GB2176647A - Manufacture of colour CRT phosphor screens - Google Patents

Abstract

In a method of forming a plurality of interspersed colour phosphor arrays on the inside of a faceplate for a colour cathode ray tube an apertured mask is first formed on the inside of the faceplate utilizing electron sensitive resist material which is exposed to one of the electron beams of the CRT via the shadow mask and is then processed to form the mask with apertures at the positions where the electron sensitive resist material was irradiated by the electron beam. A first of the colour phosphor arrays is then formed by applying a mixture of colour phosphor and photoresist on top of the mask and exposing the mixture to light through the apertures in the mask, from the opposite side of the faceplate and subsequently processing the mixture to leave the required phosphor array. The entire process is then repeated for the or each of the other colour phosphor arrays. An opaque layer may be formed on the faceplate prior to the electron sensitive material and the apertures are formed in this opaque layer to provide the mask.

Description

SPECIFICATION Colour cathode ray tube This invention relates to a method of manufacturing a screen for a colour CRT.

Colour CRTs comprise a screen on which arrays of different coloured elements are provided, a shadow mask, and means for producing a plurality of electron beams corresponding to the respective colours, the shadow mask ensuring that the respective beams impinge on the screen at positions appropriate to the respective colours.

In order for the tube to operate satisfactorily, the array of coloured elements on the screen must be accurately positioned in relation to the shadow mask and the electron beam producing means. For example, the screen of a conventional colour CRT consists of coloured elements in the form of arrays of phosphors which emit coloured light when irradiated by the electron beams through the metal shadow mask. Each electron beam activates one of the arrays of phosphors on the screen which, when irradiated by an electron beam through the shadow mask hole or slit, will emit one particular colour.These phosphor dots or lines are arranged uniformly in sequence so that a first colour is emitted when the screen is irradiated by the beam from the first electron gun, a second colour is emitted when the screen is irradiated by the beam from the second electron gun, and a third colour is emitted when the screen is irradiated by the beam from the third electron gun. The rear surface of the phosphor arrays are normally made electrically conductive by an evaporated layer of aluminium. The shadow mask CRT will thus provide a multi-colour picture by the simultaneous modulation of the electron beams while scanning the screen area.

Conventional CRTs emit three colours, normally red, green and blue to encompass the major part of the visible spectrum. However it is also possible to construct a CRT having only two colours, or more than three.

The manufacture of the phosphor screen is a complex process of photochemical printing which involves tight control of processes and high dimensional accuracy so that close registry can be maintained between each electron gun and the corresponding phosphor array.

Coloured light without contamination, and of a high degree of colour purity, must be emitted by the completed CRT when operating.

In the course of manufacture of the known phosphor screen, the glass faceplate and shadow mask are mounted on a lighthouse and each colour phosphor, mixed with a photo resist sensitive to Ultra Violet light (U.V), is exposed in succession to a U.V source, precisely positioned in relation to the screen to correspond to where the electron beam will be positioned in the completed CRT. The lighthouse is a standard piece of equipment and has means for mounting the faceplate and the shadow mask of the CRT on the lighthouse so that the faceplate can be exposed, through the shadow mask, to an U.V source contained within the lighthouse. There will either be three spaced U.V sources simulating the positioning of three electron beams in a CRT or, alternatively, means may be provided for rotating the faceplate appropriately relative to a single U.V source in the lighthouse.After each exposure to U.V the faceplate and shadow mask are removed from the lighthouse and the screen is developed by washing with water (or the appropriate solvent) so that only the irradiated portions of the phosphor material remain. In this manner three interlocking phosphor dot arrays are formed on the faceplate so that each will be in registry with the corresponding electron gun.

Subsequently the whole CRT is assembled with the screen, the shadow mask and the electron guns within the glass envelope so that processing may take place to form the vacuum which is necessary for the CRT to operate.

However, since electron beams cannot be made to travel precisely along the same paths as light beams, misalignment of the arrays of phosphor dots or lines and the points at which the corresponding electron beams impinge on the screen occurs causing colour impurity in the finished CRT. The lighthouse must therefore be set up with optical correction means to reduce alignment errors between the position of the phosphor dots or lines in each array on the screen and the points at which the corresponding electron beams impinge on the screen. The optical correcting means may comprise a Schmidt-type correction lens. Such errors are further reduced by reducing the size of the individual phosphor dots or lines, with the addition of a black border around each dot or line which does not emit light, thus reducing the light output of the CRT.

