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BACKGROUND OF THE INVENTION
This invention relates to a method for forming a substrate having high definition and high light-shielding capability. More particularly, it relates to a method for forming a substrate which has a light-shielding layer and which may be utilized
for preparing a color filter employed in particular for a liquid crystal display device, and to a substrate having the light-shielding layer.
This invention also relates to a counterelectrode substrate for a thin film transistor (referred to as TFT) array substrate for black and white display which is superior in high definition and light-shielding capability and which may be prepared
by a simple process, and to a liquid crystal display device for black and white display (LCD).
As a typical example of the color liquid crystal display device, there has hitherto been known a TFT (thin film transistor)--active matrix color liquid crystal device. The device includes a substrate arranged on an inner polarizing plate, a thin
film transistor (TFT) and a pixel electrode driven by the TFT arranged on the substrate, and an inner alignment layer contacted with both the TFT and the pixel electrode. On its outer side, the device includes a liquid crystal layer having an outer
alignment layer and an outer polarizing plate arranged on the outermost side. Between the outer alignment layer and the outer polarizing plate, there is mounted a color filter having a black light-shielding layer (black matrix) and colored layers, such
as red-hued layer, green-hued layer and blue-hued layer on a transparent electrically conductive layer formed on the transparent substrate. For producing the color filter, there are currently proposed a dyeing method, a dye-pigment dispersion method, a
printing method, an electrodeposition method and a transfer method. With any of these known methods, the precision in the arraying of the respective colored layers, such as red-hued, green-hued and blue-hued layers, herein occasionally abbreviated to R,
G, B and BL layers, respectively, is of utmost importance. Above all, the black-hued layer, that is the light-shielding layer, needs to be positioned not only in registry with the counterelectrode substrate to avoid light leakage in the vicinity of the
pixel electrode but also without voids between the other colored layers and the light-shielding layer significantly influences the picture quality such as contrast. Consequently, the current practice is to produce the light-shielding layer with high
precision and to form other colored layers so as to be overlapped to some extent with the light-shielding layer. For example, an evaporated film of metal such as chromium is patterned using a photolithographic technique to produce a black matrix and the
color layers of R, G and B layers are formed with small amounts of overlap with the black matrix at the boundary regions thereof.
With the thin film transistor (TFT) display device for black and white display, which has a construction similar to the above-mentioned color liquid crystal display device, a transparent substrate formed with a light-shielding layer (black
matrix) is provided in place of the R, G, and B layers of a color filter, and functions as a counterelectrode substrate. In preparing the counterelectrode substrate, a resist is coated on a metal chromium layer formed by sputtering on a glass substrate,
and a black matrix is formed by light exposure, development, etching and film exfoliation. A transparent ITO film is subsequently formed by sputtering on the entire surface.
However, if a light-shielding layer is formed of metal, manufacture-related problems are presented in that the evaporation method or lithography is susceptible to pinholes and involves a complicated process, and that the light-shielding film
formed of metal has high light reflectance and leads to inferior viewing properties of the display device. Above all, with the TFT black and white display device, the vacuum process needs to be carried out twice in order to produce the black matrix and
the electrode. If the method of overlapping the boundary regions of the colored layers is employed for the preparation of the color filter, it is not possible to produce a color filter having superior surface planarity which is strongly desired when the
color filter is used for the color liquid crystal display device.
In order to overcome these problems, a method of employing a photosensitive resin composition admixed with pigments of black or the like dark or thick colors has been proposed in the Japanese Laid-Open Patent Publications Nos. 63-314501,
1-293306 and 5-34514. Specifically, a method of forming a photosensitive resin composition previously colored in a dark color on a transparent substrate, exposing via a pattern mask only the portions of the resin composition required as a
light-shielding layer, for curing the resin composition and developing and removing only the unexposed portions of the resin composition, a method of forming a layer of a photosensitive resin composition previously colored to have a thick color on a
substrate on which R, G and B layers have been formed, exposing the reverse substrate surface, that is the substrate surface not having the layer of the photosensitive resin composition, to light for curing the photosensitive resin composition and
developing and removing only the unexposed portion, and a combination of these methods, are disclosed. However, the photosensitive resin composition colored to have a dark color hue exhibits high light absorption so that curing cannot proceed to a
sufficient depth on exposure to light. Consequently, the photosensitive resin composition colored to have a thick hue tends to be removed during removal by development so that the light-shielding layer having a high light-shielding capability can hardly
be produced. In addition, the photosensitive resin composition having its exposed portion cured by photopolymerization is frequently employed. In light exposure in atmospheric air, curing is obstructed significantly by oxygen contained in atmospheric
air, such that complex preventative measures such as provision of an oxygen interrupting film or employing an atmosphere free of oxygen, such as vacuum or an argon atmosphere, are needed in carrying out the light exposure. Although it may be envisaged
to eliminate such cumbersome operations by increasing the amount of light exposure to an extreme degree, reflection, scattering or leakage of light is increased, while the substrate temperature tends to be raised, thus presenting difficulties in the
formation of the high-precision light-shielding layer.
