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(11) | EP 0 322 200 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Phosphor screen preparation |
| (57) The method forms a plurality of interspersed colour phosphor arrays on the inside
of a faceplate (1) of a CRT. An opaque layer (30) if formed on the faceplate. An apertured
mask is formed in the opaque layer utilizing electron sensitive resist (31) which
is firstly exposed to one of the electron beams of the CRT via the shadow mask and
is then processed to form the apertured mask. A first phosphor array is then formed
by putting a layer (33) of phosphor and photosensitive binder on the mask and exposing
it to light through the apertures in the mask, and performing subsequent processing
to leave the required phosphor array and the resist. The entire process is then repeated
using a further resist layer (34) for the or each of the other colour phosphor arrays
(36'), each further resist layer covering the previously laid-down phosphor to protect
it from contamination by the next phosphor. |
[0002] A colour cathode ray tube (CRT) comprises a screen on which arrays of different phosphors which emit differently coloured light in response to electron beams, are accurately positioned on a screen of the CRT with respect to holes (or slits) in a shadow mask. An electron gun arrangement produces a different electron beam for each colour. Many methods of making the arrays of phosphors are known.
[0003] U.S. Patent 4251610 (Tektronix Inc) discloses an illustrative method in which an opaque layer is first formed on the inner surface of the screen. Assuming three differently "coloured" phosphors are to be used, three arrays of holes are produced in the opaque layer. This is done by coating the opaque layer with photo resist, and illuminating the coating through a shadow mask with light from a light source which simulates the three electron beams, developing the resist, etching the layer using the developed resist as a mask and then removing the resist.
[0004] Phosphor of one colour e.g. green is put into one of the arrays using the following process:-
[0005] A coating of photo resist is put over the entire opaque layer and the holes. The costing is exposed to ultra-violet (UV) light from a light source simulating the green electron beam through the shadow mask. The coating is then developed to expose the "green" array of holes leaving the two other arrays covered by resist. "Green" phosphor and photosensitive binder are put onto the screen. The phosphor and binder are exposed to ultra violet light through the screen to fix the green phosphor in the green holes. Excess phosphor and binder is removed and the photoresist is removed.
[0006] The process is repeated for the other colours. Each repetition begins with a new coating of photo resist which covers the previously laid-down phosphor, thus protecting the previously laid-down phosphor from cross-contamination.
[0007] Light beams travel in straight lines whereas electron beams follow paths which are controlled by magnetic and electrostatic fields. In order to accurately form the arrays of holes in the opaque layer, it is known to use the electron beams themselves. That is known inter alia, for example, from British Patent Application GB 2 176 647A (Rank Electronic Tubes Ltd), published on 31 December 1986.
[0008] In one illustrative method disclosed in GB 2 176 647A an opaque layer is first formed on the inner surface of the screen of a CRT. An array of phosphor is laid down using the following process:-
[0009] The layer is coated with electron beam sensitive resist which is exposed to e.g. the "green" electron beam through a shadow mask. The electron beam resist is developed and the layer etched to produce an array of "green" apertures or holes thereon. "Green" phosphor plus photosensitive binder is applied and is exposed to UV light through the screen to fix the phosphor in the apertures. The electron beam resist is removed. A new coating of electron beam resist is then laid down and the process repeated for the or each other coloured phosphor. Although the method is effective, there is in practice some cross contamination of the phosphors which reduces colour purity.
[0010] In another method proposed in GB 2 176 647A, three arrays of apertures are initially produced in the opaque layer using electron beam exposure of an electron beam sensitive resist which is removed. Then another electron beam resist is laid down exposed using e.g. the "green" electron beam, developed, and "green" phosphor and photosensitive binder fixed in the "green" apertures by UV irradiated through the screen. The or each other colour phosphor is fixed in its array of apertures in similar fashion. As in Tektronix USP 4251610, the UV light irradiates all the apertures in the opaque layer. In Tektronix, a layer of photo resist blocks the UV light in the apertures other than the apertures associated with the colour of phosphor being laid down, because a layer of photo resist is thick. Electron beam resist however, is not normally thick enough to completely block the UV light. Consequently, the resist can be partly sensitized over the other holes which can cause contamination of the other holes on development.
[0011] In accordance with one aspect of the present invention, it is desired to prepare a phosphor screen with reduced cross-contamination of the phosphors whilst having accurate placement of the phosphors.
