Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2001-06-29
2003-02-18
Chowdhury, Tarifur R. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S106000, C349S113000, C349S187000
Reexamination Certificate
active
06522377
ABSTRACT:
This application claims the benefit of Korean Patent Application No. 2000-63567 filed on Oct. 27, 2000, which is hereby incorporated by reference for all purposes as if fully set forth herein.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a transflective liquid crystal display (LCD) device implementing a color filter having varying thickness.
2. Discussion of the Related Art
As an information-oriented society rapidly develops, display devices are accordingly developed. The display device processes and displays a great deal of information. A cathode ray tube (CRT) has served as a mainstream of the display device field. However, to meet the needs of the times, a flat panel display device having small size, light weight, and low power consumption is a subject of research.
A thin film transistor (TFT) liquid crystal display (LCD) device is an example of a flat panel display device. The TFT LCD device is very thin and provides superior color display properties. For operation, a thin film transistor serves as a switching element of the TFT LCD device. The thin film transistor of the TFT LCD device switches a pixel such that the pixel controls a transmittance of light which is incident from a back light of the TFT LCD device. An amorphous silicon layer is widely used for a silicon (active) layer of the TFT, because it can be formed on a large, but relatively cheap, glass substrate at a relatively low temperature. The above-mentioned amorphous silicon TFT (a-Si:TFT) is frequently used for thin film transistors.
In general, the LCD devices are divided into transmissive LCD devices and reflective LCD devices according to whether the display uses an included or an external light source.
A typical transmissive LCD device includes a liquid crystal panel and a back light. The liquid crystal panel includes upper and lower substrates with a liquid crystal layer interposed in between. The upper substrate includes a color filter, and the lower substrate includes thin film transistors (TFT) as switching elements. An upper polarizer is arranged on the liquid crystal panel, and a lower polarizer is arranged between the liquid crystal panel and the back light. However, since the transmissive LCD transmits at most about 7% of the incident rays of light from the back light, it is very inefficient in terms of its power consumption.
For this reason, the transmissive LCD device requires a high back light brightness, and thus electric power consumed by the back light increases. A relatively heavy battery is needed to supply sufficient power to the back light of such a device. However, the battery rapidly discharges.
Unlike a transmissive LCD device, a reflective LCD device uses an ambient rays of light incident from a natural light source or an external artificial light source. Because of its low power consumption, the reflective LCD device is widely used for an electric organizer, a personal digital assistant (PDA), or the like that needs a portable display device.
For the above-mentioned reflective LCD device, an opaque material having a reflective property is selected for a pixel electrode such that the reflective pixel electrode can reflect ambient light. As mentioned previously, in the case of the transmissive LCD device, a transparent conductive material is selected for the pixel electrode such that the incident rays from a back light can pass there-through.
The reflective LCD device, however, is useless when the weather or exterior light source is dark. Accordingly, a transflective LCD device has been developed to compensate for the reflective LCD device and the transmissive LCD device. The transflective LCD device can selectively provide the reflective or transmissive mode, depending on needs of users.
FIG. 1
is a partial cross-sectional view illustrating a transflective LCD device
50
according to a related art. For the sake of convenience, just one pixel portion of the transflective LCD device
50
is shown. The transflective LCD device
50
includes an upper plate
10
(color filter substrate), a lower plate
30
(TFT array substrate), an interposed liquid crystal layer
20
therebetween, and a back light
45
disposed below the lower plate
30
.
Each of the upper and lower plates
10
and
30
includes a transparent substrate
1
. For the upper plate
10
, a color filter
12
is formed on the lower surface of the transparent substrate
1
, and an upper transparent electrode
14
is formed on the color filter
12
. The upper transparent electrode
14
serves as a common electrode. In addition, a half wave plate (HWP)
18
is formed as a retardation film on the upper surface of the transparent substrate
1
, and an upper polarizer
16
is formed on the HWP
18
. The HWP provides a phase difference of “&lgr;/2” such that right-circularly polarized rays incident thereon are changed to left-circularly polarized when they pass therethrough. The upper polarizer
16
serves as a filter selectively transmitting some rays of incident light. That is to say, the upper polarizer
16
has an optical polarizing axis in one direction, and only the rays having the same orientation as the direction of the optical polarizing axis can pass through the upper polarizer
16
.
The HWP
18
serves to improve a viewing angle quality by compensating for phase differences occurring due to users' various viewing angles. Alternatively, a couple of quarter wave plates, which may be respectively formed for the lower and upper plate
10
and
30
, can provide the same optical effect as the HWP
18
provides. However, if the HWP
18
is used, only a single HWP
18
is employed. Therefore, the HWP
18
has advantages in cost and processing time.
Still referring to
FIG. 1
, an insulating layer
33
is formed on the upper surface of the transparent substrate
1
of the lower plate
30
, and a lower transparent electrode
32
is formed on the insulating layer
33
. A passivation layer
34
and a reflective electrode
36
are sequentially formed on the lower transparent electrode
32
, and a transmitting hole
31
is formed passing through the passivation layer
34
and the reflective electrode
36
. In addition, a lower polarizer
40
is formed on the lower surface of the transparent substrate
1
of the lower plate
30
.
When an electric field is applied across the liquid crystal layer
20
, molecules of the liquid crystal layer
20
align according to the electric field. Then, the liquid crystal layer
20
refracts rays of light passing there-through such that a desired image is displayed.
The above-explained transflective LCD device has a transmissive portion “t” that corresponds to a portion of the lower transparent electrode
32
exposed via the transmitting hole
31
, and a reflective portion “r” that corresponds to the reflective electrode
36
. The transmissive portion “t” has a first cell gap “d
1
” between the common electrode
14
and the reflective electrode
36
. Whereas, the reflective portion “r” has a second cell gap “d
2
” between the common electrode
14
and the lower transparent electrode
32
. The first cell gap “d
1
” is designed to be larger than the second cell gap “d
2
” such that incident rays of light have the same efficiency for the transmissive and reflective modes. Specifically, the first cell gap “d
1
” is preferably about two times as large as the second cell gap “d
2
.”
The liquid crystal layer
20
provides a phase difference to light, and the phase difference of the liquid crystal layer
20
is usually determined depending on a refractive index and a cell gap thereof. For the above-mentioned LCD device, however, the liquid crystal layer
20
exhibits the same refractive index throughout the reflective and transmissive portions. Therefore, only the cell gap is the main factor to determine any difference between the phase difference of the liquid crystal layer
20
in the reflective or transmissive portion. Specifically, if the first cell gap “g
1
” is two times as large as the second cell g
Kim Woong-Kwon
Yu Sang-Hee
Chowdhury Tarifur R.
LG. Philips LCD Co. Ltd.
McKenna Long & Aldridge LLP
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