradiation (see Figures 7 to 10 (c) ) produces a non-flat or roughened surface 25b, forming a plurality of tips 27b, which may produce enhanced field emission. It is believed that these tips are produced due to the preselected hydrogen content of the amorphous silicon layer 15a.

Referring to Figure 3 (c) , in use, electron emission is produced from emission sites 26b across surface 25b, not only regions associated with the tips 27b.

In summary, in the second embodiment the layer 15b of thin film amorphous silicon with standard hydrogen content in the region of 8% to 10%, is provided on the metal cathode layer lib of a thin film metal, e.g. aluminium, chromium, molybdenum, etc. The backplate 5b is processed with an excimer laser resulting in changes in internal structure (e.g. formation of conducting particles 20b) , and changes in surface morphology and roughness. In use, application of an external electric field induces a high internal electric field inducing filament conduction within the layer 15a of film and subsequent field emission of electrons.

The field emission backplate 5b may be used in the device 110 of Figure 2 in place of the backplate 5a of the first embodiment.

Turning now to Figures 4 (a) to 4 (c) , there is shown a series of diagrams showing steps in a method of manufacturing of a field emission backplate 5c according to a third embodiment of the present invention. Like parts again are designated by the same numerals as in Figures 1 (a) to 1 (c) , but suffixed with the letter "c" instead of with the letter "a" .

Referring to Figure • 4 (b) , a layer 30c, for example of metal, particularly for example, ^aluminium - which may be patterned - is deposited upon the amorphous semiconductor (silicon) layer 15c. Typically the further

layer 30c is of the order of 50 nm in thickness.

Deposition of the further layer 30c causes material of layer 15c to transfer to the further layer 30c, and for material of further layer 30c to transfer to the layer 15a. In other words, semiconductor to flow to the metal and vice versa, e.g. silicon to flow to aluminium and aluminium to flow to silicon. This gives rise to creation of conducting particulates 20c, which typically comprise a mixture of silicon and aluminium.

Referring to Figure 4 (c) , the further layer 30c can be removed and surface 25c exposed. As shown in Figure 4 (c) , the surface 25c may be roughened and provide a plurality of tips 27b similar to the embodiment of Figures 3 (a) to 3 (c) .

It will be appreciated that in other implementations a semiconductor material other than silicon can be used, and also a metal other than aluminium can be used.

It will also be appreciated that between the steps illustrated in Figures 4 (b) and 4 (c) the backplate 5c is further irradiated by a laser (see Figures 7 to 10 (c) ) and optionally heated in other ways, e.g. rapid thermal annealing or vacuum thermal annealing, for example, while the further layer 30c is still on the backplate 5c. Such laser irradiation - preferably after deposition of the layer 30c, or alternatively, before deposition of the layer 30c - is believed to assist in conditioning the

backplate 5c and efficient formation of the crystallites

20c.

In summary, in the third embodiment of Figures 4 (a) to 4 (c) , the layer 15c of thin film amorphous silicon containing hydrogen is provided on the metal cathode layer lie comprising a thin film metal, e.g. molybdenum, chromium, or the like, and coated with layer 30a comprising an ultra thin layer of metal, such as aluminium, and thereafter laser processed (see Figures 7 to 10 (c) ) , e.g. with an excimer laser, resulting in changes in internal and external structure, and mixed phase conducting particulates 20c (possibly crystallites) or islands are formed. Application of an external electric field produces a high internal electric field, which induces filamentary conduction within the layer 15c and subsequent field emission of electrons.

With reference to Figures 4 (d) there is shown a modification to the third embodiment, wherein the backplate 5c is vacuum annealed at a predetermined pressure, for example, at 10 ^"6 mbar, for a predetermined time period, for example, of the order of 30 minutes. The layer 30c is then evaporated off to expose the islands or tips 27c. It will be appreciated that in this embodiment, laser irradiation also occurs.

Referring now to Figures 5 (a) to 5 (e) there is shown a field emission backplate 5d according to a fourth

embodiment of the present invention. Like parts again are designated by the same numerals as in Figures 1 (a) to l(c), but suffixed with the letter "d" instead of with the letter "a" .