[0012] According to said one aspect, there is provided a method of forming a plurality of different interspersed arrays of phosphers on the inside of a faceplate for a colour cathode ray tube, in which method, for each array, a mask is formed by irradiating an electron sensitive resist layer with an electron gun in a demountable tube substantially at the same position with respect to the array as the electron gun which will energise that array in the completed tube, and phosphor is fixed in place in the mask, wherein the masks are formed sequentially and each mask and phosphor array is covered by the resist layer used for making the next mask before the next mask and phosphor array is formed, all the resist layers being retained until all the phosphor arrays are formed.
[0013] In accordance with another aspect of the present invention, it is desired to provide a method of preparing a phosphor screen in which difficulties of removing excess phosphor are at least reduced, whilst accurately placing different phosphors of good colour purity with respect to the electron beams which, in operation of the screen, will energise them.
[0014] According to said another aspect there is provided 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) forming an opaque layer on the inside of the faceplate;
B) providing a first layer of electron sensitive resist on the opaque layer;
C) assembling the faceplate into a demountable tube including a shadow mask and electron gun means for generating a plurality of electron beams and pumping down to vacuum;
D) irradiating said layer of resist through the shadow mask with one of said beams to activate the layer of resist in positions corresponding to openings in the shadow mask and said one beam;
E) disassembling the faceplate from the tube;
F) processing the faceplate utilizing the activated electron resist material to form in the opaque layer a mask having apertures at said positions;
G) providing phosphor and photosensitive binder on the formed mask;
H) exposing said phosphor and photosensitive binder 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;
I) further processing the faceplate to form a first said array of phosphors of a first colour in register with said apertures;
J) providing a second layer of electron sensitive resist over the said first layer and over the said first array of phosphors; and
K) repeating steps C) to I) above using a second one of said beams, to form a second said array of phosphors of a second colour.
[0015] For a better understanding of the present invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:
Figure 1 is a side view in cross-section of a "demountable" cathode-ray-tube (CRT) for use in the present invention, and
Figures 2-1 to 2-13 are partial schematic views in cross-section of a faceplate of the CRT illustrating different stages in the manufacture of a cathode-ray-tube screen.
[0016] 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 whihc is connected by a tube 5 to a vacuum pump system 6. 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.
[0017] The process of manufacture of the phosphor screen will now be described with reference to Figures 2-1 to 2-13.
[0018] The faceplate 1 of the CRT is separated from the shadow mask 2 and an opaque coating 30 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 first layer of electron sensitive resist 31 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.
[0019] 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 31 is exposed, through the shadow mask 2, to a scanning electron beam from the direction indicated by the arrows in Figure 2-3 from a single electron gun in the electron gun housing 8, for example the gun responsible for the green content on the screen. Those parts of the resist 31 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 31 is treated with developer, whereupon the exposed dots or lines, as appropriate, are removed and the unexposed areas remain. This stage is illustrated in Figure 2-4, in which the opaque layer 30 on the faceplate 1 has an apertured layer of developed resist 31 superimposed thereon.
[0020] The exposed parts of the opaque layer 13 are then etched away through the apertures or holes in the resist 14, and the unexposed parts of the opaque layer 13 are allowed to remain, as shown in Figure 2-5, thus forming a mask having apertures 32 therein. If the opaque layer 30 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 30 then this can be etched using dilute caustic soda.
[0021] A first phosphor material which emits a first colour, e.g. green, when bombarded by electrons, is then mixed with an U.V. sensitive photoresist, for example, polyvinylalcohol sensitised with ammonium bichromate, to form a mixture 33 which is slurried or settled onto the faceplate layers. This stage is shown in figure 2-6. The faceplate 1 is exposed to U.V. from a source shone through the glass faceplate 1 in the direction of the arrows shown in Figure 2-7 so that the parts of the photo resist and phosphor mixture 16 which are in the apertures 32 are polymerised. Developing the resist results in hardened phosphor dots or lines 33′ remaining in the apertures 32 as shown in Figure 2-7. The remaining electron sensitive resist 31 is retained.
[0022] A second layer of electron sensitive resist 34 is now applied by spinning as before as shown in Figure 2-8. The second layer of resist 34 covers the e.g. green phosphor and the first layer of resist. 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 first and second 34 layers of resist are exposed, through the shadow mask 2 to a scanning electron beam from the direction indicated by the arrows in Figure 2-9. A different one of the electron guns is used to generate the beam in this case, for example, the gun responsible for the red content on the screen. Those parts of the first and second layers of resist 31, 34 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 first and second resist layers 31 and 34 are treated with developer, whereupon the exposed dots 35 or lines of resist 31, 34 are removed and the unexposed areas remain.
