BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a liquid crystal display device (LCD) and particularly relates to a wide image plane, color liquid crystal display device having a wide angle of visibility by an active matrix display system.
This application is based on Patent Application No. Hei 9-311782 filed in Japan, the content of which is incorporated herein by reference.
2. Background Art
In general, in a liquid crystal display device, a mode for driving the liquid crystal cell by a vertical electric field is most widely used, including a TN (twisted nematic) and an STN (super twisted nematic) modes. However, recently, a new driving mode by a transverse electric field (IPS) is now being intensively researched.
When a comparison is made between the above two driving modes in terms of an image quality of the liquid crystal cells, it is far more difficult to obtain the good image quality with the transverse electric field mode, due to the panel structure. In particular, an important factor which affects on the image quality is a spacer for maintaining the gaps of cells at a fixed space.
The spacer is a group of spherical beads for supporting a pair of substrates facing each other at a fixed spacing. One of a pair of substrates is a TFT (Thin Film Transistor) array substrate (hereinafter, called “an array substrate”) using TFTs as switching elements for driving, and another one of the pair of substrates is a color filter substrate (hereinafter, called “an opposing substrate”) on which three color layers of RGB (red, green, and blue) are coated. When assembling a display panel, these array substrate and the opposing substrate are adhered to form a cell, placing spacers therebetween. As the spacer beads, elastic organic materials such as a resin using divinylbenzene are generally used. Although inorganic materials such as silica are sometimes used, they are not the main material for the spacer because of a tendency to generate foams when the pressure is reduced.
When a comparison of a leakage of the light is made between the two types of liquid crystal panels, one driven by a vertical electric field and the other driven by a transverse electric field, the panel driven by the transverse electric field is more likely to cause light leakage than the panel driven by the vertical electric field.
One reason for the above result is based on the difference of the normal driving modes in a display operation. That is, for devices driven by the vertical electric field such as the TN mode or the STN mode devices, a normally-white mode is superior for increasing the contrast, while it is advantageous for better contrast to use a normally-black mode for devices driven by the transverse electric field. Thus, it is more likely for liquid crystal display devices driven by the transverse electric field to cause light leakage around spacers when the voltage applied to each cell is null.
The second reason is based on the difference of the driving directions of the liquid crystal. That is, when a device is driven by the vertical electric field, the liquid crystal is driven in the vertical direction perpendicular to a pair of substrates, while the liquid crystal is twisted horizontally when driven by the transverse electric filed. Consequently, a difference is caused in the orientation of liquid crystal by the direction of the electric field, especially in the direction of depth of the liquid crystal, causes an anomalous orientation around spacers by the transverse electric field to cause leakage of light.
The third reason is based on the presence of a chiral crystal. In the devices driven by the vertical electric field such as TN and STN devices, a rubbing direction of one substrate is rotated 90 degrees or 270 degrees against the rubbing direction of the orientation film formed on another substrate, and a chiral material is included in the liquid crystal for facilitating the twisted orientation at a desired direction. In contrast, in the devices driven by the transverse electric field, the rubbing directions of a pair of substrates are directions anti-parallel to each other, so that the orientation of the liquid crystal is homogeneous, and the liquid crystal does not contain the chiral material. Consequently, since the liquid crystal has a high degree of freedom in orientation when the device is driven by the transverse electric field, the liquid crystal around the spacers is thereby likely to be subjected to an anomalous orientation which can cause leakage of light when driven by the transverse electric field.
The liquid crystal molecules located around spacers are more likely to be subjected to the above anomalous orientation when the external force is applied on the cell. This is because the liquid crystal molecules are oriented around the spherical spacers by the external force.
Technical proposals for preventing deterioration of the image quality by spacers are presented in, for example, Japanese Patent Application, First Publication No. Hei 7-281195, entitled “a liquid crystal display panel”; and in Japanese Patent Application, First Publication No. Hei 7-281195, entitled “a liquid crystal display device and method of manufacturing the same”.
In the former Patent Application, as shown in FIG. 5, transparent and colored CF (Color Filter) layers 52 are disposed at an fixed intervals, black light shielding layers 53 are formed therebetween, and spherical spacers are scattered on the surface of the light shielding layer 53. Assume that distances from the CF filter layer 52 and the light shielding layer 53 to the opposing substrate 56 are LC and LB, and the diameter of spacer 54 is D, a dimensional relationship of the liquid crystal panel is represented as LC<D<LB.
The spacers on the light shielding layer are only held between the pair of the substrate 51 and 56. In contrast, spacers 54 on the color filter layer 52 fall downward toward the bottom in the liquid crystal between both substrates, when the panel is stood in an upright position. Thereby, spacers 54 are removed from the display pixel area 55 to prevent an anomalous orientation of the liquid crystal and thereby avoid the deterioration of the image quality.
An attempt of a mathematical analysis will be described. A size of the display cell of the liquid crystal display panel is generally within a range from 100 to 300 μm, and the cell gap between these cells are roughly 3 to 6 μm.
