GB2111285A - A method of forming a colour filter layer for a liquid crystal display device - Google Patents

Abstract

A dyeable layer (33) is formed upon each electrode of a plurality of electrodes arranged spaced-apart on a base plate (21) of a liquid crystal display device, the thus formed layers being dyed with a preselected dyestuff. Preferably, the dyeable layers are divided into groups (R, G, B) each group being dyed with a dye of different colour. The layers may be hardened before dyeing and may be subsequently covered by a protective membrane and a liquid crystal orienting layer (26). <IMAGE>

Description

SPECIFICATION A method of forming a colour filter layer for a liquid crystal display device This invention relates to a method of forming a colour filter layer for a liquid crystal display device.

A liquid crystal display device is formed by a pair of base plates arranged spaced-apart and parallel to one another. On the facing surfaces of the base plates are formed electrodes, usually one on each base plate. A thin colour filter layer is formed on at least one of the electrodes, and the space between the base plates is filled with a liquid crystal material, the transmittance or reflectance of the liquid crystal material being controllable by the voltage applied across the two electrodes.

Multi-colour displays are extremely useful in computer systems, television sets, in video monitors and many other apparatus. Accordingly, much research has gone into the developing muiticolour liquid crystal displays.

Figures 1 and 2 of the accompanying drawings illustrate an example of such a multicolour display, Figure 1 being a cross-section through the display on line B-B in Figure 2 and Figure 2 being a cross-section through the display on line A-A in Figure 1.

As shown in Figures 1 and 2, a pair of base plates 21 are disposed spaced-apart but parallel to one another and a respective group of fine electrodes 22 is formed on each of the facing surface. Colour filter layers 23 are deposited on each electrode in one of the groups, the layers being of photometric primary colours, that is red (R), green (G) and blue (B). The space between the base plates 21 is filled with a liquid crystal material 24 and sealed by means of a spacer 25 disposed on the periphery of the plates. As shown in Figure 2, the other group of electrodes may comprise a single electrode 22.

Referring to Figure 1 , the multi-colour display is divided into a number of zones 11 each corresponding to an element of an original picture. Each zone comprises, a red colour filter layer, a green colour filter layer and a blue colour filter layer, for example layers 23R, 23G and 23B, to form a set of three primary colours. In use of the display device, a voltage is applied across the two groups of electrodes 22 to allow a multicolour image to be produced on the base plate 21 by virtue of controlled changes in the degree of transmittance or reflectance displayed by the liquid crystal material 24.

The quality of such a multi-colour display depends upon the structure and nature of the colour filter layers.

Thus, the colour filter layers should be thin enough, initially, to prevent having to increase the drive voltage applied to the device because of the thickness of the colour filter layers interposed between the electrodes. The layer thickness should also be small in comparison with the thickness or depth of the liquid crystal material.

On the basis of reported test results, the colour~ filter layers should preferably be of 0.6 ,um or less in thickness. Also, the thickness of the layers should not vary because any unevenness would result in a lack of uniformity of the drive voltage across the electrodes layers of different tints. In general, the variation in thickness should be kept below 1% of the liquid crystal layer thickness.

It is also desirable that the colour filter layers formed on different ones of the group of electrodes may be coloured with different tints and with sufficient accuracy if a distinct multicolour image of an intricate picture is to be produced.

Moreover, the colours of the filter layers should be pure and the tints thereof should be well balanced with one another for high fidelity reproduction.

A practical mass production system which operates at low cost is also desirable.

None of the previously proposed methods has succeeded in producing colour filter layers which satisfy all of the above requirements. Thus, for example, although screen printing of the colour filter layers may reduce the production costs it cannot provide thin filter layers and does not, moreover, produce the required evenness in the filter layers. In the previously proposed "multilayer membrane interference" method, the electrodes are vacuum metalized repeatedly with suitable metal oxides. However, the thus produced colour filter layers inevitably have a high thickness and this method is not practical because of the high production costs.

It is an object of the present invention to provide a method of forming a colour filter layer for a liquid crystal display device which overcomes or at least mitigates the disadvantages of previously proposed methods.

According to one aspect of the present invention, there is provide a method of forming a colour filter layer for a liquid crystal display device, the method comprising forming a dyeable layer upon an electrode provided on a base plate for a liquid crystal display device and dyeing the thus formed dyeable layer with a preselected dyestuff.

