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(11) | EP 0 066 948 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Cathode ray tubes |
| (57) A cathode ray tube (1) (C.R.T.) comprises an envelope (2) having a fibre optic face
plate (4) formed of light transmitting fibres (6) in a light absorbing matrix (7).
Islets (8) of phosphor are arranged in register with the fibres (6). A thin electrically
conducting layer (9) covers the islets and inner face of the face plate. The islets
(8) are formed by covering the face plate (4) with a phosphor layer (5) then a layer
of photo resist (10). The photo resist (10) is exposed to light transmitted along
the fibres (6) thereby providing a mask for removing phosphor not in register with
a fibre (6). The photo resist may be a single layer of negative resist, or a layer
of negative resist follewed by a barrier layer and a layer of positive resist. |
[0001] A typical cathode ray tube comprises an evacuated glass envelope having a front face covered internally with a phosphor layer. An electrode structure at the rear of the envelope emits a beam of electrons which strikes the phosphor layer to cause a visible emission. External magnetic fields scan the beam over the layer and build up a desired image e.g. a television picture.
[0002] One form of phosphor layer is constructed of phosphor particles deposited e.g. by settlement from a suspension. This gives a relatively thick layer, e.g. 20 µm of particles. The particles may have a single dopant for monochrome receivers or may be doped to give emissions, of red, blue, and green colours and deposited separately through masks for use in colour receivers. Such techniques are well established for colour television receivers. A disadvantage is the relatively low brightness obtainable from a thick phosphor powder layer; the beam current cannot be increased too much otherwise the powder layer is destroyed. Another disadvantage is lack of resolution caused by sideways diffusion of emission to adjacent particles. These disadvantages are particularly troublesome in projection displays e.g. projection television and aircraft head-up- displays (H.U.D.).
[0003] To overcome these problems it has been proposed to use thin film phosphors. These have several potential advantages e.g. high resolution - not particle size limited; good contrast - the screen can be made non-reflective; and good thermal contact - allows a high beam current without over heating to give high brightness; planar geometry - well defined layers permit use of penatron principles.
[0004] Unfortunately thin film phosphors suffer from light trapping; the angle of emission of light is restricted by total internal reflection with a consequential loss of apparent brightness.
[0005] According to this invention a cathode ray tube has a fibre optic front face with a thin film of phosphor on the inside of the face, the film being formed of descrete islets of phosphor in register with an associated fibre.
[0006] According to this invention a method of manufacturing a cathode ray tube comprises the steps of ,
providing a fibre optic face plate having optically transparent fibres in a light absorbing matrix,
coating the rear surface of the plate with a thin film layer of phosphors,
depositing photo resist on the phosphor layer,
using the fibre plate as a mask to provide a photo resist mask for removing areas of phosphor not in register with a fibre to leave islets of phosphor each in register with a fibre,
removing the photo resist mask,
and depositing a thin electrically conducting layer over the islets.
[0008] The phosphor film may be ZnS or ZnSe doped with Mn, Cu on an 8 f diameter fibre optic face plate having transparent fibres in an opaque matrix.
[0009] The resist may be a negative resist that is fixed by U.V. exposure through the fibre face plate. Alternatively, in cases where the phosphor is too thick for.transmission of U.V., the resist may be applied as a composite layer of negative resist, barrier layer, and positive resist sensitive to blue light. In this case the positive resist is exposed through the fibre face plate and etched to form a mask for the negative resist which is exposed from the rear.
[0010] The invention will now be described, by way of example only, with reference to the accompanying drawings of which:-
Figure 1 is a view of a cathode ray tube incorporating a fibre face plate;
Figures 2a-e are sectional views of the face plate showing processing steps using a single negative resist;
Figures 3a-f are sectional views of an alternative process using a negative and a positive resist layer separated by a barrier layer.
[0011] Figure 1 shows a cathode ray tu e (C.R.T.) 1 for use in radar displays, or video receivers e.g. television receivers. It comprises an evacuated glass envelope 2 having at one end an electron emitting structure 3 and at the other end a front face 4 coated on its inner face with a phosphor layer 5. The front face is formed from a fibre optic face plate Figure 2, comprising a multiplicity of fine fibres 6, e.g. 8 µm diameter held in a matrix 7. Each fibre 6 is optically transparent whereas the matrix 7 is opaque. The phosphor layer 5, Figure 2e, comprises islets 8 of phosphor e.g. ZnS:Mn in register with an associated fibre 6. A thin coating 9 of aluminium covers the phosphor layer 8.
[0012] In operation a beam of electrons is emitted from the electron structure 3 and scanned across the front face 4 by conventional external field coils (not shown). The phosphor 5 glows where struck by electrons to form an observable display. Since each islet 8 of phosphor is discrete and covered by a reflecting layer of aluminium 9 all light emitted by that islet is transmitted along its associated fibre 6 to be seen by an observer. This separation of the phosphor layer 5 into islets 8 exactly in register with fibres 6 overcomes the problem of light trapping in thin film phosphor layers.