When the panel is stood in an upright position, a spacer located at the upper end of the screen area falls into the outer position of the screen without abutting the substrates. The standing angle θ of the panel is expressed as,
cos θ=cell gap/size of cells=6/100
From the above equation, a θ of 86.6° is obtained, corresponding to an angle when the spacer is considered as a “point”.
The falling speed of the spherical spacers in the liquid crystal with a thickness of 100 μm can be calculated by the following Stokes' Equation, under a condition of Rep<2.
where, ρp is a density of the spacer, which value is 1.1 to 1.3, when the spacer is made of organic materials such as divinylbenzene or styren resins; ρf is a density of the liquid crystal, which value is generally around 1.0 to 1.2; and μ is a viscosity of the liquid crystal, which is generally around 15 to 20 mm2/sec.
When values of ρp=1.3, ρf=1.0, Dp=6 μm, and μ=15 mm2/sec are substituted in the above equation, the falling speed of the spacer is obtained as 0.4 (μm/sec). This result shows that it takes 250 sec to fall a distance of 100 μm. This falling speed seems not so effective in the practical display operation.
In turn, as shown in FIG. 6, a technique to maintain cells at a fixed value without using spacers is proposed in Japanese Patent Application, First Publication No. Hei 7-28119.
This proposal is related to the TN mode liquid crystal panel, and a first projection is mounted for forming cell gaps at the black-matrix 63 formed in the TFT 62 on the arrayed substrate 61. On the opposing substrate 65 for forming CF (Color Filters), a second projection is formed by laminating the red layer 66, green layers, and the blue layers. The first projection and the second projection have a height which corresponds to a half of the cell gap Ga. Therefore, the orientation films 70 and 71 can be sufficiently subjected to the rubbing treatment, so that the TN liquid crystal molecules 72 are ensured to be lined up in a regular pattern. Thereby, it is possible to prevent the anomalous orientation and thus the light leakage by the liquid crystal panel in which it is not necessary to use the spacers.
In the above disclosure, if the height of the projection is higher than 5 μm or more, the rubbing treatment is then not sufficiently executed because a part is hidden by the projection causing irregular orientation of the liquid crystal. In order to avoid this, the height of the projections is limited to less than 3.8 μm.
In the liquid crystal panel driven by the transverse electric field, beside the problem that it is hard to arrange the liquid crystal molecules in a predetermined direction, there is an important problem concerning a force of constraint for constraining the liquid crystal molecule in a predetermined direction. If the force of constraint in a predetermined direction (hereinafter referred to as an orientation restraining force) is weak, a picture image after switching from displaying another picture image will contain the after-image of the previous image. Therefore, the orientation restraining force has an affect on the image quality.
As described hereinabove, the liquid crystal panel driven by the transverse electric field has many problems that must be solved. In contrast, the liquid crystal panel driven by the transverse electric field has the advantageous feature that the wide display image plane with a wide angle of visibility is obtained.
It is therefore an object of the present invention to provide a liquid crystal display device with a wide display image plane and with a wide angle of visibility having a good cell image quality by preventing a light leakage around the spacers for maintaining a fixed cell gap and by preventing after images due to the weak orientation restraint force.
SUMMARY OF THE INVENTION
The liquid crystal display panel of the present invention comprises:
an array substrate having switching elements disposed in an matrix form and an orientation film formed on the uppermost layer;
an opposing substrate having a light shielding layer and the color layers and an orientation film on the uppermost layer of those films and the color layers, in which the area excluding the area covered by said light shielding layer is turned into the display area; and
a liquid crystal layer formed in between said array substrate and said opposing substrate;
wherein the cell gap of said display area is formed so as to be larger than the diameter of the spacers, and the cell gap at the color layer area overlapping the light shielding layer is formed so as to be smaller than the diameter of the spacer, such that said spacers in said smaller cell gap are held at the compressed state between said array substrate.
In this construction, said color layer is formed elevated above said display area such that the cell gap forms a step between the display area and said color layers.
Furthermore, said color layer area is formed in the form of stripes and the opposing substrate is formed so as to form a step between a display cell portion and a color filter portion, wherein the color filter portion is formed higher than said display cell portion.
In contrast, a method of manufacturing the liquid crystal panel comprising the steps of:
forming an array substrate by arranging switching elements in the matrix form and forming an orientation film on the uppermost layer of said switching elements;
forming an opposing substrate by forming a light shielding film and color layers and forming a display cell portion on said color layer portion by forming the orientation film on said display cell portion; and
forming by inserting a liquid crystal in between said array substrate and said opposing substrate;
wherein a step is formed such that said cell gaps of said display cell portion on the opposing substrate are larger than that of the spacer diameter, and the cell gap of the color layers corresponding to the light shielding layer is made smaller than the spacer, and the height difference between said higher display portion and said lower color portion is formed so as to define the polarization anisotropy of said orientation film.
According to the above structure and the method of manufacturing, the cell gap of the display cell portion is formed higher than the spacer diameter as much as 0.3 μm, the liquid crystal molecules around the spacer are not subjected to the anomalous orientation, so that the light leakage is prevented. Furthermore, the after image due to the orientation restraining force is prevented in a liquid crystal display panel of a-Si TFT driven by an IPS (transverse electric field).