COLOR FILTER FOR LIQUID CRYSTAL DISPLAYS

Abstract: A color filter for a liquid crystal display includes an optically transparent support (100), a transparent conductor layer (500) formed on the support (100) and a coloring layer (200) formed on the transparent conductor layer (500) having red, green and blue pixels disposed in a predetermined pattern, wherein this coloring layer (200) has a specific dielectric constant of at least 2.0 under a measurement condition in which an A.C. voltage of 5 V and a frequency of 1 KHz to 100 KHz is applied, and each pixel of red, green and blue constituting the coloring layer has a specific dielectric constant within the range of ± 25 % of the mean value under the measurement condition described above. The coloring layer (200) is 0.4 to 1.0 µm thick. This color filter can provide a liquid crystal display wherein the electro-optical characteristics of the pixels substantially coincide with one another, a sufficient effective voltage can be applied, though it uses an electrode under-fitting structure (COI structure), and color regulation can be satisfactorily made.

Description: Field of Technology This invention relates to colour filters employed in colour liquid crystal display apparatuses. Background Art A known method of colouring liquid crystal display devices is the employment of a display in which a colour filter layer (colour layer) is disposed within a cell which has a pair of transparent electrodes disposed in parallel. The known general compositions of colour filters are of the types illustrated in Figure 4 and Figure 5, for example. The colour filter illustrated in Figure 4 includes a colour layer 200 upon a supporting member 100 which is composed of a transparent glass substrate, a film or the like. The colour layer 200 consists of picture elements of three colours, red (R), green (G), and blue (B), 200a, 200b, and 200c, which form a specified pattern (such as mosaic type, triangular type, striped type, etc.) that corresponds to the area of the driven picture elements. A black matrix layer 300 is formed between the picture elements 200a, 200b, and 200c. Moreover, a protective layer 400 consisting of transparent resin or the like is formed upon the colour layer 200, while a transparent conductive layer 500 consisting of for example indium tin oxide (ITO) is formed upon the protective layer 400. Colour filters formed in such manner are commonly combined with substrates possessing thin film transistor (TFT) elements in order to form liquid crystal displays for colour displays. Figure 5 illustrates the structure of a colour filter that is employed in a liquid crystal display apparatus that employs the super-twisted nematic (STN) mode or alternatively the driver mode with metal-insulator-metal (MIM) elements. Such colour filter has a similar structure to the aforementioned colour filter, in a structure of a supporting member 100, a colour layer 200, a black matrix layer 300, and a protective layer 400. The transparent conductive layer (ITO layer) 500 possesses a pattern which corresponds to the picture elements 200a, 200b, and 200c which form the colour layer 200. As the embodiments of colour filters described in the foregoing are formed of transparent conductive layers 500 upon colour layers 200, such embodiments are known as upper-ITO mode or alternatively ITO on Colour Filter (IOC) mode. It is rather difficult to manufacture at low cost large-sized colour filters that employ such IOC mode, and more particularly the type of colour filter illustrated in Figure 5, and which exceed 10 inches diagonally. This is because of problems with the formation of the patterns in the ITO layer which is formed upon the uppermost layer. The ITO layer is normally etched with a composition consisting of a mixture of hydrochloric acid, nitric acid, and pure water at a composition temperature of approximately from 40°C to 60°C, whereupon resist stripping is performed at a composition temperature of approximately from 40°C to 60°C by employing a stripping agent consisting of potassium hydroxide, an organic alkaline composition or the like in order to perform patterning. For this purpose, the protective layer 400 that is formed upon the colour layer 200 must have sufficient resistance to acidity and resistance to alkalinity. Moreover, the material of which the aforementioned protective layer 400 is formed must have a transmittance of at least 95% at all wavelengths and must be resistant to heat of at least 230°C. The protective layer 400 is required to have high resistance to heat because the resistivity of the ITO layer 500 must be decreased as the display is made larger, such that a high temperature of at least approximately 230°C is required in order to form the film for the ITO layer. Thus the material that is employed in order to form the protective layer 400 is required to have excellent resistance to acidity, resistance to alkalinity, transparency, and resistance to heat, such that the selection of organic materials which meet such requirements is restricted. Furthermore, IOC type colour filters are formed by the formation of an inorganic ITO layer 500 upon the protective layer 400 which is formed of organic polymers, and hence there is a high probability of the occurrence of electrical disconnection from the ITO layer caused by the discrepancies in the coefficients of linear thermal expansion between the two layers, to cause the problem of reduced yields. In order to resolve such problems of IOC type colour filters, the lower-ITO or colour filter on ITO (COI) mode has been developed wherein an ITO layer is formed upon the surface of the supporting member, and furthermore a colour layer is formed upon the ITO layer. With such COI colour filters there is no need for action to overcome the problem of patterning of the ITO layer as in the method of manufacture of IOC colour filters as described in the foregoing, and hence the range of materials from which the protective material may be selected is expanded, and moreover such mode has the advantage that it permits the prevention of the occurrence of electrical disconnection of the ITO layer caused by the discrepancies in the coefficients of linear thermal expansion between the ITO layer and the protective layer. However, a problem of the COI colour filter is that the colour layer acts as an insulating film. Thus the electrical capacitance of the colour layers that form the colour filter is of significant magnitude in comparison with the electrical capacitance of the liquid crystal layer, such that the voltage between the electrodes is divided between the liquid crystal layer and the colour layer. As a result of this, sufficient effective voltage is not applied to the liquid crystal layer to cause decrease in contrast, increase in the threshold voltage of the device, or loss of sharpness in rising edge of the transmittance-voltage characteristic curve. JP-A-2-221402 discloses a color filter according to the pre-characterizing portion of claim 1. In this prior art, the ratio of the thickness and the dielectric constant is disclosed to be equal for each of a plurality of color filter layers. In this context the document teaches a thickness of 1.3 µm for each of three filter layers of red, green and blue. JP-A-61-002131 discloses a color filter layer provided on transparent electrodes of a liquid crystal panel. The color filter layer has a substance with a high dielectric constant dispersed therein. EP-A-0 407 947 discloses a color filter employed for a liquid crystal panel. The color filter comprises a pigment film with a thickness in the range of from 0.7 to 1.7 µm. EP-A-0 340 968 discloses a color filter formed by a colored layer solely consisting of an organic pigment and obtained by