Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1998-12-23
2002-01-08
Dudek, James A. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
Reexamination Certificate
active
06337729
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an in-plane switching type liquid crystal display device and manufacturing method thereof. More specifically, the present invention relates to material of a spacers for keeping a space between panels and a method of distributing the spacers.
2. Description of the Background Art
A liquid crystal display device which advantageously has the feature of thin form, light weight, low power consumption and so on has been widely used as a display device for a watch, a pocket calculator and the like. Especially, a TN (twisted nematic) type liquid crystal display device of which active driving is implemented by a thin film transistor (TFT) or the like has been increasingly popular in the field of display devices for word processors, personal computers and the like for which CRT (cathode lay tube) has been conventionally used. The TN type liquid crystal display device, however, generally has a narrow angle of visibility, and when viewed obliquely, contrast decreases and gradation is inverted. In order to solve this problem, an in-plane switching type liquid crystal display device has been proposed.
The principle of operation of the in-plane switching type liquid crystal display device will be described in the following with reference to
FIGS. 3A and 3B
.
FIGS. 3A and 3B
are schematic illustrations showing the state of orientation of liquid crystals in a common in-plane switching type liquid crystal display device. In the figures, interdigital electrode
1
a
and
1
b
represent a plurality of electrodes arranged parallel to each other on the same substrate. Interdigital electrode
1
a
and
1
b
are formed on electrode substrate
3
. Between a counter substrate
4
and electrode substrate
3
, liquid crystal molecules
2
are arranged with an orientation film (not shown) interposed. An equipotential line
5
of an electric field is generated by a voltage applied between interdigital electrode
1
a
and
1
b
. A pair of polarization plates
7
,
8
have transmission axes in the directions of the arrows.
FIG. 3A
shows the state of orientation of liquid crystal molecules
2
when no voltage is applied between the pair of interdigital electrode
1
a
and
1
b
, and
FIG. 3B
shows the state of orientation when a voltage is applied between the interdigital electrode
1
a
and
1
b
. When the voltage is not applied, liquid crystal molecules
2
are oriented in the direction
10
shown in FIG.
3
A. At this time, when the polarization plate
7
is arranged with the transmission axis thereof aligned with the direction of orientation
10
and polarization plate
8
is arranged orthogonal thereto, an incident light beam
6
cannot pass through polarization plate
8
, and therefore the display is black (dark). When a voltage is applied between interdigital electrode
1
a
and
1
b
, an electric field is generated approximately parallel to the substrate surface, and the direction of orientation of liquid crystal molecules
2
changes as shown in FIG.
3
B. More specifically, birefringence of the liquid crystal layer changes, so that the incident light beam passes through polarization plate
8
and goes out as outgoing transmission light beam
9
, resulting in white (bright) display. In the in-plane switching type liquid crystal display device, the liquid crystal molecules
2
switch approximately parallel to the substrate surface dependent on application
on-application of the voltage. Therefore, even when the viewing direction changes, optical contribution of liquid crystal molecules
2
hardly changes. Therefore, even when viewing angle changes, the contrast ratio and display quality are not degraded. Thus, the device provides excellent viewing angle characteristics.
In an actual in-plane switching type liquid crystal display device, uniaxial orientation disorder occurs near a spacer or spacers used for keeping a space between panels, causing birefringence with respect to the incident light beam, resulting in elliptic polarization. This allows passage of light beam through the other polarization plate
8
, which passage of light is viewed as whitening in the black (dark) state. The state of light passage will be described with reference to
FIGS. 4A and 4B
. In the figure, there is a portion
11
where liquid crystal orientation is in disorder. A conventional spacer
12
is of divinyl benzene, acrylate resin or the like. Charges
13
are held by spacer
12
. In the figure, portions coirresponding to those of FIGS.
3
A and
3
B are denoted by the same reference characters.
The conventional spacer
12
has such a nature that tends to hold charges on its surface during the process of manufacturing a liquid crystal display device. When the conventional spacer
12
is distributed to a surface of either one of electrode substrate
3
or counter substrate
4
on which an orientation film (not shown) is formed, spacer
12
is charged to positive or negative polarity, through contact between spacer
12
and a carrier pipe system of a spacer distributing apparatus. Conventionally, the carrier pipe system of the spacer distributing apparatus is generally formed of a resin based material such as Teflon (trademark) or stainless steel, in view of cost and ease in molding. As spacer
12
is brought into contact with the carrier pipe system formed of a material different from that of spacer
12
, polarization occurs near the contact surface, and when the contact surface is separated, charges
13
are generated on the surface of spacer
12
. Such a phenomenon is referred to as separation charging. When a liquid crystal material is introduced in this state, liquid crystal molecules
2
in the vicinity of spacer
12
come to be in the state where monoaxial orientation is disordered, as shown in
FIG. 4A
or
4
B, dependent on the state of generation of charges
13
held by spacer
12
or on the characteristics of the liquid crystal material. Here, at the portion
11
where liquid crystal orientation is in disorder, light beam entering from the lower side of the panel is transmitted to the upper side of the panel because of the influence of birefringence of the liquid crystal layer, and viewed as a light passage (whitening). This light passage is particularly noticed which display is in black (dark) state. Contrast ratio, which is one of the display characteristics of the liquid crystal display device, that is, (brightness (transmittance) in the white (blight) state)/(brightness (transmittance) in the black (dark) state) degrades, as brightness (transmittance) in the black state increases. Further, by this light passage, the viewer has an impression that display screen is rough.
In order to provide a liquid crystal display device presenting images of uniform high quality without light passage, Japanese Patent Laying-Open No. 9-160051 proposes a method in which the spacers for defining a space between a pair of electrode substrate are directly fixed on a prescribed positions of the orientation film formed on the electrode substrate, for example on positions of electrode region or on a light shielding film region of the other orientation film. According to this method, the spacers can be distributed and fixed with a narrow pitch and high density only on that orientation film positions which correspond to non-open positions of the liquid crystal display device. Therefore, it is expected that a liquid crystal display device of high quality free of unwanted light leakage, unsatisfactory display or dependency of angle can be obtained. In order to selectively arrange the spacers at prescribed positions, however, it is necessary to add new process steps to the conventional steps of manufacturing. According to the aforementioned Japanese Patent Laying-Open No. 9-160051, it is necessary to newly add the step of irradiating a thermosetting material or the orientation film formed of a thermosetting material, having the function of fixing the spacer material when irradiated with light beam or heat, with ultraviolet ray using a mask patter
Dudek James A.
McDermott & Will & Emery
Mitsubishi Denki & Kabushiki Kaisha
Schechter Andrew
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