Liquid crystal display having reduced ITO shading material and method of manufacturing same

A liquid crystal display and a method for manufacturing the same wherein the light transmissivity of selected regions of the indium tin oxide film forming the common electrode on a first substrate is decreased and the regions are made substantially non-reflecting by reducing the oxygen content of the film. The shading material thus produced does not reflect light to the thin film transistors which control the picture elements on a second, facing substrate and therefore does not affect their operation. Reduction is accomplished by providing a protective mask for areas that are not to be reduced, immersing the substrate containing the thin film with the protective mask applied thereto in an electrolyte, and applying a voltage sufficient to reduce the indium tin oxide in regions unprotected by the mask. The mask, which may be a photoresist, is then removed and the substrate may be incorporated into the display.

FIELD OF THE INVENTION 
The present invention relates to liquid crystal displays. More 
particularly, it relates to liquid crystal displays having a shading 
member, and a method of manufacturing such displays. 
BACKGROUND ART 
U.S. Pat. No. 4,568,149 discloses that a shading member composed of, for 
example, aluminum may be provided between adjacent color filters (that is, 
between adjacent picture elements) on a transparent electrode formed of 
indium-tin-oxide (hereinafter abbreviated to ITO) or a mixture of indium 
oxide (In.sub.2 O.sub.3) and tin oxide (SnO.sub.2). 
The method taught by this patent requires a large number of manufacturing 
steps, including a film forming step for making a transparent electrode, a 
film forming step for making a shading member, and an etching step for 
making the shading member. Moreover, the shading member projecting from 
the transparent electrode makes an injection of liquid crystal material 
difficult. Further, as the shading member is formed of a metal such as 
aluminum, the light from a back light is reflected by the shading member, 
and enters the thin film transistor provided on the substrate which faces 
the substrate on which the shading member is disposed. This may adversely 
affect the characteristics of the thin film transistor. 
U.S. Pat. No. 4,733,948 discloses that a shading member composed of, for 
example, aluminum may be provided at a position between adjacent color 
filters on a transparent electrode disposed on one of two opposing 
substrates and facing a thin film transistor disposed on the other 
substrate. 
The methods disclosed in this patent, as is the case for U.S. Pat. No. 
4,568,149, requires a large number of steps including a film forming step 
for making a transparent electrode, a film forming step for making a 
shading member, and an etching step for forming the shading member. 
Japanese Published Unexamined Patent Application (PUPA) 62-135809 discloses 
that a photoresist may be applied to a transparent conductive film 
composed of ITO which is provided on a transparent substrate, followed by 
exposure and development, thereby patterning the photoresist. Then the 
part of the transparent conductive film not coated with the photoresist is 
removed by etching, and using the patterned photoresist as it is as a 
masking material, a shading layer is formed of metal oxides in the region 
where no transparent conductive film is provided on the substrate. 
The method disclosed in PUPA 62-135809 does not require the etching step 
for forming the shading layer, but does require the etching step for 
forming a transparent electrode, and also necessitates a film forming step 
for forming a transparent electrode and another film forming step for 
forming a shading layer. 
Japanese PUPA 63-74033 discloses applying a photoresist on an ITO film; 
forming a pattern by exposure and development through a mask, then 
removing the part of the ITO film not coated with the photoresist by 
etching, and reducing the remaining ITO film as it is with the photorsist 
deposited thereon in a hydrogen plasma, thereby lowering the resistance of 
each side of the ITO film which has been patterned. 
PUPA 63-74033 relates to the reduction of and ITO electrode, but is 
intended to lower the resistance of the ITO electrode, and does not 
suggest the use of the material obtained by reducing ITO as a shading 
material. 
SUMMARY OF THE INVENTION 
It is a principal object of the invention to provide a liquid crystal 
display which permits reducing the number of manufacturing steps for 
forming the shading material, and diminishes the reflection of light by 
the shading material. 
It is another object of the invention to provide a liquid crystal display 
which does not exhibit a displacement in the level between the transparent 
electrode and the shading member. 
It is still another object of the invention to provide a manufacturing 
method for a liquid crystal display which permits reduction in the number 
of manufacturing steps, for forming the transparent electrodes and the 
shading member. 
In accordance with the invention, the material obtained by reducing ITO is 
used as a shading material, for example, between picture elements of a 
liquid crystal display. As the ITO is reduced, its light transmission 
diminishes to a low enough level for the product to be used as a shading 
material. Because the region adjacent to the transparent electrodes each 
of which composes a picture element electrode is generally where the 
shading is needed, the necessary shading member may be formed by reducing 
the region of ITO which is not protected with a mask, while protecting 
with the mask the ITO region which is to be used as the transparent 
electrode. Because this shading member is to exist in a single layer film 
of ITO, no difference in level occurs between the shading member and the 
transparent electrode. In addition, there is no need to separately form a 
film for forming the shading member, thus reducing the number of 
manufacturing steps. Furthermore, because the material obtained by 
reducing ITO causes only a small amount of light reflection, use of this 
material as a shading member does not adversely affect the characteristics 
of the thin film transistor facing the shading member because light is not 
reflected to the transistor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, between a thin film transistor (TFT) array substrate 
10 and a facing substrate 12 which are arranged in a spaced parallel 
relationship to each other, a nematic liquid crystal material 14 is 
filled. The TFT array substrate 10 and the facing substrate 12 are both 
formed of a transparent glass. On the TFT array substrate 10, a thin film 
transistor (TFT) is formed for each picture element. That is, on the TFT 
array substrate 10, a gate electrode 18 is formed. A gate insulation film 
20 formed on the gate electrode 18. On the gate insulation film 20, an 
amorphous silicon semiconductor layer 22 is formed. A drain electrode 24 
and a source electrode 26 are connected to the semiconductor layer 22. The 
drain electrode 24 and the source electrode 26 are insulated from each 
other. 