a wet process. The colored layer is said to have good color purity even if the film thickness is 1 µm or less. The objective of the present invention is to provide colour filters having high capacitance colour layers which are capable of supplying an effective voltage sufficient to provide high contrast while providing sufficiently thin films for COI mode colour filters. Disclosure of the Invention This objective is achieved with a color filter as claimed in claim 1. Preferred embodiments of the invention are subject-matter of the dependent claims. Colour layers The inventors have confirmed that liquid crystal display apparatuses which employ COI colour filters provide satisfactory white balance in the colour filters, but nevertheless the colour adjustment for displays which employ such colour filters is not satisfactory. Upon investigation of the causes of this, the inventors concluded that, in order to obtain good colour characteristics in displays which employ COI colour filters, it is not sufficient simply to adjust the white balance of the colour filter, but that the capacitance of the red (R), green (G) and blue (B) picture elements must be adjusted also. Moreover the inventors concluded that when pigments are employed as the colouring agents, the type of pigment selected greatly influences the relative dielectric constant of the colour layer. For this reason, the present invention is characterised by the selection of relative dielectric constant of pigments within specified ranges in order to adjust the capacitances of the R, G and B picture elements, so that the relative dielectric constant of the colour layer which include the pigments are equal. The appropriate relative dielectric constant will be described in more detail subsequently on the basis of data to be presented, but when a 5 V, 1 kHz to 100 kHz alternating current is applied, the dielectric constant of the colour layer should be at least 2.0, and moreover the R, G and B colour picture elements which form the colour layer must have relative dielectric constant of within a range of ±25% of the aforementioned mean value for relative dielectric constant. Moreover, the relative dielectric constant of the R, G and B colour picture elements should preferably be as similar as possible, and each of the picture elements should preferably have relative dielectric constant of within a range of ±20% of the aforementioned mean value for relative dielectric constant. By making the relative dielectric constant of each of the picture elements similar, the thicknesses of each of the picture element films are also made uniform. Differences in height between the picture elements of the colour layer cause such problems as incorrect alignments in an alignment layer formed thereon and incorrect cell gaps. Moreover, when the relative dielectric constant of the colour layer is specified at specific frequencies, when a voltage of 5 V and a frequency of 1 kHz is applied, the relative dielectric constant of each of the R, G, and B picture elements should preferably be within the range of 3.5 ± 0.5, and moreover at a voltage of 5 V and at a frequency of 100 kHz the relative dielectric constant of each of the R, G, and B picture elements should preferably be within the range of 3.0 ± 0.5. Moreover it is of great importance that the film that forms the aforementioned colour layer in the colour filter for liquid crystal displays in the present invention be very thin, having a thickness of from 0.4 µm to 1.0 µm, and preferably a thickness of from 0.6 µm to 0.9 µm. By the formation of a very thin film colour layer having a thickness of 1 µm or less, the capacity division of the colour layer which functions as an insulating film is minimised such as to permit a sufficient effective voltage to be applied to the liquid crystal layer. In this manner it is possible to provide a high capacitance thin film colour filter with which it is possible to achieve a contrast of at least 50:1 in TFT and MIM driven liquid crystal apparatuses, and it is possible to achieve a contrast of at least 20:1 in STN type liquid crystal apparatuses. Moreover, the difference in the thickness of the R, G, and B colour cells should preferably be as small as possible as described in the foregoing, and for example should preferably be not more than approximately 0.25 µm. The colour filters for liquid crystal displays of the present invention contain from 40 percent by weight to 50 percent by weight of pigment in the colour layers. The colour filters for liquid crystal displays of the present invention are formed of very thin films having thicknesses of from 0.4 µm to 1.0 µm as described in the foregoing. In order to achieve satisfactory colouration characteristics in such a thin colour layer, the colour layer must contain a rather higher than normal proportion of pigment. Thus colour layers containing pigment of the prior art normally contain from 20 percent by weight to 30 percent by weight of pigment and such colour layer films are approximately from 1.7 µm to 2.0 µm in thickness, whereas the colour layer films of the present invention are less than 1.0 µm in thickness and the levels of pigment are quite high, being from 40 percent by weight to 50 percent by weight which is approximately double the levels used in the prior art. Pigments are employed as the colouring agent in the present invention because they provide improved resistance to light, and more especially to ultraviolet light, and because they provide improved resistance to heat. If dyestuffs are employed as the colouring agent, dyestuffs are relatively more chemically active than pigments and hence are degraded by ultraviolet light and the like, and therefore do not possess adequate resistance to light and resistance to heat. Pigments The following is a description of the pigments that are employed in the present invention. It is a characteristic of the present invention that the relative dielectric constant of the R, G and B picture elements of the colour layer of the present invention should be uniform, but this is not readily achieved. It is known that the relative dielectric constant of the picture element is more greatly influenced by the relative dielectric constant of the pigment itself than by the relative dielectric constant of the components such as the polymers, copolymers, crosslinking agents, crosslinking initiators contained together with the pigment. However, in practice it is very difficult to select pigments having completely identical relative dielectric constant because the R, G and B pigments have different chemical compositions and different electrical properties. This is because there are at present relatively few types of pigment that are employed in such colour filters. A further factor which restricts the selection of pigments is the problem of the depolarising effect possessed by pigments. The depolarizing effect is : as pigments consist of relatively larger molecules than dyestuffs, light projected onto pigment molecules is polarised by the elliptical polarisation effect, and when the colour filter is inserted between two polarising plates, from several percent to several tens of percent of the projected light vanishes in the colour layer. The extent of such depolarisation effect varies from pigment to pigment, but in practice there are very few pigments with low levels of the depolarisation effect. For the purposes of the present invention, many pigments were examined for their relative dielectric constant and for their depolarisation effects, and the plurality of pigments described in the following were successfully selected. The most difficult of the pigments to select was the green pigment. The green pigments have higher relative dielectric constant than the