Method of manufacturing a liquid crystal display

The present invention provides a method of manufacturing a matrix liquid crystal panel provided with picture element electrodes arranged in a matrix and a black mask covering spaces between the picture element electrodes, capable of forming the black mask in accurate resister relative to the picture element electrodes through simple processes. In manufacturing the liquid crystal panel, a conductive transparent layer is formed over one of the major surfaces of a substrate, a positive resist film is formed in a pattern corresponding to the arrangement of the picture element electrodes, the conductive transparent layer is patterned, using the positive resist film as a mask, a negative resist film containing pigment is formed over the major surface of the substrate so as to cover the positive resist film and spaces between the picture element electrodes, the negative resist film is exposed to light projected from behind the other major surface of the substrate so that only portions of the negative resist film coating the spaces between the picture element electrodes are exposed to light and polymerized, and then portions of the negative resist film screened from the light by the positive type of resist material film are removed to pattern the negative resist film in a black mask.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method of manufacturing a liquid crystal 
panel of a matrix type having picture element electrodes arranged in rows 
and columns and, more specifically, to a method of manufacturing a liquid 
crystal panel having picture element electrodes arranged in rows and 
columns, and a black matrix or a black mask for shading blank spaces 
formed between the picture element electrodes. 
2. Description of the Prior Art 
A liquid crystal panel having transparent electrodes is provided with a 
black mask formed on a substrate to prevent the degradation of the 
contrast of an image displayed thereon by light transmitted through blank 
spaces between the transparent electrodes. Methods of forming a black mask 
are classified roughly into three categories. A method of a first category 
forms a black mask by patterning a metal thin film, such as a chromium 
thin film, by a photolithographic etching process. A method of a second 
category forms a black mask by spreading a photoresist material containing 
particles of black pigment dispensed therein in a photoresist film over 
the surface of a substrate and patterning the photoresist film by a 
photographic process. A method of a third category forms a black mask by 
printing black ink over the surface of a substrate in a film by an offset 
printing process and subjecting the film of the black ink to a heating 
process. The method of the third category is disclosed in, for example, 
Japanese Patent Laid-open (Kokai) No. She 63-180933. 
The method of the first category comprises a film forming process for 
forming a metal thin film, such as a chromium thin film, over the surface 
of a substrate by sputtering or vacuum evaporation, a photoresist film 
forming process for forming a photoresist film over the metal thin film, a 
photographic process for forming a photoresist mask by patterning the 
photoresist film, an etching process for etching the metal thin film in 
the pattern of a black mask, and a coating process for forming an 
overcoating layer or an insulating layer over the black mask of the metal 
thin film to insulate the black mask from the transparent electrodes to be 
formed over the metal thin film. Since the number of processes of the 
method of the first category is relatively large, the manufacturing cost 
of the black mask is relatively high. 
The method of the second category also needs many processes. For example, a 
photoresist material containing particles of pigment dispersed therein is 
spread over the surface of a glass substrate by a spin coating process or 
a printing process to form a photoresist film. After prebaking the 
photoresist film, an oxygen-shielding material, such as PVA, is applied to 
the photoresist film to prevent free radicals produced in the photoresist 
film when the photoresist film is exposed to light from being deactivated 
through reaction between the free radicals and oxygen, the photoresist 
film is prebaked again, the photoresist film is exposed, using a 
photomask, the exposed photoresist film is processed by a photographic 
process, and then the photoresist film is subjected to a postbaking 
process to finish a black mask. Then, the black mask is coated with a 
overcoating layer. 
Then, a transparent conductive film is formed over the coating layer, a 
photoresist film is formed over the transparent conductive layer, and then 
the photoresist film is patterned in a positive photoresist mask, using a 
photomask. The photomask must be correctly registered relative to the 
black mask. If the photomask is registered incorrectly, gaps will be 
formed between the black mask and transparent electrodes formed by 
patterning the transparent conductive film. Light transmitted through the 
gaps deteriorates the contrast of an image displayed on the liquid crystal 
panel having such incorrectly patterned transparent electrodes. A 
technique proposed previously to prevent forming such gaps forms the black 
mask by a relatively thick lines so that the lines forming the black mask 
over lap the edges of the transparent electrodes. Although the black mask 
having such relatively thick lines facilitates registering the photomask 
relative to the black mask, the numerical aperture of the picture element 
electrodes is reduced. 
