Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2000-03-22
2004-03-30
Ngo, Julie-Huyen L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S043000, C349S129000, C349S130000, C349S138000
Reexamination Certificate
active
06714271
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) and its manufacturing method.
2. Description of the Related Art
Conventionally, twisted nematic-type (TN-type) LCDs or electrically controlled birefringence-type (ECB-type) LCDs have been in wide use. However, because of the uniformly oriented alignment of liquid crystal molecules inside a pixel at the time of voltage application, the color tone changes depending on the viewing angle. In order to reduce this problem relating to the viewing angle, there are techniques to differentiate the oriented alignments of the liquid crystal molecules within each pixel or to divide the region of liquid crystal molecules into differently oriented regions. Using this oriented alignment division, the visual property of each region is compensated with those of the surrounding regions so that a display with a wide-angle field of view can be produced. In particular, when the vertical orientation method (one of the ECB-type methods) is employed, a faster response speed than the TN system can be obtained. Furthermore, due to the improved black display, attention has been focused on the vertically oriented alignment division techniques.
Techniques such as rubbing and optical orientation have been used to orient the liquid crystal molecules. However, because rubbing utilizes a treatment where the alignment layer comes into contact with a cloth in a wiping-like manner, problems can easily develop due to molecules breaking down from static electricity or the substrate being contaminated with foreign dust particles. On the other hand, because the optical orientation process utilizes a non-contact treatment to orient the alignment layer, the above problems related to rubbing techniques do not develop. Therefore, the optical orientation process is seen as a superior technique.
Techniques for performing oriented alignment divisions using the optical vertical orientation method are described in Japanese Patent Application Laid-open Nos. Hei 9-211468 and Hei 10-142608. In each of these techniques, a photo mask is used, and an optical orientation treatment is performed on each of the orienting regions based on their respective irradiation condition.
Here is a brief explanation of the techniques described in the above Japanese Patent Application Laid-open No. Hei 9-211468. First, the principle of this optical orientation technique will be explained using FIGS.
1
(A) to
1
(C). As shown in these figures, top of the substrate
1
is coated with an ultraviolet (UV) ray inducting alignment layer (in this case a vertical alignment layer made of a polyimide). On the surface of the alignment layer
18
are CH chains
19
, and the average direction of these CH chains
19
is parallel to the normal of the substrate
1
. However, the directions of the respective CH chains are dispersed (varied). In other words, it is considered that they face all directions within each imaginary plane parallel to the substrate surface. If UV rays irradiate from the direction
22
diagonal to the alignment layer
18
, parts of the CH chains with their directions being parallel to the direction that the UV rays travel, absorb the UV rays, and thus are cut off or disintegrate. The degree of UV ray absorption depends on the direction of each CH chain on the alignment layer
18
as well as the direction of, the UV ray irradiance, with the highest level of absorption occurring when the direction
23
of the electric field of the UV ray coincides with the direction of the CH chain.
After the UV ray irradiation has occurred, a certain direction (marked with an ‘X’ in FIG.
1
(A)) of CH chain is cut off or disintegrated on the alignment layer
18
, whereas the other (marked with an ‘O’ in FIG.
1
(A)) is left intact. Then, as shown in FIG.
1
(B), the direction
21
parallel to the remaining CH chains leans away from the normal of the substrate
1
.
As shown in FIG.
1
(C), when the liquid crystal molecules
24
come into contact with the surface of the oriented film
18
, they are slanted so as to run along in the same directions as CH chains
19
. The same treatment as described above is applied to two substrates, and in order for the slant of the liquid crystal molecules on the surface of the alignment layer of each substrate to be somewhat parallel to each other, the two substrates are arranged to face each other maintaining a fixed interval between them. The liquid crystal filling between the substrates is liquid crystal with a negative dielectric anisotropy. Applying a certain voltage to the electrode forces the angle of the slant of the liquid crystal molecules
24
relative to the normal of the substrate to increase on their own from the initial angles, eventually becoming parallel to the substrate. In short, a uniform orientation is able to be obtained.
Next, the process of oriented alignment division will be explained. As stated above, the oriented direction is dependent upon the condition of the irradiating light: mainly the irradiating angle and the condition of the light polarization. Accordingly, in order to differentiate the oriented direction of each region, it is necessary to change the condition of the irradiating light in each region. It is common for a photo mask to be used to allow a specific light to hit a specified region, but not allow it to hit the other regions. The process of oriented alignment division can be executed by repeatedly and selectively irradiating light using the photo mask in proportion to the number of oriented alignment divisions.
Furthermore, in Japanese Patent Application Laid-open Hei 10-142608, other techniques besides the above techniques relating to alignment layer materials, the condition of light irradiance (light polarization, angle of light irradiance, volume of light irradiated), orientation of the liquid crystal, and mode of liquid crystal are mentioned, however, the process of oriented alignment division is the same as in the above technique.
Nevertheless, with the conventional technique described above, a problem arises where the alignment at the boundary between the divided regions easily becomes unstable. The reason for this is that regions of over-exposure and under-exposure tend to develop depending on how accurately the photo mask is matched. Also, because the liquid crystal cannot be regulated on these areas of over-exposure and under-exposure, it is easy for a poor orientation to occur. Moreover, those areas of poor orientation may be the core cause of the normally oriented regions of liquid crystal molecules becoming disturbed. This problem will be further discussed below using FIGS.
2
(A) and
2
(B) and FIGS.
3
(A) and
3
(B).
As described above, an optical orientation process is performed through the irradiation of UV rays diagonal to the alignment layer. Accordingly, a photo mask is used when an oriented alignment division is performed. In other words, as shown in FIG.
2
(A), when subjecting the left region of the alignment layer
26
, which is on top of the substrate
25
, to an optical orientation process, the right region is masked by the photo mask
27
(the first condition). Under this first condition, UV rays
31
are irradiated from the diagonal direction
29
. Next, as shown in FIG.
2
(B), when optically orienting the right region of the alignment layer
26
, which is placed on top of the substrate
25
, the left region is masked by the photo mask
28
(the second condition). Under this second condition, UV rays
32
are irradiated from the diagonal direction
30
. Through this process, the liquid crystal molecules in the left region on the surface of the alignment layer
26
are slanted to the left, whereas the liquid crystal molecules in the right region are slanted to the right. As a result, two orienting regions with different orientations are formed.
However, the following problems occur on the boundary of each region of the alignment layer that has gone through an oriented alignment division. FIG.
3
(A) shows the case where the differing con
Hirai Yoshihiko
Matsuyama Hiroaki
McGinn & Gibb PLLC
NEC Corporation
Ngo Julie-Huyen L.
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