[0013] To provide islets 8 of phosphor in register with the fibres 6 photo lithographic techniques are employed using the fibre optic plate as a mask. The steps of processing are shown in Figures 2a-e.
[0014] A fibre optic plate 4 is coated on its inner face with a thin film of phosphor 5. The phosphor may be ZnS, or ZnSe deposited by vapour deposition, sputtering, evaporation, etc., and of a thickness 0.5 7 µm. A layer 10 of negative photo resist e.g. WAYCOAT I.C. resist (Trade name) is deposited on the phosphor layer, baked at 60°C for 10 minutes to remove solvent, and exposed to ultra violet (U.V.) light through the fibre optic plate to harden those parts of the resist in register with the fibres. The matrix material between the fibres is opaque to ultra violet. I
[0015] Unexposed parts of the resist are removed by etching with e.g. Xylene for about 1 minute and baked at 100C for 15 minutes, leaving islets 11 of hard resist Figure 2b These islets 11 of hard resist form a mask for ion beam etching of the exposed phosphor. Etching is continued until the matrix 7 of the plate 4 is exposed and the phosphor layer 5 is in the form of discrete islets 8 of phosphor Figure 2c. The resist 11 is removed by etching in MICROSTRIP (Trade name) for about 20 seconds, Figure 2d followed by rinsing in de-ionised water and drying. A thin e.g. 0.03 µm layer 9 of aluminium is evaporated over the islets 8 Figure 2e.
[0016] A limitation of the above, Figure 2, process is the need for an ultra violet transparent phosphor layer 5. A layer of e.g. ZnSe of 2 µm thickness absorbs too much ultra violet. The available negative resists require ultra violet light for hardening. To overcome this problem layers of negative and positive resist may be used as shown in Figures 3a-f.
[0017] A fibre optic plate 4, Figure 3a, is coated with a layer 35 of ZnS, ZnSe or other suitable phosphor to a typical thickness of 2 µm. A 0.3 µm thick layer 16 of negative resist followed by a thin (e.g. 0.3 µm) layer 17 of a barrier material, and a 1.5 µm thick layer 18 of positive resist is deposited on the phosphor 35. The barrier layer 17, e.g. of Poly Vinyl Alcohol (P.V.A.), is necessary to prevent an adverse reaction between the positive and negative resists which inhibits hardening. The positive resist may be Shipley Y 1350 A2.
[0018] The positive resist 18 is exposed through the fibre plate 4 using blue light which is not absorbed by the phosphor to any substantial degree but is absorbed by the matrix material 7. As a result the positive resist 18 is softened in area in register with the fibres and these exposed areas are removed e.g. by A2 developer. The remaining positive resist 19, Figure 3b, is used as a mask through which the negative resist 16 is exposed to ultra violet. The positive resist mask-19, barrier layer 17, and unexposed negative resist are sequentially removed with acetone, water, and Xylene. This leaves a mask 20 of negative resist, Figure 3c, for the ion beam etching of the exposed phosphor layer 5, Figure 3d. The negative resist mask 20 is removed, Figure 3e, to leave islets 15 of phosphor in register with fibres 6. A thin film of aluminium 21 is evaporated onto the phosphor, Figure 3f.
[0019] The completed, coated, fibre plate is then fused to a glass envelope to form a cathode ray tube.
[0020] As described above a monochrome cathode ray tube is produced. The above technique may be extended to colour television receivers using three differently doped phosphors with three different masks on the front of the fibre plate to form groups of three different colours.
[0021] The invention may also be extended to provide a two colour display using the penetron principles in which two different electron energies are used with two differently doped phosphor layers. The lower energy beam excites the rear most layer whilst the higher energy penetrates to the front layer and excites its emission. In such a device the single phosphor layer of Figures 2, or 3 would be replaced by two layers of different dopents.
1. A method of manufacturing a cathode ray tube comprising the steps of
providing a fibre optic face plate having optically
transparent fibres in a light absorbing matrix, coating the rear surface of the plate with a thin film layer of phosphor,
depositing photo resist on the phosphor layer,
using the fibre plate as a mask to provide a photo resist mask for removing areas of phosphor not in register with a fibre to leave islets of phosphor each in register with a fibre,
removing the photo resist mask,
and depositing a thin electrically conducting layer over the islets.
2. The method of claim 1 wherein the photo resist is a negative photo resist which
is exposed to ultra violet light transmitted along the fibres.
3. The method of claim 1 wherein the photo resist is a layer of negative photo resist
on the phosphor layer followed by a barrier layer and a layer of positive resist,
the positive resist being exposed to light transmitted along the fibres to form a
mask for selectively exposing the photo negative resist.
4. The method of claim 1 wherein the phosphor is deposited by evaporation, sputtering,
or vapour growth techniques.
5. The method of claim 1 wherein the phosphor is ZnS or ZnSe doped with Mn or Cu.
6. A cathode ray tube produced by the method of claim 1 comprising an envelope having
a fibre optic face plate formed of light transmitting fibres held in a light absorbing
matrix the plate being coated on its inner surface with islets of phosphor each in
register with a fibre and covered with a thin electrically conducting layer.