The method of the third category forms a black mask simply by printing 
black ink by offset printing and hence the number of processes of the 
method of the third category is relatively small. However, the printing 
accuracy of offset printing is lower than the accuracy of patterning the 
transparent conductive film. Therefore, the method of the third category, 
similarly to the method of the second category is obliged to sacrifice the 
numerical aperture of the picture element electrodes to compensate errors 
in registering the printed black mask relative to the pattern of the 
transparent electrodes. 
SUMMARY OF THE INVENTION 
The present invention has been made in view of the foregoing problems in 
the prior art methods and it is therefore an object of the present 
invention to provide a method of manufacturing a liquid crystal panel, 
comprising a relatively small number of processes and capable of forming 
black mask in correct register relative to the pattern of transparent 
electrodes. 
The present invention forms a black mask after perfectly covering blank 
spaces between picture element electrodes. Accordingly, the black mask has 
a high light-shielding ratio and enhances the contrast of an image 
displayed on the liquid crystal panel. Furthermore, since the present 
invention uses a positive resist mask for patterning a transparent 
conductive film to form the picture element electrodes for forming the 
black mask, a process of registering a mask for forming the black mask 
relative to the pattern of the picture element electrodes becomes 
unnecessary. 
According to one aspect of the present invention there is provided a method 
of manufacturing a liquid crystal panel comprising the step of: disposing 
a conductive transparent layer on an insulating substrate; patterning the 
conductive transparent layer to form a transparent electrode pattern, each 
transparent electrode having a resist film thereon; forming a resist layer 
over the resist film; and patterning the resist layer to define a 
light-shielding layer between adjacent transparent electrodes by exposing 
from a back side of the insulating substrate. 
According to another aspect of the present invention there is provided a 
method of manufacturing an electro-optical device which comprises a first 
and second substrates and an electro-optical material layer sandwiched 
between the substrates, comprising the step of: disposing a conductive 
transparent layer on the first substrate: patterning the conductive 
transparent layer to form a transparent electrode pattern, each 
transparent electrode having a resist film thereon; forming a resist layer 
over the resist film; and patterning the resist layer to define a 
light-shielding layer between adjacent transparent electrode by exposing 
from the back side of the first substrate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1A, a transparent conductive film of ITO or the like is 
formed over the surface of a glass substrate 1, i.e., a insulating 
substrate, a positive photoresist mask 3 is formed over the transparent 
conductive film, and then the transparent conductive film is etched 
selectively to form transparent electrodes 2. A photosensitive resist film 
4 (FIG. 1B) of a photosensitive resin containing black pigment or a 
colored photosensitive resin is formed over the surface of the glass 
substrate 1 so as to coat the exposed portions of the surface of the glass 
substrate 1, the picture element electrodes 2 and the positive photoresist 
mask 3. Then the photosensitive resist film 4 is exposed by irradiating 
the glass substrate 1 from behind the back surface of the glass substrate 
1, in which the positive photoresist mask 3 serves as a mask for coating 
portions of the photosensitive resist film 4. Portions of the 
photosensitive resist film 4 not screened by the positive photoresist mask 
3 are exposed to light and hardens. Portions of the photosensitive resist 
film 4 other than the portions of the same exposed to light and hardened 
are removed by a photographic process to form a black mask 5 (FIG. 1C). 
Then, the positive resist mask 3 is removed to complete a picture element 
electrode (FIG. 1D). Another picture element electrode is formed by the 
same processes. 