The field emission backplate 5d forms a three terminal device having self-aligned gates for each tip 27d. This field emission backplate 5d is constructed in a manner illustrated in Figures 5 (a) to 5 (e) .

In Figure 5 (a) there is shown a backplate 5d formed of a substrate 10a, metal cathode layer Hd and a thin film of amorphous silicon 15d. The thin film silicon 15d is conditioned in the manner described hereinbefore with reference to Figures 3 (a) to 3 (c) or Figures 4 (a) to 4(d) .

The first step of forming the self aligned gates involves forming by deposition a thin SiN (Silicon Nitride) insulator 238d (using PECVD) upon the exposed surface of silicon completely encapsulating each of the tips 27d, as is illustrated in Figure 5 (b) .

The second step of the process, the results of which are shown in Figure 5 (c) , involves a layer of metal 24Od, in this case chromium, being deposited on top of the SiN layer 238d by thermal evaporation.

In 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 27d losing its metal and the SiN insulator layer 238d being exposed, as is shown in Figure 5 (d) .

As is shown in Figure 5 (c) , the SiN insulator 238d is then etched leaving a supporting metal ring 241d around the exposed tip 27d, which acts as a gate.

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

Referring to Figure 6 (a) to 6 (c) , the aforementioned process can be improved by applying a planarising agent 237e, i.e. a liquid which upon heating or solvent evaporation becomes a thin planar film to the backplate 5e after the second step of the process, resulting in an arrangement as illustrated in Figure 6 (a) . This shows the planarising agent 237e coating the backplate 5e leaving the tips 27e standing proud.

The step of etching the arrangement by plasma means thus results in the arrangement shown in Figure 6 (b) .

The SiN insulator is then etched as before, leaving a space between the metal layer and the tip 27e, as is shown in Figure 6 (c) . By utilising the planarising agent 237e 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 27d;27e 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.

Referring now to Figure 7 there is shown a first side profile of a first laser beam for use in a method according to any of the preceding embodiments of the present invention.

The method of forming or manufacturing a field emission backplate 5a - e comprises: providing a backplate material - layer 15a - e; and laser processing the backplate material, wherein an output 30Of of the laser comprises a sloping or ramped leading edge 305f.

It is believed by the present Inventors that the sloping leading edge 305f serves to dehydrogenate the backplate material 15a-e, e.g. amorphous silicon (Si:H), in a controlled manner. The leading edge slope may be used to control the initial rate of explosive release of hydrogen/dehydrogenation in a controlled manner. The present Inventors have found that the shape of the laser output 30Of plays a role in determining the surface

morphology and crystal fraction of the field emission backplate.

The output of the laser also has a ramped or sloping trailing edge 31Of.

In this implementation the laser output 30Of has a mid-portion 315f of substantially continuous intensity or power between the leading edge 305f and the trailing edge 31Of of the at least one pulse. In such implementation the laser output 30Of is said to be "flat topped". In another implementation the laser output can be substantially Gaussian.

In this implementation the laser output 30Of is symmetrical. However, it will be understood that the laser output 30Of can be asymmetrical, e.g. the leading and trailing edges (305f; 31Of) can be the same or different .

The step of providing a backplate or backplate material comprises providing a planar , body of (hydrogenated) irradiating and/or amorphous semiconductor based material upon a substrate.

The step of processing the material comprises laser crystallising at least a portion of the amorphous semiconductor based material . Upon laser irradiation/crystallising the amorphous semiconductor based material a plurality of emitter sites are typically formed .

In this implementation there are provided a plurality of pulses of laser radiation. The/each laser pulse is provided at a frequency of around 1 to 100 Hz, e.g. 5 to 50 Hz, e.g. around 20 Hz. A length of the/each pulse is around 1 to 100 nm, e.g. 5 to 50 nm, e.g. around 20 nm. Adjacent pulses are separated by greater than the thermal time constant of the backplate material .

A length of leading edge is between 1 and 5 mm, e.g. around 3 mm. A length of the trailing edge is between 1 and 5 mm, e.g. around 3 mm. A peak energy of the laser output is between 100 and 500 mJ/cm ² , e.g. around 200 mJ/cm ² . A length of the mid-portion is between 1 and 10 mm, e.g. 4 mm. The laser is beneficially an eximer laser.