On the other hand, if the photosensitive resin composition containing black-hued or nearly black-hued pigment is employed as a black matrix for a counterelectrode substrate for a TFT array substrate, and a transparent substrate having a
transparent electrode is employed as a counterelectrode substrate, it is necessary to provide a transparent electrode by sputtering on the overall surface, because the black matrix itself lacks electrical conductivity. The reason is that, if the liquid
crystal on the black matrix is not responsive to electrical voltage, the liquid crystal portion in the vicinity of pixels undergoes light leakage during voltage-on time with the normally white system employed in the TFT array system, thus lowering the
contrast.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method for forming a substrate having a light-shielding layer by which the light-shielding layer having superior definition and light-shielding properties may be easily formed with sufficient
accuracy, and which may be particularly useful for forming a color filter, and a substrate having such light-shielding layer.
It is another object of the present invention to provide a method for forming a substrate having a light-shielding layer whereby the light-shielding layer having superior definition and light-shielding properties as well as low reflectance and
electrical conductivity may be easily formed with sufficient accuracy by employing specified carbon black as the pigment.
It is a further object of the present invention to provide a counterelectrode substrate for a TFT array substrate for black and white display which is superior in definition and light-shielding properties and which may be formed by a simplified
process, and a liquid crystal display device (LCD) for black and white display employing the counterelectrode substrate.
According to the present invention, there is provided a method for preparing a substrate having a light-shielding layer comprising the steps of i) forming a positive photosensitive coating film on a transparent electrically conductive layer
formed on a transparent substrate, ii) exposing the positive photosensitive coating film via a mask exhibiting light transmitting properties only at portions thereof registering with the light-shielding layers iii) removing and developing the portions of
the coating film exposed to light for exposing the transparent electrically conductive layer and electrodepositing a dark-hued coating on at least a portion of the exposed transparent electrically conductive layer for forming the light-shielding layer,
and iv) heating the light-shielding layer.
According to the present invention, there is also provided a method for preparing a substrate having a light-shielding layer comprising the steps of i) forming a negative photosensitive coating film on a transparent electrically conductive layer
formed on a transparent substrate, ii) exposing the negative photosensitive coating film via a mask exhibiting light transmitting properties only at portions thereof other than those registering with the light-shielding layer, iii) removing and
developing the portions of the coating film not exposed to light for exposing the transparent electrically conductive layer and electrodepositing a dark-hued coating on at least the exposed transparent electrically conductive layer for forming the
light-shielding layer, and iv) heating the light-shielding layer.
According to the present invention, there is also provided a substrate having a light-shielding layer comprising a transparent substrate, a transparent electrically conductive layer formed on the transparent substrate and a light-shielding layer
formed on the transparent electrically conductive layer, wherein the light-shielding layer contains carbon black having a maximum particle size of 1 .mu.m or less dispersed within a polymer matrix and wherein the light-shielding layer has a volume
resistivity of 1.times.10.sup.2 ohm.multidot.cm or higher.
