Improved field emission backplate

(54) Title of the invention: Improved field emission backplate (51) IntCI⁷: HOI J 9/02//HOI J 1/304

<td> (21)</td><td> Application No:</td><td> 0119659.1</td>

<td> (22)</td><td> Date of Filing:</td><td> 11.08.2001</td>

<td> (43)</td><td> Date A Publication:</td><td> 12.02.2003</td>

(52) UK CL (Edition X ):

HID DPD D17D D24 D38 (56) Documents Cited:

EP 0578428 Al JP 070141984 A (58) Field of Search:

As for published application 2378570 A viz:

UK CL (Edition T) HID DPD

INT CL⁷ H01J 1/304 9/02

Other: Online: WPI, EPODOC, JAPIO,

INSPEC updated as appropriate (72) Inventor(s):

Mervyn John Rose Ravi Silva John Shannon (73) Proprietor(s):

The University Court of The University Of Dundee (Incorporated in the United Kingdom) Nethergate, Perth Road, DUNDEE,

DD1 4HN, United Kingdom (74) Agent and/or Address for Service:

Marks &amp; Clerk

Royal Exchange Square, GLASGOW, G1 3AE, United Kingdom

1/2

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Emission Current (A)

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IMPROVED FIELD EMISSION BACKPLATE

FIELD OF INVENTION

The present invention relates to a method of forming a field emission backplate and related device, and in particular, though not exclusively, to a field emission backplate comprising a plurality of emitter or emission sites or silicon tips formed by laser crystallisation,

e.g. for use in a display.

BACKGROUND TO INVENTION

Flat panel displays are of immense importance in electronics. In current developments, Active Matrix

Liquid Crystal Displays (AMLCD) are beginning to challenge 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. However, 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 the display, inefficient use of input electrical power is made, often with well over 70%, of the energy being lost as non—useful energy.

Field emission displays (FEDs), based on conventional Spindt tip technology, promised a solution to flat panel display problems. FEDs are essentially flat cathode ray tube (CRT) devices. However, rather than one electron gun firing electrons at a phosphor on a screen through a shadow mask, a FED has tens or hundreds of individual tips in each display pixel. The tips are known as Spindt tips, after the inventor Cap Spindt. The process of fabrication relies on defining a pattern of holes in a gate metal by leaving a well beneath the metal. A sacrificial layer (usually nickel) is then evaporated on the surface at an oblique angle to ensure the well is not filled. The emitter material (usually tungsten or molybdenum) is then evaporated through the holes into the well. As the evaporate metal builds up on the surface the sacrificial layer it closes the hole as the thickness increases, and has the effect of providing an emitter tip in the well. The top metal is then removed by etching the sacrificial layer, leaving the tip, the well, and the original gate metal. This forms a back plate of Spindt tips. A top plate containing a patterned phosphor is then placed using spacers. The final device is evacuated to allow the emitted electrons a long mean free path.

The principle of field emission from micro-tips is well understood and is governed by Fowler - Nordheim tunneling. The emission current, and therefore brightness of the display depends on the current density, the number of tips and their sharpness, i.e.

I = J_FnDQ!

where n = number of tips, a the tip sharpness, and J_FN the

Fowler-Nordheim tunnel current density.

The tips will provide a sharp electron source that will provide hot electron injection into, for example, a phosphor.

Unfortunately, the extreme complication in fabrication has limited the use of this technology.

Additionally, crystal silicon emitters are limited by the wafer size.

Other thin-film materials may also be used for field emission. Carbon is the main contender with diamond, diamond like carbon and carbon nano-tubes also suitable.

Diamond seemed a good choice - although it is difficult to fabricate and also the mechanism of a supposed negative electron affinity which diamond was claimed to have, has now been questioned.

An object of the present invention is to obviate or mitigate at least one of the aforementioned problems.

SUMMARY OF INVENTION

According to a first aspect of the present invention there is provided 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.

Preferably the planar body of amorphous semiconductor based material is provided by depositing a thin film of material upon a substrate.

Conveniently, the semiconductor based material is silicon or an alloy thereof.

Preferably the laser crystallising is performed using an excimer laser or Nd:YAG laser.

Conveniently, the excimer laser is a KrF laser.

According to a second aspect of the invention there is provided 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 .

Conveniently, the semiconductor based material is silicon or an alloy thereof.

According to a third aspect of the invention there is provided a field emission device comprising a field emission backplate comprising a plurality of emitter sites formed by laser crystallising of a planar body or thin film of amorphous semiconductor based material.

Preferably the field emission device is a vacuum device wherein the emitter sites of the backplate act as emission source in the devices, in use.

Conveniently, the field emission device further comprises a substrate, an evacuated space and a transparent window, e.g. thin film transparent metal or metallised phosphor, 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.