red pigments or the blue pigments such that the selection of the green pigments presented great difficulty. The green pigments are described first. The preferred green pigment is a partially brominated phthalocyanine green which is prepared by the introduction of from one to thirteen bromine atoms into a copper phthalocyanine. Moreover the aforementioned partially brominated phthalocyanine green may preferably be a brominated chlorinated phthalocyanine green into which chlorine atoms have been introduced in addition to the bromine atoms. The brominated chlorinated phthalocyanine green 6Y (C.I. Pigment Green 36) described by the following formula (I) is the most preferable embodiment of such brominated chlorinated phthalocyanine green: The phthalocyanine green (C.I. Pigment Green 7, C.I. 74260) described by the following formula (II) which consists of a copper phthalocyanine into which 15 chlorine atoms have been introduced is also a preferred embodiment in addition to the aforementioned partially brominated phthalocyanine green. Such pigments are characterised by the provision of green colours with good transparency. However, chlorinated phthalocyanine greens are known to have greater dielectric constants than the relative dielectric constant of the red and blue pigments. In general, anthraquinone type pigments are used for the red pigment, and phthalocyanine blue type pigments are used for the blue pigments. Small amounts of yellow pigment are often added to the red pigment and small amounts of violet pigment are often added to the blue pigment, but the amounts so added are small, and the effects of the addition of such pigments on the relative dielectric constant of the pigment as a whole are small. It has been confirmed experimentally that when such red or blue pigments are employed and when a 5 V AC, 1 kHz current is applied, the relative dielectric constant of such pigments is approximately between 3 and 4, while the relative dielectric constant of the aforementioned chlorinated phthalocyanine green is approximately double that of such pigments, being between 6 and 8. Consequently, when the aforementioned phthalocyanine green is employed as a green pigment, the capacitance of each of the picture elements can only be made uniform if a green colour film approximately twice as thick as the films of the red or blue colour layer is formed. On the other hand, if the aforementioned brominated chlorinated phthalocyanine green 6Y (C.I. Pigment Green 36) is employed, it has been confirmed that the relative dielectric constant of such film is approximately within the range of from 3.0 to 4.0 at a frequency of around 1 kHz. Consequently, the use of such partially brominated phthalocyanine green 6Y, makes possible uniform relative dielectric constant for the red, blue and green picture elements which form the colour layer. Even though the partially brominated phthalocyanine green 6Y has a low relative dielectric constant, it has slightly reduced transmittance. And hence a mixture of the partially brominated phthalocyanine green 6Y with chlorinated phthalocyanine green (C.I. Pigment Green 7), which has a high transparency, is preferred. When both partially brominated phthalocyanine green 6Y and chlorinated phthalocyanine green (C.I. Pigment Green 7) are mixed together, the relative dielectric constant can be reduced to within the range envisaged by the present invention without compromising on the transparency of the picture elements. Such chlorinated phthalocyanine green should preferably be mixed together with the partially brominated phthalocyanine green 6Y in a ratio of from 1:1 to 1:2. Moreover a yellow pigment having a low relative dielectric constant may be mixed with the chlorinated phthalocyanine green (Pigment Green 7 C.I. 74260) in place of the partially brominated phthalocyanine green to form the green pigment. In such case, the yellow pigment selected should have a relative dielectric constant of from 2.0 to 3.0 under for example 5 V and 1 kHz. When the yellow pigment is mixed with the chlorinated phthalocyanine green, the ratio of blending should preferably be adjusted to within a CIE standard cholorimetric system when preparing a film of from 0.8 µm to 1.0 µm thick, in order to adjust the relative dielectric constant of the green picture elements to between 3.0 and 4.0. Yellow pigments generally have a high depolarisation effect such that large amounts of yellow pigments should preferably not be added. Hence disazo yellow HR (Pigment Yellow 83 C.I. 21108) which has a low depolarisation effect should preferably be employed as the yellow pigment. The following is a description of exemplifications of the red and blue pigments. It is necessary to employ red and blue pigments that have low depolarisation effects. There are few such pigments, however such red pigments include for example PV Red HF 2B (Pigment Red 208, C.I. 12514) and disazo yellow HR (Pigment Yellow 83, C.I. 21 108), and such pigments should preferably be mixed together. Blue pigments include for example phthalocyanine blue R (Pigment Blue 15:1 and 15:2, C.I. 74160 and C.I. 74250) or alternatively phthalocyanine blue G (Pigment Blue 15:3 and 15:4, C.I. 74160), and dioxazine violet (Pigment Violet 23 C.I. 51319), and such pigments should preferably be mixed together. Moreover, for blue pigments, one or a plurality selected from phthalocyanine blue R and phthalocyanine blue G should preferably be mixed together with dioxazine violet. The pigments described in the foregoing may preferably be employed as the pigments for the present invention for reasons of their relative dielectric constant and for their depolarisation effects. Black matrix layer The following is a description of the black matrix layer that is formed between the red, green and blue picture elements. In COI colour filters, a black matrix layer is formed between the transparent conductive layer and the liquid crystal layer, and such black matrix layer functions as an insulating layer in the same manner as the aforementioned colour layer and is one of the factors that determine the contrast of the liquid crystal display. For this reason it is important to regulate the relative dielectric constant, the resistivity, and the thickness of the film in order to control the drop in the effective voltage in the same manner as for the aforementioned colour layer. First, the resistivity is examined. The black matrix layer must have a certain degree of conductivity in order to reduce the voltage drop in the black matrix layer and in order to improve the relay characteristics for the voltage to the liquid crystal layer, such that it is necessary to specify an upper limit for resistivity. However, if the resistivity of the black matrix layer is too low, two problems arise, namely, first, short circuit problems arise between adjoining transparent electrodes in the colour filter which forms the patterns for the transparent conductive layer, and secondly, short circuit problems may arise between the opposing electrodes which enclose the liquid crystal layer. More particularly, if the thickness of the black matrix layer film is relatively greater than the thickness of the colour layer film, the gap between the black matrix layer and the wiring electrodes in the opposed element substrate may be narrowed which may readily lead to short circuit problems. For this reason it is necessary to specify a lower limit for the resistivity of the black matrix layer. The research performed by the inventors of the present invention has shown that the optimum range for the resistivity of the black matrix layer is from 1.0 x 10 2 Ω·m to 1.2 x 10 6 Ω·m under an alternating current of 5 V and with a frequency of between 100 Hz and 100 kHz. In order to obtain a resistivity