The two picture element electrodes are put together so that a gap of a 
predetermined thickness is formed between the picture element electrodes 
and the picture element electrodes of one of the picture element electrode 
are in register respectively with the corresponding picture element 
electrodes of the other picture element electrode, and a liquid crystal is 
sealed in the gap between the two picture element electrodes to complete a 
liquid crystal plate. 
The black mask 5 can be formed in a desired thickness by regulating the 
radiant exposure on the photosensitive resist film 4. For example, it is 
possible to form the black mask 5 in a thickness that will make the 
transmissivity of the liquid crystal, which is dependent on .DELTA.n.d, 
where .DELTA.n is the quantity of double refraction of the liquid crystal 
and d is the thickness of the liquid crystal layer, substantially zero. 
The black mask 5 may be formed in a thickness corresponding to the 
distance between the inner surfaces of the glass substrates 1 of the 
opposite picture element electrodes to use the black mask as a spacer. 
Steps of forming a black mask, which are the essential steps among those of 
the method of manufacturing a liquid crystal plate in accordance with the 
present invention, will be described hereinafter with reference to FIG. 2. 
A transparent conductive film of a transparent conductive material, such as 
ITO, is formed over the surface of a glass substrate by sputtering or 
vacuum evaporation. A film of a positive type of resist material, such as 
a mixture of novolac resin and a quinoneazide compound (sensitive 
material) (PMER manufactured Tokyo Ohka K. K.), which material has the 
withstand temperature on the order of 130.degree. C., is formed over the 
transparent conductive film, the positive resist film is patterned to form 
a positive resist mask, and then the transparent conductive film is etched 
in step S1 to form picture element electrodes. 
In step S2, the positive resist mask is baked at a temperature on the order 
of 240.degree. C. for one hour to harden the positive resist masks namely, 
to promote the polymerization of the resin, such as the phenol resin, and 
to carbonize the sensitive material and the dye. When baked, the positive 
resist mask turns reddish brown. The baked positive resist mask can be 
easily removed. 
In step S3, a negative resist film of a negative type of resist material 
containing black pigment is formed over the entire surface of the glass 
substrate by spin coating or printing. Any suitable photosensitive color 
resist material capable of screening light may be used instead of the 
negative type of resist material containing black pigment. The 
photosensitive negative type of resist material may be a photosensitive 
acrylic resin of a photosensitive polyimide resin. In step S4, the 
negative resist film is prebaked. 
In step S5, the glass substrate is irradiated from behind the back surface 
thereof by ultraviolet rays, for example, i rays of 365 nm in wavelength, 
using the reddish brown positive resist mask to expose portions of the 
negative resist film corresponding to spaces between the transparent 
electrodes at an exposure intensity of, for example, 1.67 mW/cm2. If 
necessary, the negative resist film may be subjected to postexposure 
baking (PEB) to promote the polymerization of monomer radicals produced by 
exposure. In PEB, the negative resist film is heated at a temperature in 
the range of 90.degree. C. to 110.degree. C. for six minutes. 
In step S6, the negative resist film is developed by using, for example, an 
alkali solution, such as an aqueous solution of sodium carbonate, of a 
concentration of about 1%. Since the solution velocity of the unhardened 
negative type of resist material in the 1% alkali solution is higher by 
far than that of the baked positive type of resist material, the positive 
resist mask will not be dissolved together with the negative resist film. 
Consequently, the negative resist film is patterned so as to fill up the 
spaces between the transparent electrodes. 
In step S7, the patterned negative resist film is subjected to postbaking 
to eliminate the solvent so that the patterned negative resist film is 
perfectly resinified. A negative type of resist material containing an 
acrylic resin as a principal component is resinified or hardened perfectly 
when heated at 240.degree. C. for one hour for postbaking. The postbaked 
negative resist film is resistant to alkali. 