According to the present invention, there is provided a liquid crystal display device for black and white display comprising a first polarizing plate, a substrate arranged on the first polarizing plate, a thin film transistor, and a pixel
electrode driven by the thin film transistor, the thin film transistor and the pixel electrode both being formed on an outer surface of the substrate opposite to the first polarizing plate, an inner alignment layer in contact with the thin film
transistor and the pixel electrode, a liquid crystal layer being in contact with the inner alignment layer and having an outer alignment layer on its opposite side surface, a second polarizing plate arranged on an outermost side surface of the display
device and a counterelectrode substrate arranged between the outer alignment layer and the second polarizing plate, the counterelectrode substrate having a transparent substrate in contact with the second polarizing plate and a transparent electrically
conductive layer electrodeposited on the other side of the transparent substrate, a light-shielding layer of a dark-hued colored layer formed on the electrically conductive layer being contacted with the outer alignment layer.
BRIEF DESCRIPTION
OF THE DRAWING
The sole figure is a schematic cross-sectional view for a black and white liquid crystal display device according to the present invention.
DESCRIPTION OF THE INVENTION
The present invention will be explained in detail hereinbelow.
With the method of the present invention, the step of forming a positive photosensitive coating film on a transparent electrically conductive layer (transparent electrode) on the transparent substrate, referred to hereinafter as a step 1-A, is
first performed. Alternatively, with the method of the present invention, the step of forming a negative photosensitive coating film on a transparent electrically conductive layer on the transparent substrate, referred to hereinafter as a step 1-B, is
first performed.
There is no limitation to the materials of the transparent substrate, provided that it is transparent and plate-shaped. Specifically, these materials may include quartz, various glasses, various transparent resins (plastics), such as polyester,
polyphenylene sulfide, epoxy resins, acrylic resins, polymethylpentene, polyimides, polycarbonates, polyamides, polysulfone, polyether, polystyrene, acrylonitrile-styrene copolymers or cellulose triacetate. In view of the properties desired of the color
filter and so forth, as an ultimate product, the surface of the transparent substrate is desirably smooth and occasionally ground before use.
The transparent electrically conductive layer formed on the transparent substrate is preferably formed of tin oxide, indium oxide, indium-tin oxide or antimony oxide, and usually has a film thickness of 20 to 300 nm. The transparent electrically
conductive layer may be formed by spraying, chemical vapor deposition (CVD), sputtering or vacuum evaporation.
There is no particular limitation to the photoresist forming the positive photosensitive coating film, provided that the light exposed portion is soluble in a developing solution and thereby removed. The photoresist may be enumerated by
compounds containing quinone diazido groups, compounds having diazomeldrum's acid or nitrobenzyl esters, a composition containing these compounds, and chemically amplified compositions employing a compound generating an acid by light (acid-generating
agent by light). More specific examples include a composition prepared by suitably mixing with a resin having a film-forming function a reaction product between a compound having a hydroxyl group and a quinone diazidosulfonic acid derivative or a
quinonediazido compound having an isocyanate group. There is no limitation to the mixing ratio which may be suitably selected depending on conditions of the light exposure and development. Examples of the photoresist may further include chemically
amplified compositions composed of a resin having a quinonediazido group or a composition containing a resin having a quinonediazido group, an acid-generating agent by light selected from aryl sulfonium salts, aryl iodonium slats, halomethyl triazine,
esters of sulfonic acid and tosylates having an o-nitrobenzyl group, and polyhydroxystyrene having a t-butoxycarbonyl group or a tetrahydropyranyl group introduced therein. Various commercially available positive photoresists may also be employed.
There is no limitation to the photoresist forming the negative photosensitive coating film, provided that the light exposed portion thereof is not removed by a developing solution and only the unexposed portion thereof is insoluble in the
developing solution. For example, a prepolymer or a resin having the molecular weight ranging in general between 500 and 10,000 and containing an ethylenic double bond capable of being cross-linked by light, such as (meth)acryloyl group, e.g. acryloyl
or methacryloyl group, and/or cinnamoyl group, in its molecule, which prepolymer or resin may be dissolved or dispersed in water along with a photopolymerization initiator and occasionally with a dye and/or a pigment.
