The present invention relates to a method of manufacturing a phosphor screen of a cathode ray tube (CRT).

Furthermore, the invention relates to a CRT comprising said phosphor screen.

Many different display devices, such as a television receiver, commonly utilize cathode ray tubes having luminescent phosphor screens. A conventional cathode ray tube (CRT) is shown in Fig 1. In general, a CRT comprises a glass panel 1, which on its inside has a phosphor screen 2. In front of the screen 2 is generally provided a perforated shadow mask 3, also called a color selection electrode. A funneled part 4 is bonded to the panel 1 to form a vacuum bulb. An electron gun 5 is installed inside a neck 6 at the rear end of the funneled part 4, and a deflection unit 7 is provided on the outside of the neck 6 and the funneled part 4. Electron beams emitted from the electron gun 5 are deflected by the deflection unit 7 to land at precise locations on the phosphor screen 2, thus forming a pixel, a plurality of which form a picture.

However, it should be noted that CRTs without shadow masks, sometimes referred to as beam indexing CRTs or intelligent tracking CRTs, are also known (not shown or further described) ). Fast intelligent tracking CRTs are also referred to as FIT tubes.

Principally, there are two different categories of intelligent tracking CRTs, i. e. single beam systems with only one electron gun and multi-beam systems with several electron guns.

For a color CRT, the phosphor screen includes red, green and blue phosphor layers (e. g. phosphor stripes or dots).

In general, the phosphor screen comprises a structured light absorbing carbon (graphite) layer, also called a black matrix, which is formed between the respective phosphor elements, and a reflective metal back layer, typically made of aluminium. By segregating the red, blue and green light-emitting phosphor elements, the black matrix increases the contrast of the picture.

Electrons emitted from the electron gun pass through the perforations in the shadow mask (if a shadow mask is present) to selectively excite the separated phosphor elements. When the emitted electrons hit and excite the phosphor (or an activator element incorporated in the phosphor), light energy in the form of photons are emitted, which travel

in various directions out of the phosphor, upwards, downwards and sidewards. Light emitted in the direction of the viewing glass panel is seen by the viewer. However, some light is emitted in other directions away from the viewing panel. The reflective metal back layer then acts to redirect or reflect this light towards the viewing panel.

In current TV tubes, a flat reflective aluminium layer is often used. This flat layer reflects the light back to the openings in the black matrix, i. e. this light is seen by the viewer, and to the light-absorbing black matrix. Since the phosphor lines or dots are not optically separated when a flat metal layer is used, color contamination problems, for instance red light, may occur in a green color line, and internal light reflection may occur.

Furthermore, light emitted sideward is lost with regard to luminance and contrast performance (LCP).

It is known in the art to use a dome-shaped reflective metal back layer to overcome said problems associated with a flat metal back layer. In comparison with a planar reflective back layer, a dome-shaped metal back layer improves the display brightness and luminance since the internal reflection and absorption of light is decreased. Furthermore, the risk of color contamination is eliminated. For instance, US 5 097 175, US 5 547 411 and US 5 489 816 describe such dome-shaped reflective metal back layers forming optically separated light cavities.

Color contamination might also be reduced using pigmented phosphor, i. e. blue light, for instance, will then be absorbed by the red pigment in the red light emitting phosphor.

There are also other factors affecting color contamination, such as the spot size of the electron beam, the light scattering power of the phosphor, the size of the phosphor lines or dots, and the reflectivity of the reflective metal back layer.

The reflective metal back layer is usually applied by an evaporation method using a vacuum chamber. However, it is impossible to evaporate aluminium directly on the phosphor. If too many phosphor particles are covered with an aluminium mirror, the total amount of emitted light will be reduced. Therefore, a thin layer of, for instance, an aqueous poly (vinyl alcohol) solution is generally first applied on the phosphor. Thereafter, an organic solution, such as a solution of polyacrylate in toluene, is applied on said thin water layer.

This organic phase does not mix with the water phase and a thin skin, a pellicle, is formed on the water phase. This thin skin should be in contact only with the highest points of the

phosphor lines (or dots). Aluminium is then evaporated on this organic layer and, after annealing, a flat, almost free-hanging aluminium back layer, which is only adhered to the highest points of the phosphor is obtained.

The location of the skin is important for the result. If the skin is placed too high up, i. e. a too small amount of the phosphor is in contact with the organic phase, the back layer does not adhere. If the skin is placed too low down, too much aluminium is evaporated into the phosphor and emitted light is lost.

This"skin-forming step"is both cumbersome and not very robust and trustworthy, which means that the result is not always satisfactory and screens must sometimes be remade. Thus, the process is quite expensive as the yield with regard to working screens is rather low. Furthermore, organic solvents are required which is undesired from economical and environmental aspects.

Additionally, the evaporation step is also quite expensive as a large vacuum chamber is needed.

Thus, the overall evaporation method is both a rather cumbersome and expensive method.

An object of the present invention is to alleviate the above problems and to provide an easily performed, high-yield, inexpensive and environmentally more friendly method of manufacturing a phosphor screen of a cathode ray tube, in particular a method of manufacturing a reflective metal back layer.

According to a first aspect of the invention, this and other objects are achieved with a method of manufacturing a phosphor screen of a cathode ray tube (CRT), which comprises providing a reflective metal back layer by applying a metal flake suspension on top of a structured black matrix layer and phosphor, which are applied on an inner surface of a CRT panel, wherein the suspension comprises metal flakes having an average dimension of at least two times the phosphor particle dimension.

The metal flake suspension is preferably applied by spin coating, spraying or any other suitable application method.

The metal flakes are preferably smaller than 500 im as the stability of the metal flake suspension decreases as the flake size increases.

The phosphor particles usually have an average diameter of about 5-6 im and the distances between the particles are on average about 3 m, which means that the metal

flakes generally should be larger than 10 nm. Flakes smaller than 10 um should preferably be excluded from the suspension as these flakes will penetrate the phosphor lines (or dots) and occur between the phosphor particles, thus reducing the amount of emitted light.

More preferably the dimensions of the metal flakes range from 10 urn to 100 m, the average dimensions being for example 30 x 30 pm and the flake thickness being about 30 nm.

The flakes are suspended in water or any other solvent, such as ethanol. Water is preferred from environmental and economical points of view.

The metal flakes are preferably aluminium flakes.

An aluminium back layer is preferred since aluminium is a metal having a low atomic number, i. e. few protons in the nucleus, which means that fewer electrons in the electron beam will be stopped by the back layer than in the case of a metal having a higher atomic number. Furthermore, aluminium flakes are preferred since these flakes are highly reflective, stable in contact with phosphor, and flexible. The flakes will settle both on the phosphor line (or dot) and between the phosphor lines (or dots). Thus, the lines (or dots) will be optically separated by the metal flakes. Because they are so thin and flexible the flakes will adopt the same shape as the structure underneath and provide optically separated light cavities having light-emitting phosphor within. The advantages of optically separated light cavities are given in the introduction. Most preferably, the reflective metal back layer is dome-shaped, but the layer might also have a rectangular shape or any other shape providing optically separated light cavities.

Advantageously, the metal flake suspension further comprises a binder which comprises at least one polymer, or a combination of polymers, having good film-forming properties. The binder provides a metal flake layer, which is less sensitive and more mechanically stable during handling.

Furthermore, the binder preferably comprises at least one polymer, or a combination of polymers, having acidic groups, preferably carboxyl groups. The acidic groups of the binder will react with dissolved aluminium salts and form a cross-linked polymer matrix of metal complexes. This cross-linked polymer matrix provides an insoluble (in water and/or in an organic solvent, such as ethanol) and mechanically strong layer. Thus, additional layers may be applied on top of the back layer using aqueous and/or organic suspensions.

Electron beam bombardment through the back layer generates negative charge in the phosphor, which, if not removed, will eventually repel the electron beam. The graphite

in the black matrix may contribute to the removal of negative charge, but it may also be preferred that the metal back layer is conductive.

The aluminium flakes used in the invention have a poor electrical conductivity as the flakes are covered with oxides acting as an insulator. Thus, it may be preferred to apply a conductive layer or conductive particles on top of the back layer.

Such a conductive layer may be formed by evaporating a thin (about 20 nm) metal layer, preferably an aluminium layer, on top of the back layer.

However, the conductivity may also be improved by applying other materials, such as silver, graphite, or gold particles. Such conductive particles may be applied by e. g. spin coating, spraying or any other suitable application method, a suspension, preferably an aqueous suspension, of the particles on top of the metal back layer.