The present invention provides a method of forming a plurality of interspersed colour phosphor arrays on the inside of a faceplate for a colour cathode ray tube, comprising: A) providing a layer of electron sensitive resist material on the inside of the faceplate; B) assembling the faceplate into a tube including a shadow mask and electron gun means for generating a plurality of electron beams; C) irradiating said material through the shadow mask with one of said beams to activate the material in positions corresponding to openings in the shadow mask and said one beam; D) disassembling the faceplate from the tube; E) processing the faceplate utilizing the acti vated electron resist material to form on the faceplate a mask having apertures at said positions; F) providing phosphor and photoresist material on the formed mask;; G) exposing said photoresist material through said apertures to light from the other side of the faceplate so that portions thereof located in register with the apertures in the mask are activated; H) further processing the faceplate to form a first said array of phosphors of a first colour in register with said apertures; and I) repeating steps B) to H) above using a different one of said beams, to form a second said array of phosphors of a second colour.

Accordingly, since the positions at which the colour phosphor arrays are formed are defined by electron beams passing through the shadow mask, rather than by light which is intended to simulate the electron beams, substantially improved accuracy is achieved.

Applicants' acknowledge that proposals have been made in UK specification Nos.

1,085,470, 1,300,480 and 1,177,598 for the formation of the colour phosphor arrays utilizing electron sensitive resist material and electron beams. However, the processes disclosed in these documents differ from the process of the present invention.

The invention is described further by way of example with reference to the accompanying drawings, in which: Figure 1 is a side view in cross-section of a "demountable" cathode ray tube for use in the present invention; Figures 2-13 are partial views in cross-section of the facepiate of the cathode ray tube illustrating different stages in the manufacture of a cathode ray tube screen.

Figure 1 illustrates a "demountable" CRT system, which accurately simulates the completed CRT, using the actual CRT faceplate 1 and its matching shadow mask 2 and shadow mask frame 3. These are mounted in the envelope 4, which may be made of metal, glass or ceramic and which is connected by a tube to a vacuum pump system (not shown). Inside the neck of the envelope 4 is mounted a multiple electron gun 8 and on the exterior of the neck 7 are mounted the CRT scan coils 9 by which the electron beams emitted by the electron gun 8 can be made to scan by the application of the appropriate voltage waveforms.

For clarity only two electron beams are shown although any number of electron beams and their corresponding phosphor arrays may be used. In use, the whole assembly within the envelope 4 is vacuum tight, the faceplate 1 being joined to the envelope 4 by means of a gasket 10.

The process of manufacture of the phosphor screen will now be described.

The faceplate 1 of the CRT is separated from the shadow mask 2 and an opaque coating is applied. The coating may be reflective, for example aluminium, or may be a black coating, for example black chromium applied by evaporation in a gassy atmosphere by methods which are well known and long established. A thin layer of electron sensitive resist 14 is then applied to the faceplate 1 for example by spinning. The resist layer is then hardened by baking. (The type of resist may be described as "positive" or "negative" according to whether it softens or hardens respectively under the action of an electron beam; for convenience the method here described is the process for the "positive" resist). Various types of electron sensitive resist may be used, for example, one comprising copolymers of methylmethacrylic acid and methacryloyl chloride and other methacrylates i.e PM type resists.Alternatively, PMMA types of electron sensitive resist comprising polymethylmethacrylates in cellusolve acetate may be used. A particular commercially available electron sensitive resist which may be used is ISOFINE E-B positive resist grade P7.

The faceplate 1 and the shadow mask 2 are then assembled together and placed on the demountable CRT as in Figure 1. Appropriate voltages are applied and the resist 14 is exposed, through the shadow mask, to a scanning electron beam from the direction indicated by the arrows in Figure 2 from a single electron gun in the electron gun housing 8, for example the gun responsible for the red content on the screen. Those parts of the resist 14 which receive the electron beam through the shadow mask holes become softened.

