Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2001-02-01
2002-07-02
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S109000, C349S110000, C349S187000
Reexamination Certificate
active
06414739
ABSTRACT:
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 &mgr;m, and the cell gap between these cells are roughly 3 to 6 &mgr;m.
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 &thgr; of the panel is expressed as,
cos &thgr;=cell gap/size of cells=6/100
From the above equation, a &thgr; 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 &mgr;m can be calculated by the following Stokes' Equation, under a condition of Rep<2.
Vt
=
(
ρ
p
-
ρ
f
)
gDp
2
/
18
µ
=
0.4
(
µm
/
sec
)
where, &rgr;p 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; &rgr;f is a density of the liquid crystal, which value is generally around 1.0 to 1.2; and &mgr; is a viscosity of the liquid crystal, which is generally around 15 to 20 mm
2
/sec.
When values of &rgr;p=1.3, &rgr;f=1.0, Dp=6 &mgr;m, and &mgr;=15 mm
2
/sec are substituted in the above equation, the falling speed of the spacer is obtained as 0.4 (&mgr;m/sec). This result shows that it takes 250 sec to fall a distance of 100 &mgr;m. 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 pro
Chowdhury Tarifur R.
NEC Corporation
Sikes William L.
Sughrue & Mion, PLLC
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