In step S8, the positive resist mask is removed by using, for example, an 
alkali solution (aqueous solution of potassium hydroxide) of a 
concentration of on the order of 15%. Since the withstand temperature of 
the positive type of resist material is on the order of 130.degree. C. and 
the positive resist mask is carbonized and the adhesion of the same to the 
transparent electrodes is reduced when heated at 240.degree. C. for 
postbaking, the positive resist mask can be easily removed when treated by 
the alkali solution. Since the negative resist film is perfectly 
resinified by postbaking, the negative resist film withstands the alkali 
treatment. Thus, the black mask can be formed so as to cover only the 
spaces between the transparent electrodes by using the positive resist 
mask used for forming the transparent electrodes. Although the method has 
been described as applied to manufacturing a monochromatic liquid crystal 
panel, the present invention is applicable also to manufacturing a color 
liquid crystal panel. When manufacturing a color liquid crystal panel, a 
color filter is formed over the transparent conductive film before 
patterning the transparent conductive film. 
The black mask needs to be formed in a predetermined thickness for some 
liquid crystal panel. A method of forming the black mask in a desired 
thickness will be described with reference to FIG. 3. 
The thickness of the black mask is dependent on the radiant exposure, i.e., 
the product of exposure intensity and exposure time, on the resist film of 
a negative type of resist material containing black pigment. When the 
resist film is exposed to ultraviolet rays, free radicals are produced and 
polymerization of monomer radicals occurs in the resist film. Since the 
resist film contains black pigment, ultraviolet rays are not transmitted 
through the resist film, free radicals are produced only in the irradiated 
surface of the resist film and chain reaction for polymerization between 
the monomer radicals starts from the surface of the resist film. Since the 
resist film is exposed to ultraviolet rays projected from behind the back 
surface of the glass substrate, chain reaction for the polymerization 
between monomer radicals starts from the surface of the resist film 
contiguous with the front surface of the glass substrate and propagates 
toward the other surface of the resist film. The depth of propagation of 
the chain reaction, namely, the thickness of the black mask, is 
proportional to exposure intensity and exposure time. 
FIG. 3 shows the measured variation in the thickness of a sample black mask 
completed by simply exposing the resist mask to ultraviolet rays and a 
sample black mask completed by subjecting the resist film to PEB after 
exposure with exposure time when exposure intensity was 1.67 mW/cm.sup.2. 
Heat applied by PEB to the resist film promotes the chain reaction between 
the monomer radicals. As shown in FIG. 3, the thickness of the sample 
black mask completed by subjecting the resist film to PEB after exposure 
is greater than that of the other sample black mask. It is impossible to 
form the black mask in a thickness as large as that of the resist film as 
formed on the glass substrate, because free radicals in the surface of the 
colored resist film react with oxygen and the free radicals are 
deactivated. As shown in FIG. 3, the maximum thickness of the sample black 
mask completed by subjecting the resist film to PEB after exposure is 1.1 
.mu.m when the thickness of the resist film as formed is 1.35 .mu.m. It is 
inferred that the free radicals in the surface layer of 0.25 .mu.m in 
thickness were deactivated by reaction with oxygen. 
FIG. 4 shows a supertwist nematic liquid crystal panel of a matrix type in 
a first example having a glass substrate provided with a black mask formed 
by the method of the present invention. This liquid crystal panel 
comprises a first glass substrate 41, a second glass substrate 42 and a 
liquid crystal layer 43 of a supertwist nematic construction sealed in the 
space between the first glass substrate 41 and the second glass substrate 
42. The space between the glass substrates 41 and 42 is sealed by a 
sealing member 44. Row electrodes 45 are formed at predetermined intervals 
on the inner surface of the first glass substrate 41 by patterning a 
conductive transparent layer, and column electrodes 46 are formed so as to 
extend perpendicularly to the row electrodes 45 on the inner surface of 
the second glass substrate 42. A black mask 47 is formed by the method of 
the present invention on the inner surface of the first glass substrate 41 
so as to cover spaces between the adjacent row electrodes 45, and another 
black mask, not shown, is formed by the method of the present invention on 
the inner surface of the second glass substrate 42 so as to cover spaces 
between the adjacent column electrodes 46. 