As shown in FIG. 2, the gate electrode 18 is connected to an address line 
18A, and the drain electrode 24 to a data line 24D. The source electrode 
26 is connected to a picture element electrode 28 consisting of ITO. 
On the facing substrate 12, a common electrode 40 of ITO and a shading 
material or black matrix 42 consisting of a material obtained by reducing 
ITO are formed. The common electrode 40 and the shading member 42 are in 
the same plane (and of the same thickness), and there is thus no 
difference in level between them. As shown in FIG. 2, the shading member 
42 is formed in a region which covers an area not occupied by the picture 
element electrodes 28 on the TFT array substrate 10. Thus, the shading 
member 42 is formed in the region facing the address line 18A, data line 
24D and the amorphous silicon semiconductor layer 22. 
On the semiconductor layer 20, drain electrode 24, source electrode 26 and 
the picture element electrodes on the TFT array substrate 10, there is 
formed an orientation film 30 for orienting the molecular axis of the 
liquid crystal material in a predetermined direction. Another orientation 
film 50 for orienting the molecular axis of the liquid crystal material in 
a predetermined direction is formed on the common electrode 40 and on the 
shading member 42. 
A backlight 60 is provided on the side of the TFT array substrate 10. The 
shading member 42 formed of a material obtained by reducing ITO not only 
prevents the light incident on the side of the facing substrate 12 from 
entering the semiconductor layer 22, but also, because of the small 
reflection factor of the shading member 42, does not allow the light 
received from the backlight 60 through the transparent picture element 
electrodes 28 to reflect and enter the semiconductor layer 22. 
FIG. 3 shows an embodiment of the method for forming the shading member or 
black matrix 42 in the ITO single layer film. FIG. 4(a) to FIG. 4(d) 
repesent the respective steps shown in FIG. 3. An ITO film 40 is formed on 
substrate 12 (step 102 of FIG. 3 and FIG. 4(a)). Then, photoresist 200 is 
coated on the overall surface of the ITO film 40 (step 104 of FIG. 3 and 
FIG. 4(b)). Then, by using a photomask which covers only the region 
corresponding to the picture element electrodes 28, (which, in other 
words, permits light to pass through the region corresponding to the 
semiconductor layer 22, the address line 18A and the data line 24D), the 
photoresist 200 is exposed, and developed. Then, only the part of the 
photoresist which has been exposed remains unremoved (step 106 of FIG. 3 
and FIG. 4(c)). 
Then, the glass substrate 12 having thereon the ITO film 40 patterned with 
the photoresist 200 as described above is immersed in a conductive 
solution 202 containing hydrogen H, as shown in FIG. 4(d). The cathode of 
a DC power source 210 is connected to the ITO film 40. An anodic electrode 
206 connected to the anode of source 210 is immersed in the conductive 
solution 202, and voltage is applied between the ITO film 40 and anodic 
electrode 206. Then, the following reactions take place in the ITO film 40 
(the cathode): 
EQU In.sub.2 O.sub.3 +3H.sub.2 .fwdarw.2In+3H.sub.2 O 
EQU SnO.sub.2 +2H.sub.2 .fwdarw.Sn+2H.sub.2 O 
EQU (ITO=In.sub.2 O.sub.3 +SnO.sub.2) 
In this manner, the percentage of metal components in that part of the ITO 
which is not coated with the photoresist 200 increase, or the ITO is 
reduced (step 108 of FIG. 3). As a result, the light transmission in that 
part diminishes, thus providing a shading member 42. 
The reduction of the ITO film may be performed by applying a voltage of 
-20V to the ITO film (5% by weight of SnO.sub.2 and 95% by weight of 
InO.sub.3) for 3 minutes, using a 0.03% by weight citric acid solution as 
the conductive solution or electrolyte containing hydrogen. As shown in 
FIG. 5, a 10% to 20% transmissivity may be attained in the visible ray 
wavelength region (about 400 nm to 700 nm). These values are low enough 
for the material to be used as a shading member. 
In addition, no difference in level occurred between the common electrode 
40 and the shading member 42; the surface of each was disposed in the same 
plane. 
The reduction is believed to depend on both voltage and time. Thus, if a 
sufficiently long time is expended, an even lower voltage, for example, as 
low as 10V, may be applied to the ITO film. 
Further, any solution which is generally useable for an anodic oxidization 
process may be used as the hydrogen containing conductive solution. 
The smaller the amount of oxygen in the ITO film, the lower the light 
transmission. By expending a sufficiently long time for reduction, the 
light transmissivity may be brought near 0%, but as it approaches 0%, the 
black material changes to a material having a metallic luster, which 
reflects the light from backlight 60, thus illuminating the semiconductor 
layer 22, and thereby producing photo-leakage current in the thin film 
transistors. It is for this reason that a light transmissivity of about 10 
to 20% for the shading member 42 is considered appropriate. 
Since the ITO film is used not only as the raw material of the shading 
member 42, but also as the transparent common electrode 40, the low 
resistance thereof should be maintained without any significant 
modification. From this standpoint, the composition of ITO (defined as 
In.sub.2 O.sub.3 :SnO.sub.2) should desirably range from (95% by weight: 
5% by weight) to (85% by weight: 15% by weight). 
Thus, as described herein, this invention advantageously provides for 
diminishing the reflectivity of the shading member, without causing any 
difference in the level between the shading member and the transparent 
electrode. Further, it permits reducing the number of steps for 
manufacturing the transparent electrode and the shading member. 
While this invention has been described in connection with a specific 
embodiment, it will be understood that those with skill in the art may be 
able to develop variations of the disclosed embodiment without departing 
from the spirit of the invention or the scope of the following claims: