Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2001-06-28
2004-01-20
Ton, Toan (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S180000
Reexamination Certificate
active
06680768
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a liquid crystal display device and, more particularly, to a liquid crystal display device having a wide viewing angle by carrying out display with molecule axes in liquid crystal molecules rotated in a plane which is substantially parallel to a surface of a substrate.
One of liquid crystal display device is a color TFT type liquid crystal display device in which thin film transistors (TFTs) are used as switching elements and which is capable of carrying out a color display.
A conventional liquid crystal display device is disclosed in Japanese Unexamined Patent Publication No. Hei 6-160878, namely, JP-A 6-160878. That is, JP-A 6-160878 obtains an active matrix type liquid crystal display device having such features that the contrast is high, visual angle property is good, a multi-level display can be easily performed, and that the display is bright and the cost is reduced. The active matrix type liquid crystal display device has such a structure that an electric field parallel to a substrate surface is impressed on a liquid crystal composition layer by a thin film transistor provided with a drain electrode and a common electrode which are extending over a plurality of picture elements and a source electrode extending in a direction same as that of the drain and the common electrodes.
In the manner which will later be described in conjunction with
FIGS. 1A
,
1
B,
2
A, and
2
B in more detail, the conventional liquid crystal display device comprises first and second glass substrates spaced apart in a longitudinal direction. The first and the second glass substrates are substantially flat in shape. The first glass substrate has a first principal surface while the second glass substrate has a second principal surface opposed to the first principal surface via a space. In the manner known in the art, a plurality of thin film transistors (TFTs) are formed on the first principal surface of the first glass substrate in an array or matrix fashion. On the other hands, a plurality of color filters (CFs) are formed on the second principal surface in the manner known in the art.
The first glass substrate has a first back surface opposed to the first principal surface. A first polarizer is attached to the first back surface of the first glass substrate. Likewise, the second glass substrate has a second back surface opposed to the second principal surface. A second polarizer is attached to the second back surface of the second glass substrate. The first and the second polarizers may have first and second light transmission axes, respectively, which are perpendicular to each other.
A plurality of pixel electrodes are formed on the first principal surface of the first glass substrate in an array or matrix fashion. The pixel electrodes are equal in number to the thin film transistors and are called display electrodes. Each pixel electrode corresponds to a pixel and extends in a vertical direction or a panel up-and-down direction. Similarly, a plurality of common electrodes are formed on the second principal surface of the second glass substrate in the manner known in the art. Each common electrode extends in the vertical direction with its position shifted from that of the pixel electrode.
On the first principal surface of the first glass substrate, a first alignment layer
36
is formed or applied so as cover the pixel electrodes and the thin film transistors therewith. Likewise, a second alignment layer is formed or applied on the second principal surface of the second glass substrate so as to cover the common electrodes and the color filters therewith.
In addition, the first and the second glass substrates put a liquid crystal layer between the first and the second alignment layers. The liquid crystal layer includes a plurality of liquid crystal molecules. Each liquid crystal molecule has a long axis and a short axis. The long and short axes of each liquid crystal molecule lies in a plane which is substantially parallel to the surfaces of the first and the second glass substrates and which is defined by both of the vertical direction and a lateral direction. The long axis of each liquid crystal molecule extends in a direction which is called a director in the art. Both of the first and the second alignment layers are subjected to alignment treatment so that the director of each liquid crystal molecule inclines to one side from the vertical direction of the pixel electrodes and the common electrodes by a predetermined angle. This is because there is a chance that a display is poor due to occurrence of domain in the boundaries at which two deformations occur in the opposite direction if the director of each liquid crystal molecule is parallel to the vertical direction. At any rate, the liquid crystal molecules are uniformly aligned upon no generation of the electric field between the pixel electrodes and the common electrodes.
When a lateral electric field generates between the pixel electrode and the common electrode, each liquid crystal molecule rotate in the plane which is substantially parallel to the first and the second principal surfaces of the first and the second glass substrates and so that the director of each liquid crystal molecule is turned in a different direction on no generation of the lateral electric field.
Although the pixel electrodes and the common electrodes are formed on the substrates which are different from each other, JP-A 6-160878 teaches that the pixel electrodes and the common electrodes may be formed on the same substrate.
As described above, the liquid crystal display device of the IPS mode carries out display by rotating each liquid crystal molecule in a plane which is substantially parallel to the surfaces of the substrates caused by generation of the lateral electric field in a plane which is substantially parallel to the surfaces of the substrates. Accordingly, when a person sees or watches the liquid crystal display device from the front, the person's eyes basically receive light passing through the short axis of each liquid crystal molecule alone although the person moves his or her observing point in a predetermined view range. As a result, the liquid crystal display device of the IPS mode has no dependency on a visual field angle caused by a “rising way” of each liquid crystal molecule and it is possible for the liquid crystal display device to achieve a wider viewing angle in comparison with a liquid crystal display device of a twisted nematic (TN) mode. This is because the person's eyes receive light passing through the long axis as well as the short axis of each liquid crystal molecule in the liquid crystal display device of the TN mode when the person moves his or her observing point in the predetermined view range.
By the way, from a point of view in use conditions or the like, it is necessary for the liquid crystal display device that a contrast ratio related to the viewing angle in the panel right-and-left direction or the lateral direction is symmetrical and a contrast ratio related to the viewing angle in the panel up-and-down direction or the vertical direction is also symmetrical. In other words, it is necessary that the liquid crystal display device has not only a right-to-left symmetrical characteristic of the viewing angle in the right-and-left direction but also a up-and-down symmetrical characteristic of the viewing angle in the panel up-and-down direction. Furthermore, it is desirable that the liquid crystal display device has a symmetrical characteristic of the viewing angle in all of the up-and-down direction and the right-and-left direction. Herein, the contrast ratio (CR) is the ratio of a light transmittance in a state where white is displayed to a light transmittance in a state where black is displayed. In addition, the characteristic of the viewing angle (or an equivalent CR distribution) is a distribution characteristic indicating that how is the contrast ratio distributed for each viewing angle where the person looks in at the panel of the liquid crystal displa
Fujimaki Eriko
Matsumoto Kimikazu
Matsuyama Hiroaki
Nishida Shin-ichi
NEC LCD Technologies Ltd.
Sughrue & Mion, PLLC
Ton Toan
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