The step of providing a backplate material comprises providing a backplate comprising hydrogenated amorphous silicon (Si :H) . The backplate material is provided on a substrate, e.g. a metallised substrate, e.g. aluminium, chromium, molybdenum or any other suitable metal or metallic material.

In the step of processing the at least one laser output 30Of is caused to pass over or scan at least a part of the backplate material in a first direction and then in a second direction, the first and second directions preferably being substantially opposing (or orthogonal) directions to one another.

In the step of processing the laser output 30Of is also optionally and beneficially caused to pass over or scan at least part of the backplate material in a third direction perpendicular to the first direction and then in a fourth direction, the third and fourth directions preferably being substantially opposing (or orthogonal) directions to one another.

The laser output passes or scans at a speed or velocity of around 1 to 50 mm/sec, e.g. around 20 mm/sec. Scanning can be provided by moving the backplate material relative to the laser output (or vice versa) .

The laser output (pulse or beam) comprises a two dimensional output. The laser output (pulse or beam) has a first intensity profile (see Figure 7) across the two dimensional area in a scanning direction.

The first intensity profile comprises the leading edge, the optional mid-portion and/the trailing edge. The first intensity profile is provided in a scanning direction of the laser output.

The laser output (pulse or beam) also has a second intensity profile across the two dimensional area - in a direction perpendicular to the scanning or first direction. The second intensity profile (not shown) typically comprises a square wave, top hat, Gaussian or semi-Gaussian profile.

During laser irradiation and/orcrystallisation, the laser output is normally pulsed during relative movement between the laser output and the backplate material . Consequently the backplate material is subjected to a number of shots or pulses of laser radiation.

The step of processing the backplate or backplate material can at least partially form a plurality of conductive or conducting particulates or particles within the backplate material . The at least partial formation of the conducting particulates is conveniently termed "conditioning" of the backplate.

The step of providing a backplate material comprises: providing a planar body of amorphous semiconductor material upon substrate 10a-e.

The step of processing the material comprises : laser processing or laser irradiating or crystallising at least a portion of the (hydrogenated) amorphous semiconductor material, such that the plurality of conducting particulates are formed.

The method comprises the step of: selecting a level of hydrogenation of the (hydrogenated) amorphous semiconductor material such that when laser irradiated a surface of the amorphous semiconductor material remains substantially flat or planar. Alternatively, the method comprises the step of: selecting a level of hydrogenation of the amorphous semiconductor material such that when

laser irradiated a surface of the amorphous semiconductor material becomes substantially non-flat or roughened, e.g. comprising a plurality of tips, e.g. miσrotips.

The planar body of (hydrogenated) amorphous semiconductor based material is provided by depositing a thin film of material upon a substrate, e.g. by Plasma Enhanced Chemical Vapour Deposition (PECVD) . The semiconductor based material is typically silicon or an alloy thereof .

The step of performing laser processing can be carried out at a wavelength of around 240nm to 510nm, e.g. 248nm to 300nm, or 525nm to 540nm, e.g. substantially 248nm or 532nm. The step of laser processing can use an excimer laser or Nd: YAG laser at a suitable wavelength thereof. This step can be carried out in air, vacuum or an inert gas atmosphere. The excimer laser can be a KrF laser.

It is desirable to create a fine grain structure, and explosive modification and/or crystallisation of the backplate material can provide such.

Thus the present invention provides a method of manufacturing a field emission backplate 5a-e comprising: providing a backplate or backplate material; processing the material by exposing the material to, or irradiating with, laser radiation 30Of, wherein an

output of the laser has a ramped or sloping leading edge 305f .

The field emission backplate 5a-e formed according to the invention finds use as an element of a display- device .

The invention also provides a field emission backplate formed or manufactured by the methods according to the embodiments hereinbefore described.

The field emission backplate so formed comprises a plurality of emitter sites formed by laser irradiation/ crystallisation of a planar body or thin film of amorphous semiconductor based material .

The field emission backplate comprises a plurality of conductive or conducting particulates or particles, wherein the conductive or conducting particulates are provided within the backplate.

The field emission backplate at least initially comprises a layer of amorphous semiconductor material .