within the range specified in the foregoing, the conductivity of the black matrix layer must be controlled by the addition of carbon as well as such colored pigments as the red, green, blue which form the black pigment. For this purpose, from 10 percent by weight to 20 percent by weight to the total solids component of the pigment resist should be added in carbon. Carbon has been added as a black pigment to the black matrix layer in the prior art, but such carbon has been added to the black matrix layer essentially in order to improve the shading property of the black matrix layer. Normally, 30 percent by weight of the material of the black matrix layer in terms of carbon has been added in order to achieve such objective. In practice, if any greater amount of carbon is added, pattern film residue is caused at development, and problems are caused with the adhesion of the black matrix layer to the supporting member. Therefore, carbon is added as much amount as possible in order to improve shading properly without causing problems in patterning to form black matrix resist in general use these days. However the addition of carbon as in the prior art is not intended to improve the electrical properties, and more particularly the electrical conductivity of the black matrix layer, as is envisaged by the present invention. The electrical conductivity of the black matrix layer is greatly influenced by the amount of carbon that is added, but it has also been confirmed that the firing temperature also affects the electrical conductivity of the black matrix layer. For this reason, the material that forms the black matrix layer should be fired at a temperature of from 180°C to 250°C. The relative dielectric constant of the black matrix layer should preferably be at least 3.0, for the same reasons as stated in the foregoing for the colour layer, namely in order to prevent capacity division due to the black matrix layer. Moreover, the thickness of the black matrix layer should preferably differ by not more than 0.5 µm from the thickness of the films of the red, green and blue picture elements of the colour layer. By the specification in this manner of the range of relative dielectric constant of the colour layer and the range of relative dielectric constant of the black matrix layer, it is possible to achieve a liquid crystal display in which decline of effective voltage is restrained, and which provides good write characteristics with adequate contrast. The following is a description of the colour filter which is characterised by the form of the black matrix layer. In a COI colour filter wherein a colour layer is formed upon a transparent conductive layer, as distinct from an IOC colour filter wherein a transparent conductive layer is formed upon a colour layer, no resin layer referred to as a protective layer or a leveling layer is formed in order to prevent an interposition of an insulating layer between the liquid crystal layer and the transparent conductive layer as far as possible. For this reason, the colour layer is either disposed in direct contact with the alignment layer, or a thin film of several hundred Angstrom in thickness is formed between the alignment layer and the colour layer in order to improve the film-forming properties of the alignment layer. As a result, the irregularities of the surfaces of the colour layer and the black matrix layer opposite to the liquid crystal layer are allowed to remain in COI colour filters. The presence of such irregularities prevents the rubbing medium from reaching at spots where height changes in the irregular regions thereby prohibiting good alignment treatment, while foreign matter and dirt generated by such rubbing accumulates at the spots where height changes in the irregular regions such as to degrade the alignment, while such differing heights of the irregularities prevent uniform alignment, and so forth. More particularly, when the resist that contains high concentrations of pigment as described in the present invention is exposed to light, photoreactions occur in a layer to be exposed to light preferentially at the side closest to the photomask as illustrated in Figure 15A (upper portion of the layer exposed to light), and such portion is sufficiently exposed to the light, but the side more distant from the photomask in the layer to be exposed to light (lower portion of the layer exposed to light) is reached by light to much less extent, such that the hardening reaction of the resist in the direction of the thickness of the layer exposed to light becomes uneven. As a consequence of this, what is known as the reverse taper phenomenon occurs, wherein the lower portion of layer exposed to light is excessively removed with reference to the upper portion when the layer is developed. This phenomenon is particularly prominent when black-coloured resist is employed in the formation of the black matrix layer. As a result of this, the problems of poor alignment described in the foregoing are exacerbated. For example, as illustrated in Figure 15B, when rubbing treatment is applied in the direction of the arrow X, incorrect alignment is caused in the area A indicated by hatching in the picture elements 200a, 200b and 200c. Variations in the voltage that is applied to the liquid crystals in a location at which incorrect alignment has occurred result in the molecules of liquid crystal that are arrayed at such parts in which incorrect alignment has occurred to exhibit different behaviour from that of those molecules of liquid crystals that are arrayed at such parts with correct alignments, such that the quality of the display is degraded. Thus when voltage is applied in normally white mode and when voltage is not applied in normally black mode, the light passes through the panel only at those locations at which incorrect alignment has occurred, which leads to a decrease in contrast. Moreover, for the long run, the molecules of liquid crystals which are correctly aligned by the rubbing treatment may be influenced gradually by the molecules of liquid crystals in the locations at which poor alignment has occurred, such that the contrast is gradually decreased to degrade the long term reliability of the panel. In order to overcome such problems, in the colour filter for liquid crystal displays of the present invention, the incorrect alignments caused by the irregularities that are formed in the colour layer and in the black matrix layer are suppressed by causing the side surfaces which form the black matrix layer (projecting layer) which projects above the colour layer to slant inwards such as to present a tapered appearance. The tapered surfaces which form the side surfaces should preferably be formed at an angle of between 30° and 89° , and more preferably such as to form an angle of between 45° and 70° , with reference to the supporting member. Such tapering of the projecting layer of the black matrix layer may be formed by for example exposure to light with the setting of a light exposure gap of from 200 µm to several millimetres between the layer that is exposed to the light and the photomask. By means of the formation of tapered surfaces in the projecting layers of the black matrix layer, incorrect alignment of the portions exhibiting differing heights in alignment treatment can be largely suppressed. Brief Description of the Drawings Figure 1 is a cross-sectional view which illustrates schematically the COI colour filter of the first embodiment of the present invention. Figure 2 is a cross-sectional view which illustrates a further example of the constitution of the colour filter of the first embodiment of the present invention. Figure 3 is a cross-sectional view which illustrates a further example of the constitution of