Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
2000-03-30
2002-09-10
Flynn, Nathan (Department: 2826)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C349S110000, C257S072000
Reexamination Certificate
active
06449022
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a pixel structure comprising thin film transistors (TFT) such as a liquid-crystal projector. In particular, it relates to improvement in light shielding in an active matrix type of liquid-crystal display for a light valve where liquid crystal is switched by a TFT. This invention also relates to a method for manufacturing the pixel structure.
2. Description of the Prior Art
A variety of displays using a liquid-crystal panel has been recently developed as a wall-hung or projection-type TV or a display for an OA device. Among liquid-crystal panels, an active matrix type of liquid-crystal display where a thin film transistor as an active element is incorporated in a liquid-crystal display is promising in providing a high-quality display for an OA device or a display for a high definition because increase in the number of scanning lines does not adversely affect contrast or a response speed. It may allow large display to be easily achieved in a projection-type of liquid-crystal display such as liquid-crystal projection.
In a common active matrix type of liquid-crystal display for a light valve used for liquid-crystal projection, an intense light is radiated to a small device for switching liquid crystal by a TFT to turn ON/OFF per a pixel; a transmitted light is controlled, depending on image data; and the transmitted light is extendedly projected on a screen via an optical system such as a lens. When an active layer in the TFT is made of polysilicon (p-Si), a leak current during OFF may be generated in a channel of the TFT by photoexcitation not only due to an incident light but also due to a reflected light from an optical system such as a lens.
In such a conventional active matrix type of liquid-crystal display for a light valve, gate lines
8
and data lines
10
are arranged in a matrix form, i.e., they are orthogonal to each other. A transparent electrode such as ITO
18
is formed in a region delimited by the gate and the data lines while a TFT is formed in a crossing of a gate line
8
with a data line
10
as illustrated in FIG.
11
.
FIG. 12
is an enlarged view of the area encircled in
FIG. 11
, showing a TFT-forming area. In the data line
10
, there is formed a data-line/TFT contact
16
for supplying a signal to a source electrode
13
. A drain electrode
14
is connected to ITO
18
as a pixel electrode via an ITO/TFT contact
17
. An LDD region
15
is formed between the channel (the area covered by the gate line) of the TFT and a source-drain region.
FIG. 13
shows cross sections (a) and (b) taken on lines F-F′ and G-G′ of
FIG. 12
, respectively. In the figure, a film for blocking a light from a rear face (referred to as a “back shield film”)
3
via a base insulating film
2
and a black matrix
12
above the TFT are formed on a transparent insulating substrate such as a glass substrate
1
. Thus, when a light enters through a liquid-crystal layer from the side of an opposed substrate to the. TFT, the black matrix
12
blocks the incident light (IL) while the back shield film
3
blocks a reflected light (RL) from an optical system.
The black matrix
12
may be formed on the same substrate as the TFT via an interlayer film as illustrated in
FIG. 13
or on the opposed substrate to the TFT via the liquid-crystal layer. When the black matrix
12
is formed on the opposed substrate to the TFT, it must be larger than the back shield film
3
in the light of misalignment by about 10 &mgr;m as a precision in superposing the two substrates. As a result, an opening ratio cannot be increased.
To date, the black matrix is, therefore, always formed on the same substrate as the TFT. In such a structure, a large margin as described above is not necessary because a higher alignment precision can be achieved employing a semiconductor device manufacturing method. However, since positional relationship between two shield films and the TFT is not taken into consideration, a light due to irregular reflection within a panel is not been sufficiently blocked. In particular, in the gate-line forming region as shown in FIG.
13
(
b
), there are formed the back shield film
3
and also the black matrix
12
, which are adequate to block a light. On the other hand, in a region around the pixel electrode as shown in FIG.
13
(
a
), the back shield film
3
and the black matrix
12
are limited in their widths for improving a pixel opening ratio. An incident light from the edge of the black matrix
12
is, therefore, reflected on the surface of the back shield film
3
in the region around the pixel electrode in the polysilicon channel between the source and the drain electrodes
13
,
14
and the LDD region
15
, and a reflected light from the edge of the back shield film
3
enters into the LDD region
15
. These reflected lights might cause current leak. Of course, the incidence and the reflected lights contain not only directional components parallel to the gate line as described above, but also various directional components, among of which may enter the channel region under the gate line.
Of course, although the widths of the back shield film and the black matrix can be increased to prevent the incidence or reflected light from entering the channel, it leads to reduction in a pixel-opening ratio.
SUMMARY OF THE INVENTION
Thus, an object of this invention is to provide a pixel structure for a light valve where a pixel opening-ratio can be as large as possible, while preventing an incident light from a substrate surface or a reflected light from an optical system from entering the channel.
This invention provides a TFT (thin film transistor) based pixel structure comprising a back shield film formed on a transparent insulating substrate; a TFT consisting of a polysilicon channel formed on the back shield film with an interlayer film formed therebetween, a gate insulating film and a gate electrode connected to a gate line; a data line for transmitting a data signal to the TFT; and a black matrix for blocking an incident light to the TFT, wherein a dummy contact hole not reaching the back shield film is formed at least in the interlayer film on the back shield film near the lateral face of the TFT along the longitudinal direction of the channel within the region delimited by the back shield film and the black matrix, and a film made of at least an interconnection material is formed on the side wall of the dummy contact hole.
In the above pixel structure, it is preferable to form the dummy contact hole before forming the gate line and to deposit a gate-line material in the dummy contact hole simultaneously with forming the gate line, or to form the dummy contact hole before forming the data line and to deposit a data-line material in the dummy contact hole simultaneously with forming the data line.
This invention also provides a TFT-based pixel structure where the back shield film is formed on the transparent insulating substrate in a matrix form and is wider than the other interconnections only in the region to which the channel and the LDD are projected.
This invention also provides a pixel structure where the TFT is formed in the crossing of the gate line with the data line, and the dummy contact hole is formed at four corners of the crossing.
This invention also provides a method for manufacturing a TFT (thin film transistor)-based pixel structure comprising forming a back shield film, the first interlayer film, a polysilicon to be a channel of the TFT, a gate insulating film, a gate line including a gate electrode, the second interlayer film, a data line, the third interlayer film and a black matrix on a transparent insulating substrate in sequence, wherein after forming the gate insulating film and before forming the gate line, a dummy contact hole not reaching the back shield film is formed in the gate insulating film and the first interlayer film on the back shield film near the lateral face of the TFT along the longitudinal direction of the channel within the region delimited by the back
Fukata Tamaki
Honbo Nobuaki
Flynn Nathan
Hayes & Soloway P.C.
Mondt Johannes P
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