Additionally, it shall be noted that evaporation of a getter layer, such as metallic barium or calcium, also improves the conductivity. However, the main purpose of the getter is to remove the residual gases from the final vacuum bulb. Thus, a substantial amount of getter is consumed during this gas removal. An additional amount of getter is therefore preferably applied if additional conductivity is to be obtained by the getter layer.

The method according to the invention is easily performed, inexpensive and easy to introduce as a replacement for the conventional evaporation method in the ordinary production process of CRTs. Furthermore, the yield, as described in the introduction, is improved.

According to a second aspect of the invention, said object and other objects are achieved with a cathode ray tube (CRT) which comprises a phosphor screen manufactured according to the method described herein.

A further advantage of the present invention is that the luminance and contrast performance (LCP) of the phoshor screen is improved as compared to a phosphor screen having a conventional evaporated, flat metal back layer.

Other features and advantages of the present invention will become apparent from the embodiments described hereinafter.

Fig 1 schematically shows the construction of a commonly used CRT.

Fig 2 schematically shows a part of a phosphor screen manufactured in accordance with an embodiment of the present invention.

A part of a phosphor screen 2 comprising a reflective metal back layer 10 manufactured according to an embodiment of the invention is shown in Fig 2.

The phosphor screen 2 in Fig 2 is present on an inner surface of a glass panel 1, and comprises a structured black matrix layer 8 located on the panel 1.

Phosphor 9 is provided in the openings of the black matrix 8.

A reflective dome-shaped metal back layer 10 (top layer), manufactured according to the present invention, covers the phosphor 9 and is in contact with at least a part of the black matrix 8.

On top of the reflective metal back layer 10 is applied, according to the embodiment shown in Fig 2, a conductive layer 11, which improves the conductivity of the reflective metal back layer 10.

The term"layer"as used herein means a continuous or discontinuous coating.

By a discontinuous layer is meant that the layer is broken in accordance with a pattern, i. e. a structured layer which provides lines or dots without layer.

The light generated by the phosphor 9 will be reflected by the reflective metal back layer 10 and emitted through the openings in the black matrix 8.

The phosphor screen 2 shown in Fig 2 is preferably made in a process implementing a method according to an embodiment of the invention. The manufacturing steps are outlined below.

A structured black matrix 8 is made using conventional techniques which are known to persons skilled in the art.

The structured black matrix 8 is, for instance, obtained by first applying a standard, water-based photosensitised resist, for instance comprising poly (vinyl pyrrolidone) (PVP) and photosensitive bisazide, on an inner surface of CRT glass panel 1. This layer is subsequently exposed to UV light through a shadow mask as a result of which the exposed areas are cured. The unexposed and unhardened areas are washed away with water and the remaining pattern is dried. Thus, a resist pattern is formed.

On top of the entire surface including both the photocured pattern and the uncovered areas of the panel, a graphite suspension is applied and dried, thus forming a non- structured black matrix layer.

The photoresist pattern is then swollen by a photoresist swelling agent, such as sulfamic acid or nitric acid. The black matrix is partially broken open in accordance with a pattern by the swollen resist dots. These swollen resist dots and the black matrix overlying

the swollen resist are then jointly removed by a high-pressure water jet. Thus, a structured black matrix 8 having dots or stripes corresponding to the perforations of the shadow mask is obtained.

Said dots or stripes are then filled with phosphor 9 of said three colors in a manner known to persons skilled in the art.

A reflective metal back layer 10 is then applied, in accordance with the present invention, by e. g. spin coating a metal flake suspension on top of the phosphor 9 and the structured black matrix layer 8. The suspension comprises metal flakes having an average dimension of at least two times the phosphor particle dimension.

The phosphor particle is in this embodiment about 5-6 urn, which means that the aluminium flakes of this embodiment are larger than 10 um, but smaller than 100 um.

The metal flake suspension may also be applied by spraying or any other suitable application method. Spin coating at low speed may be referred to as flow coating.

The flakes settle both on the phosphor line (or dot) and between the phosphor lines (or dots), thus the lines (or dots) are optically separated. Because the flakes are so thin and flexible they will adopt the same shape as the black matrix 8 and phosphor 9 underneath and provide optically separated light cavities having light-emitting phosphor 9 within.

The reflective metal back layer 10 shown in Fig 2 is dome-shaped. This dome- shaped reflective metal back layer 10 improves the luminance and contrast performance (LCP) by at least about 5% as compared to a phosphor screen having a conventional evaporated, flat metal back layer.