the colour filter of the first embodiment of the present invention. Figure 4 is a cross-sectional view which illustrates schematically the constitution of an IOC colour filter. Figure 5 is a cross-sectional view which illustrates schematically a further constitution of an IOC colour filter. Figure 6 is a diagram which illustrates the transmittance - drive voltage curve of the colour filter of the first embodiment of the present invention. Figure 7 is a diagram which illustrates the transmittance - drive voltage curve of a comparative colour filter. Figure 8 is a cross-sectional view which illustrates the constitution of the sample employed to determine the relationship between the relative dielectric constant and the frequency. Figure 9 is a diagram which illustrates the relationship between the relative dielectric constant of colour filters constituted with the employment of different pigments and the frequency. Figure 10 is a diagram which illustrates the relationship between the relative dielectric constant of the black matrix layer with the employment of different pigments and the frequency. Figure 11 is a diagram which illustrates the relationship between the resistivity for the pigments employed in Figure 10 and the frequency. Figure 12 is a cross-sectional view which illustrates schematically the colour filter of the second embodiment of the present invention. Figure 13 is a cross-sectional view which illustrates a variant form of the second embodiment of the present invention. Figure 14 is a cross-sectional view which illustrates a further variant form of the second embodiment of the present invention. Figure 15A is a view which illustrates the tapered shape of the colour layer and the black matrix layer. Figure 15B is an explanatory diagram which illustrates the area of poor alignment when the colour filter illustrated in Figure 15A is subjected to the rubbing treatment. Best Mode for Carrying Out the Invention First Embodiment Figure 1 is a cross-sectional view which illustrates schematically the colour filter of the present invention. Such colour filter 1000 consists of a transparent supporting member 100, a transparent conductive layer 500 which is formed upon the supporting member 100, a colour layer 200 which is formed upon the transparent conductive layer 500, a black matrix layer 300, and a protective layer 400 which covers the peripheries of the colour layer 200 and the black matrix layer 300. The supporting member 100 is formed of a transparent glass substrate (for example, 7059 Glass manufactured by the Corning Company). The colour layer 200 consists of red (R) picture elements 200A, green (G) picture elements 200B and blue (B) picture elements 200C in specified patterns such as mosaic type, triangular type, and striped type. The picture elements 200A, 200B, and 200C are between 0.4 µm and 1.0 µm in thickness, and preferably between 0.6 µm and 0.9 µm in thickness. If the film is less than 0.4 µm thick, it is impossible to achieve satisfactory colour properties, while if the film is more than 1.0 µm thick, the resistance of the colour layer prevents the achievement of a sufficient effective voltage. Moreover, the colour layer 200 has a relative dielectric constant of at least 2.0 and preferably between 3.0 and 5.0 when a 5 V alternating current of a frequency of between 1 kHz and 100 kHz is applied. Moreover, the relative dielectric constant of the picture elements 200A, 200B and 200C under the aforementioned conditions are within a range of ±25% from the mean value for the three picture elements, and preferably within a range of ±20% from the mean value for the three picture elements. The reason for such requirement will be described in greater detail subsequently, however substantial differences in the relative dielectric constant of each of picture elements causes great variation in the capacitance of the picture elements, which prevents good colour adjustment. Moreover, if the relative dielectric constant of the colour layer 200 is less than 2.0, the electrical capacitance of the colour layer becomes very large such that capacity division occurs in the electric field between the picture element electrodes of the substrate on which the picture elements are formed and the picture element electrodes of the colour filter such that the effective voltage in the liquid crystal layer is reduced. This causes a decline in the contrast ratio and causes poorer image quality. The black matrix layer 300 includes red, green and blue pigments and carbon as colouring agents. When a 5 V alternating current at a frequency of between 100 Hz and 100 kHz is applied to the black matrix layer 300, the resistivity of the black matrix layer 300 is between 1.0 x 10 2 Ω·m and 1.2 x 10 6 Ω·m and the relative dielectric constant of the black matrix layer 300 when an abovementioned alternating current is applied is at least 3.0. If the resistivity of the black matrix layer 300 is less than 1.0 x 10 2 Ω·m, the electrical conductivity of the black matrix layer is increased excessively, and short circuits may occur with adjacent transparent electrodes, or short circuits may occur with other opposed electrodes which sandwich the liquid crystal layer which is not illustrated. Moreover, if the resistivity of the black matrix layer 300 exceeds 1.2 x 10 6 Ω·m, excessive voltage drop occurs in the black matrix layer and the addressing characteristics are degraded. More particularly, if the black matrix layer is thicker than the colour layer 200, the gap between the black matrix layer and the opposing wiring electrodes of the picture element substrate becomes narrow, which may readily lead to short circuits. Moreover, in order to achieve the range of resistivity specified in the foregoing, the proportion of carbon in the black matrix layer 300 should be between 10 percent by weight to 20 percent by weight with reference to the black matrix layer 300, and preferably between 10 percent by weight and 15 percent by weight with reference to the black matrix layer 300. Moreover, the black matrix layer 300 should preferably be between 0.5 µm and 1.5 µm thick, and more particularly, the difference in thickness between the black matrix layer 300 and the picture elements 200A, 200B and 200C which form the colour layer 200 should preferably not exceed 0.5 µm. The protective layer 400 is generally formed of a transparent resin such as acrylic resin, and is approximately between 70 nm (700 Å) and 100 nm (1000 Å) thick. The following describes the method of manufacture of the colour filter 1000 which is illustrated in Figure 1. A layer of indium tin oxide (ITO) of approximately between 50 nm (500 Å) and 300 nm (3000 Å) in thickness is formed by employing the sputtering method upon the glass supporting member 100. Next, the ITO layer is patterned by normal photolithography, and the transparent conductive layer 500 is formed. Next, a colour layer 200 is formed by employing photoresist containing dispersed pigments by the normal photolithographic process and by development technology. The red, green and blue colour elements which form the colour layer 200 may be formed in any sequence whatsoever, but the following example describes a method of manufacture in which the picture elements are formed in the sequence red, green and blue. First, resist containing red pigment is applied to the supporting member 100 and to the transparent conductive layer 500 by means of the spin coat method, or the roll coat method, or an alternative film-forming apparatus suitable for the present invention. In this case, the red pigments described in the foregoing may be employed as the red pigment, and more specifically, disazo pigments (for example, Chromophthal Red DRN manufactured by CIBA-Geigy), azo lake pigments (for example, Lake Red C manufactured by Dainichi Seika), or pyrazolone pigments. Once the resist has been applied, the material is prebaked for between 10 minutes and 20 minutes at a temperature of between 60°C and 70°C, and is then exposed to light through the photomask. The material is exposed to approximately between 100 mJ and 200 mJ of light. Next, the material is developed by showering or by dipping in an alkaline composition. After development, the resist layer is baked for approximately between 30 minutes and 60 minutes at a temperature of approximately between 150°C and 200°C with a clean oven, a hot plate or the like. The green pigment that is employed to form the green picture element 200B may be a green pigment described in the foregoing, and more specifically phthalocyanine green to which bromine atoms has been introduced (Pigment Green 36, C.I. 74265) or the like may appropriately be employed. The coating process, prebake process and the developing process are similar to those employed for the aforementioned red. The green picture elements are exposed to approximately between 250 mJ and 500 mJ of light in the exposure process. Blue pigments such as copper phthalocyanine pigments and threne pigments (for example, Chromophthal Blue A3R manufactured by CIBA-Geigy) may be employed for the formation of the blue picture element 200C. The blue picture elements are exposed to approximately between 70 mJ and 150 mJ of light in the exposure process. The other manufacturing processes for the blue picture element 200C are similar to those for the red picture element and the green picture element. Such red, green and blue pigment resists contain from approximately 15 percent by weight to 20 percent by weight of solids with reference to the total volume of resist composition, and preferably between 11 percent by weight and 12 percent by weight of solids with reference to the total volume of resist composition, and such pigments should be comprised preferably from 40 percent by weight to 50 percent by weight of the solids. The following describes the formation of the black matrix layer 300. The resist that is employed to form the black matrix layer contains from 30 percent by weight to 60 percent by weight of red, green and blue pigments, and from 10 percent by weight to 20 percent by weight of carbon black, with reference to the total solids in the resist composition. The black matrix layer 300 may be formed before the colour layer 200 is formed. Next, a protective layer 400 is formed by the application by for example the spin coating method of a heat curing acrylic resin, which is then baked for approximately 30 minutes at a temperature of 180°C. Next, a heat curing resin consisting principally of a commercially available polyamic acid type PI is applied upon the protective layer 400 by employing the flexographic method, whereupon the material is baked for from one hour to two hours at a temperature of approximately 190°C in order to form the alignment film which is not illustrated. Next, the results of the measurement of the electro-optical properties of the colour filter illustrated in Figure 1 are described. Experiment 1 First, the correlations between the transmittance and the drive voltages for each of the picture elements 200A, 200B and 200C which form the colour layer 200 in the colour filter 1000 are examined. The results are shown in Figure 6. The transmittance - drive voltage curves were produced by the measurement of the variations in the transmittance when an LC meter was employed in order to vary the voltage with a liquid crystal panel which employed the colour filter illustrated in Figure 1 prepared. Table 1 shows the pigments, relative dielectric constant, contents of pigment, and the thickness of the colour layer employed for the samples. The relative dielectric constant shown in Table 1 are values for a voltage of 5 V and a frequency of 10 kHz. The content of pigment is shown as a proportion of the total solids in the resist compositions (percent by weight). As can be seen from Figure 6, the red, green and blue transmittance - drive voltage curves for the liquid crystal apparatus which employed the colour filter of the present invention are nearly identical, and thus it is known that each of the picture elements has almost identical electro-optical properties. Thus good colour characteristics can be obtained because the same transmittance is obtained at a given drive voltage. In the present case, satisfactory colour characteristics were defined as the range that satisfied x ≥ 0.60 and y ≤ 0.35 for red, 0.33 ≥ x ≥ 0.29 and y ≥ 0.58 for green, and x ≤ 0.15 and y ≤ 0.14 for blue, when a CIE standard cholorimetric system light source C was employed. Sample Type of pigment Relative dielectric constant Content of pigment (wt%) Thickness of film (µm) a (Red) Pigment Red 208 (C.I.12514) Pigment Yellow 83 (C.I.21108) 3.0 45 0.7 b (Blue) Pigment Blue 15:1 (C.I.74160) Pigment Violet 23 (C.I.51319) 3.5 45 0.6 c (Green1) Pigment Green 36 (C.I.74265) Pigment Yellow 83 (C.I.21108) 3.2 45 0.85 d (Green2) Pigment Green 7 (C.I.74260) Pigment Yellow 83 (C.I.21 108) 4.8 45 0.85 e (Green3) Pigment Green 7 (C.I.74260) Pigment Green 36 (C.I.74265) Pigment Yellow 83 (C.I.21108) 4.2 45 0.85 The colour filter of the present invention provides such good electro-optical properties because the colour picture elements, of which the colour layer is formed, have uniform relative dielectric constant. Comparative Experiment 1 Next, the same measurements as were performed for Experiment 1 were performed when the colour layer exceeded the range of ±25% of the mean value for the relative dielectric constant of the colours, and the relationship between the transmittance and drive voltages were measured. The results are shown in Figure 7. As can be seen from Figure 7, there is considerable variation in the transmittance - drive voltage curves in the case of the present comparative experiment, such that good electro-optical properties were not obtained. This is because, focusing on curve d which shows the greatest discrepancy from the other curves, the curve indicates that the green picture element had a rather greater relative dielectric constant (in this case, approximately 27% greater than the mean relative dielectric constant) than did the other picture elements: red picture element and blue picture element, which together with the green picture element form the colour layer. When the transmittance - drive voltage curves for the colours red, green and blue exhibit such a degree of variability, the transmittance of the picture elements for each colour will vary for a given drive voltage, such that good electro-optical properties cannot be obtained. Experiment 2 Next, measurements were performed in order to determine how the transmittance - drive voltage curves vary according to type of pigment employed. The types of pigment employed are shown in Table 1. The method of measurement is described first. Figure 8 illustrates the sample employed for the purpose of measurement. In the sample illustrated in Figure 8, an ITO layer 500 with a film thickness of approximately 100 nm (1000 Å) was formed upon the glass substrate 100, and resist containing dispersed pigment was coated upon the ITO layer 500, whereupon the material was subjected to heat treatment for from 10 minutes to 20 minutes at a temperature of approximately from 60°C to 70°C in order to cure the resist layer, which formed a colour layer 200 having a thickness of approximately 1.0 µm. Moreover an electrically conductive paste consisting of a mixture of an organic solvent and silver powder was coated upon the colour layer 200, which was then cured in order to form the conductive layer 600. An LCR meter was employed with