Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2000-09-18
2004-09-21
Dudek, James A. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S111000, C349S044000
Reexamination Certificate
active
06795143
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an active matrix liquid crystal display device utilizing a switching element such as a thin film transistor (hereinafter called TFT), and the method for manufacturing the same, and more specifically, to a shading means for shading the switching element, and the method for manufacturing the same.
DESCRIPTION OF THE RELATED ART
Liquid crystal display devices are known for their advanced characteristics such as being light-weight, having reduced thickness, and exhibiting low power consumption, and active research and development is performed in the field. A liquid crystal display comprises “pixel elements” arranged in matrix, which are formed by placing liquid crystal molecules in between transparent electrodes. When an arbitrary voltage is provided between the transparent electrodes corresponding to each pixel element, the alignment of the liquid crystal molecules in the pixel element is changed, and the degree of polarization of the light passing through the liquid crystal is varied, which leads to controlling the transmission rate of the light. The liquid crystal display device is divided into two types based on operation principles, that is, the simple matrix type and the active matrix type. Since the active matrix liquid crystal display device utilizes active elements, TFTs, switching elements for individual pixel elements, independent signals can be transmitted to each pixel element, and the device provides improved resolution and a clear display image.
A TFT utilizing amorphous silicon thin film is often used as the switching element for the active matrix liquid crystal display device. Moreover, a recently proposed technique refers to a TFT that utilizes a polysilicon thin film formed either by heat treating an amorphous silicon thin film in a temperature over 600° C., or by providing a laser crystallization in which a pulse laser (such as an excimer laser) is radiated to the thin film for recrystallization. The polysilicon thin film is advantageous in that it has higher mobility compared to the amorphous silicon thin film, which allows for not only the switching elements for the pixels but also the driving circuit for driving the switching elements of the pixels to be formed on the same substrate using the polysilicon TFT.
As mentioned above, the liquid crystal display device controls the transmission rate of the light passing through the liquid crystal by changing the degree of polarization of the light passing through the liquid crystal, but the device itself is not equipped with a light emitting member. Therefore, a light source of some sort must be provided to the device. For example, in the case of a transmission-type liquid crystal display device, a lighting device, a so-called light, is placed on the back side of the liquid crystal display, and the light transmitted through the device enables images to be displayed. In the case of a projector, a metal halide lamp and the like are used as the light source, and image is projected by combining the liquid crystal display device with a lens system. Moreover, in case of a reflection-type display, the incident light provided from the exterior is reflected by a reflecting electrode in order to display an image.
In general, if light is radiated to a semiconductor, such as silicon, and light absorption occurs, electrons are excited to the conductive band and positive holes are excited to the valence band, generating electron-hole pairs and causing a so-called photoelectric effect. The same could be said for the amorphous silicon thin film or the polysilicon thin film utilized as the pixel switching elements. By radiating light thereto, electron-hole pairs are generated in the thin film. Accordingly, when light is radiated to the TFT using either the amorphous silicon thin film or the polysilicon thin film as the active layer, photocurrent is caused by the electron-hole pairs, which increases the leak current during the off-state of the TFT. This leads to deteriorating the contrast and the like of the liquid crystal display.
In the case of a reflection-type liquid crystal display device, the reflecting electrode mainly composed of a metal film connected to the TFT is arranged to cover the TFT, so that no incident light from the exterior reaches the TFT directly. Accordingly, TFT leak current is prevented from increasing. However, in the case of a transmission-type liquid crystal display device, the TFT is not only exposed constantly to the strong light from the back light, but some incident light other than that from the back light also tends to reach the TFT. Moreover, in the case of projectors, the light reflected by the lens may reach the TFT. Accordingly, various inventions are proposed that aim at preventing incident light from reaching the TFT.
For example, as shown in
FIG. 11
, shading film
63
and shading film
64
are provided above and under the switching electrode
62
via insulation layers, in order to block the light coming from above and under the switching element (Japanese Patent Application Laid-Open Publication No. 58-159516). This is effective in reducing leak current, and in improving display characteristics.
According to another proposal, as shown in
FIG. 12
, in an adhered SOI substrate, an upper shading layer
66
and a lower shading layer
67
are provided above and under a MOSFET
65
, in order to block the direct incident light coming from above and under the MOSFET, and to also block the light reflected by the back surface of the substrate, thereby effectively preventing an increase of TFT leak current (Japanese Patent Application Laid-Open Publication No. 10-293320).
According to yet another proposal, as shown in
FIG. 13
, by providing a shading film
69
under the switching element
68
and providing a black matrix
70
formed of silicon thin film and silicide film on the opposing substrate, not only the direct incident light is blocked, but also the reflection of light within the liquid crystal display device is restrained, since the fine unevenness provided to the surface reduces the reflection rate and diffuses light (Japanese Patent Application Laid-Open Publication No. 10-319435).
According to the above method, shading layers are provided above and under the TFT so as to prevent incident light coming in from the exterior from reaching the semiconductor film or active layer of the TFT, and most of the incident light fails to reach the semiconductor film. However, the incident angle of the light coming into the liquid crystal display device is not always perpendicular the substrate, but has a certain degree of dispersion, and the light entering the display device may be repeatedly reflected within the device. When light reaches the TFT according to these reasons, the light causes problems such as an increase of TFT leak current.
As shown in FIG.
10
(
a
), light (A) and light (B) are blocked by the upper shading layer
54
and the lower shading layer
51
, and they will not reach the TFT
55
. However, the oblique incident light (C) coming from the side of upper shading layer
54
is reflected by the lower shading layer
51
, and reaches the TFT
55
. Moreover, the oblique incident light (D) coming from the side of upper shading layer
54
side is reflected by the lower shading layer
51
, then reflected by the upper shading layer
54
, before reaching the TFT
55
. Similarly, the incident light (E) and (F) coming from the side of lower shading layer
51
also reaches the TFT
55
after being reflected one or more times. Therefore, according to the proposal of Japanese Patent Application Laid-Open Publication No. 58-159516, light traveling as mentioned above will reach the transistor causing an increase of leak current.
Moreover, as shown in FIG.
10
(
b
), when the upper shading layer
60
is larger than the lower shading layer
57
, the oblique incident light (C), (D) and (G) coming from the side of upper shading layer
60
is blocked by the upper shading layer, but on the other hand the oblique incident light (E), (F) and (I) comin
Dudek James A.
Nixon & Vanderhye PC
Sharp Kabushiki Kaisha
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