A binder is preferably used in the aluminium flake suspension. The binder is a polymer, or a combination of polymers, which is soluble in water, or the solvent used, e. g. ethanol, and which does not form insoluble metal complexes in the suspension. The polymer should have good film-forming properties, and it should leave no residue after a final anneal step at about 450°C.

The binder provides a metal flake layer 10, which is less sensitive and more mechanically stable during handling.

Furthermore, the binder may in some cases, for reasons given below, preferably comprise at least one polymer, or a combination of polymers, having acidic groups, preferably carboxyl groups. The acidic groups of the binder will react with dissolved aluminium salts and form a cross-linked polymer matrix of metal complexes. This cross- linked polymer matrix provides an insoluble (in water and/or in an organic solvent, such as

ethanol) and mechanically strong layer. Thus, additional layers may be applied on top of the back layer using aqueous and/or organic suspensions.

If, using an aqueous metal flake suspension, it is desired to obtain a water- insoluble and mechanically more stable flake layer 10, a preferred binder composition is a combination of a good film-forming polymer, such as poly (vinyl pyrrolidone), and a polymer having acidic groups, such as carboxyl (-COOH), sulfonic acid (-S03H), phosphor containing acidic groups, e. g. phosphinic acid groups (-PO (OH) 2), or phenol (-C6H40H) groups. If an additional layer is to be applied using an aqueous suspension it is preferred that the metal flake layer 10 is water-insoluble. From environmental and economical points of view it is preferred to use aqueous suspensions in all process steps.

The pH of the suspension is preferably increased to about pH 10 to increase the amount of dissolved aluminium salts. The acidic groups of the binder will then react with this increased amount of dissolved aluminium salts and form a cross-linked polymer matrix of metal complexes. Thus, a water insoluble and mechanically strong back layer 10 is formed.

The acidic groups are preferably carboxyl groups, since polymers comprising carboxyl groups are less inclined to form insoluble metal complexes, in the metal flake suspension, in comparison to other acidic polymers. Furthermore, films formed of polymers comprising carboxyl groups are often more mechanically stable than films formed of other acidic polymers. In addition, sulfonic and phosphinic acid groups are often not completely removed during the annealing step.

Examples of polymers having good film-forming properties and which may be used in aqueous suspensions are poly (vinyl pyrrolidone), poly (vinyl alcohol), poly (acryl amide), hydroxyethyl cellulose, and polyurethane.

Examples of polymers having acidic groups and which may be used in aqueous suspensions are polyacrylates comprising carboxyl groups, such as Rohagit SD-15, carboxymethyl cellulose, cellulose acetate trimellitate, polyacrylic acid (in combination with other polymers), and polyurethanes with carboxyl groups.

An example of a polymer with good film-forming properties and having acidic groups, and which may be used in an aqueous suspension, is poly (acrylamide-co-acrylic acid).

If a water-insoluble metal flake layer 10 is required for reasons given above, the metal flakes may instead be suspended in ethanol and a binder soluble in ethanol (and

insoluble in water) may be used. Thus, it is not required to form the cross-linked polymer matrix of metal complexes by using a polymer having acidic groups as described above.

Examples of polymers having good film-forming properties and which may be used in ethanol suspensions are polybutyral, poly (vinyl pyrrolidone), and poly (vinyl acetate).

However, it is of course also possible to form said insoluble cross-linked polymer matrix of metal complexes using an ethanol flake suspension, if this should be desired for any reason. For instance, it might be desired if an additional layer is to be applied using an ethanol suspension.

Examples of polymers having acidic groups and which might be used in an ethanol suspension are co-polymers comprising methacrylic acid, such as poly (tert-butyl- acrylate-co-ethylacrylate-co-methacrylic acid).

Examples of polymers having good film-forming properties and comprising acidic groups, and which might be used in ethanol suspensions, are poly (vinyl acetate-co- crotonic acid) and poly (styrene-co-maleic acid).

The amount of acidic groups is also important. A polymer with a high degree of carboxyl groups, such as poly (acrylic acid), often gives insoluble metal complexes and an unstable suspension. The concentration of carboxyl groups should be about 5-80% (w/w), preferably about 20-40% (w/w). Below 20% (w/w) the layers become more fragile.

The aluminium flakes used in the invention have a poor electrical conductivity as the flakes are covered with oxides acting as an insulator.