the samples formed in this manner in order to measure their capacitance. Such measurements and the measurements of the thicknesses of the films upon the sample were employed to calculate the relative dielectric constant of the sample. For such measurements of capacitance, the voltage was set at 5 V and the frequency was varied between 100 Hz and 100 kHz. The results are illustrated in Figure 9. As can be seen from Figure 9, the values for each of the pigments declined until approximately a frequency of 10 kHz, whereupon the values showed virtually no change. A most important point that is evident from Figure 9 is that those samples in which bromine was introduced into the phthalocyanine green (samples c and e) exhibited lower values for relative dielectric constant than did the sample in which bromine was not introduced into the phthalocyanine green (sample d). Consequently it is confirmed through the present experiment that the employment of phthalocyanine green to which bromine atoms have been introduced enables the establishment of the relative dielectric constant within the range of approximately from 3.0 to 6.0, and that the relative dielectric constant of the three colours R, G and B can be made almost uniform within this range. Moreover, Figure 6 and Figure 9 show that, at a voltage of 5 V and at a frequency of 1 kHz, the relative dielectric constant of the red, green and blue picture elements are within the range 3.5 ± 0.5, and that moreover it is desirable to employ pigments which will provide relative dielectric constant of the red, green and blue picture elements which are within the range 3.0 ± 0.5 at a voltage of 5 V and at a frequency of 100 kHz. Next, the relative dielectric constant - frequency characteristics of the black matrix layer were measured. The results are illustrated in Figure 10. As the method of measurement was identical to that employed for the aforementioned colour layer (see Figure 8 and Figure 9), a detailed description of the method of measurement is omitted. Four samples A to D were employed for this experiment. A black resist employing an acrylic resin (Black CK 3001 manufactured by Fuji Hanto) was selected for each of the samples A to D. Samples A, B and C had carbon contents of 12 percent by weight, and sample D had a carbon content of 20 percent by weight. When an alternating current of 5 V and a frequency of from 100 Hz to 100 kHz was applied to such samples A to D, they were found to have resistivities of within the range of from 1.0 x 10 2 to 1.2 x 10 6 Ω·m which is within the range shown by the present invention, as illustrated in Figure 11. From Figure 10, it may be considered that a relative dielectric constant for the black matrix layer of at least 3.0 is satisfactory. When samples which employed a black resist with a carbon content of 30 percent by weight were tested, short circuits were observed to occur between opposing electrodes, and such samples were turned out to be unfit for practical use. It can be confirmed that there is such short circuiting by optical defects in the form of dots when electricity is applied to a liquid crystal panel. Figure 2 and Figure 3 illustrate variant forms of the colour filter that is illustrated in Figure 1, and these colour filters differ from the colour filter 1000 in the constitution of the black matrix layer. A colour filter 2000 in Figure 2 has a black matrix layer 300 which is composed of overlaid blue and red colour layers, while a colour filter 3000 in Figure 3 has a black matrix layer 300 which is composed of overlaid green, red and blue colour layers. As the constitutions of the colour filters are in all other regards identical to that of the colour filter illustrated in Figure 1, no further details of such variant forms of the colour filters are provided. Second Embodiment Figure 12 is a cross-sectional view which illustrates schematically the colour filter of the second embodiment of the present invention. A colour filter 4000 possesses a supporting member 100, a transparent conductive layer 500, a colour layer 200, and a black matrix layer 300, and such basic constitution, particularly the basic constitution of the colour layer 200, is identical with that of First Embodiment. Thus the thickness of the colour layer 200 is from 0.4 µm to 1.0 µm, and preferably from 0.6 µm to 0.9 µm, and when an alternating current of 5 V and a frequency of from 1 kHz to 100 kHz is applied to the colour layer, the relative dielectric constant of the colour layer is at least 2.0, and preferably from 3.0 to 5.0. Moreover, the relative dielectric constant of the picture elements 200A, 200B and 200C are all within a range of ± 25% of the mean value for the three colour elements, and preferably within a range of ± 20% of the mean value for the three colour elements. The characteristic of Second embodiment is in the projecting portion of the black matrix layer 300, that is to say in the upper layer which forms the black matrix layer 300, a part of the layer (projecting layer) 300A which projects above the colour layer 200 is formed with side surfaces which tapered upwards. Such tapered side surfaces are formed at an angle of from 30° to 89° with reference to the supporting member 100, and preferably at an angle of from 45° to 70° with reference to the supporting member 100. If the angle of incline of such side surfaces exceeds 89°, the angle of that portion of the projecting layer 300A which is different in height becomes steep, and may readily lead to poor alignment. Moreover, if the angle of incline of the side walls is less than 30° , the film becomes too thin at the edge, such that such portion does not possess adequate light blocking properties. Moreover, if the film is too thin, the control of the width of the patterns during manufacture becomes difficult, and the reproducibility of the dimensions envisaged at design becomes poor. The following describes the method by which the taper is added to the projecting layer 300A of the black matrix layer 300. Various methods have been considered in this regard, and a typical such method involves the provision of a gap between the photomask and the surface which is exposed to light. More specifically, an interval of from 0 µm to 30 µm is normally disposed between the surface which is exposed to light and the photomask, but this interval is expanded to between 200 µm and several hundred mm before exposure to light. If such method is employed, when the material is exposed to light, the light diffraction effect causes the light exposure pattern to be greater than the design value of the photomask, such that the layer of expanded portion is formed into the tapered shape illustrated in Figure 12 by development. Alternative methods of forming such taper include for example the employment of surface reforming agents or surfactants in order to increase the cohesion between the resist layer that is exposed to light and the transparent conductive layer, thereby ensuring a certain degree of tapering. The optimum angle of incline of the aforementioned side surface of the projecting layer 300A may be selected having regard to rubbing conditions and to the direction of rubbing. Figure 13 and Figure 14 illustrate variant forms of the present embodiment. In the colour filter 5000 in Figure 13, the black matrix layer 300 is formed after the formation of the colour layer 200, and Figure 14 illustrates the constitution of the colour filter 6000 wherein the black matrix layer is formed by means of the overlaying of red and blue colour layers without the use of black pigment. Such type of black matrix layer 300 is formed with a tapered shape in the side surfaces of the blue colour layer (300A) which is formed above the upper surface line of the colour layer 200.