The prepolymer or the resin may be enumerated by the prepolymers, such as epoxy (meth)acrylate, urethane (meth)acrylate or polyester (meth)acrylate; cationic resins which are produced by introducing onium groups, such as amino groups, ammonium or
sulfonium and the above-mentioned photosensitive groups into acrylic resins, epoxy resins, urethane resins or polybutadiene resins and which are dissolved and/or dispersed in an organic solvent or solubilized and/or dispersed in water with acids such as
formic acid, acetic acid, propionic acid or lactic acid or with acidic substances; and anionic resins which are produced by introducing carboxyl groups and the above-mentioned photosensitive groups into acrylic resins, polyester resins, maleinated oil
resins, polybutadiene resins or epoxy resins, and which are dissolved and/or dispersed in an organic solvent or solubilized and/or dispersed in water with basic substances such as triethylamine, diethylamine or ammonia. Prepolymers or resins capable of
being solubilized and/or dispersed in water are especially preferred for simplifying the process or preventing environmental pollution.
Low molecular (meth)acrylates may be added to the negative photosensitive coating material for adjusting the viscosity and photosensitivity of the coating film. These (meth)acrylates may be enumerated by 2-hydroxyethyl (meth)acrylate,
2-phenoxyethyl (meth)acrylate, 3-phenoxy-2-hydroxypropyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, tricyclodecane (meth)acrylate, hexanediol di(meth)acrylate, trimethylolpropane triacrylate, pentaerythritol triacrylate, dipentaerythritol hexaacrylate
and tris(acryloyloxyethyl) isocyanurate. These may also be used in a mixture. The mixing ratio of these (meth)acrylates is preferably 0 to 50 parts by weight and more preferably 0 to 30 parts by weight to 100 parts by weight of the resin for the
negative photosensitive coating material. If the mixing ratio exceeds 50 parts by weight, the coating film undesirably tends to be viscid.
As the photopolymerization initiator, any material known in the art may be employed. Examples of the known photopolymerization initiator include benzoin and ethers thereof, benzyl alkyl ketals, benzophenone derivatives, anthraquinone derivatives
and thioxanthone derivatives. The photopolymerization initiator may be admixed with a suitable sensitizer if so desired. The photopolymerization initiator is added preferably in an amount of 0.1 to 30 parts by weight and more preferably in an amount of
0.5 to 20 parts by weight to 100 parts by weight of the resin for the negative photosensitive coating material. If the photopolymerization initiator is added in the amount less than 0.1 part by weight, photocurability becomes insufficient. If it
exceeds 30 parts by weight, curing proceeds excessively so that the film strength becomes insufficient with economic demerits.
The negative photosensitive coating material may be prepared by a method comprising mixing a resin for the photosensitive coating material, a photopolymerization initiator, an organic solvent and/or water and, if necessary, various assistant
agents, such as dyes and/or pigments, acidic or basic substances, dispersion assistant agents for dyes or pigments, levelling agents for improving smoothness of the coating films, viscosity adjustment agents or anti-foaming agents, and sufficiently
dispersing the ingredients, using a well-known dispersion means, such as a sand mill or an attriter.
There is no particular limitation to the organic solvents employed for dispersing or dissolving the above-mentioned positive or negative photoresists or compositions thereof capable of forming the photosensitive coating films. Examples of the
solvents may include glycolethers, such as ethylene glycol monobutylether, ethylene glycol monohexylether, ethylene glycol monophenylether, propylene glycol monomethyl ether, propylene glycol monophenylether, diethylene glycol dimethylether or
triethylene glycol dimethylether; ketones, such as acetone, methylethyl ketone, methylisobutyl ketone, cyclohexanone, isophorone, or N-methyl pyrrolidone; ethers, such as dibutyl ether, dioxane, or tetrahydrofuran; alcohols, such as methoxybutanol,
diacetone alcohol, octanol, butanol, or isopropanol; hydrocarbons, such as toluene, xylene, cyclohexane or hexane; esters, such as ethyl acetate, butyl acetate, 2-methoxy ethyl acetate, 2-methoxy propyl acetate or ethyl benzoate; and acid amides, such as
dimethyl formamide, N,N-dimethyl acetamide or dimethyl sulfoxide. These organic solvents may be used alone or in combination.
The positive photosensitive coating film or the negative photosensitive coating film may be formed on the transparent electrically conductive layer by any known methods, such as immersion, spraying, spin coating, roll coating, screen printing or
electrodeposition.