Therefore, a conductive layer 11 (or conductive particles) is preferably applied on top of the back layer.

This conductive layer 11 may be formed by evaporating a thin (about 20 nm) metal layer, preferably an aluminium layer, on top of the back layer 10.

However, if the vacuum chamber is to be completely avoided, or for any other reasons, conductivity may also be achieved by application of, for instance, silver particles, graphite, gold or any other conductive material. However, materials having a higher atomic number than aluminium will reduce the electron beam intensity and thus decrease the amount of emitted light.

However, a conductive layer 11 of silver particles may be applied by e. g. spin coating, spraying, or any other suitable application method, a suspension of silver particles on top of the metal back layer 10. The silver particles may be suspended in water or any other solvent, such as ethanol. Water is preferred from environmental and economical points of view. As outlined above, it is important that the back layer 10 comprising metal flakes is

insoluble in contact with water if an aqueous silver particle suspension is to be applied on top of the back layer 10.

Thus, if metal evaporation is not used to form the conductive layer, it is advantageous to incorporate a binder in the metal flake suspension to form a water insoluble metal back layer 10.

Furthermore, if any other type of additional layer is to be applied (by any technique other than evaporation) on top of the back layer 10, it is advantageous to incorporate a binder in the metal flake suspension to form a less fragile metal flake layer 10.

An example of an additional layer, apart from a conductive layer, is the phosphor particles in FIT tubes which may be applied on top of a reflective metal back layer, such as the above disclosed metal flake layer 10.

In one of the last steps in the production of the CRT, the CRT, including said reflective metal back layer 10 and said conductive layer 11, is annealed at about 450°C to pyrolyse all organic compounds. The binder is then pyrolysed leaving a pure metal back layer 10. Thus, the metal back layer 10 is reduced about 2-3 times due to the annealing. The thickness reduction is of course dependent on the amount of binder used in the metal flake suspension.

Preferably, the metal flake layer 10, before annealing, has a thickness of from about 300 nm to about 1 um, and a reflection over about 80%, such as 80-90%.

After annealing, it is preferred that the back layer thickness, i. e. the thickness of the final back layer 10, is at most about 300 nm. (The layer thickness within a layer may vary somewhat. ) Thicker layers are more optically dense, but also stop more electrons and consequently the amount of generated light is reduced. Too thin layers have too many holes or openings in the layer and consequently a too low reflectivity.

The invention will now be further illustrated by means of the following non- limiting example.

4.10 g of a 7.5% (w/w) aluminium flake suspension comprising 13.5% (w/w) polyurethane in water (Eckart Ultrastar Aqua FP-4100) was mixed with 1.49 g of a 5.2% (w/w) solution of Mowiol 40-88 (Clariant), 0.56 g 3.0% (w/w) Rohagit SD-15 (Rohm & Haas), 0.27 g 11.0% (w/w) Tween 20 (Aldrich), 27.13 g water. 0.23 g 5% (w/w) ammonia was added to bring the acidity of the suspension to about pH 10. This suspension was vigorously stirred for about 15 min.

Mowiol 40-88 is a poly (vinyl alcohol).

Rohagit SD-15 is a polyacrylate comprising carboxyl groups. It both contributes to the formation of aluminium complexes and increases the viscosity of the suspension.

Tween 20 is a polysorbate acting as a surface-active agent.

The average dimension of the aluminium flakes in Eckart Ultrastar Aqua FP- 4100 is about 12-14 um with a flake thickness of 30 nm.

The ratio of aluminium to polymers is in this suspension about 1: 2.3.

The suspension comprising the aluminium was then spin coated at about 125 rpm for about 30 s on top of a structured black matrix 8 (about 0.8 + 0.1 ujn) and phosphor 9 applied on an inner surface of a CRT panel 1 according to the method described above. The applied layer 10 was then dried at about 40°C in an oven for about 5 min. The aluminium flake layer 10 was about 1 pm thick.

The reflectivity of this reflective metal back layer 10 was about 85%.

A thin (about 20 nm) aluminium layer 11 was then evaporated on top of the reflective aluminium back layer 10 using a vacuum chamber.

The reflectivity of the aluminium back layer 10 is not affected by the evaporated aluminium layer. However, the electrical conductivity is largely improved.

After the final annealing, when all organic compounds, including the binder, were pyrolysed, the layer thickness became about 1/3 of its initial thickness, i.e. about 300 nm.

While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to persons skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.