The intersections of the row electrodes 45 and the column electrodes 46 
serve as pixels. The thickness dO of a portion of the liquid crystal layer 
43 corresponding to the pixel is determined so that the ratio between 
transmissivity when the pixel is selected and transmissivity when the 
pixel is not selected is a maximum. A cell gap D in a blank portion, i.e., 
a portion between the space between the adjacent row electrodes 45 and the 
column electrode 46, is equal to a value obtained by subtracting the 
thickness of the row electrodes 45 from the thickness dO. The 
transmissivity of the liquid crystal layer 43 is greatly dependent on 
.DELTA.n.d, where .DELTA.n is the quantity of double refraction of the 
liquid crystal or a quantity representing the anisotropy of the refractive 
index of the liquid crystal and d is the thickness of the liquid crystal 
layer 43. If no portion of the black mask is not formed in a space 
corresponding to the blank portion, the product of the quantity of double 
refraction of the liquid crystal and the thickness of the liquid crystal 
layer in the blank portion is .DELTA.n.D. However, in general, it is 
impossible to make .DELTA.n.D an optimum value and hence it is impossible 
to reduce the transmissivity to a minimum. On the other hand, the 
thickness d2 of the liquid crystal layer 43 in the blank portion that will 
make the transmissivity a minimum can be determined if .DELTA.n is known. 
Generally, the transmissivity of a black mask of a thickness on the order 
of 1 .mu.m is in the range of 4 to 5% and the black mask is unable to 
screen light perfectly, even if the black mask is formed by patterning a 
resist film containing black pigment. A portion of the liquid crystal 
layer 43 can be formed in the thickness d2 by adjusting the thickness d1 
of the black mask 47 to prevent the leakage of light through the blank 
portion effectively. As is obvious from FIG. 4, d1=D-d2. An image can be 
displayed on the liquid crystal panel in a satisfactorily high contrast by 
determining the thickness d1 of the black mask 47 so that the value of the 
.DELTA.n.d of the liquid crystal layer 43 makes the transmissivity of 
portions of the liquid crystal layer 43 corresponding to the spaces 
between the adjacent transparent electrodes 0%, even if the black mask is 
not able to screen light perfectly. The determination of the value of 
.DELTA.n.d by adjusting the thickness of the black mask is effective also 
when the black mask is formed by patterning a transparent photosensitive 
resin not having any property that causes double refraction, such as an 
epoxy resin, an acrylic resin or a polyimide resin. 
The liquid crystal panel of a supertwist nematic type shown in FIG. 4 is 
provided with a first phase plate 50 and an second phase plate 51 
attached, respectively, to the respective outer surfaces of the first 
glass plate 41 and the second glass substrate 42 to prevent the coloring 
of the liquid crystal panel. In the liquid crystal panel of a supertwist 
nematic type as shown in FIG. 4, the change of the alignment of molecules 
in the liquid crystal layer 43 causes the change of transmissivity. 
Therefore, a first polarizing plate 48 and a second polarizing plate 49 
are attached, respectively, to the respective outer surface of the first 
glass substrate 41 and the second glass substrate 42. 
FIG. 5 shows a liquid crystal panel in a second example manufactured by the 
method of the present invention. The liquid crystal panel comprises a 
first glass substrate 61, a second glass substrate 62, a liquid crystal 
layer 63 sealed in the space between the glass substrates 61 and 62, and a 
sealing member 64 sealing the liquid crystal layer 63 in the space between 
the glass substrates 61 and 62. Row electrodes 65 are formed at 
predetermined intervals on the inner surface of the first glass substrate 
61, and column electrodes 66 are formed at predetermined intervals on the 
inner surface of the second glass substrate 62 so as to extend 
perpendicularly to the row electrodes 65. A black mask 67 is formed on the 
inner surface of the first glass substrate 61 by the method of the present 
invention so as to cover spaces between the adjacent row electrodes 65. 