Alternatively, the field emission device further comprises a wide band-gap light emitting material, e.g. a light emitting polymer, into which electrons from the emitter sites of the backplate are emitted, in use.

Conveniently, the field emission device comprises a substrate, the field emission backplate on one side of which is formed the plurality of emitter sites, the wide band-gap light emitting material comprising a light emitting polymer, and transparent window comprising a thin film transparent metal or metallised phosphor, wherein the field emission backplate is formed upon the substrate, and one surface of the light emitting polymer is disposed on the plurality of emitter sites of the field emission backplate, the transparent window being disposed on the other surface of the light emitting polymer .

Conveniently, the field emission device is a display device .

BRIEF DESCRIPTION OF DRAWINGS

These and other aspects of the present invention will become apparent from the following description when taken along with the accompanying drawings, which show:

Figures 1A - IF a thin film semiconductor crystallised at various energies according to the present invention;

Figure 2 a field emission device according to an embodiment of the present invention; and

Figure 3 a graph of field emission current vs electric field for a field emission backplate according to the present invention.

DETAILED DESCRIPTION OF DRAWINGS

With reference to Figures 1A - IF there is shown field emission backplates 10a to lOf, each formed of an amphorous semiconductor based material, in this case ntype hydrogenated amorphous silicon, on the surface of which a plurality of emitter sites 14a - 14f are formed. The field emission backplate 10 is formed by the deposition of a thin film of approximately 100 nm of n— type hydrogenated amorphous silicon onto a substrate of, for example, aluminium by plasma enhanced chemical vapour deposition (PECVD) . The deposited thin film then undergoes laser crystallisation by an excimer laser or

Nd:YAG laser, in this case a KrF laser operating at a wavelength of 248 nm scanning at 2 mm/s in an atmosphere of oxygen and then quenched. Alternatively, Nd:YAG laser operating at a wavelength of 5-32 nm pulsed at 3 to 7 nseccnds, stepped and repeated to form a pattern is used.

This process results in the surface of the silicon having a rough texture. The energy absorbed by the silicon influences the extent of roughening of the surface as can be seen in Figures 1A - IF. Figure 1A shows the effect of a small amount of absorbed energy, approximately 100 mJ/cm² energy, while Figure IF shows rounded tips achieved by absorbed energy in the region of 300 mJ/cm². In each

<td> case, each of the sites</td><td> or tips</td><td> 14a -</td><td> 14f acts as an</td>

<td> emitter site. In a field</td><td> emission</td><td> device</td><td> an emitter site</td>

<td> emits electrons at low</td><td> fields</td><td> in a</td><td> field emission</td>

configuration. The backplate 10 results in emission currents in excess of 10’⁵A and low field threshold of around lOV^m.

An example field emission device 18, having a field emission backplate 20, formed as described with reference to Figures 1A - IF, is shown in Figure 2. The device 18 shown is a triode device having a field emission backplate 20 with a substrate 21 of aluminium and a thin film 22 of n-type hydrogenated amorphous silicon which has been treated by an excimer laser and thus has a plurality of emitter sites 24 upon a surface thereof. An insulating material such as silicon nitride, has been disposed on the crystallised silicon, and subsequently etched thus providing spacer elements 26. Upon each of the spacer elements 26 is disposed a thin film 28 of phosphor, e.g. metallised phosphor, and the device 18 is completed with a layer of glass 3 0 thus giving a three terminal gate control arrangement. An area 31 between the glass 30 and the emitter sites 24 are evacuated which allows emission to be controlled using low voltages.

This is important for effective spacial control when the device 18 is used in displays.

Emission currents measured in such a device 18 having a vacuum below 5 x 10'⁶ mbars are shown in Figure 3, which is a graphical representation of emission current vs electric field. It is also estimated that a beta factor for the described device 18 is in excess of

450, with this figure including both geometric enhancement and internal enhancement.

The device 18 hereinbefore described is suitable for many display applications due to low power consumption and being relatively simple to fabricate. Such devices may also be used as cathodes for high power transistors for microwave amplifiers in the satellite and mobile communication markets.

Various modifications can be made to the invention as herein before described without departing from the scope of the invention. For example, TFT control circuitry can be fabricated in the same manner as the described field emission backplate 20, either at pixel level or via integrated peripheral drivers. It is possible that the field emission device 18 having a field emission backplate 20 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 example given is an n-type hydrogenated amorphous silicon. However, the semiconductor may alternatively be germanium or germanium alloy or similar. The substrate on which the thin film semiconductor is disposed has been described as being aluminium. However, such may be formed of various other types of metal such as molybdenum, chromium or similar. The use of KrF (Krypton

Fluorine) excimer laser is described. However, any excimer laser may be used. Further, the use of a Nd:YAG laser at 532 nm is described. However, any other wavelength of Nd:YAG laser can be used.