Claims: A color filter for a liquid crystal display comprising: an optically transparent supporting member (100); a transparent conductive layer (500) formed upon said supporting member; and a color layer (200) of a specified pattern of red, green and blue picture elements which is formed upon said transparent conductive layer wherein pigments are employed as the coloring agents to constitute said picture elements of said color layer, wherein said color layer has a relative dielectric constant of at least 2.0 under measurement conditions of an alternating current of a voltage of 5 V and a frequency of from 1 kHz to 100 kHz, and each of said red, green and blue picture elements which form said color layer has relative dielectric constant of within a range of ±25% of the mean value of said red, green and blue picture elements for the relative dielectric constant under said measurement conditions, characterized in that said color layer (200) has a thickness of from 0.4 µm to 1.0 µm and contains pigments in the range of from 40 percent by weight to 50 percent by weight. A color filter as defined in claim 1, wherein each of said red, green and blue picture elements (200A, 200B, 200C) has a relative dielectric constant under said measurement conditions of within a range of ± 20% of the mean value for the relative dielectric constant. A color filter as defined in claim 1 or 2, wherein each of said red, green and blue picture elements (200A, 200B, 200C) has a relative dielectric constant of within a range of 3.5 ± 0.5 under an alternating current of 5 V and a frequency of 1 kHz, and has a dielectric constant of within a range of 3.0 ± 0.5 under an alternating current of 5 V and a frequency of 100 kHz. A color filter as defined in claim 1, 2 or 3, wherein the differences in thickness between the red, green and blue picture elements (200A, 200B, 200C) which form said color layer (200) are within 0.25 µm. A color filter as defined in any one of the preceding claims, wherein a partially brominated phthalocyanine green into which bromine atoms have been introduced is employed as the pigment for said green picture element. A color filter as defined in claim 5, wherein the molecules of said partially brominated phthalocyanine green pigment include from one to thirteen bromine atoms. A color filter as defined in claim 5, wherein a brominated and chlorinate phthalocyanine green into which chlorine atoms have been introduced in addition to bromine atoms is employed as the pigment for said green picture element. A color filter as defined in claim 5, wherein a chlorinated phthalocyanine green into which from 14 to 16 chlorine atoms have been introduced (Pigment Green 7, C.I. 74260) is employed in addition to said partially brominated phthalocyanine green as the pigment for said green picture element. A color filter as defined in claim 5, wherein phthalocyanine green 6Y (Pigment Green 36, C.I.74265) is employed in addition to said partially brominated phthalocyanine green as the pigment for said green picture element. A color filter as defined in claim 5, wherein a chlorinated phthalocyanine green into which from 14 to 16 chlorine atoms have been introduced (Pigment Green 7, C.I. 74260) and disazo yellow HR (Pigment Yellow 83, C.I. 21108) are employed as the pigment for said green picture element. A color filter as defined in claim 10, wherein the chromaticity of said green picture element by the CIE standard colormetric system when measured employing a C light source is 0.33 ≥ x ≥ 0.29 and y ≥ 0.58 when the thickness of the green picture element is between 0.8 µm and 1.0 µm. A color filter as defined in any one of claims 5 to 11, wherein the pigment for said red picture element is composed of a mixture of PC Red HF 2B (Pigment Red 208, C.I. 12514) and disazo yellow HR (Pigment Yellow 83, C.I. 21108). A color filter as defined in any one of claims 5 to 11, wherein the pigment for said blue picture element is composed of a mixture of at least one selected from phthalocyanine blue R (Pigment Blue 15:1, 15:2, C.I. 74160, C.I. 74250) and phthalocyanine blue G (Pigment Blue 15:3, 15:4, C.I. 74160), and dioxazine violet (Pigment Violet 23, C.I. 51319). A color filter as defined in claim 1, wherein a black matrix layer formed between said red, green and blue picture elements has an impedance of which the resistance component value is from 1.0 x 10 2 Ω·m to 1.2 x 10 6 Ω·m under an alternating current of 5 V and a frequency of from 100 Hz to 100 kHz, and a relative dielectric constant of 3.0 or more at the aforementioned voltage and range of frequencies. A color filter as defined in claim 14, wherein said black matrix layer comprises a pigment and carbon as a coloring agent. A color filter as defined in claim 15, wherein said carbon is contained within the range of from 10 percent by weight to 20 percent by weight of said black matrix layer. A color filter as defined in claim 14, 15 or 16, wherein said black matrix layer varies in thickness from the thicknesses of the red, green and blue picture elements which form said color layer by at most 0.5 µm. A color filter as defined in claim 1, wherein a black matrix layer is disposed between the red, green and blue picture elements which form said color layer, and a layer which forms a portion of said black matrix layer and which protrudes above said color layer has side surfaces which are tapered off towards the top end. A color filter as defined on claim 18, wherein said tapered side surfaces are at an angle of from 30° to 89° to said supporting member. A color filter as defined in claim 19, wherein said tapered side surfaces are at an angle of from 45° to 70° to said supporting member. A liquid crystal display device having a color filter according to any one of the preceding claims.

Inventors: ARUGA HISASHI, KOBAYASHI TAKASHI, SAWATA RYUJI, TAKIZAWA KEIJI, ARUGA, HISASHI, KOBAYASHI, TAKASHI, SAWATA, RYUJI, TAKIZAWA, KEIJI
Applicants: SEIKO EPSON CORP, SEIKO EPSON CORPORATION
Application year: 1993
Main classification: G02B5
IPC codes: G02F1, G02B5