According to a second aspect of the present invention, there is provided a method of forming a colour filter layer for a liquid crystal display device, the method comprising forming a dyeable layer upon each electrode of a plurality of electrodes arranged spaced apart on a base plate for a liquid crystal display device and dyeing the thus formed dyeable layers with a preselected dyestuff.

Preferably, the step of dyeing the dyeable layers comprises dividing the dyeable layers into a number of groups and dyeing each group of dyeable layers with a preselected dyestuff of different colour and conveniently, the groups of dyeable layers are selected so that a repetitive pattern of differently coloured dyed layers is produced.

Desirably, the or each dyeable layer is hardened before dyeing and usuaily, the or each dyeable layer comprises an organic polymer or compound, the polymer or compound containing a quaternary ammonium salt and/or an amino group.

In a preferred embodiment the or each dyed layer is covered with an orientation member so that in use of a liquid crystal display device incorporating such a dyed layer, the orientation of liquid crystals appearing when a voltage is applied across the electrodes is improved and also the migration of dyestuff molecules into the liquid crystal material may be prevented particularly by use of a permanent protective membrane between the dyed layers and the orientation membrane.

For a better understanding of the present invention, and to show how the same may be put into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a horizontal cross-section through a multi-colour liquid crystal display, taken along line B-B in Figure 2.

Figure 2 is a vertical cross-section through the device of Figure 1, taken along the line A-A in Figure 1; Figures 3 to 9 illustrate respectively, in partial vertical section, the steps involved in a method embodying the invention; and Figure 10 is a graph of colour purity against colour balance for colour filter layers formed by the method illustrated in Figures 3 to 9.

Referring now to the drawings, Figures 3 to 9 illustrate a method of forming colour filter layers for a multicolour liquid crystal display device. As shown in Figure 3, an electroconductive layer of approximateiy 0.1 ,t4m in thickness is formed in a conventional manner on a glass base plate 21 by vacuum metallizing of a metal or metal oxide such as tin dioxide (SnO2) or indium oxide (In203). The layer is then etched at regular intervals by a photo-etching process to form a plurality of fine electrodes 22.

Then as shown in Figure 4, the surfaces of the electrodes 22 and the surface of the glass between the electrodes are covered with a dyeable agent 31 up to a thickness of 0.6 ym.

Any suitable apparatus such as a spinner may be utilized to apply the dyeable agent.

The dyeable agent comprises a film-forming substance and a photosensitive substance both dissolved in a suitable solvent such as water. The film-forming substance may be a water soluble protein, for example gelatin, or a resin, for example polyvinyl alcohol (hereinafter designated as "PVA"). The photosensitive substance is, for example, chosen from bichromates or diazocompounds.

After the dyeable layer 31 has dried, regions of the dyeable layer directly above the electrode surfaces are exposed to light (U) through an appropriate mask 32. The dyeable layer portions on the electrodes 22 are hardened or cured by exposure to the light (U), rendering them insoluble. The remaining uncured portions of the dyeable layer are then dissolved off from the glass base plate to produce the arrangement shown in Figure 5.

The abovementioned film-forming substance, for example a protein or PVA, is advantageous because it forms a thin and even layer which can easily be dyed to deep colours. However, where an even thinner layer as well a much better dyeability are needed, the dyeable agent 31 may contain an additive such as poly-dimethyl ally ammonium chloride, methyl glucose chitosamine or any other organic polymer (or compound) which contains one or more quaternary ammonium groups and/or amino groups.

The thus formed dyeable layers 33 are then covered with a lipophilic positive photoresist 34, as shown in Figure 6. After the photoresist has dried, any one 35 of previously prepared screen masks for each filter colours such as red, green and blue is selected to expose to the light (U) the dyeable layers that are to be dyed to produce that particular filter Thus, as shown in Figure 6, a red filter screen mask 35 is used to expose to the light (U) only those dyeable layers 33R which are to become red filter layers. Only the photoresist 34 above the layers 33R remains soluble because of the positiveness of the photoresist and this is then dissolved off by a development process so as to leave temporary protective layers 36 over the filter layers which are, for example, to be green and blue colour filter layers.

