Semiconductor device manufacturing: process – Making field effect device having pair of active regions... – On insulating substrate or layer
Reexamination Certificate
1999-05-13
2001-01-23
Niebling, John F. (Department: 2812)
Semiconductor device manufacturing: process
Making field effect device having pair of active regions...
On insulating substrate or layer
C438S166000, C438S030000
Reexamination Certificate
active
06177301
ABSTRACT:
This application claims the benefit of Korean Patent Application No. 98-21287, filed on Jun. 9, 1998, which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of fabricating a thin film transistor for a liquid crystal display device.
2. Discussion of the Related Art
A liquid crystal display (LCD) device is fabricated by forming various devices on a transparent insulated substrate, such as a glass substrate. To fabricate a thin film transistor (TFT) on a weak heat-resistant or heat-sensitive substrate, such as a glass substrate, an amorphous silicon layer or a polycrystalline silicon layer is first formed on the substrate. An active layer for the TFT is then formed by etching the silicon layer. Characteristics of charge carrier mobility are excellent when a polycrystalline silicon layer is formed on the glass substrate. Accordingly, when a polycrystalline silicon layer is used, devices for a driver and a pixel array of the LCD device are fabricated on the same substrate simultaneously.
FIG. 1
is a schematic drawing of an LCD device on which a pixel array and a driver circuit are simultaneously formed on the same substrate. The pixel array
11
lies in the middle of the substrate
100
, and a gate driver
12
and a data driver
13
are located near two sides of the pixel array
11
on the substrate
100
. The pixel array
11
comprises a plurality of pixel cells
14
defined by a plurality of gate lines
15
connected to the gate driver
12
and a plurality of data lines
16
connected to the data driver
13
. The gate driver
12
drives the pixel cells. The data driver
13
supplies the cells with data signals. The gate driver
12
and the data driver
13
receive external signals through external signal input terminals
17
and supply them to pixel cells
14
. The drivers use inverters of complementary TFTs to generate proper signals for the pixel cells
14
.
FIG. 2
is a schematic drawing of silicon grains
21
in a polycrystalline silicon layer which is formed according to a related art. According to
FIG. 2
, an amorphous silicon layer is formed on a substrate. A laser beam is applied to the amorphous silicon layer and moves by a predetermined distance for each laser pulse. Accordingly, the substrate is scanned by the laser beam, during which the portion of the silicon layer irradiated with the laser beam is melted and crystallized by solidification. By controlling laser energy properly, the portion of the silicon irradiated with the laser beam is almost completely melted, with an unmelted portion remaining at an interface between the silicon layer and the substrate.
In the portion of the silicon layer irradiated with the laser beam, silicon grains grow laterally, rather than in the direction of the thickness of the layer, by using other portions of the silicon that remain unmelted as seeds. Depending on a state of the amorphous silicon layer and the laser energy supply, the seeds are located at an interface between the substrate. Therefore, a polycrystalline silicon layer having grains of irregular sizes at random locations is formed.
When fabricating a TFT of a coplanar type, an active layer is formed by patterning a polycrystalline layer formed on the insulated substrate using, e.g., photolithography. A gate insulating layer and a gate electrode are then formed on the active layer, and a source and a drain region are formed by doping the active layer with impurities to form a TFT for the LCD device.
FIG. 3
shows a schematic layout of a polycrystalline silicon TFT according to the related art. The grain pattern of the active layer has no significance on the channel direction because the silicon grains in the active layer are scattered irregularly on the substrate. Referring to
FIG. 3
, a plurality of silicon grains
34
are included in an active layer
30
of the TFT. The charges moving from the source region
31
through the channel region
32
toward the drain region
33
of the TFT are greatly affected by the grain boundaries in the channel region. Hence, the charge carrier mobility of the polycrystalline silicon TFT is much smaller than that of a single crystalline silicon TFT. Moreover, physical characteristics of the TFT formed on the substrate are irregular since the polycrystalline silicon layer contains irregular grains.
Accordingly, circuit malfunctioning may occur in the gate and data drivers which include TFTs, causing external signals to be transferred unevenly to the gate and data lines. In addition, the TFTs formed irregularly in the pixel array may cause the image characteristics of the LCD to deteriorate.
SUMMARY OF THE INVENTION
Accordingly, the present invention is directed to a method of fabricating thin film transistors for a liquid crystal display device that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a method of fabricating TFTs for an LCD by which physical characteristics of TFTs on a substrate become uniform.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practicing the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purposes of the present invention, as embodied and broadly described, a method of fabricating a thin film transistor (TFT) for a liquid crystal display device having a driver and a pixel array formed on a substrate comprises the steps of forming a polycrystalline silicon layer on the substrate by growing silicon grains in a first direction from an amorphous silicon layer using a sequential lateral solidification technique; forming an active layer by patterning said polycrystalline silicon layer, said active layer defining a channel of the TFT having a direction inclined at a predetermined angle with respect to said first direction; and forming the TFT on the active layer.
According to another embodiment of the present invention, a method of fabricating a liquid crystal display (LCD) device having a driver and a pixel array formed on a substrate comprises the steps of forming a polycrystalline silicon layer on the substrate by growing silicon grains in a first direction from an amorphous silicon layer using a sequential lateral solidification technique; forming an active layer by patterning said polycrystalline silicon layer, said active layer defining a plurality of channel regions having a second direction inclined at a predetermined angle with respect to said first direction; and forming a plurality of thin film transistors (TFTs) on the active layer, the channel regions of the active layer forming the channels of the TFTs, wherein the TFTs are devices for the driver and the pixel array of the LCD device.
In another aspect of the present invention, a liquid crystal display (LCD) device comprises a gate drive circuit having a plurality of thin film transistors (TFTs); a data drive circuit having a plurality of TFTs; a plurality of gate lines and data lines connected to the gate and data drive circuits, respectively; and a pixel array having a plurality of pixel cells defined by the gate lines and data lines, each pixel cell having a TFT, wherein each TFT has a channel region formed of a thin polycrystalline silicon layer by a sequential lateral solidification technique, the polycrystalline silicon layer having elongated silicon grains grown laterally in a predetermined direction and having substantially no grain boundaries in the direction of the thickness of the layer, and wherein the TFTs define a channel direction which is inclined at a predetermined angle with respect to the growth direction of the silicon grains.
It is to be understood that both the foregoing general description
LG.Philips LCD Co. , Ltd.
Long Aldridge & Norman
Niebling John F.
Simkovic Viktor
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