The faceplate assembly is then removed from the demountable CRT and the resist layer 14 is treated with solvent developer, whereupon the exposed dots or lines, as appropriate, are removed and the unexposed areas remain.

This stage is illustrated in Figure 3, in which the opaque layer 13 on the faceplate 1 has an apertured layer of developed resist 14 superimposed thereon.

The exposed parts of the opaque layer 13 are then etched away through the holes in the resist 14, and the unexposed parts of the opaque layer 13 are allowed to remain, as shown in Figure 4, thus forming a mask having holes 15 therein. If the opaque layer 13 is black chromium then the etching agent used can be a mixture of ammonium ferric nitrate, perchloric acid and demineralised water. If aluminium forms the opaque layer 13 then this can be etched using dilute caustic soda.

A first phosphor material which emits a first colour, e.g red, when bombarded by electrons, is then mixed with an U.V sensitive photoresist, for example, polyvinylalcohol sensitised with ammonium bichromate, to form a mixture 16 which is slurried or settled onto the faceplate layers and dried. This stage is shown in Figure 5. The faceplate 1 is exposed to U.V from a diffuse source shone through the glass faceplate 1 in the direction of the arrows shown in Figure 5 so that the parts of the photo resist and phosphor mixture 16 which are in the holes 15 are polymerised.

Developing the resist results in hardened phosphor dots or lines 16' remaining in the holes 15 as shown in Figure 6. Then the remaining electron sensitive resist is also dissolved away with the appropriate solvent, for example, acetone, as shown in Figure 7.

Another layer of electron sensitive resist 14' is now applied by spinning as before as shown in Figure 8. The faceplate 1 together with the shadow mask 2 are then assembled together and placed on the demountable CRT shown in Figure 1 and the resist 14' is exposed, through the shadow mask, to a scanning electron beam from the direction indicated by the arrows in Figure 8. A different one of the electron guns is used to generate the beam in this case, for example, the gun responsible for the blue content on the screen.

Those parts of the resist 14' which receive the electron beam through the shadow mask holes become softened and, as before, the faceplate assembly is then removed from the demountable CRT and the resist layer 14' is treated with solvent developer, whereupon the exposed dots or lines of resist 14' are removed and the unexposed areas remain.

The further exposed parts of the opaque layer 13 are then etched away through the holes in the resist 14', and the unexposed parts of the opaque layer 13 are allowed to remain, as shown in Figure 10. There are holes 18 between each area of the opaque layer 13 and superimposed resist 14'.

A second phosphor material which emits a second colour, for example blue when bombarded by electrons, is then mixed with U.V sensitive photoresist such as bichromated polyvinylalcohol to form a mixture 19 which is slurried onto the faceplate layers and dried as before. This stage is shown in Figure 11. The faceplate 1 is exposed to U.V from a diffuse source shone through the glass faceplate 1 in the direction of the arrows indicated in Figure 11 so that the parts of the photoresist and phosphor mixture 19 which are in the holes 18 are polymerised. Developing the photoresist results in hardened phosphor dots or lines 19' remaining in the holes 18 as shown in Figure 12.The remaining electron sensitive resist 14' is then dissolved away using acetone resulting in the arrangement shown in Figure 13 in which arrays of the first and second phosphors 16' and 19' are arranged in interspersed relation on the faceplate 1 in registry with the points at which the first and second electron beams, respectively, irradiated the faceplate 1 during use.

Finally, the interstitial opaque coating 13 which remains may be removed, or left in position to enhance contrast in the complete CRT.

The process can be repeated for a third phosphor which emits a third colour, for example green, if required.

An alternative embodiment of the invention comprises the following steps: (1) Coating the inside of the faceplate with an opaque black deposit for example by evaporation.

(2) Coating this black deposit with an electron sensitive resist material, for example by centrifuging.

(3) Scanning the faceplate in a demountable vacuum system through the shadow mask with the three electron beams simultaneously or in sequence.

(4) Developing all exposed elements of the photoresist layer so that an array of holes remains.

(5) Etching the black metallic deposit through the resist holes, thus making a complete array of holes in the black layer.