Next, the exposed dyeable layers 33R are dyed with a suitable red dyestuff solution to a desired depth of colour. Red filter layers are thus formed on the selected electrodes. Finally, the protective layers 36 are removed by use of a suitable solvent.

The steps illustrated in Figures 6 and 7 are then repeated first for a second colour (for example green) and then for a third colour (for example blue) in order to form, for example, green and blue filter layers on predetermined electrodes.

The dyestuffs are preferably purified, particuiarly to exclude usual additives ordinarily contained in the dyestuffs manufactured for textile fibre dyeing, before use to enable deeper dyeing of the layers 33 so that the layers 33 may be made much thinner.

The thus dyed layers 23 may be heated if any unfavourable swelling thereof is observed after dyeing. Such heating will compress the swelled layers making them thinner and stronger.

An orientation membrane 26 may be formed, if necessary, over the colour filter layers 33. The membrane 26 improves the orientation of the liquid crystal material 24 and is also effective in preventing dyestuff molecules from migrating into the liquid crystal material. Suitable substances for forming the orientation membrane are organic substances such as polyimide resins, polyacrylic resins, polyepoxy resins, PVA and organosilane resins and inorganic insulating substances such as SiO2. These substances may be applied to the colour filter layers 33 by means of the spinner or the vacuum metallizing method both of which are suitable for obtaining a membrane thickness of not greater than 0.1 ssm.

A rubbing or smoothing treatment is finally conducted on the surfaces of the filter layers or the orientation membrane, as is usual for liquid crystal display devices, and thereafter a sealant 25 is screen-printed onto the periphery of the base plates 21. The sealant 25 acts as a spacer means and seals the space between the plates which are to be bonded to each other Liquid crystal material 24 will then be introduced into the space, as shown in Figure 9. Figure 10 is a graph of colour purity against colour balance, illustrating the high colour purity and good colour balance of the filter layers produced in accordance with the above method.

An example of the use of the above method to produce particular colour filter layers for a liquid crystal display device will now be described.

Transparent electrodes 22 of In203 are formed on a base plate by one of the methods common in the manufacture of electrodes for liquid crystal crystal devices. A coating solution prepared by adding 1 part by weight of ammonium chromate to 30 parts by weight of a glue or adhesive solution whose viscosity is adjusted to 40 centipoise is applied to the electrodes using a spinner apparatus to form a dyeable layer. The coating solution further contains a small amount of methyl glucose chitosamine. The thickness of the layer of coating solution produced is controlled to be 1 ym. The layer is dried and then exposed to light via a screen mask 32 so that portions of the dyeable layer 31 disposed over the electrodes 22 are hardened or cured forming the dyeable layers 33 (Figure 5).The remaining uncured regions of the layer are dissolved off in a hot water bath at 500C.

Next, a lipophilic positive photoresist "OFPR" (a product of Tokyo Ouka Ltd.) 34 is coated onto the dyeable layers 33 using the spinner and dried (Figure 6). Layers 35R that are to be dyed red in colour are selectively exposed to light through a mask so that the photoresist over the layers 35R decomposes and can then be dissolved away by means of a developing solution leaving protective layers 36 covering the remaining layers (Figure 7).

The dyeable layers 35R are dyed using a red dye solution comprising 1.5 parts Kayakalan Orange RL (a dye produced by Ni hon Kayaku Ltd.) purified with methanol, 2.0 parts Aminyl Brilliant Red F4B (a dye produced by Sumitomo Kagaku Kogyo Ltd.), 0.2 parts Amylazin (a diazo-compound produced by Daiichi Kogyo Seiyaku Ltd.), 4 parts sodium chloride and 100 parts water. The dyeing is conducted at 50CC for 20 minutes by keeping the base plate immersed in the solution.

Thereafter the protective layers 36 are removed from the other dyeable layers 33 which are to be successively dyed green and blue.

The steps shown in Figures 6 and 7 are then repeated using first a green colour layer filter mask and then a blue colour layer filter mask.

The green dye solution comprises 1.0 parts Suminol Milling Yellow MR (a dye produced by Sumitomo Kagaku Kogyo Ltd.) purified with methanol, 0.125 parts Sandolan Brilliant blue N5GM (a dye produced by Sandoz Ltd.), 1.0 parts citric acid and 110 parts water. The dyeing is carried out at 600C for 20 minutes by keeping the base plate immersed in the solution.