Similarly, another black mask, not shown, is formed on the inner surface 
of the second glass substrate 62 by the method of the present invention so 
as to cover spaces between the adjacent column electrodes 66. Recesses are 
formed in the component lines of one of the black masks at the 
intersections of the respective component lines of the black masks so that 
the respective component lines of the black masks may not interfere with 
each other. The thickness of the black mask 67 is equal to the 
predetermined thickness of the liquid crystal layer 63; that is, the black 
mask 67 serves as a spacer for determining the thickness of the space 
between the respective inner surfaces of the first glass substrate 61 and 
the second glass substrate 62. Therefore, spacing particles need not be 
spread over the surface of the first glass substrate 61, a spacing 
particle spreading process is omitted, the deterioration of the contrast 
of an image displayed on the liquid crystal panel due to the diffusion of 
light caused by spacing particles can be avoided, and the disturbance of 
the alignment of the liquid crystal due to the effect of an electric field 
that occurs when the pixels are very small and the distance between the 
adjacent transparent electrodes is very small can be avoided. 
FIG. 6 shows a liquid crystal panel in a third example manufactured by the 
method of the present invention. The liquid crystal panel comprises a 
first glass substrate 72, a second glass substrate 73, a liquid crystal 
layer 74 sealed in the space between the first glass substrate 72 and the 
second glass substrate 73, and a sealing member 71 sealing the liquid 
crystal layer 74 in the space between the first glass substrate 72 and the 
second glass substrate 73. Row electrodes 75 are formed on the inner 
surface of the first glass substrate 72, a black mask 76 is formed on the 
inner surface of the first glass substrate 72 by the method of the present 
invention so as to cover spaces between the adjacent row electrodes 75, 
column electrodes 77 are formed on the inner surface of the second glass 
substrate 73, and a black mask, not shown, is formed on the inner surface 
of the second glass substrate 73 by the method of the present invention so 
as to cover spaces between the column electrodes 77. A color resist 
material forming the black mask 76 contains spacing particles 78, such as 
micropearls or glass fibers. The outside diameter of the spacing particles 
78 is equal to the thickness of the space between the respective inner 
surfaces of the first glass substrate 72 and the second glass substrate 
73. Since the black mask 76 contains the spacing particles 78, a spacing 
particle spreading process is omitted and the deterioration of the 
contrast of an image displayed on the liquid crystal panel due to the 
diffusion of light by the spacing particles spread in portions 
corresponding to the pixels can be avoided. Furthermore, the 
transmissivity of the liquid crystal layer 74 can be made 0% by forming 
the colored resist film in an appropriate thickness. 
FIG. 7 shows an active matrix liquid crystal panel in a fourth example 
manufactured by the method of the present invention. Separate transparent 
electrodes 81, conductive material are formed in a matrix on the inner 
surface of a insulating substrate, such as a quartz substrate. A thin-film 
transistor 82 is disposed in connection with each picture element 
electrode 81. Signal lines 83 are extended between adjacent columns of the 
picture element electrodes 81, and scanning lines 84 are extended between 
adjacent rows of picture element electrodes 81. Each thin-film transistor 
82 has a source region S connected to the signal line 83, a drain region D 
connected to the corresponding picture element electrode 81, and a gate 
electrode G continuous with the scanning line 84. A black mask 85 is 
formed by patterning a resist film by the method of the present invention 
so as to cover spaces between the adjacent picture element electrodes 81. 
In forming the black mask 85, a positive resist mask used for forming the 
picture element electrodes 81 is used as a mask. Therefore, any additional 
mask for forming the black mask 85 need not be formed and a process of 
registering a mask for forming the black mask 85 is not necessary. Since 
the black mask 85 need not be provided with margin overlapping the picture 
element electrodes 81, the picture element electrodes 81 have a relatively 
large aperture ratio. 
Although the invention has been described in its preferred form with a 
certain degree of particularity, obviously many changes and variations are 
possible therein. It is therefore to be understood that the present 
invention may be practiced otherwise than as specifically described herein 
without departing from the scope and spirit thereof.