[0023] The further exposed parts of the opaque layer 13 are then etched away through the apertures 35 in the resist layers 31 and 34, and the unexposed parts of the opaque layer 30 are allowed to remain, as shown in Figure 2-11.
[0024] A second phosphor material which emits a second colour, for example red when bombarded by electrons, is then mixed with U.V. sensitive photoresist such as bichromated polyvinylalcohol to form a mixture which is slurried onto the faceplate layers as before. The faceplate is exposed to U.V. from a diffuse source shone through the glass faceplate 1 so that the parts of the photoresist and phosphor mixture which are in the apertures 35 are polymerised. Developing the photoresist results in hardened phosphor dots or lines 36 remaining in the apertures 35 as shown in Figure 2-12. The first and second layers of electron sensitive resist 14′ are retained.
[0025] The process is repeated by applying a third layer of resist over the first and second layers and over the red and green phosphers. The third layer is exposed to the "blue" electron gun through the shadow mask, and developed, the opaque layer etched and "blue" phosphor fixed in the apertures as described above. The result is shown in Figure 2-13.
[0026] As shown in Figure 2-13 arrays of the red, green and blue phosphors 33′, 36 and 37 are arranged in interspersed relation on the faceplate 1 in registry with the points at which the red, green and blue electron beams, respectively, irradiate the faceplate 1 during use.
[0027] The second repeat of the process may be omitted if required to produce a screen with only two colours of phosphor. Similarly, further repeats may be necessary to produce a screen with more than three colours.
[0028] By retaining the resist layers, each phosphor to be fixed in position is protected from cross-contamination by the subsequent phosphors.
[0029] Because the sets of holes 32, 35 etc are made one set at a time and the opaque layer thus acts as a mask, it is possible to use the electron-sensitive resists which are normally too thin to act as masks to the UV light themselves even when two or more layers of resist are present.
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) forming an opaque layer on the inside of the faceplate;
B) providing a first layer of electron sensitive resist on the opaque layer;
C) assembling the faceplate into a demountable tube including a shadow mask and electron gun means for generating a plurality of electron beams and pumping down to a vacuum;
D) irradiating said layer of resist through the shadow mask with one of said beams to activate the layer of resist in positions corresponding to openings in the shadow mask and said one beam;
E) disassembling the faceplate from the demountable tube;
F) processing the faceplate utilizing the activated electron resist material to form in the opaque layer a mask having apertures at said positions;
G) providing phosphor and photosensitive binder on the formed mask;
H) exposing said phosphor and photosensitive binder 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;
I) further processing the faceplate to form a first said array of phosphors of a first colour in register with said apertures; and
J) repeating steps C) to I) above using a second one of said beams, to form a second
said array of phosphors of a second colour;
characterised by the steps of:
K) providing a second layer of electron sensitive resist over the said first layer and over the said first array of phosphors between the first step I) and the second step C).
2. A method according to claim 1, further comprising the step of providing a third
layer of electron sensitive resist over the said first and second layers of resist
and over the said first and second arrays of phosphors, and repeating steps C) to
I) using a third one of said beams to form a third said array of phosphors of a third
colour.
3. A method according to claim 1 or 2, further comprising the step of removing the
layers of electron sensitive resist after all the arrays of phosphors are formed on
the faceplate.
4. A method according to any preceding claim, wherein said opaque layer is a metallic
layer.
5. A method according to any preceding claim, wherein the said opaque layer is black.
6. A method of forming a plurality of different interspersed arrays of phosphors on
the inside of a faceplate for a colour cathode ray tube, in which method, for each
array, a mask is formed by irradiating an electron sensitive resist layer with an
electron gun in a demountable tube substantially at the same position with respect
to the array as the electron gun which will energise that array in the completed CRT,
and phosphor is fixed in place in the mask, wherein the masks are formed sequentially
and characterised in that each mask and phosphor array is covered by the resist layer
used for making the next mask before the next mask and phosphor array is formed, all
the resist layers being retained until all the phosphor arrays are formed.
7. A method according to claim 6, further comprising forming an opaque layer on the
faceplate, the said masks being formed on the opaque layer using the said resist layers,
the said mask being used to form respective arrays of apertures in the opaque layer,
and the phosphors being mixed with photosensitive binder and being fixed in the respective
arrays of holes by irradiating them with light through the face plate.
8. A faceplate made by the method of any preceding claim.
9. A cathode ray tube having a faceplate according to claim 8.