Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1999-06-23
2002-06-04
Ton, Toan (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S110000, C349S129000, C349S191000
Reexamination Certificate
active
06400440
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to liquid crystal displays, and more particularly to structures for introducing pretilt to liquid crystal materials for the displays.
2. Description of the Related Art
Flat panel displays have become increasingly important in the computer industry and in other industries where the display of information is important. These types of displays provide unique opportunities for lowering the weight, size, power consumption and eventually the cost of displaying information.
Liquid crystal displays seem to hold the most promise as the technology which will eventually be utilized in almost all practical flat panel displays. Considerable success has been achieved in small size color televisions and in monochrome flat panel displays as well as larger sizes used in color notebook, laptop computers or desk top monitors. However, unlike the cathode ray tube display, which exhibits good viewing quality from a variety of angles, conventional liquid crystal displays suffer from a loss of contrast or contrast reversal when viewed at an angle beyond about 15 degrees from the normal to the plane of the display. This is due to the interaction of light with the molecules of the liquid crystal material in the liquid crystal display cells. Light traveling through these display cells at other than a normal angle of incidence interacts with the liquid crystal display molecules in a manner different from that of light traveling with normal incidence. The contrast between a light transmissive (white) state and a non-transmissive state (black) at other than the normal angle is decreased, thus making such displays less desirable for use in many applications, such as flat panel television screens and large computer screens.
There have been various attempts to solve this problem. One method is discussed in U.S. Pat. No. 5,309,264, commonly assigned to the assignee of the present invention, wherein a pattern of openings is formed in the common electrode. Such openings cause the display elements of the display to have more than one liquid crystal domain. This is an elegant approach; however, to provide sufficient optical performance, the width of such openings is required to be about twice that of the cell gap or larger. Importantly, for high resolution displays (such as ≧120 pixels per inch), the width of a given display element may be on the order of twice or more that of the cell gap. In this case, this method becomes ineffective.
Another approach to solving this problem is to use an in-plane switching LC mode. This has the disadvantage that closely spaced electrodes are needed to provide the required lateral electric fields. The needed electrodes reduce the yield, aperture ratio, and scale poorly to higher resolution displays.
There have been various attempts to provide liquid crystal displays with a wide viewing angle without degrading the contrast ratio or brightness. The wide viewing angle must also be provided at low cost. One method is discussed in application Ser. No. 08/960,826, which is commonly assigned to the assignee of the present invention and incorporated herein by reference.
Referring to
FIG. 1
, a top view of a conventional liquid crystal display device
30
is shown wherein a pixel electrode
26
is formed below the pixels (6 are shown) of display
30
. The pixels are formed between gate lines
32
(3 shown) and data lines
31
(4 shown).
FIG. 2
illustrates a partial cross-section of the conventional liquid crystal display device
30
of FIG.
1
. Device
30
includes a first substrate
25
and a second substrate
27
formed of a transparent material such as glass. The two substrates are arranged so as to be parallel to one another with a high degree of precision. Typically, the substrates
25
,
27
are separated from one another by a distance of approximately one to twenty microns, and are sealed at their edges (not shown) so as to define a closed interior space there between. First substrate
25
has deposited thereon an array of pixel electrodes
26
which define pixels of the liquid crystal display. Also formed on substrate
25
, in selected areas not having electrode films deposited thereon, are semiconductor devices such as diodes or thin film transistors (TFTs)
37
. As is well known in the art, there are one or more TFTs
37
for each pixel. TFTs
37
are each controlled by a conductive gate line
32
(not shown) and a conductive data line
31
, which are typically deposited on substrate
25
in a manner so as not to be electrically connected to electrodes
26
except that the source of each TFT
37
is electrically connected to one respective electrode
26
. Gate lines
32
(not shown) and data lines
31
are also electrically insulated from one another at crossover regions. The second substrate
27
typically has deposited thereon a color matrix layer
23
. The color matrix layer
23
typically has a black matrix material
23
-
1
interleaved with R, G, or B color matrix material
23
-
2
and is frequently underneath the R, G, B color matrix material. The black matrix material
23
-
1
is disposed opposite the TFTs
37
, data line
31
and gate line
32
(not shown) to block the devices from ambient incident light and prevent light leakage outside the pixel area. The color matrix material
23
-
2
is disposed opposite the pixel electrode
26
. In addition, a continuous electrode
28
is typically formed on the color matrix layer
23
or a transparent overcoat layer. The continuous electrode
28
is preferably formed of a thin transparent layer of a conductive material, such as indium tin oxide (ITO) or other suitable material.
A liquid crystal material
36
fills the space between substrates
25
and
27
. The nature of the material depends on the mode of operation of liquid crystal display
30
.
The interior surfaces of the liquid crystal display may be coated with respective alignment layers
38
and
40
to provide boundary conditions for the molecules of liquid crystal material
36
.
The exterior surfaces of substrates
25
and
27
may have respective optical compensating films
42
and
44
disposed thereon. Finally, respective polarizing films
46
and
48
may be applied over compensation films
42
and
44
(if compensating films are used), respectively, or applied over substrate
25
and
27
(if compensating films are not used), respectively.
Conventional liquid crystal displays of the type illustrated in
FIG. 2
are illuminated by a light source (not shown) that is located below the panel (the substrate
25
side) and viewed from above the panel (the substrate
27
side).
Liquid crystal cells typically are characterized by a pixel area and cell gap. The pixel area of a given cell is defined by the width W and the length L of the pixel electrode pattern of the cell as illustrated in FIG.
1
. In addition, the cell gap is defined by the distance between the alignment layers
38
,
40
as shown in FIG.
2
.
As illustrated in
FIG. 3A
, in the case of a homeotropic type LCD, liquid crystal (LC) molecules near the electrodes
26
and
28
are aligned so that the long axes of the LC molecules are almost perpendicular to the electrode surfaces when no electric field is applied between the pixel electrode
26
and the electrode
28
. The molecules have a small pretilt angle, typically one to fifteen degrees of tilt, away from the substrate normal. As illustrated in
FIG. 3B
, when an electric field is applied between the electrodes
26
and
28
of the homeotropic liquid crystal display cell, the molecules are caused to be oriented in a direction substantially perpendicular to the electric field.
Homeotropic liquid crystal cells require a liquid crystal material that exhibits negative dielectric anisotropy, such as ZLI-4788, ZLI-2857 or 95-465MLC manufactured by E. Merck Darmstadt of Germany and available in the United States through EM Industries. The alignment of the LC molecules of the homeotropic cells is typically provided by rubbing alignment layers
38
,
40
. An example of such
Colgan Evan G.
Lien Shui-Chih A.
Schleupen Kai R.
Schechter Andrew
Ton Toan
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