Each conducting particulate comprises a point or locality e.g. of crytallisation, e.g. a crystallite, within the layer of amorphous semiconductor material . The conducting particulates each comprise semiconductor and/or metallic material. The amorphous semiconductor material comprises amorphous silicon or an alloy thereof, e.g. hydrogenated amorphous silicon, Si:H. The layer of amorphous semiconductor material is provided on a

substrate. The substrate can be made from glass, or alternatively from a ceramic or metallic material.

Filaments or pathways, e.g. conductive or conducting filaments, can be provided between the conducting particulates, at least in use. Such filaments can provide a means for electron transport through the backplate to emitter sites on a surface of the backplate, which ^■ surface can comprise a surface of the amorphous semiconductor material .

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

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

In use, electrons can move through the filaments between conducting particulates and to an emitter site. The electrons can 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 particulate is sufficiently small .

In one embodiment the surface of the amorphous semiconductor material is substantially flat. However,

in an alternative embodiment the surface of the amorphous semiconductor material is substantially non-flat or roughened, comprising a plurality of tips, e.g. microtips .

The emitter sites can be provided anywhere upon or across the surface. For example, in the alternative embodiment the emitter sites can be provided on the tips and/or between adjacent tips.

The invention also provides a field emission device comprising a field emission backplate as hereinbefore described.

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

The device can 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 can alternatively comprise an electro-luminescent material or (wide band-gap) light emitting material into which electrons from the emission sites are emitted, in use.

Such a field emission device can further comprise a substrate, the light emitting material, and a transparent window, wherein electrons from the emission sites are

emitted into the light emitting material. The light emitting material can be a light emitting polymer. The electro-luminescent or light emitting material can have a fluorescent material chemically attached thereto. The transparent window can be a thin film transparent metal .

One surface of the light emitting material can be disposed on a surface of the field emission backplate, and the transparent window can be disposed on another surface of the light emitting material.

In preferable implementations the device is a display device.

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

It has been established that processing of thin film hydrogenated amorphous silicon by an excimer laser leads to field emission, and can be used to form a backplate for a filed emission display. The Inventors have established that the laser energy is crucial to the nature of this field emission, and the resultant internal structure may be controlled by the laser fluence. The shape of the laser beam can play a role in the formation of the backplate and in the nature of the resultant material .

Amorphous silicon, which is an alloy of silicon and hydrogen, has a random network structure, which results

in metastability. The defect density in amorphous silicon (a-Si:H) is dynamic and is determined by the link between the electro-chemical potential (EF) and the hydrogen chemical potential . The link is the mobility of hydrogen and involvement thereof in the creation of dangling bonds - metastability.

In laser processing of amorphous silicon for production of polysilicon thin film transistors, for example, it is normal to dehydrogenate the silicon by annealing (thermal anneal) at : 400 ⁰ C — 450 ⁰ C for around 1 to 2 hours, in pure N ₂ .

Alternatively, low energy laser fluence can be used to give the same residual hydrogen content. If dehydrogenation is not used, explosive evolution of H ₂ occurs during laser crytallisation badly damaging the a- Si/pSi surface. a-Si around 8% H as deposited is greater than 3% after dehydrogenation.

For the purposes of preparing a backplate for field emission, the hydrogen is kept in the silicon without pre-annealing, and the explosive release of the hydrogen can play a role in determining surface roughness. This surface roughness can be a beneficial result of the process for field emission.

The release of this hydrogen, however, can be controlled by carefully defining the laser beam output shape. In this case, a sloping leading and

trailing/falling edge of the laser beam profile plays a role in surface morphology and roughness and on the resultant internal structure.

Referring now to Figure 8 there are shown side views of first profiles of second and third laser outputs 30Og, 30Oh capable of being used in the preceding embodiments of Figures 1 (a) to 6 (c) like parts of the profiles of Figure 8 are indicated by the same numerals as that of the profile of Figure 7, but suffixed by the letters "g" and "h" respectively.

As can be seen from Figure 8, laser output 30Og is symmetrical (Gaussian) , while laser output 30Oh is asymmetric, having a longer leading edge 305h than the trailing edge 31Oh.

