Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2002-10-02
2004-06-08
Prenty, Mark V. (Department: 2822)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S347000, C257S344000
Reexamination Certificate
active
06747325
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a thin film transistor, and more particularly to a lightly doped drain (LDD) structure of the thin film transistor. The present invention also relates to a process for producing such an LDD structure of a thin film transistor.
BACKGROUND OF THE INVENTION
With the increasing development of integrated circuits, electronic devices have a tendency toward miniaturization. As is known, TFTs (Thin Film Transistors) are widely used as basic elements for controlling pixels of a TFT liquid crystal display (TFT-LCD). As a result of miniaturization, a channel between a source region and a drain region in each TFT unit will become narrower and narrower. Therefore, a short channel effect is likely to occur. Such short channel effect possibly causes the TFT unit to be undesirably turned on even when the gate voltage is zero. The switch function of the transistor is thus failed. In addition, the electric field intensity at the channel increases due to the short distance. Therefore, hot electrons in the vicinity of the drain region have a higher energy compared with the energy gap of the semiconductor. The electrons in valence bands might be promoted to conduction bands when being collided by the hot electrons, thereby producing many electron-hole pairs. Such phenomenon is also referred as a “hot electron effect”.
In a TFT-LCD, the TFT units are typically formed on a glass substrate. Since the glass substrate is generally not heat resistant, the process for producing TFTs on the LCD glass plate should be a low-temperature manufacturing process. In order to minimize the hot electron effect, a low-temperature polysilicon thin film transistor (LTPS-TFT) having LDD (lightly doped drain) structures was developed. In these LTPS-TFTs, a gate-drain overlapped LDD (GO-LD) structure was widely employed.
A process for producing such an N-type LTPS-TFT is illustrated with reference to FIGS.
1
(
a
) to
1
(
g
). In FIG.
1
(
a
), a silicon-oxide buffer layer
11
and an intrinsic amorphous silicon (i-a-Si) layer are sequentially formed on a glass substrate
10
. Then, the i-a-Si layer is converted to an intrinsic polysilcon (i-poly-Si) layer
12
by a laser annealing procedure. Then, by a micro-photolithography and etching procedure, the i-poly-Si layer
12
is partially etched to form a desired polysilicon structure
120
, as can be seen in FIG.
1
(
b
). In FIG.
1
(
c
), a photoresist layer is formed on the polysilicon structure
120
and properly patterned to be a mask
13
. Then, two N-type regions
121
and
122
are formed on a portion of the polysilicon structure
12
exposed from the mask
13
by an ion implantation procedure. The two N-type regions
121
and
122
serve as source/drain regions of an N-channel TFT. After the photoresist mask
13
is removed, a gate insulator
14
, for example made of silicon dioxide, is formed on the resulting structure of FIG.
1
(
c
), as shown in FIG.
1
(
d
). In FIG.
1
(
e
), a gate electrode
15
is then formed on the gate insulator
14
by sputtering and patterning a gate conductive layer on the resulting structure of FIG.
1
(
d
). Then, by a lightly ion implantation procedure with the gate electrode
15
serving as a mask to provide trace N-type dopants into the polysilicon structure
120
, two LDD (lightly doped drain) regions
123
and
124
are formed immediately adjacent to the drain/source regions
121
and
122
, respectively. In FIG.
1
(
f
), an interlayer dielectric layer
17
is formed on the resulting structure of FIG.
1
(
e
). Then, a proper number of contact holes directing to the gate electrode and source/drain regions are created. Afterwards, as shown in FIG.
1
(
g
), a conductive layer is sputtered on the resulting structure of FIG.
1
(
f
), fills the contact holes, and then patterned to form a gate conductive line
190
and a source/drain conductive line
191
.
The gate-drain overlapped LDD (GO-LD) structure results in a reduced electric field intensity in the vicinity of the drain region so as to slightly diminish the influence of the hot electron effect. However, with the increasing demand of high resolution of the display, the circuitry is more and more complicated than ever. In other words, the number of the electronic devices increases significantly so as to reduce the space of a single electronic device. Accordingly, the channels of transistors will become narrower and narrower. Furthermore, the LDD regions shorten the channel to an extent, and thus depletion regions in the vicinity of the source/drain regions will be relative close and even reachable to each other. Therefore, current leakage and punch-through problems may occur so as to deteriorate the electronic devices. The above-described effects will be even significant with the increasing development toward miniaturization.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a thin film transistor having diminished hot electron, current leakage and punch-through effects.
It is another object of the present invention to provide a process for producing a thin film transistor having a particular LDD structure to reduce hot electron, current leakage and punch-through effects.
In accordance an aspect of the present invention, there is provided a thin film transistor. The thin film transistor comprises a semiconductor layer, source/drain structures, a single LDD structure, a gate structure and an insulator layer. The semiconductor layer is formed of a semiconductor material such as polycrystalline silicon, and the semiconductor layer is disposed on a glass substrate. The source/drain structures are formed apart from each other in the semiconductor layer. The single LDD structure is disposed between the source/drain structures, and having a first side adjacent to a first one of the source/drain structures and a second side spaced from a second one of the source/drain structures by essentially the semiconductor material. The gate structure is formed over the semiconductor layer. The insulator layer is disposed between the semiconductor layer and the gate electrode for insulating the gate electrode from the source/drain structures and the LDD structure.
In an embodiment, the single LDD structure is a gate-drain overlapped LDD. The first one of the source/drain structures is the drain structure, and the second one of the source/drain structures is the source structure.
In an embodiment, the thin film transistor is of an N-type, and the LDD structure contains a doping material selected from a group consisting of P ions, As ions, PH
x
ions, AsH
x
ions and a combination thereof.
In accordance with another aspect of the present invention, there is provided a thin film transistor. The thin film transistor comprises a semiconductor layer, source/drain structures, a first LDD structure, a second LDD structure, a gate structure and an insulator layer. The semiconductor layer is formed of a semiconductor material. The source/drain structures is formed apart from each other in the semiconductor layer. The first LDD structure is disposed between the source/drain structures, and having a first side adjacent to a first one of the source/drain structures and a second side opposed to the first side. The second LDD structure having a third side adjacent to the second side of the first LDD structure, and a fourth side spaced from a second one of the source/drain structures by essentially the semiconductor material. The gate structure formed over the semiconductor layer. The insulator layer is disposed between the semiconductor layer and the gate electrode for insulating the gate electrode from the source/drain structures and the LDD structures.
In an embodiment, each of the first and second LDD structures is a gate-drain overlapped LDD. The thin film transistor is of an N-type, the first LDD structure contains a doping material selected from a group consisting of P ions, As ions, PH
x
ions, AsH
x
ions and a combination thereof, and the second LDD structure contains more than one doping material selected from a gro
Prenty Mark V.
Toppoly Optoelectronics Corp.
Volpe and Koenig P.C.
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