There is no particular limitation to the film thickness of the photosensitive coating film, which may be suitably selected depending on the properties required of the color filter as the ultimate product. Above all, in relation to the shape of
the light-shielding layer, it is desirable for the film thickness of the photosensitive coating film to be usually 0.5 to 20 times and preferably 1 to 10 times that of the light-shielding layer. If the film thickness of the photosensitive coating film
is less than the above range, there may be a large possibility that the thick-colored layer which is to form the light-shielding layer by electrodeposition during the next step is formed on the photosensitive coating film, or the light-shielding layer is
formed which has a line width broader than that of the pattern prescribed by the photosensitive coating film. If the film thickness of the photosensitive coating film is larger than the above range, there may be a high possibility that problems are
presented in the resolution of the photosensitive coating film or the electrodeposition or the coating material for electrodeposition can hardly enter the opening during electrodeposition so that the light-shielding layer cannot be formed in a desired
manner.
For adjusting the film thickness, the coating conditions may suitably be selected depending on the particular coating method. In the case of spin coatings for example the film thickness can easily be controlled by adjusting the viscosity of the
coating liquid or the number of revolutions of a spinner, while in case of electrodeposition, for example the conditions of electrodeposition such as coating voltage, electrodeposition time or liquid temperature may be changed to adjust the film
thickness.
In the method of the present invention, the positive photosensitive coating film is exposed to light via a mask exhibiting light-transmitting properties only at portions thereof registering with the light-shielding layer. The process step is
referred to hereinafter as a step (2-a). Alternatively, the negative photosensitive coating film is exposed to light via a mask exhibiting light-transmitting properties only at portions thereof other than those registering with the light-shielding
layer. The process step is referred to hereinafter as a step (2-b).
The mask exhibiting light transmitting properties at the portions thereof registering with the light-shielding layer, or the mask exhibiting light transmitting properties at the portions thereof other than those registering with the
light-shielding layer, is a photomask usually employed in photolithography, and needs only to be patterned to have a desired shape.
The light exposure may be effected using a light source capable of generating a large quantity of ultraviolet rays, such as a high-pressure mercury lamp, an ultra-high pressure mercury lamp, xenon lamp, a metal halide lamp, an excimer laser or a
synchrotron light radiator. If necessary, a radiation source other than the ultraviolet ray generator may be employed. The light exposure conditions may be suitably selected depending on the photosensitive coating film, the method or the apparatus for
light exposure. For example, there is no particular limitation to the amount of light exposure which may be suitably selected depending on the light source or the photosensitive coating film. The amount of light exposure is usually 0.1 to 1000
mJ/cm.sup.2 and preferably 5 to 500 mJ/cm.sup.2.
With the method of the present invention, when the positive photosensitive coating film is employed, the portions of the coating film exposed to light are removed and developed for laying or exposing the transparent electrically conductive layer
to the outside, and a dark-hued coating is electrodeposited on at least the exposed transparent electrically conductive layer said to the outside for forming the light-shielding layer. The above process step is referred to hereinafter as the step (3-a). With the method of the present invention, when the negative photosensitive coating film is employed, the portions of the coating film not exposed to light are removed and developed for laying or exposing the transparent electrically conductive layer to
the outside, and a dark-hued coating is electrodeposited on at least the exposed transparent electrically conductive layer laid to the outside for forming the light-shielding layer. The above process step is referred to hereinafter as the step (3-b).
There is no particular limitation to the development conditions in the steps (3-a) or (3-b) and any conditions may be suitably employed depending on the amount of light exposure in the steps (2-a) or (2-b), solubility of the photosensitive
coating film in the developing solution, the kind or the concentration of the developing solution, the developing time or developing temperature. There is no particular limitation to the developing solution if it is capable of :removing the exposed or
unexposed portions of the photosensitive coating film of the step (3-a) or (3-b) by development, such that the developing solution may be suitably selected depending on the types or the light exposed state of the photosensitive coating film.
In the step (3-a), an aqueous solution of a basic substance is usually employed as the developing solution. Examples of the basic substance include sodium carbonate, sodium hydrogen carbonate, sodium metasilicate, tetraalkyl ammonium hydroxide,
such as tetramethyl ammonium hydroxide, sodium hydroxide, potassium hydroxide or ammonia. For the developing solution, an organic solvent, such as alcohols, glycol ethers, ketones, hydrocarbons or chlorinated hydrocarbons, may be employed either as a
mixture or in combination with the above-mentioned aqueous developing solutions. Of these, the aqueous developing solution is preferred.