In Figure 8 there is provided an asymmetric shorter pulse profile 30Oh, as opposed to the Gaussian profile. The pulse profile retains the desirable gradual leading edge of the Gaussian pulse for controlled evolution of hydrogen, while increasing the peak energy. The resultant backplate will have increased surface roughness along with higher crystalline volumes, which may be beneficial .

This may provide significant reduction of crystallisation laser energy density required to yield a given crystalline volume by modifying the Gaussian pulse

profile, while retaining the controlled evolution of hydrogen from a-Si:H films.

Higher peak energy is realised by adjusting two parameters of the laser pulse: the length of the laser pulse and the off-setting of the peak. Off-setting of the peak allows one to attain gradual increase of laser energy on the leading edge 305h. The gradual increase of energy helps control the evolution of hydrogen during crystallisation, which is an advantage of using a Gaussian pulse profile.

Referring now to Figures 9 (a) to (f) there are shown side views of first profiles of fourth to tenth laser outputs 30Oi - j capable of being used in the preceding embodiments of Figures 1 (a) to 6 (c) . Like parts of the profiles of Figures 9 (a) - (f) are indicated by the same numerals as that of the profile of Figure 7, but suffixed by the letters "i" to "o" respectively.

In the scanning direction, laser output 30Oi is symmetrical; laser output 30Oj is symmetrical; laser output 300k is non-symmetrical ; laser output 3001 is nonsymmetrical; laser output 300m is symmetrical; laser output 30On is non-symmetrical or optionally nonsymmetrical (see dashed line) , while laser output 30Oo is non- symmetrical .

Referring now to Figures 10 (a) to (c) there are shown perspective views showing first and second profiles

of eleventh to thirteenth laser outputs 30Op - r, respectively, capable of being used in the preceding embodiments of Figures 1 (a) to (6(c) . Like parts of the profiles of Figures 10 (a) - (c) are indicated by the same numerals as that of the profile of Figure 7, but suffixed by the letters "p" to "r" respectively.

As can be seen from Figures 10 (a) to (c) the laser outputs having a first profile 32Op - r in the scanning direction, which is symmetrical and a second profile 325p - r perpendicular to the scanning direction, which is of a "top hat" shape.

Figure 10 (c) shows what may be typically a more accurate output, in use, with "noise" or variation carried thereon.

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 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 summary of invention either individually or combined.

For example, during laser treatment of the thin film of amorphous silicon the use of a single laser pulse has been described in locally crystallising the region.

However, a number of pulses may alternatively be used - possibly allowing energies as low as 20 mJ/cm ² to be used. Additionally, it has been described how the crystallisation of larger line or dot structures can be used to grow spacers during the selective etch and growth process of the tips. However, silicon can also be grown in blocks on an insulator and thin film transistor devices for active address delineated in the same process. The depositing of the thin film of amorphous silicon, which may be intrinsic or doped n-type, has been described by PECVD. However, the thin film may also be deposited by sputtering, evaporation or other such means. The substrate on which the thin film silicon has been deposited has been described as aluminium. However, such may alternatively be a metal such as molybdenum, chromium, or similar. Also in the example given in the description a Nd:YAG laser having 532 nm wavelength is used to maximise absorption in silicon. However, other wavelengths can be used, and in particular other wavelengths to maximise absorption in other appropriate semiconductor based materials can be used. The use of a transparent metal to form a diode configuration field emission device is described. However, a suitable conducting polymer may alternatively be used.

TFT control circuitry can be fabricated in the same manner as the described field emission backplate, either

at pixel level or via integrated peripheral drivers . It is possible that the field emission device having a field emission backplate is formed such that emitter sites inject directly into a wide band-gap light emitting material (not shown) to produce light emission. Such arrangements would be particularly useful in the case of the thin film semiconductor not being of n-type, and there being no low barrier metal that enables electrons to be injected. The thin film semiconductor of the examples given is an n-type hydrogenated amorphous silicon. However, the semiconductor may alternatively be germanium or germanium alloy or similar.

Devices such as those shown hereinbefore are particularly suitable for many display applications, e.g. due to their having low power consumption and being relatively simple to fabricate. The devices/backplates of the invention may also be used elsewhere, e.g. as a cathode (s) for high power transistors for microwave amplifiers, e.g. in the satellite and mobile communication markets.