Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2001-05-08
2003-12-02
Ton, Toan (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S113000
Reexamination Certificate
active
06657689
ABSTRACT:
This application claims the benefit of Korean Application No. 2000-24481 filed on May 8, 2000, which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly, to a transflective liquid crystal display and method of fabricating the same. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for a high contrast ratio.
2. Discussion of the Related Art
In general, a liquid crystal display (LCD) is classified as a transmission type and a reflection type depending on implementing an internal or external light source. The transmission type has a liquid crystal display panel, which does not emit light itself, and has a backlight as a light-illuminating section.
The backlight is disposed at the rear or one side of the panel. The amount of the light from the backlight that passes through the liquid crystal panel is controlled by the liquid crystal panel in order to implement an image display. In other words, the light from the backlight varies and displays images according to the arrangement of the liquid crystal molecules. However, the backlight of the transmission type LCD consumes 50% or more of the total power consumed by the LCD device. Providing a backlight therefore increases power consumption.
In order to overcome the above problem, a reflection type LCD has been selected for portable information apparatuses that are often used outdoors or carried with users. Such a reflection type LCD is provided with a reflector formed on one of a pair of substrates. Thus, ambient light is reflected from the surface of the reflector. The reflection type LCD using the reflection of ambient light is disadvantageous in that a visibility of the display is extremely poor when surrounding environment is dark.
In order to overcome the above problems, a construction which realizes both a transmissive mode display and a reflective mode display in one liquid crystal display device has been proposed. This is so called a transflective liquid crystal display device. The transflective liquid crystal display (LCD) device alternatively acts as a transmissive LCD device and a reflective LCD device. Due to the fact that a transflective LCD device can make use of both internal and external light sources, it can be operated in bright ambient light as well as has a low power consumption.
FIG. 1
shows a typical transflective liquid crystal display (LCD) device
11
. The transflective LCD device
11
includes upper and lower substrates
15
and
21
with an interposed liquid crystal
23
. The upper and lower substrates
15
and
21
are sometimes respectively referred to as a color filter substrate and an array substrate.
On the surface facing into the lower substrate
21
, the upper substrate
15
includes a black matrix
16
and a color filter layer
17
. The color filter layer
17
includes a matrix array of red (R), green (G), and blue (B) color filters that are formed, such that each color filter is divided by the black matrix
16
. The upper substrate
15
also includes a common electrode
13
over the color filter layer
17
and the black matrix
16
.
On the surface facing into the upper substrate
15
, the lower substrate
21
includes an array of thin film transistors (designated as TFT “T” in
FIG. 1
) that act as switching devices. The array of thin film transistors is formed to correspond to the matrix of color filters. A plurality of gate and data lines
25
and
27
are positioned and crossed over each other. A TFT is located near at each crossing portion of the gate and data lines
25
and
27
. The lower substrate
21
also includes a plurality of pixel electrodes
19
in the area between the gate and data lines
25
and
27
. Such an area is often referred to as pixel regions “P”, as shown in FIG.
1
.
Each pixel electrode
19
includes a transparent portion
19
a
and a reflective portion
19
b
. The transparent portion
19
a
is usually formed of a transparent conductive material having good light transmissivity, such as indium tin oxide (ITO). Alternatively, the transparent portion
19
a
may be a hole. Moreover, a conductive metallic material having a superior light reflectivity is used for the reflective portion
19
b.
FIG. 2
, a schematic cross-sectional view of a transflective LCD device
57
illustrating an operation of such devices. For convenience, the color filters
17
(shown in
FIG. 1
) are not shown in
FIG. 2
because it does not affect the polarization state of light. As shown in
FIG. 2
, the transflective LCD device
57
includes lower and upper substrates
21
and
15
and an liquid crystal layer
23
having optical anisotropy is interposed therebetween.
The upper substrate
15
includes a common electrode
13
on its surface facing into the lower substrate
21
. On the other surface of the upper substrate
15
, an upper quarter wave plate (QWP)
45
(often referred to as a retardation film), which has a phase difference &lgr;/4, and an upper polarizer
55
are formed in series.
The lower substrate
21
includes a transparent electrode
50
on its surface facing into the upper substrate
15
. A passivation layer
48
and a reflective electrode
19
b
are formed in series on the transparent electrode
50
. The reflective electrode
19
b
and the transparent electrode
50
act together as a pixel electrode (the reference numeral
19
of FIG.
1
). The passivation layer
48
and the reflective electrode
19
b
also have a transmitting hole
19
a.
Various configurations and structures may be implemented for the pixel electrode in the transflective LCD device. However, the passivation layer
48
should be formed between the transparent electrode
50
and the reflective electrode
19
b.
In order to form a pixel electrode, a transparent conductive material such as ITO (indium tin oxide) or IZO (indium zinc oxide) is deposited on the lower substrate
21
and then patterned into the transparent electrode
50
.
Next, the passivation layer
48
is formed on the transparent electrode
50
. The conductive metallic material having superior reflectivity, such as aluminum (Al) or the like, is deposited on the passivation layer
48
and then patterned to form a reflective electrode
19
b
. In this patterning process, the transmitting hole
19
a
as a transparent portion is formed at the central portion of the reflective electrode
19
b
. Moreover the central portion of the passivation layer
48
corresponding to the hole
19
a
is also patterned to expose the central portion of the transparent electrode
50
.
Accordingly, the transparent electrode
50
and the reflective electrode
19
b
serve as a pixel electrode. Moreover, this structure makes different cell gaps “d
1
” and “d
2
” between the common electrode
13
and the pixel electrode (the reflective electrode
19
b
and the transparent electrode
50
). “d
1
” denotes the first cell gap between the common electrode
13
and the reflective electrode
19
b
while “d
2
” denotes the second cell gap between the common electrode
13
and the transparent electrode
50
.
On the other surface of the lower substrate
21
, a lower quarter wave plate
54
and a lower polarizer
52
are formed in series. Moreover, a backlight device
41
is arranged below the lower polarizer
52
.
In a homogeneous liquid crystal or twisted nematic (TN), its molecules are oriented in the vertical direction when a voltage is applied (V
on
=5V) and used as a liquid crystal layer
23
. When an optical retardation “&Dgr;n·d
1
” of a first cell gap is &lgr;/4 (&lgr;=550 nm) and a second cell gap “d
2
” is twice as large as the first cell gap “d
1
” as described by equations (1) and (2), an optical retardation “&Dgr;n·d
2
” of the second cell gap “d
2
” is shown in equation (3).
&Dgr;
n·d
1
=&lgr;/4 (1)
d
2
≅2
d
1
(2)
∴&Dgr;
n·d
2
≅&lgr;/2 (3)
In the above equations, &Dgr;n is birefringence, d
1
denotes the first cell gap between the
LG. Philips LCD Co. Ltd.
Morgan & Lewis & Bockius, LLP
Rude Timothy L
Ton Toan
LandOfFree
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