(6) Removing the remaining resist layer with a solvent.

(7) Coating the black deposit with further electron sensitive resist material, for example by centrifuging.

(8) Irradiating the electron sensitive resist layer in a demountable vacuum system through the shadow mask with a scanning electron beam from a first of the electron guns.

(9) Developing the exposed elements of the resist layer so that an array of holes remains in the resist layer, thus forming a mask.

(10) Slurrying a phosphor material mixed with an ultra violet sensitised photoresist (such as bichromated polyvinyl alcohol) on the back of the mask.

(11) Exposing the phosphor/PVA layer through the holes in the mask to ultra violet light applied from the opposite side of the faceplate.

(12) Developing the phosphor/PVA layer to produce an array of phosphor dots which can emit one colour.

(13) Repeating the above from step 7, utilizing a different electron gun and forming a phosphor dot which can emit a second colour.

(14) Repeating again from step 7 using the third electron gun and forming an array of phosphor dots which can emit a third colour.

It could be noted that during the repeats, instead of repeating step 7, in which a fresh layer of electron sensitive resist material is applied, the electron sensitive resist material previously applied may be allowed to remain so that the repeat would begin at step 8. It should also be noted that a similar modification could be made in the embodiment described with reference to the drawings. That is to say instead of applying the fresh layer of electron sensitive resist material 14' shown in Fig. 8, the prevously applied such layer may be allowed to remain.

In all the methods described above it is en visaged that a conductive layer, for example, of aluminium, may be coated onto the faceplate so as to overlie the phosphor layers.

This is a common feature of colour CRTs and, among other things, enhances brightness during use.

Claims (9)

1. A method of forming a plurality of interspersed colour phosphor arrays on the inside of a faceplate for a colour cathode ray tube, comprising; A) providing a layer of electron sensitive resist material on the inside of the faceplate; B) assembling the faceplate into a tube including a shadow mask and electron gun means for generating a plurality of electron beams; C) irradiating said material through the shadow mask with one of said beams to activate the material in positions corresponding to openings in the shadow mask and said one beam; D) disassembling the faceplate from the tube; E) processing the faceplate utilizing the activated electron resist material to form on the faceplate a mask having apertures at said positions; F) providing phosphor and photoresist material on the formed mask;; G) exposing said photoresist material through said apertures to light from the other side of the faceplate so that portions thereof located in register with the apertures in the mask are activated; H) further processing the faceplate to form a first said array of phosphors of a first colour in register with said apertures; and I) repeating steps B) to H) above using a different one of said beams, to form a second said array of phosphors of a second colour.

2. A method according to claim 1, wherein, prior to step A), an opaque layer is formed on the inside of the faceplate and the electron sensitive resist material is provided on the opaque layer; and wherein, in step E), said apertures are formed in said opaque layer.

3. A method according to claim 2, wherein during processing of the faceplate said electron sensitive resist material provided in step A) is substantially completely removed; and further electron sensitive resist material is provided on the inside of the faceplate prior to repeating steps B) to H).

4. A method according to claim 1, wherein, prior to step A): (i) an opaque layer is provided on the inside of the faceplate and coated with a layer of electron sensitive resist material; (ii) the faceplate is assembled with the tube and electron gun means; (iii) the electron sensitive resist material provided in step (i) is irradiated through the shadow mask with each of said beams and thereby activated at positions corresponding with the openings in the shadow mask and each of said beams; (iv) the faceplate is disassembled from the tube and processed utilizing the activated electron resist material provided in step (i) to form apertures in the opaque material at said positions corresponding to the shadow mask openings and each of the electron beams; and wherein after completion of step (iv), steps A) to I) are carried out.

5. A method according to claim 2, 3 or 4, wherein said opaque layer is a black metallic layer.

6. A method according to any preceding claim, wherein said photoresist material is sensitive to ultraviolet light, and said light in step G) is ultraviolet light.

7. A method of forming a plurality of interspersed colour phosphor arrays on the inside of a faceplate for a colour cathode ray tube, substantially as herein described with reference to the accompanying drawings.

8. A faceplate when made according to the method of any preceding claim.

9. A cathode ray tube having a faceplate according to claim 8.