The blue dyeing solution comprises 1.0 parts Sandolan Cyanine NG 360% (a dye produced by Sandoz Ltd.) and 100 parts water, and the dyeing is carried out at 50"C for 20 minutes again by the steeping method.

The dyeable layers 33 which initially have a thickness of 0.1 m sweil to have a larger thickness of approximately 0.3 m when all the dyeing processes have been finished. In order to reduce or eliminate this swelling, the layers are heated at 150 C for 20 minutes compressing the layers to about 0.2 ym in thickness.

An orientation membrane 26 is then formed over the thus prepared colour filter layers 23. A polyimide resin solution diluted with N-methyl-2pyrrolidone and Dimethyl Acetamide (DMAC) is utilized for this purpose and is applied to the surface of the filter layers using the spinner. The orientation membrane has a thickness of 0.1 m and is heated at 2000C for 20 minutes.

If the direct application of the polyimide solution is likely to cause dyestuff to migrate from the filter layers into the orientation membrane, a permanent protective membrane (not shown) may be placed between the orientation membrane and the filter layers. The permanent protective membrane may be formed, before formation of the orientation membrane, from an organic substance such as "Polydule" (produced by Mikuni Paint Ltd.) which is impermeable to the dyes.

Although in the arrangement described above the dyeable layers 33 and the orientation membrane 26, respectively, comprise an adhesive solution and a polyimide resin solution, other suitable materials may be used. Thus, either the layers 33 or the membrane 26 may be water soluble while the other may be oil soluble, or both the layers and the membrane may be water or oil soluble. When the layers 33 and the membrane 26 are water soluble, the solution from which the orientation membrane 26 is produced may be applied and dried rapidly so as to prevent any migration of dyestuff molecules which may otherwise take place with some combinations of materials. Also, a negative photoresist may be used in the forming of the aforementioned temporary protective layer in place of the above described positive photoresist and of course a method embodying the invention may be used where the substances used for, and the shapes of, the above-described base plates and electrodes vary according to the application required. Indeed, although the electrodes described above are in the form of vacuum metallized strips, a method in accordance with the present invention may also be used where each electrode is in the form of a thin film transistor.

In a method in accordance with the invention a colour filter layer may be made thin and can accurately be dyed with clear distinction between colours. The variation in thickness across the layers is less than +0.04 um. The colour purity as well as the colour balance are also satisfactory despite the simple production process which enables mass production with low production costs.

Claims (12)

Claims

1. A method of forming a colour filter layer for a liquid crystal display device, the method comprising forming a dyeable layer upon an electrode provided on a base plate for a liquid crystal display device and dyeing the thus formed dyeable layer with a preselected dyestuff.

2. A method of forming a colour filter layer for a liquid crystal display device, the method comprising forming a dyeable layer upon each electrode of a plurality of electrodes arranged spaced apart on a base plate for a liquid crystal display device and dyeing the thus formed dyeable layers with a preselected dyestuff.

3. A method according to claim 2, wherein the step of dyeing the dyeable layers comprises dividing the dyeable layers into a number of groups and dyeing each group of dyeable layers with a preselected dyestuff of different colour.

4. A method according to claim 3, wherein the groups of dyeable layers are selected so that a repetitive pattern of differently coloured dyed layers is produced.

5. A method according to claim 1, 2, 3 or 4, wherein the or each dyeable layer is hardened before dyeing.

6. A method according to claim 1, 2, 3, 4 or 5, wherein the or each dyeable layer comprises an organic polymer or compound, the polymer or compound containing a quaternary ammonium salt and/or an amino group.

7. A method according to any preceding claim, including covering the or each dyed layer with an orientation membrane.

8. A method according to claim 7, including covering the dyed layers with a permanent protective membrane prior to the orientation membrane.

9. A method of forming a colour filter layer for a liquid crystal display device substantially as hereinbefore described with reference to Figures 3 to 10 of the accompanying drawings.

10. A colour filter layer for a liquid crystal display device whenever produced in accordance with any one of claims 1 to 9.

11. A liquid crystal display device whenever incorporating a colour filter layer in accordance with claim 10.

12. Any novel feature or combination of features described herein.