If a cationic resin is used as an ingredient of the photosensitive coating material, the developing solution employed in the step (3-b) may be an aqueous solution containing an acidic substance dissolved therein. The acidic substance may be
enumerated by organic acids, such as formic acid, acetic acid, propionic acid or lactic acid, and inorganic acids, such as hydrochloric acid or phosphoric acid. If an anionic resin is used as an ingredient of the negative photosensitive coating
material, a developing solution containing a basic substance dissolved in water may be employed. The basic substance includes sodium carbonate, sodium hydrogen carbonate, sodium metasilicate, tetraalkylammonium hydroxide, sodium hydroxide and potassium
hydroxide. As the developing solution, an organic solvent, such as alcohols, glycol ethers, glycols, ketones, hydrocarbons or chlorinated hydrocarbons may be employed, either as a mixture or in combination with the aqueous developing solution. The
developing solution may be admixed with surfactants or anti-foaming agents for improving wettability or anti-foaming properties. An aqueous developing solution is preferably employed in view of toxicity or working environments.
If the aqueous solution of tetramethyl ammonium hydroxide is used in the step (3-a) for the developing solution, the developing conditions may be suitably selected from the concentration of 0.01 to 20 wt %, and preferably 0.05 to 10 wt %, the
temperature of 10.degree. to 80.degree. C. and preferably 15 to 40.degree. C., the developing time of 2 to 600 seconds and preferably 4 to 300 seconds.
If an aqueous solution of sodium carbonate is used as the developing solution in the step (3-b), the concentration of sodium carbonate may usually be in a range of 0.01 to 25 wt % and preferably in a range of 0.05 to 15 wt %, the temperature may
be in a range of 10.degree. to 70.degree. C., and the developing time may be suitably selected from a range of 5 to 600 seconds and preferably from a range of from 5 to 300 seconds. If an aqueous solution of lactic acid is used in the step (3-b), the
concentration of lactic acid may usually be in a range of 0.01 to 50 wt % and preferably in a range of 0.05 to 25 wt %, the temperature may be in a range of 10.degree. to 70.degree. C. and preferably in a range of 15.degree. to 50.degree. C., and the
developing time may be suitably selected from a range of 2 to 600 seconds and preferably from a range of from 4 to 400 seconds.
The dark-hued colored coating material employed in the step (3-a) or (3-b) may be exemplified by a water-soluble or -dispersible coating material containing (a) a dye and/or a pigment having a thick hue, such as black, thick indigo or thick
brown, (b) a binder resin for electrodeposition and occasionally (c) a curing agent.
The dye and/or the pigment may be exemplified by, for example, carbon black, graphite, vanadium trioxide, manganese dioxide, molybdenum disulfide, triiron tetraoxide, Aniline Black, Sudan Black B, Acid Black 1 and 52, Fast Black K Salt, Nigrosin
or mixtures thereof. The dyes and/or pigments shown in detail in "COLOR INDEX", third issue, may also be employed.
If the substrate having the light-shielding layer formed by the method of the present invention is employed as the black matrix for the color filter, there is no particular limitation to the dyes and/or the pigments and any dyes and/or pigments
enumerated above may be employed. If the substrate having the light-shielding layer formed by the method of the present invention is employed as the light-shielding layer for the counterelectrode substrate for a TFT array substrate for black and white
display, there is also no particular limitation to the dye and/or pigment contained therein and any of the above-enumerated dyes and/or pigments may be employed. Of these, however, the pigments, above all, those in the form of electrically conductive
fine particles, are particularly preferred. As the electrically conductive fine particles, at least one of carbon black, metal oxides, such as tin oxide, ITO, indium oxide, titanium oxide, ruthenium oxide, or vanadium oxide, and metals, such as gold,
platinum, palladium, silver alloys, copper or nickel, may preferably be employed. Needless to say, two or more of these components may be used as a mixture in order to strike a balance between light-shielding properties and electrical conductivity. It
is preferred for the substrate having the light-shielding properties of the present invention to have the volume resistivity of 1.times.10.sup.2 ohm.multidot.cm or higher and above all the volume resistivity of 1.times.10.sup.2 to 1.times.10.sup.12
ohm.multidot.cm and the surface resistivity of 1.times.10.sup.2 to 1.times.10.sup.14 ohm/.quadrature.. If the volume resistivity is lower than 1.times.10.sup.2 ohm.multidot.cm, the dye and/or the pigment tends to be raised in concentration such that a
film having satisfactory physical properties, and above all high adherence cannot be produced. If the volume resistivity is higher than 1.times.10.sup.12 ohm.multidot.cm, the dye and/or the pigment tends to have low concentration, thus leading to
insufficient light-shielding properties. Similarly, if the surface resistivity is lower than 1.times.10.sup.2 ohm/.quadrature., the dye and/or the pigment tends to be raised in concentration and hence poor in adherence, whereas, if the surface
resistivity is higher than 1.times.10.sup.14 ohm/.quadrature., sufficient light-shielding properties occasionally cannot be achieved.
If the black matrix portion of the light-shielding layer is used as a counterelectrode substrate for a TFT array substrate for black and white display, light modulation due to the liquid crystal orientation on the black matrix has the effect of
preventing light leakage, so that higher electrical conductivity of the black matrix portion, that is the light-shielding portion, is desired. Specifically, the volume resistivity of 1.times.10.sup.2 to 1.times.10.sup.9 ohm.multidot.cm and preferably
1.times.10.sup.2 to 1.times.10.sup.6 ohm.multidot.cm and the surface resistivity of 1.times.10.sup.2 to 1.times.10.sup.9 ohm/.quadrature. and preferably 1.times.10.sup.2 to 1.times.10.sup.6 ohm/.quadrature. is desired. If the volume resistivity is
lower than 1.times.10.sup.2 ohm.multidot.cm, the electrically conductive fine particles usually need to be used in a higher concentration, which leads undesirably to inferior adhesivity of the light-shielding layer to the ITO substrate. If the volume
resistivity is larger than 1.times.10.sup.9 ohm.multidot.cm, the liquid crystal display device becomes undesirably inferior in viewing properties. If the surface resistivity is lower than 1 .times.10.sup.2 ohm/.quadrature., the electrically conductive
fine particles usually need to be used in a higher concentration, which leads undesirably to inferior adhesivity of the light-shielding layer to the ITO substrate. If the surface resistivity is larger than 1.times.10.sup.9 /.quadrature., the liquid
crystal display device may lack viewing properties. The lowering of viewing properties is due to the fact that the liquid crystal in contact with the light-shielding layer is not responsive to the voltage application. That is, the TFT-LCD is usually
employed with normally white so that the light polarized by the polarizing plate undergoes birefringence at the liquid crystal in contact with the light-shielding layer, thus undesirably leading to light leakage in the vicinity of the pixels and hence to
lowered contrast and inferior viewability of the liquid crystal display device. Accordingly, in the black matrix of the present invention, it is not necessary to further form an ITO relative to the black matrix.
If, on the other hand, the black matrix is used as the light-shielding layer of the color filter for the liquid crystal display device manufactured by the electrodeposition method, it is preferred for the black matrix portion to have low
electrical conductivity in order to prevent the occurrence of the phenomenon of electrodeposition of the electrodeposited layers of, for example, R, G and B on the black matrix portion.
Specifically, it is desirable for the black matrix portion of the light-shielding layer to have the volume resistivity of 1.times.10.sup.6 to 1.times.10.sup.12 ohm.multidot.cm and the surface resistivity of 1.times.10.sup.4 ohm/.quadrature. to
1.times.10.sup.14 ohm/.quadrature.. If the volume resistivity is lower than 1.times.10.sup.6 ohm.multidot.cm, the phenomenon of over-coating, that is the phenomenon of the colored coating material being electrodeposited on the light-shielding layer
undesirably tends to be produced. Above the volume resistivity of 1.times.10.sup.12 ohm.multidot.cm, light-shielding properties are extremely lowered, thus leading to lowered contrast of the color filter and lowered protective characteristics of TFT.
Below the surface resistivity of 1.times.10.sup.4 ohm/.quadrature., the phenomenon where the colored coating material may be electrodeposited on the light-shielding layer (over-coating) may occur. If the surface resistivity is higher than
1.times.10.sup.14 ohm/.quadrature., the light-shielding properties are significantly lowered, so that characteristics of the color filter, such as contrast, or protective characteristics of the TFT, are undesirably lowered.
Of the above pigments, carbon black is preferred because it exhibits high light-shielding properties even if used in small quantities. In view of light-shielding properties and stability of the electrodeposition coating material, it is necessary
for the carbon black to have the maximum particle size of not larger than 1 .mu.m. As for the maximum particle size of the carbon black on dispersion thereof, as measured by the light-scattering particle size distribution measurement device manufactured
by OTSUKA ELECTRONICS CO., LTD. under the trade name of "PAR-III", the number average particle size (dn) is desirably not more than 1 .mu.m and more desirably not more than 0.5 .mu.m, while the weight average particle size (dr)/number average particle
size (dn) is desirably not more than 2.5. It is more preferred for dn and the ratio dv/dn to be not more than 0.3 Nm and not more than 2, respectively. If the maximum particle size exceeds 1 .mu.m, the electrodeposition coating material tends to be
inferior in stability, while the resulting light-shielding layer tends to be inferior in smoothness or definition. The maximum particle size of not more than 1 .mu.m is desirable because the electrical conductivity of the light-shielding layer becomes
more readily controllable by suitably selecting the pigment/binder resin mixing ratio or the heat treatment conditions as later explained.
In general, if the electrically conductive material, such as carbon black, is used as a pigment in a dark-hued coating material, the dark-hued layer exhibits electrical conductivity. It is possible with the present invention to increase the
electrical conductivity of the dark-hued layer to such an extent that the light-shielding layer can be utilized as the counterelectrode for a TFT array substrate, or to decrease the electrical conductivity of the dark-hued layer to such an extent that
the light-shielding layer can be satisfactorily employed as a light-shielding layer for the color filter manufactured by the electrodeposition method. The black-hued black matrix exhibiting high light-shielding properties may be produced if the
above-mentioned dye and/or the pigment is used in an amount usually of 2 to 300 parts by weight and preferably 3 to 100 parts by weight to 100 parts by weight of the electrodeposition binder resin.
If carbon black is employed as the pigment, it is possible to accord stability and reliability to the light-shielding layer on curing by heating and hence to produce the black-hued black matrix exhibiting light-shielding properties by employing
usually 10 to 80 parts by weight and preferably 20 to 60 parts by weight of the carbon black to 100 parts by weight of the electrodeposition binder resin. The black matrix in which carbon black is dispersed in this manner in the polymer matrix exhibits
light reflectance which is lower than when the black matrix is manufactured by metal sputtering or the like.
For producing a counterelectrode for a TFT array substrate for black and white display using carbon black, the electrically conductive fine particles are preferably added in an amount of 30 to 80 parts by weight to 100 parts by weight of the
electrodeposition binder resin. If the particles are added in an amount less than 30 parts by weight, the resulting black matrix is low in light-shielding properties and electrical conductivity, whereas, if the particles are added in an amount exceeding
80 parts by weight, the resulting black matrix is lowered in planarity and definition.
As the above-mentioned electrodeposition binder resin, the resins having groups which become cationic or anionic groups when dissolved and/or dispersed in water may be employed. If a transparent substrate having an ITO transparent electrically
conductive layer thereon is used as the substrate for electrodeposition, it is not desirable to perform cationic electrodeposition using a resin containing the groups which become cationic groups, because the ITO layer becomes oxidized by the acid
contained in the electrodeposition solution. Thus it is preferred to perform anionic electrodeposition using a resin containing the groups which become anionic groups. If cationic electrodeposition is to be preformed, a substrate which is not oxidized
with the acid contained in the electrodeposition solution, such as stainless steel or platinum, needs to be employed, and the light-shielding layer produced by electrodeposition needs to be transferred onto an actually employed transparent substrate,
such as glass. As the resins containing cationic groups, those produced by introducing amino groups or onium groups, such as ammonium, sulfonium or phosphonium, into acrylic, epoxy, urethane, polybutadiene of polyamide resins and rendering the resulting
material soluble or dispersible in water with acids such as formic acid, acetic aid, propionic acid or lactic acid, or with acidic substances.
The anionic resins may be enumerated by, for example, acrylic resin, polyester resin, maleinated oil resin, polybu | | |
